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Fatigue crack propagation behavior of underfill materials in microelectronic packaging
Materials Science and Engineering A314 (2001) 194– 200 www.elsevier.com/locate/msea
Fatigue crack propagation behavior of underfill materials in microelectronic packaging Jieping Zhang Intel Corporation, CH5 -158, 5000 W. Chandler Bl6d. Chandler, AZ 85226, USA
Abstract Cracks formed in underfill materials under loading are often observed to continue their propagation into traces in the substrate, which causes electrical failures in microelectronic devices. Hence, a fracture mechanics-based technique was used to characterize the fatigue crack propagation behavior of six different underfill materials under two different environmental conditions, i.e., ambient and 85°C/85% relative humidity (RH). Under the ambient condition, there was a well-defined threshold in each material studied, and the fatigue crack growth rate was found to have a power-law dependence on stress intensity range. However, under the 85°C/85% RH condition, near-threshold instability was observed. The crack growth rate at the near threshold region was found to oscillate from low to high after a period of time during cycling. The near-threshold instability is believed to be the result of interaction between the crack tip and moisture. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Underfill materials; Crack growth rate; Relative humidity
1. Introduction Flip –chip (face down) microelectronic technology [1 –8], a method for interconnecting high I/O count, area array solder bumps on silicon chips to base carriers, i.e., substrates, has been evolving for three decades. In the chip attach process, the solder bump, typically a 95Pb/5Sn alloy, is aligned with respect to a copper pad on the substrate and all solder joints are made simultaneously by reflowing in a reducing or inert atmosphere. The surface tension of the liquid solder enables self-centering of all solder joints during the reflow and the joint is typically of a convex shape. This configuration of the flip –chip jointing is referred to as a ‘‘control collapse chip connection’’, i.e., C4 [6], as schematically illustrated in Fig. 1. The flip – chip technology offers many advantages, such as higher packing density, shorter interconnection length, better electrical performance and better manufacturability, in comparison with the traditional face up wire bonding and tape-automated bonding technologies. A major challenge of the flip – chip technology is solder joint reliability due to the mismatch of the coefficient of thermal expansion (CTE) between the silicon chip and the substrate, and the large distance from the neutral point (DNP) for large chips. Recently, a dramatic improvement of the C4 solder
bump fatigue life was reported by many investigators [8–21] by using particle-filled epoxy resin with low CTE to fill the gap (25 –75 mm) between the silicon chip and the substrate by capillary action. This epoxy-based material, called underfill encapsulant, has a CTE that closely matches that of the solder. This approach, based on finite element analysis [8,13 –18] and experimental verification [20,21], decreases the shear stress and thus lowers the strain applied to C4 joints accordingly. For example, Zheng et al. [18] reported that the number of cycles to failure in the case of an encapsulated joint is about 30 times higher than that of the unencapsulated joint. Consequently, the thermal and mechanical properties of the underfill material have direct impact on the reliability of the C4 solder joints. The underfill material is generally required to have an excellent combination of mechanical and electrical properties along with corrosion resistance, good adhesion between both the silicon chip and the substrate, low viscosity, high purity, low CTE, low alpha activity and low cost [10,12]. From the reliability point of view, the following issues are most important: (1) voids between solder bumps can cause electrical shorts by the mechanism of solder protrusion, (2) interfacial delamination between the underfill material and the silicon ship or the substrate can result in electrical open cir-
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 9 1 7 - 1
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Fig. 1. Schematic of C4 flip –chip packaging in microelectronics.
cuits due to solder bump fracture, and (3) underfill fatigue cracks can propagate through the substrate, cut copper traces in the substrate and cause electrical opens. Most studies of the underfill materials have been focused on thermal and rheological properties during last decade [10,12,22– 24] in order to improve the maufacturability of underfill process, including material cost, throughput time, yield and void level. Only recently, several investigators [25– 27] began to pay attention to the interfacial delamination behavior under monotonic loading for the purpose of optimizing the systems of underfill materials and silicon passivation, and underfill material and the substrate. However, underfill fatigue cracking, which is one of the main failure modes, has not received much attention yet. The intention of this study, therefore, is to investigate the fatigue crack propagation (FCP) behavior of the underfill materials used in microelectronic packaging. The objectives of this study are as follows: (1) to evaluate the fatigue crack growth resistance of several underfill materials to assist materials selection for high volume manufacture and high reliability, (2) to understand the environmental effects on FCP behavior of the underfill materials.
