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Effect of coating adhesion and degradation on tin whisker mitigation of polyurethane-based conformal coatings
Polymer Degradation and Stability 166 (2019) 219e229
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Effect of coating adhesion and degradation on tin whisker mitigation of polyurethane-based conformal coatings Fei Dong a, Stephan J. Meschter b, Shinji Nozaki c, Takeshi Ohshima d, Takahiro Makino d, Junghyun Cho a, * a
Binghamton University (SUNY), Materials Science and Engineering Program, Binghamton, NY, 13902, USA BAE Systems, Electronics Systems, Endicott, NY, 13760, USA University of Electro-Communications, Department of Computer and Network Engineering, Chofu-shi, Tokyo, 182-8585, Japan d Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, Takasaki, Gunma, 370-1292, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2019 Accepted 17 May 2019 Available online 18 May 2019
Tin whisker growth, which may lead to short circuits and current leakage, is a serious issue in electronics components. Polyurethane-based conformal coatings have been seen as an effective candidate to mitigate the tin whisker growth. To better understand the effect of such coatings, this study exposed the tinplated copper coupons coated with i) moisture cured polyurethane (PU) and ii) dual UV/moisture cured polyurethane acrylate (PUA), for long-term (up to 5000 h) high-temperature and high-humidity (HTHH) storage (85 C and 85%RH, respectively). After HTHH storage, microstructural developments in the tin layer, coating degradation, tin surface reaction, and coating adhesion to tin surface were analyzed to compare the tin whisker mitigation behavior. In order to improve the coating adhesion, tin surface was also treated with a silane adhesion promoter before coating was applied. Furthermore, the effect of electron beam irradiation was investigated as it degraded the coating properties and adhesion. The results show that maintaining good adhesion and suppressing the degradation of mechanical properties of conformal coatings during various environmental exposures are essential for enhanced tin whisker mitigation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Conformal coatings Polyurethane Acrylate Tin whiskers Adhesion Silane Electron beam irradiation
1. Introduction Due to the restriction of lead and other toxic materials from Restriction of Hazardous Substances (RoHS) regulations, there is a significant increasing usage of pure tin or tin alloys in electronics packaging fields [1]. However, an increased number of uses of tin may lead to tin whisker formation that spontaneously grows from pure tin and alloyed tin surface with various appearances ranging from filamentary, hillocks, flowers, extrusions to odd-shaped eruptions [2e4]. The size of tin whisker can be up to hundreds of microns in length with only a few microns in width [5]. Tin whisker growth is a serious issue in the electronic packaging fields which can result in current leakage and short circuit and in turn damage the components [6]. This phenomenon would trace back to some time ago with intensive study [7,8], but the controlling
* Corresponding author. E-mail address: [email protected] (J. Cho). https://doi.org/10.1016/j.polymdegradstab.2019.05.019 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
mechanisms behind tin whisker growth are still not clearly resolved. There are many theories proposed for tin whisker growth falling into three major categories: dislocation theory mechanism [9,10], recrystallization mechanism [11,12] and compressive stress mechanism [13,14]. Franks [15] found that dislocations are pinned by lattice defects and the glide movement of dislocations may form the whisker. Ellis et al. [16] mentioned that recrystallization played an important role in tin whisker growth but his research was based on indirect evidence. Compressive stress mechanism is the most popular one to account for tin whisker growth and stress relief leading to whisker formation is the key point for this theory [17]. Mechanical induced stress is an easy way to accelerate tin whisker growth by adding a clamp to apply a stress to the tin layer [18]. Shin and Chason [19] proposed that the coefficient of thermal expansion (CTE) mismatch between tin coatings and Si substrate during heating is responsible for thin whisker growth. IMC growth is another source to induce the stress to tin system proven by Lee and Lee [20]. Besides that, compressive stress can also be built up
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through volume expansion due to the oxidation and push the whiskers to come out [21]. Many ways of mitigating the tin whisker growth have been tried through manufacturing improvement, including avoiding pure tin plating and dipping tin finished surface in solder, but the solutions do not offer a universal method to reduce the tin whisker. As another good candidate, conformal coating can effectively confine the tin whisker growth due to its comprehensive coverage protection over the tin surface [22]. Two commercially available conformal coatings (parylene and acrylics) are widely used in the industry. However, the expensive application process and low chemical resistance are the drawbacks for parylene and acrylics, respectively. Polyurethane-based conformal coatings have also been proposed for this application, due to its reworkability and better controllable mechanical properties [23,24]. Our previous work had demonstrated the effect of curing condition and electron beam irradiation on the mechanical properties of PU-based conformal coatings [25,26]. In this study, two mechanically contrasting coatings, ductile PU and rigid PUA, were selected for testing the whisker mitigation function during high-temperature high-humidity (HTHH) exposures. Under this environment, the coating showed the degradation that allowed easier penetration by the growing tin whiskers. Further, the adhesion between coating and tin surface played an important role in maintaining the coating's ability to mitigate tin whisker/nodule formation. Such performances were, however, undermined by high energy e-beam irradiation commonly found in space electronics. It will add significant concern to the long-term reliability due to its impact on the tin whisker mitigation capacity of conformal coatings. In particular, this study highlights achieving tin whisker mitigation via improved adhesion. 2. Experimental procedure 2.1. Substrates and coatings This study used a slotted copper coupon electroplated with bright tin that has shown to accelerate tin whisker growth compared to that with matte tin finish (Fig. 1a). The tin thickness of as-plated coupon is around 4e12 mm and coupon shape is in rectangular with 25.4 mm in length and 12.7 mm in width. Conformal coatings were made on one side of this coupon. The cross sectional
Fig. 1. (a) Tin plated copper coupon (b) cross section view of coupon.
view of the sample with thickness information is represented in Fig. 1b. Two types of polyurethane-based conformal coating, PU (PC18 M; Henkel Inc., Irvine, CA) and PUA (PC40UMF; Henkel Inc., Irvine, CA), were used in this study. PUA is harder and less ductile than PU [26,27]. PC18 M is a urethane prepolymer in 2methoxypropyl acetate solvent (along with small amounts of xylenes, ethylbenzene, toluene diisocyanate, toluene). PC40UMF is a mixture of 1,6-diisocyanatohexane homopolymer, 2-hydroxyethyl acrylate and isobornyl acrylate. The thickness of conformal coating was controlled to be 40e50 mm by a polyimide tape (Kapton™, 3 M Inc., St. Paul, Minnesota) walls through a doctor blade process. In order to strengthen the adhesion between conformal coating and Sn surface, (3-Aminopropyl)trimethoxysilane (APTMS) was primed on the tin surface that is also covalently bonded to the subsequent conformal coating. 2.2. High-temperature high-humidity (HTHH) coupons The sample list of various treatments on PU and PUA coupons is shown in Table 1. PU coating was prepared by moisture curing process (1 h solvent evaporation and moisture curing for 2 h @80 C, 60%RH) while PUA was by a UV/moisture dual curing process (UV20min (4800 mW/cm2, UV-A) and moisture 4 h @80 C, 60% RH). All the comparison analyses between PU and PUA coating are based upon the above curing conditions. After curing, all the samples were put into an HTHH chamber (Despatch® Ecosphere™) under 85 C/85%RH for tin whisker growth acceleration [28,29]. There were 4 time steps (0 h, 2500 h, 4000 h and 5000 h) involved in the HTHH treatment and samples were taken out for characterization at each time step. The sliane-treated Sn/Cu samples were also used to check the role of adhesion properties on the mitigation ability of conformal coatings for the tin whisker growth. In addition, the effect of electron beam irradiation on the tin whisker growth was investigated with the electron beam (e-beam) at 1 MeV (with 1 mA, 1 1016 cm2 fluence) in a nitrogen atmosphere that corresponds to a rate of irradiation of 1.65 1012 cm2s1. The detailed electron beam irradiation and the silane treatment procedure for adhesion improvement were discussed in our previous works [26,30]. 2.3. Characterization methods Optical microscopy (OM; Zeiss Axio Imager M1m) was used to check the corrosion condition of tin plated copper coupon after HTHH treatment with bright field mode. Scanning Electron Microscopy (Zeiss Supra 55 VP-SEM) was employed to characterize the coating top surface observation, such as coating coverage, degradation and whisker protrusions. The number of tin whiskers or dome events was counted within the same magnification (240) and three images of each case were taken from the area around the edge of the coupon. The error bars represent for the upper side standard deviation. The area of each image examined is around 0.8 mm2. Also, the cross sectional analysis of the sample was prepared by ultramicrotome (Leica™ EM UC7) through diamond low
Table 1 Sample list of tin whisker growth experiments. PU coating on Sn/Cu coupons
PUA coating on Sn/Cu coupons
PU on Silane-treated Sn/Cu coupons E-beam irradiated PU on Sn/Cu coupons PU on Silane-treated Sn/Cu coupons with E-beam irradiation No coating on Sn/Cu coupons (reference 1) No coating on E-beam irradiated Sn/Cu coupons (reference 2)
PUA on Silane-treated Sn/Cu coupons E-beam irradiated PUA on Sn/Cu coupons PUA on Silane-treated Sn/Cu coupons with E-beam irradiation
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Fig. 2. Coating coverage at various time steps up to 4000 h: (a) PU; (b) PUA.
