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IPC Technology Solutions White Paper on Performance-Based Printed Board OEM Acceptance Via Chain Continuity Reflow Test: The Hidden Reliability Threat — Weak Microvia Interface
Introduction
The proliferation of copper-filled stacked microvias in printed boards has resulted in the identification of a hidden reliability threat – a weak interface between the microvia target pad and the electrolytic copper fill. When thermally stressed, the generally weak interface fractures. This is manifest especially during forced convection assembly reflow. Unfortunately, the reflow-induced latent defect cannot be detected by in-circuit tests (ICT). Only after the product is deployed does the stealth threat eventually reveal itself. The latent fault is most commonly identified as an intermittent or complete failure; often well before expected wear-out. Reflowdamaged stacked microvias act much like mechanical switches that open and close with transient thermomechanical strains that accompany operation. Failures were observed between the copper fill and target pad; this is the weak interface.
In day-to-day production, printed boards must survive forced convection heating required for assembly reflow. If they cannot survive, they are worthless. The fundamental objective of assembly reflow is to secure component attachment with solder while not stressing the printed board beyond limits where damage to the printed board can be expected to occur. It is well recognized in the industry that assembly reflow subjects the printed board to the greatest thermal excursion.
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