2. Materials and experimental
2.1. Materials and specimens All underfill materials were provided by three manufacturers, A, B, and C, in the form of cured bulk blocks, and will be referred to as materials Al, A2, A3, A4, B, and C. All of them were highly filled materials
Fig. 2. Geometry of CT Specimen. All dimensions in mm. hB 1.9 mm, h =30°.
containing 64–78wt.% silica particles. Table 1 summarizes the basic chemistry, filler loading, elastic modulus, CTE and viscosity of each underfill material. Compact tension (CT) configuration for the samples was selected for this study. All specimens were cut from the molded blocks, and then machined into the geometry shown in Fig. 2 according to ASTM Standards E399 and E647. Accordingly, the thickness of the CT specimens is 5.8 mm. The width (h) of the machined notch is less than 1.9 mm, while the angle of the notch tip is 30°. Surface finish (the arithmetical average deviation from the mean plane) is better than 0.8 mm, including the two loading holes. In order to facilitate precracking, the notch was further sharpened by using a thin razor blade after the Krak-Gage (Ni–Cr metal foil) was attached to the specimen. The Krak-Gage is a thin metal foil bonded to the side of the specimen, and is used to measure crack length via an ‘‘indirect potential drop’’ method [28].
Table 1 Chemistry, filler loading, elastic modulus, CTE and viscosity of underfill materials Materials
Basic chemistry
Filler loading (silica particles) (wt.%)
Elastic modulus (GPa)
CTE (10−6 per °C)
Viscosity (Pa s)
Al A2 A3 A4 B C
Epoxy Epoxy Epoxy Epoxy–Anhydride Cyanate ester Epoxy–Anhydride
60–70 60–70 60–70 70–80 60–70 60–70
7.2 7.2 7.2 11 7.6 7.4
28 28 28 15 27 24
250 100 64 44 3.3 4.2
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2.2. Experimental FCP tests were performed using a high resolution, computer-controlled, MTS 858.02 electro-servo-hydraulic testing system, i.e., MTS 810 load frame with 485 controller along with a Krak-Gage/Fractomat system (Hartrun Corporation, St. Augustine, FL) and CrackWatch fatigue-crack growth software control. The Krak-Gage/Fractomat system was used to monitor crack length with a resolution better than 2 mm. The Fractomat controller supplies a constant current to the Krak-Gage while monitoring the potential drop across the gage. As the crack propagates through the specimen and gage, the change in the potential drop is related to the crack length. The specimens were cyclically loaded at a sinusoidal frequency of 10 Hz under automated load-shedding schemes to obtain growth rates over a wide spectrum from 10 − 3 to 10 − 8 m cycle − 1. A nominal load ratio R of 0.1 was employed. The fatigue threshold value, DKth, below which fatigue crack growth is presumed dormant, was approached by varying the applied loads such that the instantaneous value of DK changed according to the equation: DK =DK0exp[2C*(a −a0)],
(1)
where a0, DK0 are the initial values of crack length and stress intensity factor range. a is the instantaneous value of crack length, DK is the instantaneous value of stress intensity factor range, and C* is the normalized K-gradient set to − 0.1 per mm of crack extension. The crack growth rate (da/dN, where N is the number of cycles) was calculated according to an incremental polynomial procedure. Fatigue precracking was conducted under decreasing DK mode prior to the FCP tests. In an attempt to circumvent possible load interaction effects, a precracking procedure was performed such that the final Kmax during the precracking was less than the initial Kmax of the decreasing DK portion of the test. Soon after 0.5 mm of crack growth was registered in the crack gage during precracking, the FCP test formally started under the decreasing DK condition. After the crack growth rate decreased to 10 − 8 m cycle − 1, the test was continued under increasing DK condition until the crack has propagated through the gage region. The crack path analysis was conducted by scanning electron microscopy (SEM) observations. The results along with discussions of the tests outlined above are presented in Section 3.
3. Results and discussion Epoxy resins have become one of the most important structural materials of recent years. Because of their inherent brittleness and lower FCP resistance, most
researchers have focused on how to toughen the epoxy resins in order to improve their fatigue performance. Three common approaches taken to improve the fatigue resistance include (1) modification of the epoxy using compliant rubbery particles [29–35], (2) reinforcement of the epoxy using rigid inorganic fillers [36,37], and (3) modification of the epoxy using both rubbery particles and rigid fillers [34,35,38–40], namely, synergistic toughening in hybrid epoxy composites. The addition of a compliant rubbery phase can toughen the epoxy resins by promoting process zone mechanisms such as the cavitation of the rubbery particles, shear banding and plastic void growth in the matrix [29,32– 35,41–43]. Matrix shear banding and plastic void growth mechanisms reduce the effective crack driving force [34,35]. On the other hand, the addition of rigid fillers toughens epoxy polymers through crack tip pinning/crack surface bridging mechanisms [34–37,44–46]. Pinning the crack tip causes the crack front to bow between the rigid impenetrable particles, thereby absorbing more energy due to line tension effects [34,35,44]. In addition, the rigid particles bridge the two crack surfaces, which provides resistance to crack opening by applying closure forces which directly reduce the crack driving force at the crack tip [34,35]. Obviously, the objective of hybridizing rubbery particles and rigid particles in epoxy resins is to promote the simultaneous occurrence of cavitation/shear banding induced by the rubbery particles and crack tip pinning/crack surface bridging mechanisms induced by the rigid particles [34,35]. Usually, adding the modifiers (compliant rubbery particles, rigid inorganic fillers, or combination of rubbery particles and rigid fillers) to the epoxy resins can shift the FCP curves toward higher DK values (i.e., higher fatigue threshold DKth), and simultaneously decrease their gradient (i.e., smaller fatigue crack growth rate for a given DK value) [47]. As mentioned previously [8–21], epoxy resins used in electronic packaging are usually modified by inorganic particles, e.g., silica particles, mainly in order to lower the CTE of underfill materials to better match to that of solder bumps. FCP behavior of the underfill materials outlined in Table 1 is presented as follows.