speed knife (Diatome™ Histo) slicing with a precision edge, then putting the prepared sample into SEM for microstructure development evaluation of the underlying tin layer and the coating adhesion to the tin. EDX (EDAX™ Pegasus EDS) was used for identifying tin whiskers and the reaction between the coating and tin. 3. Results and discussion
shows the top view SEM image of conformal coatings around the slot hole after a series of HTHH treatments. At time 0, PU coating has a uniform coverage on the coupon while the PUA coating shows thinner or no coating area near the slot edge. And the tin-exposed area becomes larger with longer time HTHH treatments for PUA, while PU coating appears to maintain a good coverage even after long time treatment. It is clear to see that most of tin exposure with no PUA coating is near the edge of slot which becomes more susceptible to tin whisker growth.
3.1. Top view observation 3.1.1. Coating coverage HTHH treated samples were taken out of the chamber at various time steps and characterized in the areas near the slot cut where tin whisker growth was greater due to thinner/missing coating and potential stress gradient from the coating thickness change. Fig. 2
3.1.2. Tin surface oxidation Fig. 3 represents the tin surface corrosion status after 2500 h and 5000 h HTHH treatment. There is almost no corrosion in both PU and PUA at time 0 in the OM images. When the HTHH treatment went to 5000 h, a large amount of oxidation products grew over the entire tin surface of the coupon, and the degree of corrosion under
Fig. 3. Coated-tin surface corrosion developed under HTHH treatments at various time steps up to 5000 h: (a) PU with and without the silane treatment; (b) PUA with and without the silane treatment.
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Fig. 4. Coating surface degradation after HTHH treatment at various time steps: (a) PU; (b) PUA.
PUA seemed higher than that under PU. However, the corrosion is significantly reduced when the silane treatment is used that improves the adhesion between conformal coating and tin surface as has been reported in our detailed adhesion study [30]. Good adhesion means less gap or delamination between the coating and the tin surface which makes less moisture being trapped in that area resulting in less corrosion. If a thick oxide layer formed on the tin surface, it may bend the coupon and induce a stress into the tin layer. Less corrosion decreases the build-up internal stress from the growth of oxide products on the tin layer that should be beneficial for mitigating tin whisker growth. On the other hand, a thin native oxide on the tin surface may help to strengthen the adhesion between conformal coating and tin to protect the tin surface from corrosion [31].
3.1.3. Coating degradation The appearance of coating surface after long aging under HTHH exposure can be seen in Fig. 4. Although surface contamination may exist in PU or PUA case at time 0, the coating itself is quite uniform with little defects at the microscales. For PU case, a snowflake-like structure, which is likely to be a weak point for tin whisker penetration, appears to come out when the HTHH treatment was at 2500 h and this domain becomes larger with 5000 h treatment. Such a defect can be attributed to the formation of urea from the reaction of isocyanate with the presence of water as following equations, which makes PU brittle [32]. ReN¼C¼O þ H2O / ReNH2 þ CO2
(1)
ReNH2 þ R0 eN¼C¼O / ReNHeCOeNHeR’
(2)
For PUA case, the snowflake-like structure was not seen because of a smaller amount of isocyanate existed in the PUA system compared to PU coating. Instead, small spherical voids with around 200 nm diameter form within the coating at 2500 h and 5000 h exposures. Although the initial evaporation process can remove most of the very minor volatile organics included, a small amount of organic residuals may still remain in the PUA coating. The coating
system becomes rigid after UV curing which makes the volatile organics hard to evaporate during the following moisture curing step, but it can be evaporated out after a long time HTHH treatment during which a hole or void can be created from the expanded site of evaporated organics. The number of voids did not seem to show any further increase after 2500 h treatment. These void areas are the defect sites for the preferential penetration of the growing tin whiskers. 3.2. Cross sectional view observation 3.2.1. Adhesion between coating and tin The adhesion properties of PU and PUA coating on tin surface at 2500 h HTHH treatment are shown in the Fig. 5 cross sectional view. Poorer adhesion with isolated delamination was noted at the coating e tin interface in plain PU case while the silane-treated PU has a robust interface with tin layer which proves the improved adhesion (Fig. 5a). On the other hand, complete separation with a large gap shows up between PUA coating and tin layer which indicates very poor adhesion between these two layers, as shown in Fig. 5b. The silane treatment, however, dramatically increases the adhesion between these two surfaces with little delamination. PUA coating has shown much poorer adhesion on tin surface than the PU coating, which also tends to agree with the resulting decreased coating coverage of PUA, as compared to PU on the tin layer. 3.2.2. Microstructure development SneCu system can form IMC during high enough temperature aging to provide sufficient bond strength for solder application. The phase diagram of SneCu system is shown in Fig. 6. Two common IMCs, Cu6Sn5 and Cu3Sn, exist in the system with melting temperatures of 415 C and 676 C, respectively [33,34]. During the initial stage of HTHH treatment, Cu diffuses rapidly into Sn to form Cu6Sn5 with the reaction expressed in equation (3). After that, Cu3Sn can form with the reaction between Cu and Cu6Sn5 (equation (4)) through Cu atoms diffusion at Cu/Cu6Sn5 interface. Compared to Cu diffusion, Sn diffusion at Sn/Cu interface is quite slow so that it can be ignored [35].
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Fig. 5. Adhesion between conformal coating and tin surface: (a) PU without and with the silane treatment; (b) PUA without and with the silane treatment.
Fig. 6. Calculated CueSn phase diagram.
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Fig. 7. Example of tin nodule formation due to IMC growth.
6Cu þ 5Sn / Cu6Sn5
(3)
9Cu þ Cu6Sn5 / 5Cu3Sn
(4)
Fig. 7 shows a representative example of tin nodule formation due to IMC growth after 2500 h HTHH treatment. Interfacial IMC layers, Cu6Sn5 and Cu3Sn (darker contrast), continuously grow at the interface between Cu and plated Sn with some particle pull-out regions left in those layers which are probably due to the damage from microtome slicing. These interfacial IMC layers kept growing into the Sn layer, which resulted in a Sn layer with only 35% of its original thickness after the 5000 h HTHH treatment. Additionally, EDS results show that the concentration of Cu in the middle of a Sn layer is lower than the region toward the top of Sn layer. More Cu enrichment would enable bulk IMCs to form at the top area that interfaces with the conformal coating, thereby introducing internal stress potentially accelerating the tin whisker/nodule growth. An
obvious whisker-like structure grew out of the top portion of a tin layer. In addition, the particle pull-out regions at the tin protrusion area is an indirect evidence of the existence of bulk IMCs near the tin whisker/nodules. The particle pull-outs from the bottom to the top of the tin layer may indicate that Cu diffusion followed by the IMC formation is likely the origin of tin whisker/nodule growth stress. The large amount of IMC embedded in the tin layer can introduce more compressive stress and in turn result in tin whisker growth. A delamination that results in the gap between Sn layer and PU conformal coating was observed, which is due to the adhesion getting poorer between the plain PU and Sn surface after 2500 h HTHH treatment. For the samples such as plain PU/Sn and PUA/Sn, they usually display a tin layer thickening effect after long aging time under the HTHH condition, which is shown in Fig. 8. In the PU case with 2500 h treatment, an obvious tin thickening effect up to 22 mm has been observed compared to the initial thickness of 4e12 mm. The potential mechanism for this thickening behavior can be related to the diffusing Cu atoms and ‘embedded’ IMC formation. First, IMCs increases the volume of the areas where they form within a Sn layer, resulting in its thickening. It cannot, however, be responsible for the observed amount of thickening (which is beyond the volume increase due to the embedded IMCs). As a result, significant thickening can arise from the ‘secondary’ effect from such embedded IMCs that induce a strong compressive stress, which will subsequently cause the outward protrusive growth of tin. When the initial vertical growth of tin protrusion is resisted by the conformal coating, it could delaminate the coating from Sn surface and turn to grow laterally along the delaminated space between coating and tin surface. However, there would be little space for tin lateral growth at the interface in the silane-treated PU case since good adhesion was achieved; therefore, little tin thickening exists in this case. For the plain PUA and silane-treated PUA cases, the thickening effect is similar with their counterparts in PU coating. The schematics for the thickening behavior are explained in Fig. 9, where the tin nodule grows laterally into the delaminated space created between the coating and the Sn surface. The lateral growth of a tin nodule was also observed in our separate experiment [31].
