Authored By:
Christopher Hunt
National Physical Laboratory
Teddington, United Kingdom
Summary
Manufacturing high reliability lead-free circuit assemblies is challenging, and understanding the pitfalls and knowing material properties is clearly desirable. This paper will address a wide range of issues, which include material and processing properties. Examples of these phenomena include: copper dissolution, tin whisker mitigation, PTH reliability, measuring solder joint reliability, conformal coating protection, and tin pest.
Conclusions
The implementation of the RoHS directive has clearly had a material impact, but in turn the new materials have had a number of processing impacts, namely an increase in the processing temperature during assembly. These changes have been accommodated in high volume consumer markets, but for high reliability and high stress environments the reliability case is not yet proven. Processing issues such as copper dissolution can be solved by machine design modifications and minor elemental modification to the alloys to reduce copper solubility.
The PCB substrate is pushed very heard during the multiple higher temperature reflow passes. This can impact on the PTH reliability, can cause delamination and increase susceptibility to CAF. Fatigue of solder, both low and high cycle fatigue, are still areas that are being investigated, and prediction and defining acceleration factors have yet to be defined. Clearly for safety critical equipment a high level of confidence will be required for these acceleration factors.
While tin pest remains an issue more of an academic interest, that alloys of tin can transform if nucleated is of concern. The question that needs to be answered is how does nucleation initiate in this system and what are the environmental factors. Tin whiskers remains an ongoing issue as no plated tin finish is guaranteed as whisker free, hence the interest in mitigation strategies and whether coatings can mitigate against a whisker short. The test approach developed here will facilitate coating developments for this purpose. Finally symptomatic of an increase in electronics reliability across the board there is increasing interest in applying coatings, and the work presented here shows opportunities for improved evaluation techniques for coatings
Initially Published in the SMTA Proceedings
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