3.1. Fatigue crack propagation beha6ior at an ambient condition Crack growth rate (da/dN) data vs. applied crack driving force (DK) at ambient conditions for all materials listed in Table 1 are plotted in Fig. 3. Note that the da/dN vs. DK curves are sigmoidal if enough data are obtained for a given material, e.g., material C here. That is, the DK dependence of crack growth rate increases markedly at both low and high DK values. At low crack growth rates (the low DK region), the curve approaches a vertical asymptote. A fatigue threshold,
J. Zhang / Materials Science and Engineering A314 (2001) 194–200
Fig. 3. FCP data for the underfill materials. The arrows indicate fatigue threshold values for the materials.
DKth, can be operationally defined as the DK at which crack growth is presumed to be dormant. This is a critical parameter because components operate close to the fatigue threshold regime for the majority of their life [48]. In addition, below the threshold, existing cracks in the components will not grow, therefore, fatigue damage is highly unlikely. Note that all materials studied here show a well-defined fatigue threshold, e.g., materials A4 and A3 have the highest fatigue thresholds (Table 2), 0.72 and 0.70 MPa m0.5, respectively, while materials B and C have the lowest threshold of 0.5 and 0.42 MPa m0.5, respectively. Materials A1 and A2 have a very similar FCP behavior, as expected, since the only difference between materials A1 and A2 is viscosity. A linear region at an intermediate DK regime in a curve represents stable fatigue crack growth. The da/dN vs. DK data pairs tend to follow the Paris–Erdogan power law [49] in such region: da/dN =P(DK)m,
(2)
where P and m are constants, a is crack length, and N is number of cycles. Table 2 shows P and m values for all materials in Fig. 3 as well as the R 2 for the power law fitting. Note that the R 2 are between 0.95 and 0.99, which indicates that the power-law fitting works very
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well. Thus, the fatigue life of the underfill materials can be predicted by the power law for a given material and stress level. In a real microelectronic packaging, scratches and grooves on the underfill surface and voids in the underfill materials can be treated as flaws, and the stress level can be calculated based on the properties of materials (e.g., CTE and module) and a given package using condition (e.g., temperature range); therefore, the data gathered here can be used to roughly predict underfill related packaging reliability. As can be seen from Fig. 3, the FCP resistance of materials A4 and A3 is the highest, followed by materials A1, A2, and B, whereas the FCP resistance of material C is the lowest. For example, at an arbitrary growth rate of 1× 10 − 5 m/cycle, the stress intensity range of material A4 (DK*) is 0.86 MPa m0.5, while DK* of material C is only 0.6 MPa m0.5 (Table 2). This result agrees very well with the observed underfill reliability in actual microelectronic packaging, meaning that material A4 has been observed to be more reliable than material C. Fig. 4 shows the FCP path of materials A4 and C through side-view observation in the SEM. The path for material A4 (Fig. 4(b)) is rougher than that of material C (Fig. 4(a)). Note that silica particles in material A4 are larger and irregular, while particles in material C are relatively smaller and spherical. For both cases, silica particle fracture was not observed, instead, only silica particle debonding from the epoxy was seen, as illustrated by the arrows in Fig. 4. Fig. 5 is the SEM fracture surface observation of material Al fatigued under ambient condition, which shows massive debonding between the epoxy and silica particles regardless of particle size, but no evidence of silica particle fracture in the material. The debonding process causes crack deflection and can shield the crack tip from applied stresses. Therefore, large and irregular particles can more effectively improve the FCP resistance of these materials. Of course, epoxy matrix chemistry is also believed to play an important role during fatigue process. Unfortunately the required detailed information of these epoxy resins cannot be obtained from material manufacturers. It would be interesting to see the effect of silica particles on FCP behavior by
Table 2 Constants (P and m) of the Paris–Erdogan power law, R 2, DK* values (values of DK at a growth rate of 1×10−5 m cycle−1) and DKth for the materials studied Materials
P
m
R2
DK* (MPa m0.5)
DKth (MPa m0.5)
Al A2 A3 A4 B C
8.2×10−2 4.3×10−3 9×10−5 7×10−5 7×10−4 2.8×10−3
30.7 23.1 13 13.5 13 11
0.96 0.95 0.95 0.96 0.99 0.97
0.75 0.77 0.85 0.86 0.7 0.6
0.63 0.63 0.70 0.72 0.50 0.42
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3.2. Fatigue crack propagation beha6ior at 85 °C/85% RH condition
Fig. 4. Side-view SEM observations of FCP path. (a) For material C, silica particle size is small and crack propagation path is relatively smooth. (b) For material A4, silica particles are bigger and irregular, and crack propagation path is relative rough. Arrows illustrate silica particle debonding from the epoxy.
