Dynamic Test and Modeling Methodology for BGA Solder Joint Shock ...

Dynamic Test and Modeling Methodology for BGA Solder Joint Shock Reliability Evaluation Phil Geng Intel Corporation 5200 NE Elam Young Parkway, Hillsboro, OR 97124 [email protected]

Introduction The BGA solder joint reliability under dynamic loads becomes more significant with smaller component/joint size, the leadfree solder alloy, ENIG, heavy thermal solutions (desktop PC and server), and more severe shock conditions (cell phone drop). While the traditional shock test methodologies [1, 2] are indispensable for today’s product evaluation, a more efficient methodology is needed to evaluate solder joint failure envelope under dynamic load. At system level, use-condition-based shock test and simulation were studied in details by many researchers, e.g. desktop PC [3-5] and cell phone [6, 7]. The predicted overall system behavior through the finite element analysis agrees well with the test data. For shock testing, solder joint failures may not dominant at certain shock level due to system characteristics since difference system failure modes may vary at different shock conditions [8]. For shock simulation, the solder joint reliability prediction is limited by the understanding of strain rate dependent solder material properties and failure strengh/mode/mechnism [9]. All of these concerns require a clean component level test and modeling methodology to remove the complicated system configuration effect. At component level, four-point bend test with strain measurement has become popular for solder joint reliability evaluations [9-13]. Although the test is best suited for assembly test and handling use conditions, it can be applied to shock reliability at higher load speed. The solder joint failure 0-7803-8906-9/05/$20.00 ©2005 IEEE

mode and failure mechanisms are quite consistent between the high speed flexural test and the shock test. However, the monotonic high-speed test does not represent the real shock behavior, as shown in Figure 1. The dynamic oscillation, the damping and the cyclic fatigue are all contributing to the solder joint failure, which also makes finite element modeling of strain rate effect difficult. Shock along +Z 0.3

Strain Gage 1 Strain Gage 2

0.2

Strain (%)

Abstract This work developed a dynamic test and modeling methodology for BGA solder joint shock strength evaluation. A test board and a test fixture were designed similar to a fourpoint bend test. The test setup for BGA evaluation was calibrated to a typical desktop PC motherboard under packaged shock condition. The fundamental frequency of the proposed test setup is matched to that of the motherboard through the experimental modal analysis. The BGA solder joint shock failure envelope was established through the proposed shock test and modeling. With an incremental shock sequence and an in-situ solder joint continuity monitoring setup for the shock events, the Glevel (acceleration) and shock duration at BGA solder joint failure was measured. The dynamic finite element analysis was performed with the experimental input and the test board dynamic response to the measured solder joint failure shock level was simulated. The failure strengths of the solder joints were estimated with different BGA orientations on the test board. A preliminary solder joint failures envelope under dynamic load is established, which represents key board and system levels shock conditions.

Strain Gage 3

0.1

Strain gage 4

0 0

0.05

0.1

0.15

0.2

-0.1 -0.2 -0.3 Tim e (Sec)

Figure 1. Dynamic behavior on a PWB surface near BGA four corners – strain histories [4] To avoid the above issues, a four-point bend like shock test fixture, test board and test procedure were proposed to IPC [8, 14, 15]. The test board and fixture were adjusted to the system characteristics – the resonant frequencies of the system. In this paper, a desktop PC motherboard was used to demonstrate the test process.

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Figure 2. A desktop motherboard with a dummy heatsink 2005 Electronic Components and Technology Conference

System Characterization A typical desktop motherboard was used to represent a system, as shown in Figure 2. A dummy heat sink of 600 grams was attached to the heatsink. The motherboard is mounted on a shock test fixture. Experimental modal analysis was performed on the motherboard under the shock configuration. The average fundamental modal frequency is 75 Hz. Comparing with the finite element analysis in Table 1, the experimental and numerical results agree well. The mode shape is shown in Figure 3. More detailed discussion of motherboard modal analysis and dynamic test/simulation can be found in [3-5]

match the tested system (motherboard). A detailed parts of the test fixture is shown in Figure 5. The test boards are similar to those used in a four-point bend test, as shown in Figure 6. The board form factor design is simple and usable for four-point bend test. Give the focus is the solder joint reliability of the BGA on the tested system (motherboard), all other system characteristics were eliminated from the test board, except the BGA component. The BGA can be oriented at 45 or 90 degrees from the test board. More detailed discussion of test board and test fixture designs can be found in [14].

