Wafer Level Package Solder Joint Reliability Study for Portable Electronic Devices Se-Young Jang, Tae-Sang Park, Yeon-Sung Kim, Jae-Woo Jeong, Jung-Je Bang+, Dong-Ki Kang+ Micro-Nano Research Team Institute of Mechatronics , Corporate Technology Operations Samsung Electronics. Co., Ltd. 416, Maetan-3Dong, Yeongtong-Gu, Suwon, Korea
[email protected] +
Mobile Business Division, Telecommunication Network Samsung Electronics. Co. Ltd.
Abstract Small form factor WLP (or WLCSP) type component has been increasingly applied to portable electronic devices. However, wafer level package type active components are more vulnerable to the failure than the passive or conventional BGA or QFP type active components. WLP constitute a neck point in terms of PCB assembly reliability. Therefore, it is very important to optimize the structure and material to secure high reliability of PCB assembly. Three variables are chosen for WLP reliability evaluation. They are solder ball size, PCB pad size, and underfill (use or non-use). DOE (Design of Experiment) technique is adopted to analyze variables effects on WLP solder joint lifetime. Board assembly level thermal cycling, high temperature and high humidity, and 4-point bending tests are employed as output parameters. The TC test results are compared with the finite element analysis. Solder joint cracks were mainly initiated from the triple point in IC side where solder, redistribution layer, and UBM meet together. WLP ball size was the major factor to determine solder joint lifetime whereas PCB pad size was not a critical parameter in all three reliability tests. Ironically, for the small ball sized WLPs, underfill decreased thermal fatigue lifetime. This is quite a different result compared with other previous reports. This discrepancy is presumably due to improper combination of underfill material property with high stress induced WLP passivation material, BCB (Benzocyclo Buten). Introduction Today, portable electronic devices have been converged to support multiple functions, combining telephone, camera, PDA, mp3, satellite TV, etc. However, it is required to keep their sizes comparable to simple mobiles phones or even smaller. This is why small form factor package such as WLP (or WLPCSP) are highlighted continuously. The WLP is now commonly used in mobile product components, for example EMI/ESD filer, audio amplifier, DC/DC converter, and LCD voltage regulators. However, its board assembly still has some reliability problems. Especially, Lack of stress buffer layer between IC and PCB board causes cracks at solder joint and IC during assembly process and in usage conditions. IC cracks can be relived by back side coating, but solder joint cracks remain to be solved. We have found that solder life time of WLP is occasionally very short, depending on its function, assembly location in a product, or usage conditions. It is very unusual taken its tiny size and the small number of 0-7803-8906-9/05/$20.00 ©2005 IEEE
pins into account. It hasn’t been cleared what causes these kinds of failures, and it is very critical to understand the failure mechanism in order to increase the reliability and assess the lifetime. In this study, investigated are effects of WLP solder ball size, PCB pad size and underfill condition on the failures. According to the analysis on those effects, the clues, how to optimize those combinations as well as each of 3 variables, are suggested. Experiments Small WLP of 1.5 X 1.5 mm size and 8 pins WLP is chosen for this study. DOE (Design of Experiment) technique is adopted to analyze variables effects on solder joint lifetime. The selected 3 variables are solder ball size (0.17 vs. 0.30 mm), PCB pad size (0.15 vs. 0.24 mm), and underfill (use or non-use). These three variable conditions are shown in Table 1 and the cross-sectional images of the assembled WLP with size variations are appeared in Fig. 1.
(a) Ball Size: 0.17 mm PCB pad: 0.24 mm
(b) Ball Size: 0.17 mm PCB pad: 0.14 mm
(c) Ball Size: 0.3 mm (d) Ball Size: 0.3 mm PCB pad: 0.24mm PCB pad: 0.14mm Fig. 1 Cross-sectional images of 4 design parameters Table I. DOE parameters
660
Level
WLP ball size
PCB pad size
Underfill
-1 +1
0.17 mm 0.30 mm
0.15 mm 0.24 mm
Applied No underfill
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Table II. DOE Table Control Parameters Run Number 1 2 3 4 5 6 7 8
Ball Size -1 1 -1 1 -1 1 -1 1
Pad Size -1 -1 1 1 -1 -1 1 1
Response Paramateres
Underfill
Thermal Cycles (Failure Cycles)
High Temp. High Humidity (Failure Time)
Bending Test (Failure Cycles)
-1 -1 -1 -1 1 1 1 1
The PCB is made of FR-4 with 1mm thickness and 8 layers. The PCB has ENIG (Electroless Ni/Immersion Gold) surface finish and NSMD(Non-Solder Mask Defined) type pads. Daisy chain structure and Pb-free solder ball/paste are applied for the test vehicle. The DOE table for WLP reliability test is shown in Table II. Three response parameters according to three input parameters are chosen. The response parameters are thermal cycle (-55/125 oC, 30 min/30min) life time, high temperature and humidity (85 oC /85% RH) life time, and 4-point bending cycle lifetime (0 ~ -100N, 2Hz). Every run number was conducted 3 times for each test. Due to limited test time, the lived samples after 1000 cycles TC, 1000 hr humidity/temp, 140000 cycles bending test are assumed to have the lifetime of the final stopped cycles. For convenient DOE comparison, total open resistances after reliability test were recorded as 10000 Ω.
