Authored By:
Agustin A. Vasquez, Ph.D., P.E., Raiyo F. Aspandiar, Ph.D., Pubudu Goonetilleke, Lavanya Ashok Swaminathan
Intel Corporation
OR, USA
Summary
Printed Circuit Boards (PCBs) - for Intel tester platforms such as HDBI (High Density Burn-In Tester); HDMT (High Density Modular Tester); XPV (Platform Validation Tester) uses Electroless Nickel and Immersion Gold (ENIG) as surface finish to prevent oxidation of copper pads and improve solderability. However, ENIG plated PCBs periodically exhibit two key failures during board assembly: Solder dewet or no wetting of the solder to the ENIG surface finish and Solder Joint Cracks (SJCs) or separation from the nickel layer surface causing an electrical open. The periodicity of assembled boards failures - with ENIG plated boards and its cost impact to Intel manufacturing - was the motivation to investigate an alternative PCB plating solution with a higher margin to failure under shock stress. This paper discusses the results of a comparative study for solder joint performance - under shock - for ENIG and ENEPIG surface plated tester PCBs.
Conclusions
The best cost-reduction practice for Intel manufacturing is to deliver testers with high quality and reliability. Solutions with higher margins to failure will support this goal. In the case of solder joints in tester boards, it was demonstrated that ENEPIG delivers solder joints with significant higher margin to failure compared with ENIG POR plating solution under mechanical shock. Therefore, the team strongly recommends converting all tester boards to ENEPIG surface finish based on the results of this 1.5 years long study. The main learnings of this study include:
- Solder joints formed with ENEPIG surface finish outperformed solder joints with ENIG surface finish under shock stress. Data indicates that ENIG solder joints are marginal. Therefore, ENIG solder joints should not be used on tester boards integrated to cold plates.
- The investigation assessed the performance of solder joints with SMD and NSMD lands. Both land designs are used in tester boards designs. The solder joints on SMD lands with ENIG surface finish did not survive even one drop cycle during the mechanical shock test. In contrast, joints on lands with ENEPIG surface finish showed a superior performance under shock testing for both SMD and NSMD Lands. Therefore, for tester boards integrated to cold plates, boards with ENEPIG surface finish provide a solution with a large margin for failure.
- The IMC layer on ENIG solder joints was formed badly which resulted in the insufficient mechanical strength of solder joints and caused crack or catastrophic solder joint failures at first drop. In contrast, IMC layer on ENEPIG solder joints was formed correctly which resulted on stronger mechanical strength of solder joints, outperforming ENIG solder joints.
- The use of test vehicles to assess solder joint performance is a low cost and data rich approach to assess PCB pad plating quality delivered by PCB suppliers. The use of this approach should be encouraged as part of a PCB supplier qualification as well as enabling periodic monitors to assess PCB incoming quality.
- Signal integrity analysis conducted on ENEPIG and ENIG HDBI MB production boards resulted in some differences. However, these gaps can be addressed by reworking trace paths. It is recommended that signal analysis is conducted for tester boards early on in development, when boards are being converted to ENEPIG. In this way, discovery of potential marginalities on circuit designs can be addressed earlier at a lower cost.
- The cost adder for converting all HDBI tester boards from ENIG to ENEPIG surface finish is negligible.
This paper did not address the other key failure with ENIG surface finish: poor wetting (non-wetting or de-wetting) of the solder to the ENIG surface finish on PCB lands during soldering processes. This is a recurrent problem in tester manufacturing, impacting production cost. Hence, this is an important and relevant avenue for future work.
Initially Published in the SMTA Proceedings
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