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Failure analysis of contact probe pins for SnPb and Sn applications
Microelectronics Reliability 48 (2008) 942–947
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Failure analysis of contact probe pins for SnPb and Sn applications Changsoo Jang a, Seungbae Park a,*, Bill Infantolino a, Lawrence Lehman a, Ryan Morgan b, Dipak Sengupta b a b
Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY 13902-6000, United States Analog Devices, Norwood, MA 02062, United States
a r t i c l e
i n f o
Article history: Received 3 April 2007 Received in revised form 4 March 2008 Available online 12 May 2008
a b s t r a c t A study has been conducted to investigate the failure mechanism of pogo pin-type probe contacts. Probe pins are used for electrical test of microelectronic components in manufacturing. A false rejection of parts due to high probe contact resistance results in a penalty in cost and yield. The probe pin contact bears distinctive characteristics of failure compared to the conventional contact systems such as mechanical switches and interconnects. Moreover, the transition to Pb-free component leads demands understanding of different probe failure mechanisms between a SnPb and Sn surface. The objective of this study is to understand this unique failure mechanism and the effect of lead coating metals on probe pin life. This has not been clearly elucidated to date in spite of its significant impact on yield and cost of electronic package manufacturing. A simulated probe tester with 3-axes actuation capability was devised to mimic the actual test process. The force required to penetrate the surface oxide layer and develop electrical contact was measured. Contact resistance history revealed that probe pins mating to Sn surfaces failed earlier than pins used on SnPb surfaces. Through periodic inspection of probe pins using microprobe/EDS as a function of probe actuations, the general root cause of probe pin failure was found to be probe pin tip wear out associated with the Sn oxide growth on its surface. The matte Sn surface wears the probe pin more than SnPb due to the rough and abrasive nature of the matte Sn surface. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical contact between two conductive surfaces is widely used in electrical switches and interconnections of various devices. Sn and SnPb have been used as popular surface finishes in electrical contacts for their low cost and manufacturability compared to the more expensive gold plating [1,2]. A critical failure mechanism for such a device is fretting, which is a mechanism occurring in loadbearing systems with contacting surfaces moving slightly relative to each other [3]. In particular, for Sn and SnPb, Sn oxide is thickened on a Sn-coated surface during long-term operation. Contact resistance is sharply increased by the presence of Sn oxide. The resistance can reach to the order of tens of Ohms [4]. When in contact with a Sn/SnPb plated surface, a mating metal part has to break and wipe away the Sn oxide through a scrubbing and/or plunging action to establish a metal-to-metal contact. Fretting reduces the life of switches or interconnects because Sn oxide gets more difficult to remove as it is thickened and therefore it becomes hard to make the metal to metal contact. Numerous studies have focused on the effects of various mating surface combinations and environmental/operating parameters such as contact force, wipe distance, current, and so on for higher service life of contacts [3]. * Corresponding author. Tel.: +1 607 777 3415; fax: +1 607 777 4620. E-mail address: [email protected] (S. Park). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.03.015
In electronics manufacturing, circuit integrity of an electronic device is tested by measuring electrical resistance using contact probes. The probing relies on mechanical contact similar to switches and interconnects. During its lifetime, a probe pin sees tens to hundreds of thousands of cycles. The contact resistance between the probe pin and lead surface has to be minimized to measure the actual resistance of a device circuit accurately. The degradation of probe pin/lead contact causes erroneous test results or frequent replacement of probe units, which results in higher yield loss and manufacturing cost. Although it employs similar or identical combinations of mating metals, the probe contact has several substantial differences from conventional contacts in switches and interconnects. The similarities and differences are summarized in Table 1. The primary difference lies in the continuous change of mating surfaces. A probe pin always contacts brand-new mating surfaces throughout its service life. As a consequence, failure occurs at the probe pin, whereas the Sn-containing surface fails due to fretting in conventional contacts. It has been conjectured that the contamination of probe pin tip surfaces, by Sn/SnPb transferred from component surfaces, causes the probe pin failure [5]. The concern about environment and health and relevant legislation are driving electronic products from lead bearing to lead-free materials [6]. The conventional SnPb platings used in component leads are being replaced with lead-free platings. Pure Sn has been used as one of the major lead-free plating candidates. Several
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C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947 Table 1 Summary of similar and dissimilar facts between two contacts Conventional contact Electrical connection mechanism Critical issue Critical concern Contact period Mate change Failure site
Probe contact
Pressure at contact with micro-scale relative sliding ? disruption of oxide ? electrical connection Combination of mating metals Which pair will be the best? Which metal will be the best mate of lead plating? Until failure A very short time of testing No change until failure Pad is always brand new Usually softer metal like Sn Probe metal
studies have reported that the shift from SnPb platings to Sn led to reduced probe pin life [7–9]. As a consequence, yield loss and manufacturing cost would be raised substantially. In spite of its significant impact, the cause of such deterioration of probe pin reliability has not yet been determined. This study addresses this reliability issue of the contact probe pin, applied to Sn and SnPb plated lead frames. The failure mechanism of generally used pogo-type probe pins and the cause of accelerated degradation with the Sn surface are identified through simulated probing tests. 2. Experimental setup Among several types of probe pins used for module testing, one of the most popular would be the pogo-type pin. Fig. 1 shows the pogo probe pin used in this study and its cross-sectional view. It consists of three separable components: a probing tip, a stationary circuit contact tip, and a spring in the middle. During probing, force is developed by compression of the coil spring due to the spring retainer the probe tip. The stroke distance of the probe tip relative to the other tip determines the magnitude of spring force (contact force). The probe pin uses Fe as a base metal and a noble metal such as Au or PdCo as a coating metal. Ni together with a small amount of phosphorus is used as an interposing metal in between the base metal and the coating metal. Copper strips electroplated with two different platings, matte Sn and eutectic SnPb (63Sn37Pb), were used to simulate plated leads. The thicknesses of the copper base and plated metal were 0.2 mm and around 10 lm, respectively. The cross-sectioned images of two strips are shown in Fig. 2. The Sn and SnPb platings were characterized in terms of modulus, hardness, and surface roughness using a nano-indenter (Hysitron Triboindenter). More than 25 data points for each plating type were analyzed for a statistical analysis. The results are listed in Table 2. Although deviations caused by dense grain structures of platings cannot be ignored compared with the mean values, the overall results clearly show that the Sn surface is softer and rougher than the SnPb sur-
Fig. 2. Cross-sectioned images of Sn and SnPb plated strips.