Table 1. Finite element result Finite Element Analysis Fundamental Frequency Without heatsink preload on CPU 71 Hz With heatsink preload on CPU 74 Hz

BGA

Figure 5. Details of the test fixture

Fundamental Mode

Figure 3. Mode shape – finite element analysis

Figure 6. Test boards Figure 4. Test fixture Test Fixture and Test board A four-point bend like shock test fixture was designed, as shown in Figure 4. By adjusting the mass on the loading span and the length of the supporting/loading spans, the fundamental frequencies of the system can be adjusted to

Modal Analysis for System Correlation In order to match the fundamental frequency of the tested motherboard, different masses were used, as shown in Table 2. The experimental data showed that the BGA size and orientation has relatively small effect on the fundamental frequency of the test system. Therefore, same mass can be

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used for different BGA orientations for the same system correlation.

Shock Test Procedure Two different shock procedures were tested as following: (a) Plan different shock levels and subject one test board at one shock level repeatedly until failure is detected (b) Plan an incremental shock sequence at different shock level and subject one test board to the shock sequence until failure is detected The first approach was straight forward and simple when the test boards are abundant. The second approach is more test time consuming, but need less test samples. In this paper, the second approach is discussed in details. For different shock applications, difference shock pulse shape and duration can be selected. For motherboard applications in desktop PC, a trapezoidal shock pulse with 4.3 m/s velocity change is used. The acceleration level is increased linearly in the first 8 shock events and stayed at the constant acceleration level after the 8th shock event, as shown in Figure 8. The proposed shock sequence starts from a typical packaged product shock and ended with a typical in field use shock. It includes the capability of testing both brittle solder joint shock failure and fatigue shock failure. A specific shock sequence can be defined more suitable for its specific application, and should not be confined to the proposed sequence.

Table 2. Test board fundamental frequency 37.5 mm BGA 42.5 mm BGA 42.5 mm BGA Mass (g)

90 degree orientation


96

68.8

68.3

234

53.0

53.0

54.2

397

43.3

42.5

44.5

512

37.5

37.5

37.8


Table 3. Comparison of mass effect on fundamental frequency Test fixture mass Simulated Measured (gram) Frequency (Hz) Frequency (Hz) 96

70

68.8

234

52

53

397

40.4

42.5

512

36.1

37.5

real system and the test fixture through mass and fixture spans adjustment.

Typical Incremental Shock Sequence 140 120

G-level

100 80 60 40

Incremental Shock

20

Typical Spec Level

0 0

5

10

15

20

25

30

Number of Incremental Shock Sequence

Figure 8. Incremental shock test Figure 7. Fundamental mode shape of test board with 37.5 mm BGA and 234 grams mass (FEA) Finite element modal analysis was performed to correlate to the experimental data. The mass effect on the 37.5 mm BGA was simulated, as shown in Table 3 and Figure 7. The measured and the simulated data agree very well. By choosing 96 grams heatsink, the test board and the system (motherboard) will have the same fundamental frequency, which ensure similar shock behavior. Also, by validation the FEA model with the experimental modal analysis, the finite element dynamic analysis is calibrated at high confidence level. One can match more modes between a

Shock Test – In-situ Continuity Monitoring The solder joint continuity is monitored through both the daisy chain through the joints and strain history on PCB or BGA substrate. The maximum strain in the dynamic test is contributed more by the dynamic characteristics, than by the damage characteristics of solder joints. Therefore, total solder joint open is monitored through the BGA daisy chains. A shock joint opening typically closes after the shock level is damped, as shown in Figure 9(a). The first solder joint open is generally not occur at the max strain level. Further shock of a board with the first solder joint opening will result in the repeated joint opening and closing, as shown in Figure 9(b).

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Finite Element Simulation Dynamic finite element analysis was performed on both the 45 and 90 degrees of BGA orientation. The mean acceleration in Table 4 and the velocity change (4.3 m/s) when solder joint failed were input to the FEA model, as shown in Figure 11. The Young’s modulus is assumed strain rate-independent and the yield strength is strain ratedependent.

