Journal of SMT Article

1000 Days of Testing Tin Whiskering PCB Assemblies to Determine the Suitability of Conformal Coatings to Mitigate Against Shorting

Authors: Martin Wickham, Adam Lewis and Kate Clayton
Company: National Physical Laboratory
Date Published: 10/1/2017   Volume: 30-4

Abstract: The spontaneous growth of tin (Sn) whiskers leading to electrical short circuits has been an issue across avionics, space and other high reliability applications. Commercial satellites, nuclear reactors, missile systems and automotive applications have all reported complete or partial failures believed to be associated with tin whisker growth. The UK’s National Physical Laboratory has developed a test vehicle incorporating specially plated SOIC components mounted onto PCBs which have a high propensity to develop tin whiskers. These test vehicles have been used to undertake trials on different mitigation techniques designed to inhibit Sn whisker growth. Seventeen international industrial partners have collaborated to determine the relative reliability of a range of different Sn whisker mitigation techniques including twenty conformal coatings. The experimental set up to measure the occurrence of electrical shorts has been built on a bespoke daisy chained SOIC16W, twenty-four of which are mounted on a PCB. Each SOIC is monitored for shorts between adjacent terminations by a resistance measurement. By multiplexing, the experiment has looked at over 3500 components continuously up to 1000 days, with over 48,000 opportunities for failure. The system has captured and stored the resistance state across all components every 15 minutes. The collected data permits the study of the incidence of short circuits, the length of time of each short and the number of intermittent short circuits. We will discuss the rapidity of whisker formation and the formation of intermittent shorts and their duration as the whisker continues to grow.


tin whiskers, SOIC, tin plating, intermittent, conformal coating

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