Table 2 Properties of Sn and SnPb surfaces
Modulus (GPa) Hardness (MPa) Surface roughness, Rsq (nm)
Sn
SnPb
17.9 ± 6.0 57.9 ± 26.3 205.5 ± 22.8
27.3 ± 4.7 94.3 ± 30.6 161.6 ± 16.1
face, which is consistent with conventional data found in the literature [10,11]. An automated mechanical probing stage was developed to simulate the contact probing process. Fig. 3 illustrates the schematic of the tester. It consists of a 3-axes stage with a load cell, a digital multi-meter, and a PC equipped with an A/D converter. The pin-fixture assembly, comprised of a pin/socket assembly and socket/ stage fixture, is moved forward and backward to probe normal to the surface (Y-axis). The pin-fixture assembly steps in the X–Z plane to probe a new, undisturbed plated surface with each actua-
Spring
Multimeter AD board
Stationary tip
Probing tip
Pin & fixture Pad
PC Software
z
Ni (3~4μm) Fe Au (1~2μm) Fig. 1. Shape of probing pin (top) and cross-section of its tip (bottom).
y x
As a function of time Force, displacement, & contact resistance
Fig. 3. Schematic of probing tester used in experiments.
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Contact onset force
Wire and connector
0.8
Stationary tip contact
Probing 1st 2nd 3rd 4th
0.6
Upper spring contact
5th 6th 7th 8th
Lower spring contact Probe tip-pad contact Wire and connector
Contact Resistance (Ohm)
0.4
New Pin
0.2
0.8 0.0
0.6
Contact development force
0.4
Degraded Pin
Fig. 4. Electrical resistance network in probe pin contact.
tion. The load cell measures contact force while the digital multimeter monitors total resistance. The total resistance contains not only the pure contact resistance but also numerous inherent resistance sources such as circuit wires, connectors and probe pin components as illustrated in Fig. 4. The well-known 4-wire measurement technique can enhance measurement accuracy by measuring a specific resistance element(s) within a resistance network [12]. In this experiment, however, the maximum achievable enhancement would be just elimination of the wire/connector resistance since it is practically impossible for a measurement probe to access any part of the probe pin during actuation due to the small size of the pin and the presence of the socket encasing it. Thus a simple 2-point measurement was conducted throughout the experiment. The inherent resistance was approximated to be about 80 mX in total based on a preliminary measurement of the experimental setup without the probe pin/pad contact (one of the wires connected to the pin tip directly), i.e. total resistance minus probe tip/pad contact when pin was not compressed (Fig. 4). The contact resistance of two contact spots at both ends of the spring wire may be reduced further by compression of the spring. All the resistance sources resulted in an output resistance around 80–200 mX under normal operating conditions. The probe count for each pad strip ranged from 15,000 to 100,000 cycles depending on the surface analysis schedule of the probe pin. A metallurgical analysis on the probe pin tip was conducted using an electron microprobe and energy dispersive spectroscopy (EDS) in between pad strip changes. All experiments were carried out at room temperature. The failure criterion was set as 500 mX or higher, which is normally used in packaging industry. The probing was continued until the contact resistance reached the failure criterion. 3. Results and discussions 3.1. Force versus resistance Fig. 5 shows a typical resistance variation with respect to contact force. The resistance does not drop down from infinity immediately at the commencement of contact due to the presence of Sn oxide. An interesting fact observed in the plot is that resistance does not change when the contact force exceeds a certain magnitude (contact development force). This can be explained by the characteristic of Au-solder contact. When a metal-to-metal contact is made through wiping away Sn oxide, the magnitude of the contact resistance is normally reduced to the order of mX and very little change occurs by a further in-
0.2
0.0 0.0
0.1
0.2
0.3
0.4
Contact Force (N) Fig. 5. Contact force versus contact resistance plots of new and degraded pins.
crease in contact force [13]. Consequently, once metal-to-metal contact is achieved, the resistance change by contact force increase would be negligible. The contact development force is observed to be under 0.1 N in this experiment. Even for the degraded probe pin, this force did not change significantly. The resistance measured in the experiment reflects mostly the system resistance in the case of a new probe pin. The probe pin/ pad contact resistance becomes dominant as the measured resistance increases by degradation of the probe pin. Therefore, for the sake of brevity, the measured resistance is denoted simply by the contact resistance hereafter. The contact resistance of a degraded probe pin shown in Fig. 5 is higher and more dispersed than that of a new probe pin, whereas the contact development force does not change significantly. This seems contradictory. Unchanged contact development force regardless of probe pin status could imply that the Sn oxide on a pad is sufficiently wiped out even by a degraded probe pin. This speculation is reasonable because the thickness of Sn oxide formed on a ‘‘continuously brand-new” Sn surface is extremely thin and constant, usually on the order of 5 nm [14,15]. Considering that the inherent portion of the contact resistance does not change significantly, the increased portion can be attributed to Sn oxide on the probe pin tip, which grows on the Sn transferred to the pin surface during probing. The Sn oxide on a pad can be easily broken and removed by the plunging motion of the probe pin because the metal beneath (Sn or SnPb) is soft and easily deformed. On the other hand, Sn oxide on a probe pin tip is more difficult to break and remove due to the harder base metals (Au, Ni and Fe) [16]. It plays an important role in the increase of contact resistance, and consequently, in failure [17,18]. An arbitrary status of this oxide at each contact probing, that is, neither completely covering the tip nor completely being removed away, results in a more diverging contact resistance. A metallurgical analysis of probe pin surfaces shows that a degraded pin has more Sn on its surface than a new one.