Fig. 8. Tin layer thickening phenomenon through its lateral growth.
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Fig. 9. Tin lateral growth mechanism under a hard coating with poor adhesion.
3.3. Tin whisker growth mitigation assessment In this analysis, SEM was used to assess the tin whisker growth behavior with the presence of an overlying conformal coating from the top down view of the coating surface after the HTHH treatment. The tin whisker/nodule can either penetrate through the coating or push the coating upward to form the dome shape on the coating surface. The latter case still encloses the tin whisker growth under the coating without the coating rupture.
3.3.1. Effect of coating on tin whisker growth mitigation The representative images of tin whisker or dome formation are shown on PU and PUA coated sample surfaces, compared with no coating sample, after 5000 h HTHH treatment (Fig. 10a). It can be
clearly seen that some whiskers and their aggregates form on the bare tin surface of no coating sample but very few whiskers are found in PU case since the coating can effectively resist the whisker growth by enclosing it, thereby forming a dome. In PUA case, although the number of whiskers was much lower than no coating case, some whiskers punctured through the coating due to the brittleness and defect formation in PUA coating after the long aging under the HTHH treatment. The morphology of penetrated whiskers in PUA case is thread-like structure that has grown out of an aggregate since the individual whisker is easier for penetration through the coating. On the other hand, when there is no coating, it tends to show a large odd-shaped eruption with several whiskers/ nodules growing out of it. In order to quantify the whisker or dome formation at different
Fig. 10. Effect of coating on tin whisker mitigation: (a) Representative images of tin whisker growth and the dome shape after the 5000 h HTHH treatment; (b) Statistical analysis at different time steps.
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Fig. 11. Effect of the silane treatment to improve the coating adhesion on tin whisker mitigation: (a) Representative images of tin whisker growth and dome shape after the 5000 h HTHH treatment; (b) Statistical analysis at different time steps.
time steps, the statistical analysis was conducted as shown in Fig. 10b. Compared to no coating case, less whiskers can be seen in the coated sample which means conformal coatings have the ability to block the tin whisker growth. In addition, there are more coating domes present in PU case while more coating ruptures and whiskers/nodules formed in PUA case, which indicates PU coating provides better whisker mitigation than PUA coating. Neither coatings, however, prevented the nucleation and growth of tin whiskers/ nodules. The PU coating possesses higher ductility that enables plastic deformation and enough strength to prevent puncture by the growing whiskers/nodules and causes a coating dome to form. On the other hand, the nature of brittleness and defects generated during HTHH in the PUA coating make it more easily punctured by the tin growths. 3.3.2. Effect of silane treatment on tin whisker mitigation Silane treatment improves the adhesion between tin surface and conformal coating, and in turn influences the mitigation ability of conformal coating on tin whisker growth. Fig. 11a shows the representative images of the silane-treated PU and PUA coating after the 5000 h HTHH treatment. There is less whisker/dome formation in the PU and PUA coatings, both of which were applied to the silane-treated tin surface, as compared to these coatings on the untreated tin. In addition, the coating coverage at the edge of coupon slot in silane-treated PUA case is better than the untreated PUA case which can provide more uniform mitigation of tin whisker/nodule growth.
The statistical analysis of the number of whisker/dome formations at various time steps is shown in Fig. 11b. It is seen that the number of whiskers/domes increases with HTHH treatment time, but silane-treated samples have a stronger mitigation ability on tin whisker/nodule growth than the untreated PU and PUA cases at each time step. In general, PU coating exhibited very good protection until 4000 h while it was well adhered to the tin surface, after which time tin whiskers began to delaminate and deform the coating (dome shape) by 5000 h. On the other hand, PUA coating showed very good protection until 2500 h that began to show many dome formations by 4000 h because of more delaminated areas and many tin whisker penetrations by 5000 h. Three schematic diagrams are shown in Fig. 12 to indicate three levels of adhesion cases between conformal coating and tin surface from a cross sectional view. If good adhesion is achieved, it usually results in a small number of whisker/dome formations since there is no space for moisture and little IMC accumulation so that less stress is generated from oxidation and IMCs in the tin layer. This situation is exemplified by the PU coating on the silane-treated tin surface. The second situation is a partial delamination sample at the interface and two different examples of this occurred here. For the PU coating, a large number of domes were observed due to good ductility of the coating. Once the whisker nucleates at the delaminated area and grows towards to the coating, its growth will be stopped by the conformal coating with plastic deformation that results in the dome shape. However, the PUA coating on the silanetreated tin surface is more likely to form whiskers/nodules through
Fig. 12. Three different levels of interfacial adhesion between coating and tin that can lead to the different tin whisker growth behavior.
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Fig. 13. Effect of electron beam irradiation on tin whisker growth behavior: (a) Representative images of tin whisker/dome growth after the 2500 h HTHH treatment; (b) Statistical analysis of the number of domes or whiskers.
the coating instead of domes in the isolated delaminated areas, because the defects or small cracks that might be initially present or are generated in the PUA coating over time become more brittle during HTHH treatment. The last situation is the PUA coating on the untreated tin surface, which possesses an extensive gap or delamination at the interface between PUA and tin surface for trapped moisture and IMC growth. The poor adhesion gives the whiskers /nodules a greater chance to nucleate at the tin surface whose formation is accelerated by the compressive stress from oxidation and IMC formation. Once the growth of tin protrusion is resisted by the coating, it starts to delaminate the coating during its vertical growth, but when the delamination stops, the whisker may penetrate through the weak spots of brittle coating to form the highest number of whiskers among all the cases analyzed. All the examples in various adhesion states associated with these three schematic diagrams can be found in Fig. 5. According to the statistical analysis, PU and PUA coatings on the silane-treated tin have the good mitigation performance at 2500 h, but for the long term treatments after 4000 h, only the PU coating on the silane-treated tin showed the best mitigation ability for tin whisker/nodule growth due to the PUA adhesion begins to deteriorate. 3.3.3. Effect of electron beam irradiation on tin whisker growth The electron beam irradiated PU and PUA-coated coupons, along with no coating coupons, were examined after the 2500 h HTHH treatment. Fig. 13a shows the representative images of electron beam irradiated samples compared to the corresponding control samples with no irradiation, which were exposed to the 2500 h HTHH treatment. A higher number of domes and whiskers were detected in PU and PUA, respectively, compared to their no irradiation counterparts. In particular, tin whisker aggregates tended to be larger with an e-beam irradiation as shown in the e-beam irradiated control sample surface, even though the number of the whiskers shown was similar to that on the non-irradiated control sample. This
behavior appeared to be triggered by charged domains on the tin surface induced by electric fields due to an e-beam irradiation, which accelerate the nucleation and growth rates of the nearby whiskers [36]. Larger aggregation was also observed in the e-beam irradiated PUA, which could have been further accelerated by the coating degradation during the long exposure under the HTHH treatment. The statistical results, Fig. 13b, also prove this point. The number of whisker aggregates or domes in PU and PUA coating has dramatically increased on the electron beam irradiated samples compared to the non-irradiation ones. The reason behind that is due to degradation of chemical structure of the coating during e-beam irradiation which not only influences the mechanical properties but also adhesion of the coating, and in turn impacts its mitigation capacity of tin whisker growth. In fact, the peeled-off percentages of the irradiated PU and PUA coatings in cross-cut test at time zero are 40.0% and 82.5%, respectively, which are much higher than the non-irradiated PU (12.0%) and PUA (57.0%) coatings. Hence the decreased adhesion of conformal coating on tin surface during the e-beam irradiation triggers more occasions for tin whisker nucleation, thereby resulting in a large number of domes or whiskers, depending upon the coating properties. It is also noted that the aggregated whisker cluster is treated as a single occurrence in the statistical analysis which might have led to an underestimation of the actual number of whiskers. 3.3.4. Effect of silane treatment on electron beam irradiated coupons In order to enhance the tin whisker mitigation ability of the conformal coating after electron beam irradiation, the silane treatment was applied on the tin surface because the adhesion is a key factor for tin whisker/nodule nucleation. Fig. 14a indicates the representative images of the silane-treated samples that were exposed to e-beam irradiation, compared to non-irradiated control samples after the 2500 h HTHH treatment. It is observed that the
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Fig. 14. Effect of electron beam irradiation (1 1016 cm2) and silane treatment on tin whisker growth behavior after 2500 h HTHH treatment: (a) Representative images of tin whisker/dome growth; (b) Statistical analysis of the number of domes or whiskers, along with peeled-off percent at time 0.
occurrence of tin whisker/dome formation is much reduced on the surface of both PU and PUA coatings made on the silane-treated tin surface. More quantitative data are shown in Fig. 14b, where the number of whiskers/domes in e-beam irradiated samples is much higher when the silane treatment is not used on the tin coupon surface. It indicates the silane treatment can significantly enhance the tin whisker mitigation ability of conformal coating under electron beam irradiation conditions. Cross-cut test results show that the peeled-off percentages of silane-treated PU and PUA after irradiation are only 8.9% and 10.2% at time zero, respectively. The improvement of adhesion makes the tin whisker nucleation more difficult, resulting in less whiskers/ domes on the coating. In addition, there are more whiskers/nodules in the silane-treated irradiation samples than the silane-treated non-irradiated counterparts due to the degradation of coating after electron beam irradiation which makes the coating structure become more brittle so that the coating is easier to penetrate with the growing whiskers/nodules. If PU and PUA coatings were irradiated under e-beam, both of them would form more whiskers/domes during the HTHH aging than non-irradiated counterparts. In this case, the silane treatment would provide an effective means of mitigating the tin whisker/ dome formation by maintaining the adhesion of conformal coating to Sn surface. Even PUA coatings show very little whisker penetration with the silane treatment on the tin surface, so are now comparable to the PU coatings within the data scattering in terms of the whisker/dome formation.
