Thermal Shock/Drop Test of Lead-free

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Thermal Shock/Drop Test of Lead-free

Paper has results from experimental research making a comparison of different lead-free alloy combinations including thermal shock and drop tests.
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Authored By:

Bankeem Chheda and S. Manian Ramkumar, Ph.D.
Rochester Institute of Technology-CEMA, Rochester, NY, USA

Reza Ghaffarian, Ph.D.
Jet Propulsion Laboratory, Pasadena, CA, USA

Summary

With ROHS compliance the transition to lead-free is inevitable. Several lead-free alloys are available in the market and its reliability has been the main concern. The results from this experimental research aims at making a comparison of different lead-free alloy combinations. Thermal shock and drop tests are a part of this experimental study.

The test vehicle considered for this study contains a variety of components such as ultra chip scale package (UCSP), package on package (PoP), plastic grid array (PBGA-676 & 1156), very thin chip array BGA (CVBGA), thin small outline package (TSOP-40 & 48), dual row micro-lead frame (DRMLF), micro-lead frame (MLF-36 & 72), and chip resistors (0201, 0402, 0603). The scope of this paper is limited to the performance evaluation for area array packages only. Solder ball alloy combinations for the area array packages include SAC305, SAC405, SAC105 and SnAg. The solder paste used for assembly is SAC305 with Type 3 particle size.

Three different PCB surface finishes, electroless nickel immersion gold (ENIG), SnPb hot air solder level (HASL), and immersion silver (ImAg) are used. Different solder ball alloys and surface finish combinations will provide good comparison data for investigating the assembly performance.

Preliminary investigations have been carried out on PCB assemblies subjected to mechanical shock in the as-soldered condition and also after 200 and 500 thermal shock cycles at -55 to 125°C. The mechanical shock test was conducted by subjecting the assemblies to 30-drop cycles from a height of 3ft. After each drop cycle the daisy chains were checked for continuity for the solder joint evaluation. The number of drops for the first daisy chain failure is used in analyzing the performance of the solder joints.

Since each component has many independent daisy chains, the failure of the individual daisy chains was later used in determining the location of the failure and its progression. The preliminary investigations revealed consistent failure in the corner daisy chains of the area array components attributable to the flexing of the PCB resulting in the solder joint strain when subjected to the shock event.

Initial results indicate SnAg alloys for the solder balls to be performing better than the SAC 305 and 405 alloys in the no-underfill condition, irrespective of the PCB finish. After 200 cycles of thermal shock UCSPs showed marked improvement in drop tests but were found to fail after 500 cycles of thermal shock. PoP assemblies were found to survive all 30 drops for all combinations of PCB finishes. This paper will provide a detailed analysis of these findings including the results to be investigated with corner underfilled assemblies.

Conclusions

This experimental study provided good comparisons for performance evaluation of area array packages, with and without the use of underfill. Some of the research findings are as follows.

1. PoP package survived all 30 drops of mechanical shock test for the as-soldered condition and after 200 and 500 cycles of thermal shock. Similar results were observed with and without the use of underfill.

2. The performance of UCSP was better with corner underfill when compared to without underfill.

3. UCSP with corner underfill provided better results for the as soldered and the 200 cycles of thermal shock, but it could not withstand 500 cycles of thermal shock. This was observed with and without the use of underfill.

4. Significant improvement in reliability was observed for PBGA1156 in the as-soldered condition with the use of corner underfill when compared to without corner underfill.

5. For PBGA676 the performance of SnAg solder ball was better than SAC305 and SAC405 solder balls. This result holds good with and without the use of corner underfill. This is possible due to the location of the component on the test vehicle.

6. SnAg solder balls of PBGA676 showed marked improvement with use of corner underfill than without corner underfill. However SAC305, SAC405 solder balls did not have any such improvements in performance with corner underfill.

7. Corner underfill improved the performance for CVBGA when compared to no underfill. Improvement was observed for SAC305, SAC405, and SAC105 solder balls.

8. There was a specific pattern observed in the failure of daisy chains for PBGA676 and PBGA1156 components. This daisy chain failure pattern was observed to be linked to the solder joints undergoing the most deflection during the drop.

9. Pad lifting and crack formation were the root cause for failure of the packages.

Initially Published in the SMTA Proceedings