C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947
A comparable contact development force was observed when the pin probed a SnPb surface as shown in Fig. 6. Although the SnPb surface is harder than the Sn surface, it did not make a significant difference in terms of the contact development force. It is speculated that this results from the fact that the pin metals are much harder than either Sn or SnPb. The effect of pin probing speed was also examined. The results did not show any clear evidence of a relationship between speed and resistance. This indicates that time-dependent properties of solder materials might not play a role. A preliminary study through a finite element analysis revealed that the maximum strain of the solder plating at the contact surface should be greater than 0.75% and even reach several hundred percent depending on the pin tip radius. During the plunging process, the behavior of solder should be dominated by creep at this high strain range. The creep strain rate is dependent on stress as well as temperature. For a higher speed the creep rate is increased by a higher stress. In contrast, it is decreased by a lower stress corresponding to a lower speed. This
0.5
Probing Speed 1.0 mm/sec 0.5 mm/sec 0.1 mm/s ec
0.4
0.3
Sn Pad
Contact Resistance (Ohm)
0.2
0.1
0.5 0.0
0.4
0.3
SnPb Pad 0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
Contact Force (N) Fig. 6. Contact force versus contact resistance plots of a new pin with respect to pad type and probing speed.
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compensational effect seems to result in insignificant change in contact development force. Once deformation is stabilized, the creep rate is significantly reduced in both cases (_e ¼ 0:01 1 =s at the most at room temperature based on the property data in reference [19]) so that there should be little change with respect to the probing speed in given times shorter than 1 s for all cases. 3.2. Reliability test and failure mechanism Fig. 7 shows contact resistance histories for three test probe pins. Two pins were applied to Sn surfaces and one pin to SnPb surfaces. Sharp peaks in the middle of the test indicate pad change. At each pad change, contact resistance was stabilized after tens of probe actuations. In actual manufacturing, probe pins are stabilized prior to product testing. Both probe pins for Sn failed at around 140,000 probe actuations while the other one for SnPb has lasted an additional 100,000 actuations. This is consistent with the trend found in the literature [7–9]. A metallurgical analysis was conducted to determine the composition of a probe pin tip surface. A back scattered electron (BSE) image captured by an electron microscope shows rough metallurgical information of a target spot because the brightness varies with the atomic number of a metal; the brighter the color, the greater the atomic number. In this case, the brightest metal would be Au, and then Sn, Pb, Ni, and Fe follows. An EDS analysis was also conducted to obtain detailed metallurgical information. The change of probe pin tip status with respect to probe count is shown in Fig. 8. Black spots indicate organic contaminants which have little influence on probing performance [20]. Overall status of the probe pin for SnPb looks better than that of the probe pin for Sn. Magnified images of the worst probe tips are presented in Fig. 9. In the case of SnPb, Au was still present and covered with SnPb at 30,000 cycles while the metals underneath the Au had already been exposed in the case of Sn. At 100,000, the Au tip surface of the probe pin for SnPb was also worn-off but the degree of wear was still less than the probe pin for Sn. The depth of probe pin tip wear can be approximated from obtained BSE images and the probe pin structure shown in cross-section images. A width of an exposed Fe layer observed from a BSE image corresponds to a certain amount of wear-out depth. The result is illustrated in Fig. 10. The degree of tip wear by Sn was found to be about 30% greater than that by SnPb. Sn oxide, a kind of ceramic, is thought to be responsible for abrasion of all metals comprising a probe pin, which are much harder than Sn or SnPb. Two facts could answer the question why Sn is worse. One is that the Sn surface is rougher. Polished or reflowed Sn/SnPb surfaces were observed to provide longer probe pin life [18] than matte Sn or unpolished SnPb. From the practical point of view, pol-
Fig. 7. Contact resistance histories for three test pins.
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C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947
20
Pin 1 for Sn Pad Pin 2 for Sn Pad Pin 3 for SnPb Pad
Tip Wear-Out (μm)
16
12
8
4
0 0
50
100
150
200
250
300
Actuations (x1000) Fig. 10. Calculated tip wear-out as a function of probe actuation count.
Fig. 8. BSE images of pins at particular probing counts.
Fig. 9. BSE images of worst tips before (30k and 100k) and after (200k and 300k) failure.
ishing surfaces cannot be a solution for mass production because of a cost issue. The other reason that Sn is worse is that the presence of Pb in SnPb acts as a solid lubricant. It is well known the contact life can be enhanced by supplying a lubricant [21]. PdCo has been proposed as an alternative coating for probe pin surfaces to get better performance [18]. This is true for the NiPd(Au)-plated surface, which is another plating solution for leadfree applications. There should be no oxide issue for this surface because both sides are noble. In the case of a Au-coated probe pin, however, quick wear occurs because of the harder NiPd(Au).