4. Conclusion Effects of conformal coatings were examined to investigate their mitigation ability to reduce tin whiskers growth on tin plated copper coupons after HTHH treatments (85 C/85%RH) ranging from 0 to 5000 h. Both PU and PUA coatings displayed degradation during HTHH treatment and increased underlying tin surface oxidation, but the silane-treated sample significantly reduced the tin oxidation since the good adhesion was made between coating and tin surface which reduced moisture trapping. In addition, PU has a better coating coverage than does PUA on the tin surface especially at the edge of the slot area. The coverage of PUA was, however, improved by the silane treatment on the tin surface that further increased the tin whisker mitigation ability of PUA. Cross sectional analysis showed that the silane treatment improves the adhesion between conformal coating and tin surface. PU has a better adhesion than PUA that displays an extensive delamination after HTHH treatment. The adhesion of both coatings is significantly improved after the silane treatment. Bulk IMC growth due to copper diffusion was found near the top portion of a tin layer resulting in a compressive stress which led to tin protrusion. When the adhesion is poor, the conformal coating delamination from Sn surface creates a space for lateral growth of a tin mass/whisker, leading to a tin layer thickening since the protrusive growth of tin is impinged by the conformal coating. The tin whisker growth mitigation assessment indicates that conformal coating clearly slows down tin whisker growth but that
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it does not prevent the nucleation of tin whiskers and nodules. PU is a better candidate than PUA in terms of tin whisker mitigation since the former has more ductility with enough strength, less brittleness, and higher adhesion to tin surface. In this case, tin whiskers can be confined under the PU coating while more whiskers come out of the PUA coating due to its increased brittleness and defects. In an effort to improve the coating adhesion, a silane adhesion promoter was used between tin surface and conformal coating, which produced less whiskers and domes than did non-silanetreated coupons. In particular, electron beam irradiation (1 1016 cm2) has shown significant influence on the mitigation ability of conformal coatings due to the degradation of coating adhesion and chemical structure after the irradiation. More aggregated whisker clusters were found in the non-coated case after electron beam irradiation. However, the mitigation ability of e-beam irradiated conformal coatings on tin whisker growth was significantly improved by the same silane treatment (up to 2500 h) since the coating continued to maintain good adhesion even after e-beam irradiation, which leads to less whisker nucleation and growth. Acknowledgments This work was sponsored by the Strategic Environmental Research and Development Program (SERDP, Contract #: W912HQ10-C-0052) of the U.S. Department of Defense and the New York State Energy Research and Development Authority (NYSERDA, Grant #: 58256). PU (PC18M) and PUA (PC40UMF) resins were provided from Henkel Electronics Materials, Irvine, CA, USA (David Edwards). References [1] A. Ezroj, How the European Union's WEEE & RoHS directives can help the United States develop a successful national E-waste strategy, VA. Envtl. LJ. 28 (2010) 45e72. [2] W.J. Boettinger, C.E. Johnson, L.A. Bendersky, K.W. Moon, M.E. Williams, G.R. Stafford, Whisker and hillock formation on Sn, SneCu and SnePb electrodeposits, Acta Mater. 53 (19) (2005 Nov 1) 5033e5050. [3] J.W. Osenbach, Tin whiskers: an illustrated guide to growth mechanisms and morphologies, JOM 63 (10) (2011 Oct 1) 57e60. [4] G.T. Galyon, Annotated tin whisker bibliography and anthology, IEEE Trans. Electron. Packag. Manuf. 28 (1) (2005 Jan) 94e122. [5] P. Zhang, Y. Zhang, Z. Sun, Spontaneous growth of metal whiskers on surfaces of solids: a review, J. Mater. Sci. Technol. 31 (7) (2015 Jul 1) 675e698. [6] U. Lindborg, A model for the spontaneous growth of zinc, cadmium and tin whiskers, Acta Metall. 24 (2) (1976 Feb 1) 181e186. [7] Z. Sun, H. Hashimoto, M.W. Barsoum, On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature, Acta Mater. 55 (10) (2007 Jun 1) 3387e3396. [8] B.D. Dunn, Whisker formation on electronic materials, Circuit World 2 (4) (1976 Mar 1) 32e40. [9] M.O. Peach, Mechanism of growth of whiskers on cadmium, J. Appl. Phys. 23 (12) (1952 Dec) 1401e1403. [10] F.C.X.C. Frank, On tin whiskers, Lond. Edinb. Dublin Phil. Mag. J. Sci. 44 (355) (1953 Aug 1) 854e860. [11] V.K. Glazunova, An investigation of the conditions of spontaneous growth of filiform crystals on electrolytic coatings, Zh. Prikl. Khim. 36 (3) (1963) 543e550.
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[12] T. Kakeshita, K.I. Shimizu, R. Kawanaka, T. Hasegawa, Grain size effect of electro-plated tin coatings on whisker growth, J. Mater. Sci. 17 (9) (1982 Sep 1) 2560e2566. [13] E. Buchovecky, N. Jadhav, A.F. Bower, E. Chason, Finite element modeling of stress evolution in Sn films due to growth of the Cu6Sn5 intermetallic compound, J. Electron. Mater. 38 (12) (2009 Dec 1) 2676. [14] E.J. Buchovecky, N. Du, A.F. Bower, A model of Sn whisker growth by coupled plastic flow and grain boundary diffusion, Appl. Phys. Lett. 94 (19) (2009 May 11), 191904. [15] J. Franks, Growth of whiskers in the solid phase, Acta Metall. 6 (2) (1958 Feb 1) 103e109. [16] W.C. Ellis, D.F. Gibbons, R.C. Treuting, Growth of metal whiskers from the solid, Growth Perfect. Cryst. (1958 Aug 27) 102e120. [17] G.T. Sheng, C.F. Hu, W.J. Choi, K.N. Tu, Y.Y. Bong, L. Nguyen, Tin whiskers studied by focused ion beam imaging and transmission electron microscopy, J. Appl. Phys. 92 (1) (2002 Jul 1) 64e69. [18] R.M. Fisher, L.S. Darken, K.G. Carroll, Accelerated growth of tin whiskers, Acta Metall. 2 (3) (1954 May 1) 368e373. [19] J.W. Shin, E. Chason, Stress behavior of electroplated Sn films during thermal cycling, J. Mater. Res. 24 (4) (2009 Apr) 1522e1528. [20] B.Z. Lee, D.N. Lee, Spontaneous growth mechanism of tin whiskers, Acta Mater. 46 (10) (1998 Jun 12) 3701e3714. [21] Z.M. Sun, S. Gupta, H. Ye, M.W. Barsoum, Spontaneous growth of freestanding Ga nanoribbons from Cr2GaC surfaces, J. Mater. Res. 20 (10) (2005 Oct) 2618e2621. [22] K. Mahan, Y. Sun, B. Han, S. Han, M. Osterman, Adhesion and puncture strength of polyurethane coating used to mitigate tin whisker growth, J. Electron. Packag. 136 (3) (2014 Sep 1), 031004. [23] M.F. Lucey, UV curable coatings for electronic components, IEEE Trans. Compon. Packag. Manuf. Technol. A 17 (3) (1994 Sep) 326e333. [24] W. Fox, L. Woody, Conformal coatings for tin whisker risk management, in: IPC APEX EXPO Technical Conference, Las Vegas, NV, April, 2010 Apr, pp. 6e9. [25] F. Dong, S. Maganty, S.J. Meschter, J. Cho, Effects of curing conditions on structural evolution and mechanical properties of UV-curable polyurethane acrylate coatings, Prog. Org. Coating 114 (2018 Jan 31) 58e67. [26] F. Dong, S. Maganty, S.J. Meschter, S. Nozaki, T. Ohshima, T. Makino, J. Cho, Electron beam irradiation effect on the mechanical properties of nanosilicafilled polyurethane films, Polym. Degrad. Stabil. 141 (2017 Jul 31) 45e53. [27] S. Maganty, M.P.C. Roma, S.J. Meschter, D. Starkey, M. Gomez, D.G. Edwards, A. Ekin, K. Elsken, J. Cho, Enhanced Mechanical Properties of Polyurethane Composite Coatings through Nanosilica Addition, Prog. Org. Coating 90 (2016 Jan 1) 243e251. [28] S. Meschter, P. Snugovsky, Z. Bagheri, E. Kosiba, M. Romansky, J. Kennedy, L. Snugovsky, D. Perovic, Whisker formation on SAC305 soldered assemblies, JOM 66 (11) (2014 Nov 1) 2320e2333. [29] A. Baated, K.S. Kim, K. Suganuma, S. Huang, B. Jurcik, S. Nozawa, M. Ueshima, Effects of reflow atmosphere and flux on Sn whisker growth of SneAgeCu solders, J. Mater. Sci.: Mater. Electron. 