Making the Lead-free Transition: The Enigma of Tin Whiskers in Connector Applications

摘要

Component finishers have long relied on tin-lead coatings because of lead's ability to inhibit whisker formation. Presently, the industry is forging ahead with lead-free solders and finishes due to impending legislation and environmental directives from businesses. Several lead-free alternate finishes such as tin, tin-bismuth, tin-silver, tin-zinc, Pd, Pd-Ni-Au have been proposed. To date the preferred lead-free coating is pure matte tin over a nickel under-layer. However, with this coating system there remains a risk of whisker formation and the major concern is that not all of the factors relating to whisker generation are known. This paper describes results of one aspect of our search for suitable pure tin chemistries and a definition of their process limits. This particular designed experiment investigated the critical parameters of three candidate baths and evaluated them versus the critical response variables, whisker formation. To gain a better understanding of the whisker phenomenon, we included another input variable, tin thickness, and then correlated all these input variables to numerous response variables such as coating morphology, grain size, carbon content, etc. and resulting whisker growth and density. Based on this work, (and additional related work) we have been able to select suitable chemistries and have determined their acceptable process windows.