The PdCo probe pin can survive longer because it is harder than NiPd(Au) [11]. On the other hand, when the PdCo probe pin is applied to a Sn/SnPb surface, there is only a small improvement compared with the Au probe pin because the same wear occurs at the tip due to abrasive Sn oxide. Once the thin PdCo layer is worn-off, the remainder of the probe degradation process should be almost identical to the Au case. A significant change before and after failure was observed at the circular band of the Au layer (Fig. 9). Before failure, most of the Au band area was seen as a bright area in BSE images indicating pure Au is present. However, after failure, this band area was covered with Sn or SnPb. This corroborates that the transferred Sn/SnPb on pin tips is the root cause of the probe pin failure. This is also supported by the fact that tin oxide on a rigid surface like pin tips is more difficult to remove than that on a soft surface like Sn/SnPb plated surfaces [16]. The oxide that grows on top of a sharp probe pin can be removed by a geometrical disruption mechanism as shown in Fig. 11. As the probe actuation keeps blunting the tip, the mechanism that pushes the oxide up the side of the pin gradually shifts to a horizontal segregation mechanism. Although the latter is also able to provide metal-to-metal contact, the wiping efficiency would be substantially reduced. Some portion of the Sn oxide can be chipped off while another portion remains. The remaining Sn oxide can accumulate during continuous operation and build up a high resistance barrier, which is similar to Sn fretting. The blunt tip of a degraded probe pin, and mating surface as well, would be subjected to a lower pressure than the sharp tip of a new pin when the same force is applied. It is intuitively anticipated that the contact development force should be changed accordingly. However, surprisingly, an insignificant change was observed in the force/resistance measurements (Fig. 5). The effect of pressure change can be found in the ‘‘onset” force for metalto-metal contact which is indicated by vertical lines on the plots. The onset force of the degraded pin was several times higher than that for the new pin. Based on what has been observed so far, the probing contact mechanism can be described as follows: (1) The probe pin tip breaks the oxide layer at both pin tip and pad surfaces and metal-to-metal contact is initiated (onset force). The contact resistance is suddenly reduced to near the final contact resistance. (2) Metal-to-metal contact is secured as the solder deformation proceeds. At a certain point, the contact resistance is stabilized (contact development force).
C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947
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deteriorated the efficiency of Sn oxide wiping action and increased the contact resistance. The degree of probe pin tip wear-out by the matte Sn surface was found to be about 30% higher than that by the smoother eutectic SnPb surface. The presence of Pb, which plays a role as solid lubricant, also reduced the probe pin wear. These factors collectively resulted in a longer life of the probe pin applied to the SnPb surface.
Start of probing
Acknowledgements
Disruption of oxide layers
This work was supported by the Integrated Electronics Engineering Center (IEEC) of the State University of New York at Binghamton. The authors would like to thank Dr. John Lee at The State University of New York at Binghamton for valuable discussions and advice on failure mechanisms. They also want to express their thanks to Aaron Reichman and Oberon Deichman for their excellent work on the 3-axes tester development and Hyungsuk Lee for his support in property measurement using the nanoindentor. References
Vertical segregation
Horizontal segregation
Oxide wipe-away and metal-to-metal contact Fig. 11. Electrical connection mechanisms of sharp and blunt tips.
Table 3 Factors influencing pin life
Rougher pad surface Pin tip wear-out Solid lubricant (Pb)
Beneficial
Detrimental
Likely to break and wipe away Sn oxide on tip N/A Less abrasive to pin tip
Also likely to wear tip out more effectively Harder to remove oxide Also less abrasive to oxide on tip
(3) The final contact resistance is dominated by the degree of oxide wipe away from probe pin tips which depends on wear-out or degradation of pin tips. Degree of wear-out of probe pin tips, pad surface roughness, and the lubricating function of Pb have inter-related effects on probe pin life as listed in Table 3. The blunted tip is simply detrimental to probe pin reliability. The rough pad surface makes it easier to remove Sn oxide on both surfaces while it blunts the probe pin tip more quickly. Pb makes it harder to wipe away Sn oxides while it retards probe pin tip wear. The degree of tip wear for SnPb was smaller than that for Sn at the point of failure. If both cases fail at the same degree of wear, the probe pin for SnPb should endure far longer. However, the adverse effects of Pb and a smooth surface seemed to reduce the potential life span of the probe pin. 4. Conclusions The reliability of probing with a pogo-type probe was investigated. The primary cause of the contact resistance increase was identified as Sn oxide growth on top of the Sn layer transferred to the probe pin surface during probing. The probe pin tip got blunter through wear-out as probing actuation continued. The blunt tip
[1] Mroczkowski RS. Electric connector handbook: technology and applications. New York: McGraw-Hill; 1998. [2] Wu J, Pecht MG. Contact resistance and fretting corrosion of lead-free alloy coated electrical contacts. IEEE Trans Compon Pack Technol 2006;29(2):402–10. [3] Antler M. Electrical effects of fretting connector contact materials: a review. Wear 1985;106:5–33. [4] Laverty SJ, Maguire PD. Contact resistivity of metallized tin oxide surfaces. J Electrochem Soc 2000;147(2):772–5. [5] Podpora Z. Test and burn-in socket evaluation for PBGA devices. In: Proceedings of burn-in and test socket workshop 2000, session 3, 2000. [6] Eveloy V, Ganesan S, Fukuda Y, Wu J, Pecht MG. Are you ready for lead-free electronics? IEEE Trans Compon Pack Technol 2005;28(4):884–94. [7] Sheposh BW. Challenges of contacting lead-free devices. In: Proceedings of burn-in and test socket workshop 2005, session 1, 2005. [8] Brost B, Sherry J. Socket performance over time and insertion count with Pbfree application. In: Proceedings of burn-in and test socket workshop 2006, session 2, 2006. [9] Orwoll, E. Minimizing spring probe operational cost using optimized maintenance techniques. In: Proceedings of burn-in and test socket workshop 2006, session 8. [10] Website, www.matweb.com. [11] Treibergs V. Ph-free leadframe devices and their impact on pogo pin socket performance. In: Proceedings of burn-in and test socket workshop 2004, session 8, 2004. [12] Giaimo A. A method for measuring and evaluating contact resistance in burnin