21 (10) (2010 Oct 1) 1066e1075. [30] F. Dong, S.J. Meschter, J. Cho, Improved adhesion of polyurethane-based coatings to tin surface, J. Mater. Sci: Mater. Electron. 30 (8) (2019 Apr 1) 7268e7279. [31] S. Maganty, Enhanced Conformal Coatings for Tin Whisker Mitigation, State University of New York at Binghamton, 2017. Doctoral dissertation. [32] J.W. Rosthauser, K.W. Haider, C. Steinlein, C.D. Eisenbach, Mechanical and dynamic mechanical properties of polyurethane and polyurethane/polyurea elastomers based on 4, 40 -diisocyanatodicyclohexyl methane, J. Appl. Polym. Sci. 64 (5) (1997 May 2) 957e970. [33] M. Onishi, H. Fujibuchi, Reaction-diffusion in the CueSn system, Trans. Jpn. Inst. Met. 16 (9) (1975) 539e547. [34] T.T. Luu, A. Duan, K.E. Aasmundtveit, N. Hoivik, Optimized Cu-Sn wafer-level bonding using intermetallic phase characterization, J. Electron. Mater. 42 (12) (2013 Dec 1) 3582e3592. [35] C. Han, Q. Liu, D.G. Ivey, Interfacial reactions between electrodeposited Sn-Cu, Sn-Ag-Cu solders and Cu, Ni substrates, J. Microelectron. Electron. Packag. 7 (1) (2010 Jan) 48e57. [36] V.G. Karpov, Electrostatic theory of metal whiskers, Phys. Rev. Appl. 1 (4) (2014 May 15), 044001.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Effect of coating adhesion and degradation on tin whisker mitigation of polyurethane-based conformal coatings Fei Dong a, Stephan J. Meschter b, Shinji Nozaki c, Takeshi Ohshima d, Takahiro Makino d, Junghyun Cho a, * a
Binghamton University (SUNY), Materials Science and Engineering Program, Binghamton, NY, 13902, USA BAE Systems, Electronics Systems, Endicott, NY, 13760, USA University of Electro-Communications, Department of Computer and Network Engineering, Chofu-shi, Tokyo, 182-8585, Japan d Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, Takasaki, Gunma, 370-1292, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2019 Accepted 17 May 2019 Available online 18 May 2019
Tin whisker growth, which may lead to short circuits and current leakage, is a serious issue in electronics components. Polyurethane-based conformal coatings have been seen as an effective candidate to mitigate the tin whisker growth. To better understand the effect of such coatings, this study exposed the tinplated copper coupons coated with i) moisture cured polyurethane (PU) and ii) dual UV/moisture cured polyurethane acrylate (PUA), for long-term (up to 5000 h) high-temperature and high-humidity (HTHH) storage (85 C and 85%RH, respectively). After HTHH storage, microstructural developments in the tin layer, coating degradation, tin surface reaction, and coating adhesion to tin surface were analyzed to compare the tin whisker mitigation behavior. In order to improve the coating adhesion, tin surface was also treated with a silane adhesion promoter before coating was applied. Furthermore, the effect of electron beam irradiation was investigated as it degraded the coating properties and adhesion. The results show that maintaining good adhesion and suppressing the degradation of mechanical properties of conformal coatings during various environmental exposures are essential for enhanced tin whisker mitigation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Conformal coatings Polyurethane Acrylate Tin whiskers Adhesion Silane Electron beam irradiation
1. Introduction Due to the restriction of lead and other toxic materials from Restriction of Hazardous Substances (RoHS) regulations, there is a significant increasing usage of pure tin or tin alloys in electronics packaging fields [1]. However, an increased number of uses of tin may lead to tin whisker formation that spontaneously grows from pure tin and alloyed tin surface with various appearances ranging from filamentary, hillocks, flowers, extrusions to odd-shaped eruptions [2e4]. The size of tin whisker can be up to hundreds of microns in length with only a few microns in width [5]. Tin whisker growth is a serious issue in the electronic packaging fields which can result in current leakage and short circuit and in turn damage the components [6]. This phenomenon would trace back to some time ago with intensive study [7,8], but the controlling
* Corresponding author. E-mail address: [email protected] (J. Cho). https://doi.org/10.1016/j.polymdegradstab.2019.05.019 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
mechanisms behind tin whisker growth are still not clearly resolved. There are many theories proposed for tin whisker growth falling into three major categories: dislocation theory mechanism [9,10], recrystallization mechanism [11,12] and compressive stress mechanism [13,14]. Franks [15] found that dislocations are pinned by lattice defects and the glide movement of dislocations may form the whisker. Ellis et al. [16] mentioned that recrystallization played an important role in tin whisker growth but his research was based on indirect evidence. Compressive stress mechanism is the most popular one to account for tin whisker growth and stress relief leading to whisker formation is the key point for this theory [17]. Mechanical induced stress is an easy way to accelerate tin whisker growth by adding a clamp to apply a stress to the tin layer [18]. Shin and Chason [19] proposed that the coefficient of thermal expansion (CTE) mismatch between tin coatings and Si substrate during heating is responsible for thin whisker growth. IMC growth is another source to induce the stress to tin system proven by Lee and Lee [20]. Besides that, compressive stress can also be built up
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through volume expansion due to the oxidation and push the whiskers to come out [21]. Many ways of mitigating the tin whisker growth have been tried through manufacturing improvement, including avoiding pure tin plating and dipping tin finished surface in solder, but the solutions do not offer a universal method to reduce the tin whisker. As another good candidate, conformal coating can effectively confine the tin whisker growth due to its comprehensive coverage protection over the tin surface [22]. Two commercially available conformal coatings (parylene and acrylics) are widely used in the industry. However, the expensive application process and low chemical resistance are the drawbacks for parylene and acrylics, respectively. Polyurethane-based conformal coatings have also been proposed for this application, due to its reworkability and better controllable mechanical properties [23,24]. Our previous work had demonstrated the effect of curing condition and electron beam irradiation on the mechanical properties of PU-based conformal coatings [25,26]. In this study, two mechanically contrasting coatings, ductile PU and rigid PUA, were selected for testing the whisker mitigation function during high-temperature high-humidity (HTHH) exposures. Under this environment, the coating showed the degradation that allowed easier penetration by the growing tin whiskers. Further, the adhesion between coating and tin surface played an important role in maintaining the coating's ability to mitigate tin whisker/nodule formation. Such performances were, however, undermined by high energy e-beam irradiation commonly found in space electronics. It will add significant concern to the long-term reliability due to its impact on the tin whisker mitigation capacity of conformal coatings. In particular, this study highlights achieving tin whisker mitigation via improved adhesion. 2. Experimental procedure 2.1. Substrates and coatings This study used a slotted copper coupon electroplated with bright tin that has shown to accelerate tin whisker growth compared to that with matte tin finish (Fig. 1a). The tin thickness of as-plated coupon is around 4e12 mm and coupon shape is in rectangular with 25.4 mm in length and 12.7 mm in width. Conformal coatings were made on one side of this coupon. The cross sectional
Fig. 1. (a) Tin plated copper coupon (b) cross section view of coupon.