In an actual microelectronic device, the reliability of underfill encapsulants depends mainly on three important material properties, namely, moisture resistance, the residual stress developed from the cure process (cure stress) or from the CTE mismatch (cooling stress) [23], and the applied stress (a fatigue loading) during operation. Usually, the residual stress (static loading) alone cannot cause the failure of the device. The most underfill related failures in microelectronic devices were found to be related to fatigue loading or combination of moisture and fatigue. However, understanding of the FCP behavior of underfill encapsulants under humid conditions is nonexistent. Since the 85°C/85% RH environment is often used as a reliability stress in microelectronic packaging, it was selected as an environmental test condition here. Fig. 6 shows the typical relationship between FCP rate and stress intensity range of material A2 under the 85°C/85% RH condition. For the purpose of comparison, the FCP data under an ambient condition is also plotted. Note that the da/dN vs. DK curve is shifted to a low DK region under the 85°C/85% RH condition. This indicates that FCP resistance was greatly reduced under such environmental conditions. In addition, there is no well-defined fatigue threshold under the humid condition. The fatigue crack growth rate was found to oscillate from low to high when the material was cycled for a long period of time under humid conditions even though the fatigue test was performed under the K-decreasing mode. For example, the growth rate jumped one order of magnitude in material A2, as shown in Fig. 6. This result was confirmed through repeated
Fig. 5. SEM fracture surface observation of material Al fatigued under the ambient condition. Silica particle fracture was not observed, instead, only silica particle debonding from the epoxy was seen.
testing the epoxy resins alone. Due to confidentiality reasons, the material manufacturers could not provide the pure epoxy resins for such tests.
Fig. 6. FCP data for material A2 under ambient and 85°C/85% RH conditions. The da/dN vs. DK curve was shifted to low DK region, and fatigue crack growth rate was found to oscillate from low to high under the 85°C/85% RH condition.
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tests. Such behavior, to the author’s best knowledge, has not been previously reported. Interactions between moisture and crack tip may result in this behavior. The crack-tip plastic zone may accelerate moisture absorption in the crack tip region because of the stress state. It is well known that moisture can degrade the strength of epoxy materials. Therefore, when moisture reaches a critical level to make the material weak enough, the fatigue crack then advances with a high rate. Since the test was performed under decreasing DK mode, the applied DK was automatically decreased further, which had a tendency to slow down the crack growth rate. This, in turn, led to the oscillation of the fatigue crack growth rate with time. This result implies that under moist conditions, theoretically, no matter how small an applied stress is, preexisting cracks inside the material will eventually propagate. Therefore, underfill process needs to be vigorously controlled to minimize flaws in the underfill materials in an actual microelectronic packaging. The shelf-time of microelectronic devices should also be strictly controlled in order to minimize the moisture absorption in the underfill materials. It should be mentioned here that the understanding of the FCP behavior of underfill materials is still preliminary. In order to fully understand the FCP mechanism of the underfill materials and to provide more reliable underfill materials to the microelectronic industry, there are lots of areas which need to be further explored, e.g., (1) to study the characteristics of fracture surfaces at different DK regions, especially at the oscillation region, to understand moisture interaction with epoxy resin and particles; (2) to study the FCP behavior of underfill materials with irregular shape particles or surface-treated particles under different moisture conditions to understand the effect of the reinforcement particles on the FCP behavior; and (3) to study the FCP behavior of pure epoxy resins under different moisture conditions to understand the effectiveness of the reinforcements.
4. Conclusions An investigation of FCP behavior has been carried out on underfill encapsulant materials under ambient and 85°C/85% RH conditions. This investigation leads to the following conclusions: (1) Under the ambient condition, there was a welldefined threshold existing in each material studied, and the relationship between da/dN and DK can be well described by the Paris–Erdogan power law. The size and shape of silica fillers were found to influence FCP behavior. Larger and irregular particles increased FCP resistance through crack deflection. (2) Under the 85°C/85% RH condition, nearthreshold crack propagation instability was observed.
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The crack growth rate at the near threshold region was found to oscillate from low to high after a period of time during cycling even though the test was conducted under decreasing DK mode. The near-threshold instability is believed to be the result of interactions between the crack tip and moisture.
Acknowledgements This work was supported by the Department of Assembly Technology Development of Intel Corporation. The author wishes to thank Mr. Jason Hansen for the provision of SEM pictures. Stimulating discussions with Dr. Daniel J. Belton, Dr. Keith D. Jones and Mr. Robert Starkston at Intel Corporation are gratefully acknowledged.