and test sockets. In: Proceedings of burn-in and test socket workshop 2000, session 3, 2000. [13] Blacker RW, Warwick ME, Long JB. Preliminary studies of tin and tin rich coatings as electrical contact materials. IEEE Trans Compon, Hybr, Manuf Technol 1981;4(3):294–303. [14] Burlacu D, Nguyen L, Kivilahti J. Effects of the solder oxide layer on high frequency signal propagation in pb-free interconnections. In: Proceedings of 55th electronic components and technology conference 2005. [15] Tamura K, Kimura Y, Suzuki H, Kido O, Sato T, Tanigaki T, et al. Structure and thickness of natural oxide layer on ultrafine particle. Jpn J Appl Phys Part 1 2003;42(12):7489–92. [16] Antler M. Contact resistance of oxidized metals: dependence on the mating material. IEEE Trans Compon, Hybr, Manuf Technol 1987;CHMT-10(3):420–4. [17] Bock E. Mateability of tin to gold, palladium, and silver. In: Proceedings of 40th electronic components and technology conference 1990. p. 840–4. [18] Lian CT, Jayachandrian, Dieter S. Testing of VQFN with palladium–cobalt pogo pin. In: Proceedings of burn-in and test socket workshop 2004, session 7, 2004. [19] Feustel F, Wiese S, Meusel E. Time-dependent material modeling for finite element analyses of flip chips. In: Proceedings of 50th electronic components and technology conference. New York: IEEE; 2000. p. 1548–53. [20] Souza T. Effects of contaminants on test pad surfaces. In: Proceedings of burnin and test socket workshop 2004, session 7, 2004. [21] Antler M, Aukland NR, Hardee H, Wehr-Aukland A. Recovery of severely degraded tin-lead plated connector contacts due to fretting corrosion. IEEE Trans Compon Pack Technol 1999;22(1):61–71.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Failure analysis of contact probe pins for SnPb and Sn applications Changsoo Jang a, Seungbae Park a,*, Bill Infantolino a, Lawrence Lehman a, Ryan Morgan b, Dipak Sengupta b a b
Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY 13902-6000, United States Analog Devices, Norwood, MA 02062, United States
a r t i c l e
i n f o
Article history: Received 3 April 2007 Received in revised form 4 March 2008 Available online 12 May 2008
a b s t r a c t A study has been conducted to investigate the failure mechanism of pogo pin-type probe contacts. Probe pins are used for electrical test of microelectronic components in manufacturing. A false rejection of parts due to high probe contact resistance results in a penalty in cost and yield. The probe pin contact bears distinctive characteristics of failure compared to the conventional contact systems such as mechanical switches and interconnects. Moreover, the transition to Pb-free component leads demands understanding of different probe failure mechanisms between a SnPb and Sn surface. The objective of this study is to understand this unique failure mechanism and the effect of lead coating metals on probe pin life. This has not been clearly elucidated to date in spite of its significant impact on yield and cost of electronic package manufacturing. A simulated probe tester with 3-axes actuation capability was devised to mimic the actual test process. The force required to penetrate the surface oxide layer and develop electrical contact was measured. Contact resistance history revealed that probe pins mating to Sn surfaces failed earlier than pins used on SnPb surfaces. Through periodic inspection of probe pins using microprobe/EDS as a function of probe actuations, the general root cause of probe pin failure was found to be probe pin tip wear out associated with the Sn oxide growth on its surface. The matte Sn surface wears the probe pin more than SnPb due to the rough and abrasive nature of the matte Sn surface. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical contact between two conductive surfaces is widely used in electrical switches and interconnections of various devices. Sn and SnPb have been used as popular surface finishes in electrical contacts for their low cost and manufacturability compared to the more expensive gold plating [1,2]. A critical failure mechanism for such a device is fretting, which is a mechanism occurring in loadbearing systems with contacting surfaces moving slightly relative to each other [3]. In particular, for Sn and SnPb, Sn oxide is thickened on a Sn-coated surface during long-term operation. Contact resistance is sharply increased by the presence of Sn oxide. The resistance can reach to the order of tens of Ohms [4]. When in contact with a Sn/SnPb plated surface, a mating metal part has to break and wipe away the Sn oxide through a scrubbing and/or plunging action to establish a metal-to-metal contact. Fretting reduces the life of switches or interconnects because Sn oxide gets more difficult to remove as it is thickened and therefore it becomes hard to make the metal to metal contact. Numerous studies have focused on the effects of various mating surface combinations and environmental/operating parameters such as contact force, wipe distance, current, and so on for higher service life of contacts [3]. * Corresponding author. Tel.: +1 607 777 3415; fax: +1 607 777 4620. E-mail address: [email protected] (S. Park). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.03.015
In electronics manufacturing, circuit integrity of an electronic device is tested by measuring electrical resistance using contact probes. The probing relies on mechanical contact similar to switches and interconnects. During its lifetime, a probe pin sees tens to hundreds of thousands of cycles. The contact resistance between the probe pin and lead surface has to be minimized to measure the actual resistance of a device circuit accurately. The degradation of probe pin/lead contact causes erroneous test results or frequent replacement of probe units, which results in higher yield loss and manufacturing cost. Although it employs similar or identical combinations of mating metals, the probe contact has several substantial differences from conventional contacts in switches and interconnects. The similarities and differences are summarized in Table 1. The primary difference lies in the continuous change of mating surfaces. A probe pin always contacts brand-new mating surfaces throughout its service life. As a consequence, failure occurs at the probe pin, whereas the Sn-containing surface fails due to fretting in conventional contacts. It has been conjectured that the contamination of probe pin tip surfaces, by Sn/SnPb transferred from component surfaces, causes the probe pin failure [5]. The concern about environment and health and relevant legislation are driving electronic products from lead bearing to lead-free materials [6]. The conventional SnPb platings used in component leads are being replaced with lead-free platings. Pure Sn has been used as one of the major lead-free plating candidates. Several