view of the sample with thickness information is represented in Fig. 1b. Two types of polyurethane-based conformal coating, PU (PC18 M; Henkel Inc., Irvine, CA) and PUA (PC40UMF; Henkel Inc., Irvine, CA), were used in this study. PUA is harder and less ductile than PU [26,27]. PC18 M is a urethane prepolymer in 2methoxypropyl acetate solvent (along with small amounts of xylenes, ethylbenzene, toluene diisocyanate, toluene). PC40UMF is a mixture of 1,6-diisocyanatohexane homopolymer, 2-hydroxyethyl acrylate and isobornyl acrylate. The thickness of conformal coating was controlled to be 40e50 mm by a polyimide tape (Kapton™, 3 M Inc., St. Paul, Minnesota) walls through a doctor blade process. In order to strengthen the adhesion between conformal coating and Sn surface, (3-Aminopropyl)trimethoxysilane (APTMS) was primed on the tin surface that is also covalently bonded to the subsequent conformal coating. 2.2. High-temperature high-humidity (HTHH) coupons The sample list of various treatments on PU and PUA coupons is shown in Table 1. PU coating was prepared by moisture curing process (1 h solvent evaporation and moisture curing for 2 h @80 C, 60%RH) while PUA was by a UV/moisture dual curing process (UV20min (4800 mW/cm2, UV-A) and moisture 4 h @80 C, 60% RH). All the comparison analyses between PU and PUA coating are based upon the above curing conditions. After curing, all the samples were put into an HTHH chamber (Despatch® Ecosphere™) under 85 C/85%RH for tin whisker growth acceleration [28,29]. There were 4 time steps (0 h, 2500 h, 4000 h and 5000 h) involved in the HTHH treatment and samples were taken out for characterization at each time step. The sliane-treated Sn/Cu samples were also used to check the role of adhesion properties on the mitigation ability of conformal coatings for the tin whisker growth. In addition, the effect of electron beam irradiation on the tin whisker growth was investigated with the electron beam (e-beam) at 1 MeV (with 1 mA, 1 1016 cm2 fluence) in a nitrogen atmosphere that corresponds to a rate of irradiation of 1.65 1012 cm2s1. The detailed electron beam irradiation and the silane treatment procedure for adhesion improvement were discussed in our previous works [26,30]. 2.3. Characterization methods Optical microscopy (OM; Zeiss Axio Imager M1m) was used to check the corrosion condition of tin plated copper coupon after HTHH treatment with bright field mode. Scanning Electron Microscopy (Zeiss Supra 55 VP-SEM) was employed to characterize the coating top surface observation, such as coating coverage, degradation and whisker protrusions. The number of tin whiskers or dome events was counted within the same magnification (240) and three images of each case were taken from the area around the edge of the coupon. The error bars represent for the upper side standard deviation. The area of each image examined is around 0.8 mm2. Also, the cross sectional analysis of the sample was prepared by ultramicrotome (Leica™ EM UC7) through diamond low
Table 1 Sample list of tin whisker growth experiments. PU coating on Sn/Cu coupons
PUA coating on Sn/Cu coupons
PU on Silane-treated Sn/Cu coupons E-beam irradiated PU on Sn/Cu coupons PU on Silane-treated Sn/Cu coupons with E-beam irradiation No coating on Sn/Cu coupons (reference 1) No coating on E-beam irradiated Sn/Cu coupons (reference 2)
PUA on Silane-treated Sn/Cu coupons E-beam irradiated PUA on Sn/Cu coupons PUA on Silane-treated Sn/Cu coupons with E-beam irradiation
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Fig. 2. Coating coverage at various time steps up to 4000 h: (a) PU; (b) PUA.
speed knife (Diatome™ Histo) slicing with a precision edge, then putting the prepared sample into SEM for microstructure development evaluation of the underlying tin layer and the coating adhesion to the tin. EDX (EDAX™ Pegasus EDS) was used for identifying tin whiskers and the reaction between the coating and tin. 3. Results and discussion
shows the top view SEM image of conformal coatings around the slot hole after a series of HTHH treatments. At time 0, PU coating has a uniform coverage on the coupon while the PUA coating shows thinner or no coating area near the slot edge. And the tin-exposed area becomes larger with longer time HTHH treatments for PUA, while PU coating appears to maintain a good coverage even after long time treatment. It is clear to see that most of tin exposure with no PUA coating is near the edge of slot which becomes more susceptible to tin whisker growth.
3.1. Top view observation 3.1.1. Coating coverage HTHH treated samples were taken out of the chamber at various time steps and characterized in the areas near the slot cut where tin whisker growth was greater due to thinner/missing coating and potential stress gradient from the coating thickness change. Fig. 2
3.1.2. Tin surface oxidation Fig. 3 represents the tin surface corrosion status after 2500 h and 5000 h HTHH treatment. There is almost no corrosion in both PU and PUA at time 0 in the OM images. When the HTHH treatment went to 5000 h, a large amount of oxidation products grew over the entire tin surface of the coupon, and the degree of corrosion under
Fig. 3. Coated-tin surface corrosion developed under HTHH treatments at various time steps up to 5000 h: (a) PU with and without the silane treatment; (b) PUA with and without the silane treatment.
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Fig. 4. Coating surface degradation after HTHH treatment at various time steps: (a) PU; (b) PUA.
PUA seemed higher than that under PU. However, the corrosion is significantly reduced when the silane treatment is used that improves the adhesion between conformal coating and tin surface as has been reported in our detailed adhesion study [30]. Good adhesion means less gap or delamination between the coating and the tin surface which makes less moisture being trapped in that area resulting in less corrosion. If a thick oxide layer formed on the tin surface, it may bend the coupon and induce a stress into the tin layer. Less corrosion decreases the build-up internal stress from the growth of oxide products on the tin layer that should be beneficial for mitigating tin whisker growth. On the other hand, a thin native oxide on the tin surface may help to strengthen the adhesion between conformal coating and tin to protect the tin surface from corrosion [31].
3.1.3. Coating degradation The appearance of coating surface after long aging under HTHH exposure can be seen in Fig. 4. Although surface contamination may exist in PU or PUA case at time 0, the coating itself is quite uniform with little defects at the microscales. For PU case, a snowflake-like structure, which is likely to be a weak point for tin whisker penetration, appears to come out when the HTHH treatment was at 2500 h and this domain becomes larger with 5000 h treatment. Such a defect can be attributed to the formation of urea from the reaction of isocyanate with the presence of water as following equations, which makes PU brittle [32]. ReN¼C¼O þ H2O / ReNH2 þ CO2
(1)
ReNH2 þ R0 eN¼C¼O / ReNHeCOeNHeR’
(2)
For PUA case, the snowflake-like structure was not seen because of a smaller amount of isocyanate existed in the PUA system compared to PU coating. Instead, small spherical voids with around 200 nm diameter form within the coating at 2500 h and 5000 h exposures. Although the initial evaporation process can remove most of the very minor volatile organics included, a small amount of organic residuals may still remain in the PUA coating. The coating
system becomes rigid after UV curing which makes the volatile organics hard to evaporate during the following moisture curing step, but it can be evaporated out after a long time HTHH treatment during which a hole or void can be created from the expanded site of evaporated organics. The number of voids did not seem to show any further increase after 2500 h treatment. These void areas are the defect sites for the preferential penetration of the growing tin whiskers. 3.2. Cross sectional view observation 3.2.1. Adhesion between coating and tin The adhesion properties of PU and PUA coating on tin surface at 2500 h HTHH treatment are shown in the Fig. 5 cross sectional view. Poorer adhesion with isolated delamination was noted at the coating e tin interface in plain PU case while the silane-treated PU has a robust interface with tin layer which proves the improved adhesion (Fig. 5a). On the other hand, complete separation with a large gap shows up between PUA coating and tin layer which indicates very poor adhesion between these two layers, as shown in Fig. 5b. The silane treatment, however, dramatically increases the adhesion between these two surfaces with little delamination. PUA coating has shown much poorer adhesion on tin surface than the PU coating, which also tends to agree with the resulting decreased coating coverage of PUA, as compared to PU on the tin layer. 3.2.2. Microstructure development SneCu system can form IMC during high enough temperature aging to provide sufficient bond strength for solder application. The phase diagram of SneCu system is shown in Fig. 6. Two common IMCs, Cu6Sn5 and Cu3Sn, exist in the system with melting temperatures of 415 C and 676 C, respectively [33,34]. During the initial stage of HTHH treatment, Cu diffuses rapidly into Sn to form Cu6Sn5 with the reaction expressed in equation (3). After that, Cu3Sn can form with the reaction between Cu and Cu6Sn5 (equation (4)) through Cu atoms diffusion at Cu/Cu6Sn5 interface. Compared to Cu diffusion, Sn diffusion at Sn/Cu interface is quite slow so that it can be ignored [35].
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Fig. 5. Adhesion between conformal coating and tin surface: (a) PU without and with the silane treatment; (b) PUA without and with the silane treatment.