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Fatigue crack propagation behavior of underfill materials in microelectronic packaging Jieping Zhang Intel Corporation, CH5 -158, 5000 W. Chandler Bl6d. Chandler, AZ 85226, USA
Abstract Cracks formed in underfill materials under loading are often observed to continue their propagation into traces in the substrate, which causes electrical failures in microelectronic devices. Hence, a fracture mechanics-based technique was used to characterize the fatigue crack propagation behavior of six different underfill materials under two different environmental conditions, i.e., ambient and 85°C/85% relative humidity (RH). Under the ambient condition, there was a well-defined threshold in each material studied, and the fatigue crack growth rate was found to have a power-law dependence on stress intensity range. However, under the 85°C/85% RH condition, near-threshold instability was observed. The crack growth rate at the near threshold region was found to oscillate from low to high after a period of time during cycling. The near-threshold instability is believed to be the result of interaction between the crack tip and moisture. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Underfill materials; Crack growth rate; Relative humidity
1. Introduction Flip –chip (face down) microelectronic technology [1 –8], a method for interconnecting high I/O count, area array solder bumps on silicon chips to base carriers, i.e., substrates, has been evolving for three decades. In the chip attach process, the solder bump, typically a 95Pb/5Sn alloy, is aligned with respect to a copper pad on the substrate and all solder joints are made simultaneously by reflowing in a reducing or inert atmosphere. The surface tension of the liquid solder enables self-centering of all solder joints during the reflow and the joint is typically of a convex shape. This configuration of the flip –chip jointing is referred to as a ‘‘control collapse chip connection’’, i.e., C4 [6], as schematically illustrated in Fig. 1. The flip – chip technology offers many advantages, such as higher packing density, shorter interconnection length, better electrical performance and better manufacturability, in comparison with the traditional face up wire bonding and tape-automated bonding technologies. A major challenge of the flip – chip technology is solder joint reliability due to the mismatch of the coefficient of thermal expansion (CTE) between the silicon chip and the substrate, and the large distance from the neutral point (DNP) for large chips. Recently, a dramatic improvement of the C4 solder
bump fatigue life was reported by many investigators [8–21] by using particle-filled epoxy resin with low CTE to fill the gap (25 –75 mm) between the silicon chip and the substrate by capillary action. This epoxy-based material, called underfill encapsulant, has a CTE that closely matches that of the solder. This approach, based on finite element analysis [8,13 –18] and experimental verification [20,21], decreases the shear stress and thus lowers the strain applied to C4 joints accordingly. For example, Zheng et al. [18] reported that the number of cycles to failure in the case of an encapsulated joint is about 30 times higher than that of the unencapsulated joint. Consequently, the thermal and mechanical properties of the underfill material have direct impact on the reliability of the C4 solder joints. The underfill material is generally required to have an excellent combination of mechanical and electrical properties along with corrosion resistance, good adhesion between both the silicon chip and the substrate, low viscosity, high purity, low CTE, low alpha activity and low cost [10,12]. From the reliability point of view, the following issues are most important: (1) voids between solder bumps can cause electrical shorts by the mechanism of solder protrusion, (2) interfacial delamination between the underfill material and the silicon ship or the substrate can result in electrical open cir-
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Fig. 1. Schematic of C4 flip –chip packaging in microelectronics.
cuits due to solder bump fracture, and (3) underfill fatigue cracks can propagate through the substrate, cut copper traces in the substrate and cause electrical opens. Most studies of the underfill materials have been focused on thermal and rheological properties during last decade [10,12,22– 24] in order to improve the maufacturability of underfill process, including material cost, throughput time, yield and void level. Only recently, several investigators [25– 27] began to pay attention to the interfacial delamination behavior under monotonic loading for the purpose of optimizing the systems of underfill materials and silicon passivation, and underfill material and the substrate. However, underfill fatigue cracking, which is one of the main failure modes, has not received much attention yet. The intention of this study, therefore, is to investigate the fatigue crack propagation (FCP) behavior of the underfill materials used in microelectronic packaging. The objectives of this study are as follows: (1) to evaluate the fatigue crack growth resistance of several underfill materials to assist materials selection for high volume manufacture and high reliability, (2) to understand the environmental effects on FCP behavior of the underfill materials.