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C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947 Table 1 Summary of similar and dissimilar facts between two contacts Conventional contact Electrical connection mechanism Critical issue Critical concern Contact period Mate change Failure site
Probe contact
Pressure at contact with micro-scale relative sliding ? disruption of oxide ? electrical connection Combination of mating metals Which pair will be the best? Which metal will be the best mate of lead plating? Until failure A very short time of testing No change until failure Pad is always brand new Usually softer metal like Sn Probe metal
studies have reported that the shift from SnPb platings to Sn led to reduced probe pin life [7–9]. As a consequence, yield loss and manufacturing cost would be raised substantially. In spite of its significant impact, the cause of such deterioration of probe pin reliability has not yet been determined. This study addresses this reliability issue of the contact probe pin, applied to Sn and SnPb plated lead frames. The failure mechanism of generally used pogo-type probe pins and the cause of accelerated degradation with the Sn surface are identified through simulated probing tests. 2. Experimental setup Among several types of probe pins used for module testing, one of the most popular would be the pogo-type pin. Fig. 1 shows the pogo probe pin used in this study and its cross-sectional view. It consists of three separable components: a probing tip, a stationary circuit contact tip, and a spring in the middle. During probing, force is developed by compression of the coil spring due to the spring retainer the probe tip. The stroke distance of the probe tip relative to the other tip determines the magnitude of spring force (contact force). The probe pin uses Fe as a base metal and a noble metal such as Au or PdCo as a coating metal. Ni together with a small amount of phosphorus is used as an interposing metal in between the base metal and the coating metal. Copper strips electroplated with two different platings, matte Sn and eutectic SnPb (63Sn37Pb), were used to simulate plated leads. The thicknesses of the copper base and plated metal were 0.2 mm and around 10 lm, respectively. The cross-sectioned images of two strips are shown in Fig. 2. The Sn and SnPb platings were characterized in terms of modulus, hardness, and surface roughness using a nano-indenter (Hysitron Triboindenter). More than 25 data points for each plating type were analyzed for a statistical analysis. The results are listed in Table 2. Although deviations caused by dense grain structures of platings cannot be ignored compared with the mean values, the overall results clearly show that the Sn surface is softer and rougher than the SnPb sur-
Fig. 2. Cross-sectioned images of Sn and SnPb plated strips.
Table 2 Properties of Sn and SnPb surfaces
Modulus (GPa) Hardness (MPa) Surface roughness, Rsq (nm)
Sn
SnPb
17.9 ± 6.0 57.9 ± 26.3 205.5 ± 22.8
27.3 ± 4.7 94.3 ± 30.6 161.6 ± 16.1
face, which is consistent with conventional data found in the literature [10,11]. An automated mechanical probing stage was developed to simulate the contact probing process. Fig. 3 illustrates the schematic of the tester. It consists of a 3-axes stage with a load cell, a digital multi-meter, and a PC equipped with an A/D converter. The pin-fixture assembly, comprised of a pin/socket assembly and socket/ stage fixture, is moved forward and backward to probe normal to the surface (Y-axis). The pin-fixture assembly steps in the X–Z plane to probe a new, undisturbed plated surface with each actua-
Spring
Multimeter AD board
Stationary tip
Probing tip
Pin & fixture Pad
PC Software
z
Ni (3~4μm) Fe Au (1~2μm) Fig. 1. Shape of probing pin (top) and cross-section of its tip (bottom).
y x
As a function of time Force, displacement, & contact resistance
Fig. 3. Schematic of probing tester used in experiments.
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Contact onset force
Wire and connector
0.8
Stationary tip contact
Probing 1st 2nd 3rd 4th
0.6
Upper spring contact
5th 6th 7th 8th
Lower spring contact Probe tip-pad contact Wire and connector
Contact Resistance (Ohm)
0.4
New Pin
0.2
0.8 0.0
0.6
Contact development force
0.4
Degraded Pin
Fig. 4. Electrical resistance network in probe pin contact.
tion. The load cell measures contact force while the digital multimeter monitors total resistance. The total resistance contains not only the pure contact resistance but also numerous inherent resistance sources such as circuit wires, connectors and probe pin components as illustrated in Fig. 4. The well-known 4-wire measurement technique can enhance measurement accuracy by measuring a specific resistance element(s) within a resistance network [12]. In this experiment, however, the maximum achievable enhancement would be just elimination of the wire/connector resistance since it is practically impossible for a measurement probe to access any part of the probe pin during actuation due to the small size of the pin and the presence of the socket encasing it. Thus a simple 2-point measurement was conducted throughout the experiment. The inherent resistance was approximated to be about 80 mX in total based on a preliminary measurement of the experimental setup without the probe pin/pad contact (one of the wires connected to the pin tip directly), i.e. total resistance minus probe tip/pad contact when pin was not compressed (Fig. 4). The contact resistance of two contact spots at both ends of the spring wire may be reduced further by compression of the spring. All the resistance sources resulted in an output resistance around 80–200 mX under normal operating conditions. The probe count for each pad strip ranged from 15,000 to 100,000 cycles depending on the surface analysis schedule of the probe pin. A metallurgical analysis on the probe pin tip was conducted using an electron microprobe and energy dispersive spectroscopy (EDS) in between pad strip changes. All experiments were carried out at room temperature. The failure criterion was set as 500 mX or higher, which is normally used in packaging industry. The probing was continued until the contact resistance reached the failure criterion. 3. Results and discussions 3.1. Force versus resistance Fig. 5 shows a typical resistance variation with respect to contact force. The resistance does not drop down from infinity immediately at the commencement of contact due to the presence of Sn oxide. An interesting fact observed in the plot is that resistance does not change when the contact force exceeds a certain magnitude (contact development force). This can be explained by the characteristic of Au-solder contact. When a metal-to-metal contact is made through wiping away Sn oxide, the magnitude of the contact resistance is normally reduced to the order of mX and very little change occurs by a further in-
0.2
0.0 0.0
0.1
0.2
0.3
0.4
Contact Force (N) Fig. 5. Contact force versus contact resistance plots of new and degraded pins.