Fig. 6. Calculated CueSn phase diagram.
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Fig. 7. Example of tin nodule formation due to IMC growth.
6Cu þ 5Sn / Cu6Sn5
(3)
9Cu þ Cu6Sn5 / 5Cu3Sn
(4)
Fig. 7 shows a representative example of tin nodule formation due to IMC growth after 2500 h HTHH treatment. Interfacial IMC layers, Cu6Sn5 and Cu3Sn (darker contrast), continuously grow at the interface between Cu and plated Sn with some particle pull-out regions left in those layers which are probably due to the damage from microtome slicing. These interfacial IMC layers kept growing into the Sn layer, which resulted in a Sn layer with only 35% of its original thickness after the 5000 h HTHH treatment. Additionally, EDS results show that the concentration of Cu in the middle of a Sn layer is lower than the region toward the top of Sn layer. More Cu enrichment would enable bulk IMCs to form at the top area that interfaces with the conformal coating, thereby introducing internal stress potentially accelerating the tin whisker/nodule growth. An
obvious whisker-like structure grew out of the top portion of a tin layer. In addition, the particle pull-out regions at the tin protrusion area is an indirect evidence of the existence of bulk IMCs near the tin whisker/nodules. The particle pull-outs from the bottom to the top of the tin layer may indicate that Cu diffusion followed by the IMC formation is likely the origin of tin whisker/nodule growth stress. The large amount of IMC embedded in the tin layer can introduce more compressive stress and in turn result in tin whisker growth. A delamination that results in the gap between Sn layer and PU conformal coating was observed, which is due to the adhesion getting poorer between the plain PU and Sn surface after 2500 h HTHH treatment. For the samples such as plain PU/Sn and PUA/Sn, they usually display a tin layer thickening effect after long aging time under the HTHH condition, which is shown in Fig. 8. In the PU case with 2500 h treatment, an obvious tin thickening effect up to 22 mm has been observed compared to the initial thickness of 4e12 mm. The potential mechanism for this thickening behavior can be related to the diffusing Cu atoms and ‘embedded’ IMC formation. First, IMCs increases the volume of the areas where they form within a Sn layer, resulting in its thickening. It cannot, however, be responsible for the observed amount of thickening (which is beyond the volume increase due to the embedded IMCs). As a result, significant thickening can arise from the ‘secondary’ effect from such embedded IMCs that induce a strong compressive stress, which will subsequently cause the outward protrusive growth of tin. When the initial vertical growth of tin protrusion is resisted by the conformal coating, it could delaminate the coating from Sn surface and turn to grow laterally along the delaminated space between coating and tin surface. However, there would be little space for tin lateral growth at the interface in the silane-treated PU case since good adhesion was achieved; therefore, little tin thickening exists in this case. For the plain PUA and silane-treated PUA cases, the thickening effect is similar with their counterparts in PU coating. The schematics for the thickening behavior are explained in Fig. 9, where the tin nodule grows laterally into the delaminated space created between the coating and the Sn surface. The lateral growth of a tin nodule was also observed in our separate experiment [31].
Fig. 8. Tin layer thickening phenomenon through its lateral growth.
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Fig. 9. Tin lateral growth mechanism under a hard coating with poor adhesion.
3.3. Tin whisker growth mitigation assessment In this analysis, SEM was used to assess the tin whisker growth behavior with the presence of an overlying conformal coating from the top down view of the coating surface after the HTHH treatment. The tin whisker/nodule can either penetrate through the coating or push the coating upward to form the dome shape on the coating surface. The latter case still encloses the tin whisker growth under the coating without the coating rupture.
3.3.1. Effect of coating on tin whisker growth mitigation The representative images of tin whisker or dome formation are shown on PU and PUA coated sample surfaces, compared with no coating sample, after 5000 h HTHH treatment (Fig. 10a). It can be
clearly seen that some whiskers and their aggregates form on the bare tin surface of no coating sample but very few whiskers are found in PU case since the coating can effectively resist the whisker growth by enclosing it, thereby forming a dome. In PUA case, although the number of whiskers was much lower than no coating case, some whiskers punctured through the coating due to the brittleness and defect formation in PUA coating after the long aging under the HTHH treatment. The morphology of penetrated whiskers in PUA case is thread-like structure that has grown out of an aggregate since the individual whisker is easier for penetration through the coating. On the other hand, when there is no coating, it tends to show a large odd-shaped eruption with several whiskers/ nodules growing out of it. In order to quantify the whisker or dome formation at different
Fig. 10. Effect of coating on tin whisker mitigation: (a) Representative images of tin whisker growth and the dome shape after the 5000 h HTHH treatment; (b) Statistical analysis at different time steps.
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Fig. 11. Effect of the silane treatment to improve the coating adhesion on tin whisker mitigation: (a) Representative images of tin whisker growth and dome shape after the 5000 h HTHH treatment; (b) Statistical analysis at different time steps.
time steps, the statistical analysis was conducted as shown in Fig. 10b. Compared to no coating case, less whiskers can be seen in the coated sample which means conformal coatings have the ability to block the tin whisker growth. In addition, there are more coating domes present in PU case while more coating ruptures and whiskers/nodules formed in PUA case, which indicates PU coating provides better whisker mitigation than PUA coating. Neither coatings, however, prevented the nucleation and growth of tin whiskers/ nodules. The PU coating possesses higher ductility that enables plastic deformation and enough strength to prevent puncture by the growing whiskers/nodules and causes a coating dome to form. On the other hand, the nature of brittleness and defects generated during HTHH in the PUA coating make it more easily punctured by the tin growths. 3.3.2. Effect of silane treatment on tin whisker mitigation Silane treatment improves the adhesion between tin surface and conformal coating, and in turn influences the mitigation ability of conformal coating on tin whisker growth. Fig. 11a shows the representative images of the silane-treated PU and PUA coating after the 5000 h HTHH treatment. There is less whisker/dome formation in the PU and PUA coatings, both of which were applied to the silane-treated tin surface, as compared to these coatings on the untreated tin. In addition, the coating coverage at the edge of coupon slot in silane-treated PUA case is better than the untreated PUA case which can provide more uniform mitigation of tin whisker/nodule growth.
The statistical analysis of the number of whisker/dome formations at various time steps is shown in Fig. 11b. It is seen that the number of whiskers/domes increases with HTHH treatment time, but silane-treated samples have a stronger mitigation ability on tin whisker/nodule growth than the untreated PU and PUA cases at each time step. In general, PU coating exhibited very good protection until 4000 h while it was well adhered to the tin surface, after which time tin whiskers began to delaminate and deform the coating (dome shape) by 5000 h. On the other hand, PUA coating showed very good protection until 2500 h that began to show many dome formations by 4000 h because of more delaminated areas and many tin whisker penetrations by 5000 h. Three schematic diagrams are shown in Fig. 12 to indicate three levels of adhesion cases between conformal coating and tin surface from a cross sectional view. If good adhesion is achieved, it usually results in a small number of whisker/dome formations since there is no space for moisture and little IMC accumulation so that less stress is generated from oxidation and IMCs in the tin layer. This situation is exemplified by the PU coating on the silane-treated tin surface. The second situation is a partial delamination sample at the interface and two different examples of this occurred here. For the PU coating, a large number of domes were observed due to good ductility of the coating. Once the whisker nucleates at the delaminated area and grows towards to the coating, its growth will be stopped by the conformal coating with plastic deformation that results in the dome shape. However, the PUA coating on the silanetreated tin surface is more likely to form whiskers/nodules through
Fig. 12. Three different levels of interfacial adhesion between coating and tin that can lead to the different tin whisker growth behavior.
