Authored By:
Hardeep Heer, Ryan Wong, Bryan Clark - FTG Corporation
Bill Birch, Jason Furlong - PWB Interconnect Solutions Inc.
Summary
The PWB industry needs to complete reliability testing in order to define the minimum copper wrap plating thickness requirement for confirming the reliability of PTH structures. Predicting reliability must ensure that the failure mechanism is demonstrated as a wear-out failure mode because a plating wrap failure is unpredictable. The purpose of this study was to quantify the effects of various copper wrap plating thicknesses through IST testing followed by micro sectioning to determine the failure mechanism and identify the minimum copper wrap thickness required for a reliable PWB.
Minimum copper wrap plating thickness has become an even a bigger concern since designers started designing HDI products with buried vias, microvias and through filled vias all in one design. PWBs go through multiple plating cycles requiring planarization after each plating cycle to keep the surface copper to a manageable thickness for etching.
The companies started a project to study the relationship between Copper wrap plating thickness and via reliability. The project had two phases. This paper will present findings from both Phase 1 and Phase 2.
Conclusions
Phase 1 data analysis showed that as long as there is some copper wrap plating, the main reason for failure was predominantly related to copper plating thickness in the barrel and not copper wrap plated thickness. Typically the failures were a fatigue type of failure. Since no coupon could be found which had no wrap plating at all it could not be categorically concluded that vias with no copper wrap plating would have followed the same failure mode. This led to the Phase 2 study.
The Phase 2 study showed that if coupons had IPC 6012 (C or D) Class 2 and Class 3 copper wrap plating thickness the failure mode would likely be barrel cracking, a fatigue type of failure. This was not true for Class 1 types with no wrap. Through vias with no wrap predominantly had unpredictable (butt joint) types of failure. Some sections also showed fatigue (barrel crack) failures.
This was not true for buried vias. All three types of copper wrap plating thickness coupons for buried vias had an average of 40.0 assembly cycles to failure. Even coupons with no wrap had an average of 31.3 assembly cycles to failure. Almost all failures for buried vias were fatigue related failures.
Weibull chart analysis, resistance plots, IST cycles and micro-sections analysis showed that barrel copper plating thickness has a major influence on cycles to failure rather than the copper wrap plating thickness, PTH via size and cap plating of buried vias.
It was also observed that plated through vias failed earlier compared to buried vias. There could be two factors which could cause the earlier failure. One being the length of barrel and the second being the greater volume of via fill material, which are both affected by z-axis expansion. Lead-free assembly (2450 C) accelerated the time to failure compared to leaded assembly (2300 C).
With a high degree of confidence, it can be stated that a board with Class 2 or greater wrap plating thickness is very reliable and any failure is not likely to be because of copper wrap plating thickness.
Initially Published in the SMTA Proceedings
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