crease in contact force [13]. Consequently, once metal-to-metal contact is achieved, the resistance change by contact force increase would be negligible. The contact development force is observed to be under 0.1 N in this experiment. Even for the degraded probe pin, this force did not change significantly. The resistance measured in the experiment reflects mostly the system resistance in the case of a new probe pin. The probe pin/ pad contact resistance becomes dominant as the measured resistance increases by degradation of the probe pin. Therefore, for the sake of brevity, the measured resistance is denoted simply by the contact resistance hereafter. The contact resistance of a degraded probe pin shown in Fig. 5 is higher and more dispersed than that of a new probe pin, whereas the contact development force does not change significantly. This seems contradictory. Unchanged contact development force regardless of probe pin status could imply that the Sn oxide on a pad is sufficiently wiped out even by a degraded probe pin. This speculation is reasonable because the thickness of Sn oxide formed on a ‘‘continuously brand-new” Sn surface is extremely thin and constant, usually on the order of 5 nm [14,15]. Considering that the inherent portion of the contact resistance does not change significantly, the increased portion can be attributed to Sn oxide on the probe pin tip, which grows on the Sn transferred to the pin surface during probing. The Sn oxide on a pad can be easily broken and removed by the plunging motion of the probe pin because the metal beneath (Sn or SnPb) is soft and easily deformed. On the other hand, Sn oxide on a probe pin tip is more difficult to break and remove due to the harder base metals (Au, Ni and Fe) [16]. It plays an important role in the increase of contact resistance, and consequently, in failure [17,18]. An arbitrary status of this oxide at each contact probing, that is, neither completely covering the tip nor completely being removed away, results in a more diverging contact resistance. A metallurgical analysis of probe pin surfaces shows that a degraded pin has more Sn on its surface than a new one.
C. Jang et al. / Microelectronics Reliability 48 (2008) 942–947
A comparable contact development force was observed when the pin probed a SnPb surface as shown in Fig. 6. Although the SnPb surface is harder than the Sn surface, it did not make a significant difference in terms of the contact development force. It is speculated that this results from the fact that the pin metals are much harder than either Sn or SnPb. The effect of pin probing speed was also examined. The results did not show any clear evidence of a relationship between speed and resistance. This indicates that time-dependent properties of solder materials might not play a role. A preliminary study through a finite element analysis revealed that the maximum strain of the solder plating at the contact surface should be greater than 0.75% and even reach several hundred percent depending on the pin tip radius. During the plunging process, the behavior of solder should be dominated by creep at this high strain range. The creep strain rate is dependent on stress as well as temperature. For a higher speed the creep rate is increased by a higher stress. In contrast, it is decreased by a lower stress corresponding to a lower speed. This
0.5
Probing Speed 1.0 mm/sec 0.5 mm/sec 0.1 mm/s ec
0.4
0.3
Sn Pad
Contact Resistance (Ohm)
0.2
0.1
0.5 0.0
0.4
0.3
SnPb Pad 0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
Contact Force (N) Fig. 6. Contact force versus contact resistance plots of a new pin with respect to pad type and probing speed.
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compensational effect seems to result in insignificant change in contact development force. Once deformation is stabilized, the creep rate is significantly reduced in both cases (_e ¼ 0:01 1 =s at the most at room temperature based on the property data in reference [19]) so that there should be little change with respect to the probing speed in given times shorter than 1 s for all cases. 3.2. Reliability test and failure mechanism Fig. 7 shows contact resistance histories for three test probe pins. Two pins were applied to Sn surfaces and one pin to SnPb surfaces. Sharp peaks in the middle of the test indicate pad change. At each pad change, contact resistance was stabilized after tens of probe actuations. In actual manufacturing, probe pins are stabilized prior to product testing. Both probe pins for Sn failed at around 140,000 probe actuations while the other one for SnPb has lasted an additional 100,000 actuations. This is consistent with the trend found in the literature [7–9]. A metallurgical analysis was conducted to determine the composition of a probe pin tip surface. A back scattered electron (BSE) image captured by an electron microscope shows rough metallurgical information of a target spot because the brightness varies with the atomic number of a metal; the brighter the color, the greater the atomic number. In this case, the brightest metal would be Au, and then Sn, Pb, Ni, and Fe follows. An EDS analysis was also conducted to obtain detailed metallurgical information. The change of probe pin tip status with respect to probe count is shown in Fig. 8. Black spots indicate organic contaminants which have little influence on probing performance [20]. Overall status of the probe pin for SnPb looks better than that of the probe pin for Sn. Magnified images of the worst probe tips are presented in Fig. 9. In the case of SnPb, Au was still present and covered with SnPb at 30,000 cycles while the metals underneath the Au had already been exposed in the case of Sn. At 100,000, the Au tip surface of the probe pin for SnPb was also worn-off but the degree of wear was still less than the probe pin for Sn. The depth of probe pin tip wear can be approximated from obtained BSE images and the probe pin structure shown in cross-section images. A width of an exposed Fe layer observed from a BSE image corresponds to a certain amount of wear-out depth. The result is illustrated in Fig. 10. The degree of tip wear by Sn was found to be about 30% greater than that by SnPb. Sn oxide, a kind of ceramic, is thought to be responsible for abrasion of all metals comprising a probe pin, which are much harder than Sn or SnPb. Two facts could answer the question why Sn is worse. One is that the Sn surface is rougher. Polished or reflowed Sn/SnPb surfaces were observed to provide longer probe pin life [18] than matte Sn or unpolished SnPb. From the practical point of view, pol-
Fig. 7. Contact resistance histories for three test pins.