2. Materials and experimental
2.1. Materials and specimens All underfill materials were provided by three manufacturers, A, B, and C, in the form of cured bulk blocks, and will be referred to as materials Al, A2, A3, A4, B, and C. All of them were highly filled materials
Fig. 2. Geometry of CT Specimen. All dimensions in mm. hB 1.9 mm, h =30°.
containing 64–78wt.% silica particles. Table 1 summarizes the basic chemistry, filler loading, elastic modulus, CTE and viscosity of each underfill material. Compact tension (CT) configuration for the samples was selected for this study. All specimens were cut from the molded blocks, and then machined into the geometry shown in Fig. 2 according to ASTM Standards E399 and E647. Accordingly, the thickness of the CT specimens is 5.8 mm. The width (h) of the machined notch is less than 1.9 mm, while the angle of the notch tip is 30°. Surface finish (the arithmetical average deviation from the mean plane) is better than 0.8 mm, including the two loading holes. In order to facilitate precracking, the notch was further sharpened by using a thin razor blade after the Krak-Gage (Ni–Cr metal foil) was attached to the specimen. The Krak-Gage is a thin metal foil bonded to the side of the specimen, and is used to measure crack length via an ‘‘indirect potential drop’’ method [28].
Table 1 Chemistry, filler loading, elastic modulus, CTE and viscosity of underfill materials Materials
Basic chemistry
Filler loading (silica particles) (wt.%)
Elastic modulus (GPa)
CTE (10−6 per °C)
Viscosity (Pa s)
Al A2 A3 A4 B C
Epoxy Epoxy Epoxy Epoxy–Anhydride Cyanate ester Epoxy–Anhydride
60–70 60–70 60–70 70–80 60–70 60–70
7.2 7.2 7.2 11 7.6 7.4
28 28 28 15 27 24
250 100 64 44 3.3 4.2
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2.2. Experimental FCP tests were performed using a high resolution, computer-controlled, MTS 858.02 electro-servo-hydraulic testing system, i.e., MTS 810 load frame with 485 controller along with a Krak-Gage/Fractomat system (Hartrun Corporation, St. Augustine, FL) and CrackWatch fatigue-crack growth software control. The Krak-Gage/Fractomat system was used to monitor crack length with a resolution better than 2 mm. The Fractomat controller supplies a constant current to the Krak-Gage while monitoring the potential drop across the gage. As the crack propagates through the specimen and gage, the change in the potential drop is related to the crack length. The specimens were cyclically loaded at a sinusoidal frequency of 10 Hz under automated load-shedding schemes to obtain growth rates over a wide spectrum from 10 − 3 to 10 − 8 m cycle − 1. A nominal load ratio R of 0.1 was employed. The fatigue threshold value, DKth, below which fatigue crack growth is presumed dormant, was approached by varying the applied loads such that the instantaneous value of DK changed according to the equation: DK =DK0exp[2C*(a −a0)],
(1)
where a0, DK0 are the initial values of crack length and stress intensity factor range. a is the instantaneous value of crack length, DK is the instantaneous value of stress intensity factor range, and C* is the normalized K-gradient set to − 0.1 per mm of crack extension. The crack growth rate (da/dN, where N is the number of cycles) was calculated according to an incremental polynomial procedure. Fatigue precracking was conducted under decreasing DK mode prior to the FCP tests. In an attempt to circumvent possible load interaction effects, a precracking procedure was performed such that the final Kmax during the precracking was less than the initial Kmax of the decreasing DK portion of the test. Soon after 0.5 mm of crack growth was registered in the crack gage during precracking, the FCP test formally started under the decreasing DK condition. After the crack growth rate decreased to 10 − 8 m cycle − 1, the test was continued under increasing DK condition until the crack has propagated through the gage region. The crack path analysis was conducted by scanning electron microscopy (SEM) observations. The results along with discussions of the tests outlined above are presented in Section 3.
3. Results and discussion Epoxy resins have become one of the most important structural materials of recent years. Because of their inherent brittleness and lower FCP resistance, most
researchers have focused on how to toughen the epoxy resins in order to improve their fatigue performance. Three common approaches taken to improve the fatigue resistance include (1) modification of the epoxy using compliant rubbery particles [29–35], (2) reinforcement of the epoxy using rigid inorganic fillers [36,37], and (3) modification of the epoxy using both rubbery particles and rigid fillers [34,35,38–40], namely, synergistic toughening in hybrid epoxy composites. The addition of a compliant rubbery phase can toughen the epoxy resins by promoting process zone mechanisms such as the cavitation of the rubbery particles, shear banding and plastic void growth in the matrix [29,32– 35,41–43]. Matrix shear banding and plastic void growth mechanisms reduce the effective crack driving force [34,35]. On the other hand, the addition of rigid fillers toughens epoxy polymers through crack tip pinning/crack surface bridging mechanisms [34–37,44–46]. Pinning the crack tip causes the crack front to bow between the rigid impenetrable particles, thereby absorbing more energy due to line tension effects [34,35,44]. In addition, the rigid particles bridge the two crack surfaces, which provides resistance to crack opening by applying closure forces which directly reduce the crack driving force at the crack tip [34,35]. Obviously, the objective of hybridizing rubbery particles and rigid particles in epoxy resins is to promote the simultaneous occurrence of cavitation/shear banding induced by the rubbery particles and crack tip pinning/crack surface bridging mechanisms induced by the rigid particles [34,35]. Usually, adding the modifiers (compliant rubbery particles, rigid inorganic fillers, or combination of rubbery particles and rigid fillers) to the epoxy resins can shift the FCP curves toward higher DK values (i.e., higher fatigue threshold DKth), and simultaneously decrease their gradient (i.e., smaller fatigue crack growth rate for a given DK value) [47]. As mentioned previously [8–21], epoxy resins used in electronic packaging are usually modified by inorganic particles, e.g., silica particles, mainly in order to lower the CTE of underfill materials to better match to that of solder bumps. FCP behavior of the underfill materials outlined in Table 1 is presented as follows.