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Fig. 13. Effect of electron beam irradiation on tin whisker growth behavior: (a) Representative images of tin whisker/dome growth after the 2500 h HTHH treatment; (b) Statistical analysis of the number of domes or whiskers.
the coating instead of domes in the isolated delaminated areas, because the defects or small cracks that might be initially present or are generated in the PUA coating over time become more brittle during HTHH treatment. The last situation is the PUA coating on the untreated tin surface, which possesses an extensive gap or delamination at the interface between PUA and tin surface for trapped moisture and IMC growth. The poor adhesion gives the whiskers /nodules a greater chance to nucleate at the tin surface whose formation is accelerated by the compressive stress from oxidation and IMC formation. Once the growth of tin protrusion is resisted by the coating, it starts to delaminate the coating during its vertical growth, but when the delamination stops, the whisker may penetrate through the weak spots of brittle coating to form the highest number of whiskers among all the cases analyzed. All the examples in various adhesion states associated with these three schematic diagrams can be found in Fig. 5. According to the statistical analysis, PU and PUA coatings on the silane-treated tin have the good mitigation performance at 2500 h, but for the long term treatments after 4000 h, only the PU coating on the silane-treated tin showed the best mitigation ability for tin whisker/nodule growth due to the PUA adhesion begins to deteriorate. 3.3.3. Effect of electron beam irradiation on tin whisker growth The electron beam irradiated PU and PUA-coated coupons, along with no coating coupons, were examined after the 2500 h HTHH treatment. Fig. 13a shows the representative images of electron beam irradiated samples compared to the corresponding control samples with no irradiation, which were exposed to the 2500 h HTHH treatment. A higher number of domes and whiskers were detected in PU and PUA, respectively, compared to their no irradiation counterparts. In particular, tin whisker aggregates tended to be larger with an e-beam irradiation as shown in the e-beam irradiated control sample surface, even though the number of the whiskers shown was similar to that on the non-irradiated control sample. This
behavior appeared to be triggered by charged domains on the tin surface induced by electric fields due to an e-beam irradiation, which accelerate the nucleation and growth rates of the nearby whiskers [36]. Larger aggregation was also observed in the e-beam irradiated PUA, which could have been further accelerated by the coating degradation during the long exposure under the HTHH treatment. The statistical results, Fig. 13b, also prove this point. The number of whisker aggregates or domes in PU and PUA coating has dramatically increased on the electron beam irradiated samples compared to the non-irradiation ones. The reason behind that is due to degradation of chemical structure of the coating during e-beam irradiation which not only influences the mechanical properties but also adhesion of the coating, and in turn impacts its mitigation capacity of tin whisker growth. In fact, the peeled-off percentages of the irradiated PU and PUA coatings in cross-cut test at time zero are 40.0% and 82.5%, respectively, which are much higher than the non-irradiated PU (12.0%) and PUA (57.0%) coatings. Hence the decreased adhesion of conformal coating on tin surface during the e-beam irradiation triggers more occasions for tin whisker nucleation, thereby resulting in a large number of domes or whiskers, depending upon the coating properties. It is also noted that the aggregated whisker cluster is treated as a single occurrence in the statistical analysis which might have led to an underestimation of the actual number of whiskers. 3.3.4. Effect of silane treatment on electron beam irradiated coupons In order to enhance the tin whisker mitigation ability of the conformal coating after electron beam irradiation, the silane treatment was applied on the tin surface because the adhesion is a key factor for tin whisker/nodule nucleation. Fig. 14a indicates the representative images of the silane-treated samples that were exposed to e-beam irradiation, compared to non-irradiated control samples after the 2500 h HTHH treatment. It is observed that the
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Fig. 14. Effect of electron beam irradiation (1 1016 cm2) and silane treatment on tin whisker growth behavior after 2500 h HTHH treatment: (a) Representative images of tin whisker/dome growth; (b) Statistical analysis of the number of domes or whiskers, along with peeled-off percent at time 0.
occurrence of tin whisker/dome formation is much reduced on the surface of both PU and PUA coatings made on the silane-treated tin surface. More quantitative data are shown in Fig. 14b, where the number of whiskers/domes in e-beam irradiated samples is much higher when the silane treatment is not used on the tin coupon surface. It indicates the silane treatment can significantly enhance the tin whisker mitigation ability of conformal coating under electron beam irradiation conditions. Cross-cut test results show that the peeled-off percentages of silane-treated PU and PUA after irradiation are only 8.9% and 10.2% at time zero, respectively. The improvement of adhesion makes the tin whisker nucleation more difficult, resulting in less whiskers/ domes on the coating. In addition, there are more whiskers/nodules in the silane-treated irradiation samples than the silane-treated non-irradiated counterparts due to the degradation of coating after electron beam irradiation which makes the coating structure become more brittle so that the coating is easier to penetrate with the growing whiskers/nodules. If PU and PUA coatings were irradiated under e-beam, both of them would form more whiskers/domes during the HTHH aging than non-irradiated counterparts. In this case, the silane treatment would provide an effective means of mitigating the tin whisker/ dome formation by maintaining the adhesion of conformal coating to Sn surface. Even PUA coatings show very little whisker penetration with the silane treatment on the tin surface, so are now comparable to the PU coatings within the data scattering in terms of the whisker/dome formation.
4. Conclusion Effects of conformal coatings were examined to investigate their mitigation ability to reduce tin whiskers growth on tin plated copper coupons after HTHH treatments (85 C/85%RH) ranging from 0 to 5000 h. Both PU and PUA coatings displayed degradation during HTHH treatment and increased underlying tin surface oxidation, but the silane-treated sample significantly reduced the tin oxidation since the good adhesion was made between coating and tin surface which reduced moisture trapping. In addition, PU has a better coating coverage than does PUA on the tin surface especially at the edge of the slot area. The coverage of PUA was, however, improved by the silane treatment on the tin surface that further increased the tin whisker mitigation ability of PUA. Cross sectional analysis showed that the silane treatment improves the adhesion between conformal coating and tin surface. PU has a better adhesion than PUA that displays an extensive delamination after HTHH treatment. The adhesion of both coatings is significantly improved after the silane treatment. Bulk IMC growth due to copper diffusion was found near the top portion of a tin layer resulting in a compressive stress which led to tin protrusion. When the adhesion is poor, the conformal coating delamination from Sn surface creates a space for lateral growth of a tin mass/whisker, leading to a tin layer thickening since the protrusive growth of tin is impinged by the conformal coating. The tin whisker growth mitigation assessment indicates that conformal coating clearly slows down tin whisker growth but that
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it does not prevent the nucleation of tin whiskers and nodules. PU is a better candidate than PUA in terms of tin whisker mitigation since the former has more ductility with enough strength, less brittleness, and higher adhesion to tin surface. In this case, tin whiskers can be confined under the PU coating while more whiskers come out of the PUA coating due to its increased brittleness and defects. In an effort to improve the coating adhesion, a silane adhesion promoter was used between tin surface and conformal coating, which produced less whiskers and domes than did non-silanetreated coupons. In particular, electron beam irradiation (1 1016 cm2) has shown significant influence on the mitigation ability of conformal coatings due to the degradation of coating adhesion and chemical structure after the irradiation. More aggregated whisker clusters were found in the non-coated case after electron beam irradiation. However, the mitigation ability of e-beam irradiated conformal coatings on tin whisker growth was significantly improved by the same silane treatment (up to 2500 h) since the coating continued to maintain good adhesion even after e-beam irradiation, which leads to less whisker nucleation and growth. Acknowledgments This work was sponsored by the Strategic Environmental Research and Development Program (SERDP, Contract #: W912HQ10-C-0052) of the U.S. Department of Defense and the New York State Energy Research and Development Authority (NYSERDA, Grant #: 58256). PU (PC18M) and PUA (PC40UMF) resins were provided from Henkel Electronics Materials, Irvine, CA, USA (David Edwards). References [1] A. Ezroj, How the European Union's WEEE & RoHS directives can help the United States develop a successful national E-waste strategy, VA. Envtl. LJ. 28 (2010) 45e72. [2] W.J. Boettinger, C.E. Johnson, L.A. Bendersky, K.W. Moon, M.E. Williams, G.R. Stafford, Whisker and hillock formation on Sn, SneCu and SnePb electrodeposits, Acta Mater. 53 (19) (2005 Nov 1) 5033e5050. [3] J.W. Osenbach, Tin whiskers: an illustrated guide to growth mechanisms and morphologies, JOM 63 (10) (2011 Oct 1) 57e60. [4] G.T. Galyon, Annotated tin whisker bibliography and anthology, IEEE Trans. Electron. Packag. Manuf. 28 (1) (2005 Jan) 94e122. [5] P. Zhang, Y. Zhang, Z. Sun, Spontaneous growth of metal whiskers on surfaces of solids: a review, J. Mater. Sci. Technol. 31 (7) (2015 Jul 1) 675e698. [6] U. Lindborg, A model for the spontaneous growth of zinc, cadmium and tin whiskers, Acta Metall. 24 (2) (1976 Feb 1) 181e186. [7] Z. Sun, H. Hashimoto, M.W. Barsoum, On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature, Acta Mater. 55 (10) (2007 Jun 1) 3387e3396. [8] B.D. Dunn, Whisker formation on electronic materials, Circuit World 2 (4) (1976 Mar 1) 32e40. [9] M.O. Peach, Mechanism of growth of whiskers on cadmium, J. Appl. Phys. 23 (12) (1952 Dec) 1401e1403. [10] F.C.X.C. Frank, On tin whiskers, Lond. Edinb. Dublin Phil. Mag. J. Sci. 44 (355) (1953 Aug 1) 854e860. [11] V.K. Glazunova, An investigation of the conditions of spontaneous growth of filiform crystals on electrolytic coatings, Zh. Prikl. Khim. 36 (3) (1963) 543e550.
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