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Pin 1 for Sn Pad Pin 2 for Sn Pad Pin 3 for SnPb Pad
Tip Wear-Out (μm)
16
12
8
4
0 0
50
100
150
200
250
300
Actuations (x1000) Fig. 10. Calculated tip wear-out as a function of probe actuation count.
Fig. 8. BSE images of pins at particular probing counts.
Fig. 9. BSE images of worst tips before (30k and 100k) and after (200k and 300k) failure.
ishing surfaces cannot be a solution for mass production because of a cost issue. The other reason that Sn is worse is that the presence of Pb in SnPb acts as a solid lubricant. It is well known the contact life can be enhanced by supplying a lubricant [21]. PdCo has been proposed as an alternative coating for probe pin surfaces to get better performance [18]. This is true for the NiPd(Au)-plated surface, which is another plating solution for leadfree applications. There should be no oxide issue for this surface because both sides are noble. In the case of a Au-coated probe pin, however, quick wear occurs because of the harder NiPd(Au).
The PdCo probe pin can survive longer because it is harder than NiPd(Au) [11]. On the other hand, when the PdCo probe pin is applied to a Sn/SnPb surface, there is only a small improvement compared with the Au probe pin because the same wear occurs at the tip due to abrasive Sn oxide. Once the thin PdCo layer is worn-off, the remainder of the probe degradation process should be almost identical to the Au case. A significant change before and after failure was observed at the circular band of the Au layer (Fig. 9). Before failure, most of the Au band area was seen as a bright area in BSE images indicating pure Au is present. However, after failure, this band area was covered with Sn or SnPb. This corroborates that the transferred Sn/SnPb on pin tips is the root cause of the probe pin failure. This is also supported by the fact that tin oxide on a rigid surface like pin tips is more difficult to remove than that on a soft surface like Sn/SnPb plated surfaces [16]. The oxide that grows on top of a sharp probe pin can be removed by a geometrical disruption mechanism as shown in Fig. 11. As the probe actuation keeps blunting the tip, the mechanism that pushes the oxide up the side of the pin gradually shifts to a horizontal segregation mechanism. Although the latter is also able to provide metal-to-metal contact, the wiping efficiency would be substantially reduced. Some portion of the Sn oxide can be chipped off while another portion remains. The remaining Sn oxide can accumulate during continuous operation and build up a high resistance barrier, which is similar to Sn fretting. The blunt tip of a degraded probe pin, and mating surface as well, would be subjected to a lower pressure than the sharp tip of a new pin when the same force is applied. It is intuitively anticipated that the contact development force should be changed accordingly. However, surprisingly, an insignificant change was observed in the force/resistance measurements (Fig. 5). The effect of pressure change can be found in the ‘‘onset” force for metalto-metal contact which is indicated by vertical lines on the plots. The onset force of the degraded pin was several times higher than that for the new pin. Based on what has been observed so far, the probing contact mechanism can be described as follows: (1) The probe pin tip breaks the oxide layer at both pin tip and pad surfaces and metal-to-metal contact is initiated (onset force). The contact resistance is suddenly reduced to near the final contact resistance. (2) Metal-to-metal contact is secured as the solder deformation proceeds. At a certain point, the contact resistance is stabilized (contact development force).
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deteriorated the efficiency of Sn oxide wiping action and increased the contact resistance. The degree of probe pin tip wear-out by the matte Sn surface was found to be about 30% higher than that by the smoother eutectic SnPb surface. The presence of Pb, which plays a role as solid lubricant, also reduced the probe pin wear. These factors collectively resulted in a longer life of the probe pin applied to the SnPb surface.
Start of probing
Acknowledgements
Disruption of oxide layers
This work was supported by the Integrated Electronics Engineering Center (IEEC) of the State University of New York at Binghamton. The authors would like to thank Dr. John Lee at The State University of New York at Binghamton for valuable discussions and advice on failure mechanisms. They also want to express their thanks to Aaron Reichman and Oberon Deichman for their excellent work on the 3-axes tester development and Hyungsuk Lee for his support in property measurement using the nanoindentor. References
Vertical segregation
Horizontal segregation
Oxide wipe-away and metal-to-metal contact Fig. 11. Electrical connection mechanisms of sharp and blunt tips.
Table 3 Factors influencing pin life
Rougher pad surface Pin tip wear-out Solid lubricant (Pb)
Beneficial
Detrimental
Likely to break and wipe away Sn oxide on tip N/A Less abrasive to pin tip
Also likely to wear tip out more effectively Harder to remove oxide Also less abrasive to oxide on tip
(3) The final contact resistance is dominated by the degree of oxide wipe away from probe pin tips which depends on wear-out or degradation of pin tips. Degree of wear-out of probe pin tips, pad surface roughness, and the lubricating function of Pb have inter-related effects on probe pin life as listed in Table 3. The blunted tip is simply detrimental to probe pin reliability. The rough pad surface makes it easier to remove Sn oxide on both surfaces while it blunts the probe pin tip more quickly. Pb makes it harder to wipe away Sn oxides while it retards probe pin tip wear. The degree of tip wear for SnPb was smaller than that for Sn at the point of failure. If both cases fail at the same degree of wear, the probe pin for SnPb should endure far longer. However, the adverse effects of Pb and a smooth surface seemed to reduce the potential life span of the probe pin. 4. Conclusions The reliability of probing with a pogo-type probe was investigated. The primary cause of the contact resistance increase was identified as Sn oxide growth on top of the Sn layer transferred to the probe pin surface during probing. The probe pin tip got blunter through wear-out as probing actuation continued. The blunt tip
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