3.1. Fatigue crack propagation beha6ior at an ambient condition Crack growth rate (da/dN) data vs. applied crack driving force (DK) at ambient conditions for all materials listed in Table 1 are plotted in Fig. 3. Note that the da/dN vs. DK curves are sigmoidal if enough data are obtained for a given material, e.g., material C here. That is, the DK dependence of crack growth rate increases markedly at both low and high DK values. At low crack growth rates (the low DK region), the curve approaches a vertical asymptote. A fatigue threshold,
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Fig. 3. FCP data for the underfill materials. The arrows indicate fatigue threshold values for the materials.
DKth, can be operationally defined as the DK at which crack growth is presumed to be dormant. This is a critical parameter because components operate close to the fatigue threshold regime for the majority of their life [48]. In addition, below the threshold, existing cracks in the components will not grow, therefore, fatigue damage is highly unlikely. Note that all materials studied here show a well-defined fatigue threshold, e.g., materials A4 and A3 have the highest fatigue thresholds (Table 2), 0.72 and 0.70 MPa m0.5, respectively, while materials B and C have the lowest threshold of 0.5 and 0.42 MPa m0.5, respectively. Materials A1 and A2 have a very similar FCP behavior, as expected, since the only difference between materials A1 and A2 is viscosity. A linear region at an intermediate DK regime in a curve represents stable fatigue crack growth. The da/dN vs. DK data pairs tend to follow the Paris–Erdogan power law [49] in such region: da/dN =P(DK)m,
(2)
where P and m are constants, a is crack length, and N is number of cycles. Table 2 shows P and m values for all materials in Fig. 3 as well as the R 2 for the power law fitting. Note that the R 2 are between 0.95 and 0.99, which indicates that the power-law fitting works very
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well. Thus, the fatigue life of the underfill materials can be predicted by the power law for a given material and stress level. In a real microelectronic packaging, scratches and grooves on the underfill surface and voids in the underfill materials can be treated as flaws, and the stress level can be calculated based on the properties of materials (e.g., CTE and module) and a given package using condition (e.g., temperature range); therefore, the data gathered here can be used to roughly predict underfill related packaging reliability. As can be seen from Fig. 3, the FCP resistance of materials A4 and A3 is the highest, followed by materials A1, A2, and B, whereas the FCP resistance of material C is the lowest. For example, at an arbitrary growth rate of 1× 10 − 5 m/cycle, the stress intensity range of material A4 (DK*) is 0.86 MPa m0.5, while DK* of material C is only 0.6 MPa m0.5 (Table 2). This result agrees very well with the observed underfill reliability in actual microelectronic packaging, meaning that material A4 has been observed to be more reliable than material C. Fig. 4 shows the FCP path of materials A4 and C through side-view observation in the SEM. The path for material A4 (Fig. 4(b)) is rougher than that of material C (Fig. 4(a)). Note that silica particles in material A4 are larger and irregular, while particles in material C are relatively smaller and spherical. For both cases, silica particle fracture was not observed, instead, only silica particle debonding from the epoxy was seen, as illustrated by the arrows in Fig. 4. Fig. 5 is the SEM fracture surface observation of material Al fatigued under ambient condition, which shows massive debonding between the epoxy and silica particles regardless of particle size, but no evidence of silica particle fracture in the material. The debonding process causes crack deflection and can shield the crack tip from applied stresses. Therefore, large and irregular particles can more effectively improve the FCP resistance of these materials. Of course, epoxy matrix chemistry is also believed to play an important role during fatigue process. Unfortunately the required detailed information of these epoxy resins cannot be obtained from material manufacturers. It would be interesting to see the effect of silica particles on FCP behavior by
Table 2 Constants (P and m) of the Paris–Erdogan power law, R 2, DK* values (values of DK at a growth rate of 1×10−5 m cycle−1) and DKth for the materials studied Materials
P
m
R2
DK* (MPa m0.5)
DKth (MPa m0.5)
Al A2 A3 A4 B C
8.2×10−2 4.3×10−3 9×10−5 7×10−5 7×10−4 2.8×10−3
30.7 23.1 13 13.5 13 11
0.96 0.95 0.95 0.96 0.99 0.97
0.75 0.77 0.85 0.86 0.7 0.6
0.63 0.63 0.70 0.72 0.50 0.42
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3.2. Fatigue crack propagation beha6ior at 85 °C/85% RH condition
Fig. 4. Side-view SEM observations of FCP path. (a) For material C, silica particle size is small and crack propagation path is relatively smooth. (b) For material A4, silica particles are bigger and irregular, and crack propagation path is relative rough. Arrows illustrate silica particle debonding from the epoxy.