Three main parameter effect plots are shown in Fig.3. Ball size is the largest factor for TC reliability. PCB pad size effect is not clearly observed. Amazingly, underfill shows harmful effect on TC reliability. This is a quite different result from the well-known fatigue life extension by underfilling. [1-3] According to the interaction plot in Fig.4, it can be seen that the harmful underfilling effect is only appeared with 0.17 mm ball size, not with 0.3 mm ball. Fig. 5(a) shows a typical TC failure mode of solder joint crack of 0.17 mm ball size with underfill. Most of these failure cases (0.17 ball WLP with underfill) have both solder joint crack in chip side and chip metal layer crack under the BCB layer. Fig. 5(b) represents a typical failure mode of 0.17 ball without underfill. In this case, BCB under layer crack is not detected. In this case, solder crack is also propagated from the triple point where BCB, UBM, and Solder Joint meet together. Main Effects Plot (data means) for TC
Results and Discussion A. Thermal Cycle Test Result Average daisy chain resistance changes are indicated in Fig.2. No failures are observed with 0.30 mm ball size but all conditions with 0.17 mm size ball WLP show failures during the test. Underfilled WLP with 0.17 mm balls are totally opened after 500 cycles regardless of PCB pad size. Then, no-underfilled 0.17 ball size with small PCB pad (0.15 mm) showed abrupt failure after 700 cycles and the larger PCB pad (0.24 mm) condition is followed.
7 0.1
0.3
0
5 0.1
0.2
4
No erfill d un
rfill de Un
1000
TC
850
700
550
400
Pad Size
Ball Size
Underfill
Fig. 3 Main Effects Plot for Thermal Cycles Test Interaction Plot (data means) for TCi 5 0.1
4 0.2
erf und No
fill der Un
1000
Ball Size 0.3
600
0.17 200 1000
Pad Size 0.24
600
0.15 200
Underfill
Fig. 2 Average Resistance Change with Temperature Cycles
Fig. 4 Interaction Plot for Thermal Cycle Test 661
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(a) Failure with underfilled condition shows solder crack with BCB under layer delamination
Fig.6 Average Resistance Changes during Humidity&Temperature Test Main Effects Plot (data means) for 8585
7 0.1
0.3
0
5 0.1
0.2
4
rfill de Un
l No erfil d un
990
8585
930
870
810
750
Ball Size
(b) Failure without underfill just shows solder crack Fig.5 Cross-sectional images of underfilled and no-underfilled failure after thermal cycle failure This BCB under layer crack with underfilling is also reported by Vandevelde [4] and the failure mechanism is considered in the later discussion section.
Underfill
Pad Size
Fig. 7 Main effects plot for humidity & temperature test Interaction Plot (data means) for 8585 i 5 0.1
Ball Size 0.3 0.17
4 0.2
erf und No
Und
ll erfi
1000 750 500
B. High Temperature and High Humidity Test 1000 Pad Size Fig. 6 shows the daisy chain resistance change with time 750 0.24 under 85 oC and 85 RH. % condition. All samples with small 0.15 ball size (0.17 mm) and underfill showed abrupt break after 500 500 hours storage regardless of the pad size. No other failure Underfill combinations are appeared. Note that, no failures are appeared in no-underfill condition even with 0.17 ball mm size. Main effect plot of Fig.7 clearly indicates the harmful effect of small ball size and underfilling in 8585 test. Interaction plot in Fig. 8 shows that the larger ball size Fig. 8 Interaction effects plot for humidity & temp. test (0.30mm) WLP assembly is not sensitive with underfilling D. Discussion though. Table III summarizes the DOE results. Solder ball size C. 4-Point Bending Test Bending Test result in Fig.9 shows that larger ball size and effect is the largest factor for all three reliability test items in underfilling have positive effect to increase bending this work. This is a quite reasonable result because larger reliability. Ball size effect tendency is similar to other TC and solder joint area is influenced by thermal, humidity, and 8585 reliability but underfilling effect displays the opposite bending induced stress when larger solder balls are used. But result. Underfilling increased the solder joint robustness for unexpected harmful underfill effect for TC and temp/humidity bending stress especially for small size (0.17 mm) ball test are observed. This is quite different result from previous reports [1-3]. condition. Fig. 11 shows the failed solder joint image after bending test. Both solder joint crack and BCB under layer crack (or delamination) are observed. These BCB under layer crack appears more frequently with underfilled condition. 662 2005 Electronic Components and Technology Conference
Main Effects Plot (data means) for Bending
7 0.1
0.3
0
5 0.1
0.2
4
No erfil d un
l
rfill de Un
Bending Life
160000
130000
100000
energy densities accumulated per temperature cycle are summarized in Table IV. In this table, we found that currently using low Tg underfill (underfill A) may induce more damage to solder joints than no underfill condition. Using FEM simulation, we have successfully found the underfill material property (underfill B) that relieve the high stress.