Shock #37 Shock sequence #N 6

Voltage

4 2

Strain

0 -0.1 -2

Continuity 0

0.1

0.2

0.3

0.4

-4 Time

(sec)

(a) The shock event when the first solder joint failure was observed Shock #38 Shock sequence #N+1 6

Voltage

4 2

Strain

0 -0.1 -2

Continuity 0

0.1

0.2

0.3

0.4

-4

(a)

Time

BGA 90 o orientation

(b) Next shock event after solder joint failure detected Figure 9. Characteristics of solder joint shock failure Shock Test Results For the test boards with 45o and 90o BGA orientations (Figure 6), the incremental shock test (Figure 8) was performed for each board until the solder joint open was identified. Shock accelerations at solder joint opens were summarized in Figure 10. The data variation is significant, which is one of the characteristics of solder joint shock failure. The means and standard deviations are listed in Table 4. The 45 o BGA orientation showed the lower failure threshold as expected.

Failure G Level

120

(b) BGA 45 o orientation Figure 11. FEA models

100 45 Degree

80

90 Degree

60 40

Printed Circuit Board .....

20 0 0

30

60

90

Ball Grid Array

BGA Orientation

Figure 10. Input acceleration levels to the test board/fixture when solder joints failed Table 4. Input shock level when solder joints failed 90 o BGA Failure G 45 o BGA Orientation Orientation Level Mean Std Dev

65 G 15 G

93 G 10 G

Corner Joint Figure 12. Corner solder joint The corner solder joint force histories (Figure 12) is simulated with the input shock event, which is the shock level when solder joints opened. The axial and shear forces were extracted from the FEA model, as shown in Figure 13. The 657


shear force is comparable to the axial force along the test board longitudinal direction and is negligible along the test board transverse direction. Test Coupon with 90 Degree BGA (Brookdale-G) at Corner Joint 15

Axial Force Shear Longitudinal

10

Shear Transverse

Force (N)

5 0 0

5

10

15

-5 -10 -15 Tim e (m sec)

Note that the extracted solder joint shock strengths from the test/modeling approach were demonstrated under a specific user-defined shock condition. Figure 14 showed the axial force histories of the solder joint outer rows of the BGA with the 90 o orientation. The row direction is along the short axis of the test board. The variation of the joint force in one board from the simulation is much smaller than the variation of the joint failure loads between test boards from the experiment in Figure 10. This indicates that the variation due to location is smaller than the intrinsic variation of individual solder joints. Therefore, the data from the 90 o orientation represents the lower bound of solder joint strength (minimum strength of the daisy chained joints of the outer rows); while the data of the 45 o orientation represents the mean value of solder joint strength.

(a) BGA 90 o orientation Test Coupong with 45 Degree BGA (Brookdale-G) at Corner Joint 15

Axial Force Shear Longitudinal

Force (N)

10

Shear Transverse

5 0 -5

0

5

10

15

-10 -15 Time (msec)

Figure 14. Solder joint axial force histories along the outer rows of BGA (90 o orientation)

(b) BGA 45 o orientation Figure 13. FEA models Table 5. Maximum axial load BGA Orientation 90 degree Max Axial

The strain distributions on the board and BGA substrate surfaces are shown in Figure 15. The high stress and strain areas are consistent with the area where the solder joints were investigated, i.e. the outer row for the 90 o BGA and the corner joint for the 45 o BGA. With the strain level being the indirect indication of the solder joint strength, this work focused on extracting the solder joint shock strength directly. Therefore, no further strain level analysis was performed.

45 degree

Corresponding Shear (Longitudinal of Test Coupon)

6.87 N (1.5 lbf) 7.40 N (1.7 lbf)

13.96 N (3.1 lbf) 11.58 N (2.6 lbf)

Corresponding Shear (Transverse of Test Coupon) Corresponding Time

0.76 N (0.2 lbf) 4.02 msec

~0 4.06 msec

The maximum axial loads and the corresponding shear loads and times are listed in Table 5, which are below lower bound of the input strain rate dependent yield strength. The axial loads are at the same order of the high strain rate solder ball pull data [16] and therefore, can be referenced as solder joint shock strength. The shock fatigue effect is included if brittle failure is not dominant.