In an actual microelectronic device, the reliability of underfill encapsulants depends mainly on three important material properties, namely, moisture resistance, the residual stress developed from the cure process (cure stress) or from the CTE mismatch (cooling stress) [23], and the applied stress (a fatigue loading) during operation. Usually, the residual stress (static loading) alone cannot cause the failure of the device. The most underfill related failures in microelectronic devices were found to be related to fatigue loading or combination of moisture and fatigue. However, understanding of the FCP behavior of underfill encapsulants under humid conditions is nonexistent. Since the 85°C/85% RH environment is often used as a reliability stress in microelectronic packaging, it was selected as an environmental test condition here. Fig. 6 shows the typical relationship between FCP rate and stress intensity range of material A2 under the 85°C/85% RH condition. For the purpose of comparison, the FCP data under an ambient condition is also plotted. Note that the da/dN vs. DK curve is shifted to a low DK region under the 85°C/85% RH condition. This indicates that FCP resistance was greatly reduced under such environmental conditions. In addition, there is no well-defined fatigue threshold under the humid condition. The fatigue crack growth rate was found to oscillate from low to high when the material was cycled for a long period of time under humid conditions even though the fatigue test was performed under the K-decreasing mode. For example, the growth rate jumped one order of magnitude in material A2, as shown in Fig. 6. This result was confirmed through repeated
Fig. 5. SEM fracture surface observation of material Al fatigued under the ambient condition. Silica particle fracture was not observed, instead, only silica particle debonding from the epoxy was seen.
testing the epoxy resins alone. Due to confidentiality reasons, the material manufacturers could not provide the pure epoxy resins for such tests.
Fig. 6. FCP data for material A2 under ambient and 85°C/85% RH conditions. The da/dN vs. DK curve was shifted to low DK region, and fatigue crack growth rate was found to oscillate from low to high under the 85°C/85% RH condition.
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tests. Such behavior, to the author’s best knowledge, has not been previously reported. Interactions between moisture and crack tip may result in this behavior. The crack-tip plastic zone may accelerate moisture absorption in the crack tip region because of the stress state. It is well known that moisture can degrade the strength of epoxy materials. Therefore, when moisture reaches a critical level to make the material weak enough, the fatigue crack then advances with a high rate. Since the test was performed under decreasing DK mode, the applied DK was automatically decreased further, which had a tendency to slow down the crack growth rate. This, in turn, led to the oscillation of the fatigue crack growth rate with time. This result implies that under moist conditions, theoretically, no matter how small an applied stress is, preexisting cracks inside the material will eventually propagate. Therefore, underfill process needs to be vigorously controlled to minimize flaws in the underfill materials in an actual microelectronic packaging. The shelf-time of microelectronic devices should also be strictly controlled in order to minimize the moisture absorption in the underfill materials. It should be mentioned here that the understanding of the FCP behavior of underfill materials is still preliminary. In order to fully understand the FCP mechanism of the underfill materials and to provide more reliable underfill materials to the microelectronic industry, there are lots of areas which need to be further explored, e.g., (1) to study the characteristics of fracture surfaces at different DK regions, especially at the oscillation region, to understand moisture interaction with epoxy resin and particles; (2) to study the FCP behavior of underfill materials with irregular shape particles or surface-treated particles under different moisture conditions to understand the effect of the reinforcement particles on the FCP behavior; and (3) to study the FCP behavior of pure epoxy resins under different moisture conditions to understand the effectiveness of the reinforcements.
4. Conclusions An investigation of FCP behavior has been carried out on underfill encapsulant materials under ambient and 85°C/85% RH conditions. This investigation leads to the following conclusions: (1) Under the ambient condition, there was a welldefined threshold existing in each material studied, and the relationship between da/dN and DK can be well described by the Paris–Erdogan power law. The size and shape of silica fillers were found to influence FCP behavior. Larger and irregular particles increased FCP resistance through crack deflection. (2) Under the 85°C/85% RH condition, nearthreshold crack propagation instability was observed.
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The crack growth rate at the near threshold region was found to oscillate from low to high after a period of time during cycling even though the test was conducted under decreasing DK mode. The near-threshold instability is believed to be the result of interactions between the crack tip and moisture.
Acknowledgements This work was supported by the Department of Assembly Technology Development of Intel Corporation. The author wishes to thank Mr. Jason Hansen for the provision of SEM pictures. Stimulating discussions with Dr. Daniel J. Belton, Dr. Keith D. Jones and Mr. Robert Starkston at Intel Corporation are gratefully acknowledged.
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