70000
40000
Ball Size
Underfill
Pad Size
Fig. 9 Main Effects Plot for 4-point Bending Test Interaction Plot (data means) for Bending 4 0.2
5 0.1
No erfill und
fill der Un
160000
Ball Size 0.3
80000
(a) 3D FEM modeling
0.17 0 160000
Pad Size 0.24
80000
0.15 0
Underfill
(b) Solder Ball Modeling (c) Calculated Interface Area Fig. 12 FEM Modeling of WLP Fig. 10 Inbteraction Effects Plot for 4-point Bending Test Table. IV Average viscoplastic strain energy density accumulated per Temperature Cycle (-55/125 oC) No Underfill Underfill A Underfill B WLP side 481.5 505.6 372.7 PCB side 25.2 29.1 49.5
Fig. 11 Solder joint crack with metal layer delamination after 4-point bending failure (underfilled) Table III. Summary of variables effects on reliability Thermal Cycle Temp/Humidity Bending Ball Size +++ +++ +++ Pad Size No effect + No effect Underfill ++ −− −−
Another possible reason to explain the harmful underfill effect and unexpected too short thermal fatigue lifetime of WLP is that high internal stress of BCB layer influenced the early failure. This is very plausible because most of all failed cross-sectional image have both BCB under layer crack and solder joint crack together. (We investigated more than 90 failed samples.) Vandevelde [4] reported that the existence of the BCB layer can cause higher plastic deformations locally in the solder joints, in case the UBM is directly deposited on metal layer without stress buffer layer. They also reported that BCB induced high stress becomes even higher in the presence of underfill material. (Fig.13) The similar BCB induces harmful stress effect was also reported by Lee [5]. In this background, it is strongly recommended to insert stress buffer layer between UBM and Al pad (or chip passivation layer ie. nitride or oxide) for WLP thermal fatigue reliability. However, the BCB effect on reliability was not experimentally tested in this work scope.
Due to the rework & drop resistance issue, we had applied low Tg material for WLP assembly. It is wondered if low Tg underfill induces larger stress to solder joints even more than no underfilled condition in high temperature. Fig. 12 shows 3D FEM modeling to compare thermal cycle condition with and without underfill. Calculated average viscoplastic strain 663
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References 1. A.Shubert, R.Dudek, J.Kloeser, B.Michel, H.Reichl, T.Hauck, K.Kaskoun “Experimental and Numerical Reliability Investigations of FCOB Assemblies with Process-induced Defects”, Proc 50th Electronic Components and Technology Conf., p.624, 2000 2. Pradeep Lall, Nokibul Islam, Jeff Suhling, Robert Darveaux, “Model for BGA and CSP Reliability in Automotive Applications”, Proc 53th Electronic Components and Technology Conf , 2003 3. John Lau, Dongkai Shangguam, Dennis C.Y. Lau, Terry o Fig. 13 High tensile stresses (at -55 C) are found in the T.W.Kung, S.W.Richy Lee, “Thermal Fatigue Life BCB layer near solder bumping pad [Vandevelde, reference 4] prediction Equation for Wafer0Level Chip Scale package(WLCSP) Lead-Free Solder joints on Lead-Free Conclusions Printed Circuit Board (PCB)”. Proc 54th Electronic 1. WLP ball size is the most dominant factor to determine Components and Technology Conf , 2004. solder joint reliability. It is very improtant that the WLP 4. Bart Vandevelde, Eric Beyne, “Thermal Fatigue Reliability should have enough ball size in order to be assembled Analysis of Redistributed Flip Chip Assemblies”, Proc with other large components (BGA, QFP, etc.). As long 48th Electronic Components and Technology Conf, p.37, as pin pitch is allowed, ball size as large as possible is 1998 recommended. 5. K.O.Lee, Jin Yu, J.Y.Kim, I.S.Park, “Thermo-Mechanical 2. High stress induced by BCB seems to deteriorate solder Reliability of the Benzocyclebuten(BCB) film in a joint reliability and hence to cause very early stage failure WLCSP process”, Int’l Sysposium on Electronic materials of WLP assembly in thermal cycle test. This early stage and Packaging, p.84, 2001 failure is expected to be improved if there is a stress buffer layer between solder bump UBM and final metal pad. 3. The underfill material, that is commonly applied to BGA componets to enhance drop and bending resistance, should be removed or replaced by the other materials for higher reliability of WLP. In order to figure out the right underfill material, reworkablility and mechanical robustness of whole PCB assembly as well as WLP reliability must be take into account 4. Contray to the previous reports [3], PCB pad size didn’t affect WLP assembly reliability. In this study, the other variables like solder ball size, BCB induced stress, and underfilling played key roles to induce the early failures. Acknowledgments The authors would like to express sincere appreciations to Su-Yeon Jung, Soo-Chul Lee , Sankyu Lee at Mobile Business Division for helpful advices and support.
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