Conclusions A shock test and modeling methodology is developed to evaluate solder joint strength under dynamic loads. The proposed procedure is limited to any specific shock pulse, shape or test sequence.

Acknowledgments I would like to thank to Philip H. Chen, Marco Beltman, George Daskalakis, David Shia, and Dan Tong for their key contributions to this project. Thanks to Richard Williams, Mike Lane, Gaurang Choksi, Murli Tirumala, Yun Ling, Martin Rausch, Bill Bader, Kris Frutschy, Luke Garner and Yinan Wu for their support during the course of this research. Also, the contribution of Mike Givens, Eric Salskov, Mike 658 2005 Electronic Components and Technology Conference

Reynolds, Karissa Blue, Norm Armendariz, Ellen Tan, Mike Williams and Pete Robson are gratefully acknowledged. 6. 7. 8.

9.

10. (a)

BGA 90 o orientation 11.

12.

13.

14. (b) BGA 45 o orientation Figure 15. Maximum principal strain distribution under axial force is maximal References 1. Newton, R,E., “Fragility Assessment Theory and Test Procedure”, 1968 2. Steinberg, D.S., Vibration Analysis for Electronic Equipment, 2nd Ed., John Wiley & Sons, New York, NY, 1988 3. Pitarresi, J, P. Geng, W. M. Beltman, and Y. Ling, “Dynamic Modeling and Measurement of Personal Computer Motherboard,” Proc 52th Electronic Components and Technology Conf, San Diego, CA, May, 2002 4. Geng, Phil, and W. M. Beltman, “Monitoring Motherboard Shock Response near BGA Solder Joints,” Proc SMTA International Conf, Chicago, IL, September, 2002 5. Pitarresi, James, Brian Roggeman, Satish Chaparada and Phil Geng, “Mechanical Shock Testing and Modeling of

15.

16.

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PC Motherboards,” Proc 54th Electronic Components and Technology Conf, Las Vegas, June, 2004 Zhu, Liping, et al., Drop Impact Reliability Analysis, Proc. ITHERM, Las Vegas, 2004 Lall, P., et al., “Shock induced failure prediction model for fine pitch BGAs and CSPs,” Proc. SMTAI, Chicago, 2004 Geng, Phil, “Solder Joint Shock Testing and Modeling Methodology Development,” IPC Annual Conference, IPC 6-10d Committee Meeting, Minneapolis, MN, September. 2003 Geng, Phil, Philip H. Chen, and Yun Ling, “Effect of Strain Rate on Solder Joint Failure under Mechanical Load,” Proc 52nd Electronic Components and Technology Conf, San Diego, CA, May, Pp. 974-978, 2002 Harada, K, S. Baba, Q. Wu, H. Matsushima, T. Matsunaga, Y. Uegai, and M. Kimura, “Analysis of Solder Joint Fracture under Mechanical Bending Test,” Proc 53th Electronic Components and Technology Conf, New Orleans, LA, May. 2003 Geng, Phil, Alan McAllister, Carolyn McCormick, Mitul Modi and Arnaldo Nazario, “BGA Solder Joint Strength under Flexural Load,” Proc SMTA International, Chicago, IL, September, 2004 Bansal, Anurag, Sam Yoon and Vadali Mahadev, “Flexural Strength of BGA Solder Joints with ENIG Substrate Finish under 4-Point Bend Test,” Pan Pacific Microelectronics conference, January, 2005 Geng, Phil, Mitul Modi, Carolyn McCormick, Alan McAllister, Arnaldo Nazario and Richard Williams, “A Comparative Study of BGA Solder Joint Reliability Under Four-Point Bend and Spherical Bend Tests,” Proc. IMAPS Electronics Packaging Conf., Scottsdale, AZ, 2005 Geng, Phil, Willem M. Beltman, Philip H. Chen, George Daskalakis, David Shia and Michael H. Williams, “Modal Analysis for BGA Shock Test Board and Fixture Design,” Proc 5th EPTC Conference, Singapore, December, 2003. Geng, Phil and James F. Maguire, “Dynamic Testing and Modeling for Solder Joint Reliability Evaluation,” Proc of IPC Technical Conference, Anaheim, CA, February, 2004 George Raiser and Dudi Amir, Cold ball pull test on BGA solder joints, Intel, 2004
