SMT007 Magazine

SMT-Sept2015

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70 SMT Magazine • September 2015 ArtiCle by thomas Hester and David pinsky raYTHeon coMpanY Abstract Tin plating can spontaneously grow fila- ments, called whiskers, which can cause elec- trical shorts. Alloying of tin with lead (Pb) is known to suppress whisker formation. This ar- ticle provides the findings of a lengthy project of soldering electronic components with pure tin finishes with lead-containing solder for the attachment. The object of the project was to de- termine the conditions under which sufficient alloying of the pure tin finish by lead in the solder is achieved for the purpose of whisker suppression. We have coined the term "self- mitigation" to describe this effect. The first part of the project was planned by a committee that selected commercially available surface mount components, including package styles that exhibit a variety of solderable termi- nation heights and lengths. The sample boards in the first part of the project were of two de- signs: first a simple single layer circuit card; and second, a multi-layer circuit card with internal ground planes, representing the current design practice for our production printed wiring as- semblies. Sample boards were assembled using standard production pick-and-place equipment and automated soldering processes. The original project was later expanded with multi-chip module (MCM) samples con- taining surface mount components with pure tin solder terminations. The success criteria for tin whisker self-mitigation was chosen to be the presence of at least 3 Wt% lead in all measured solder locations. Materials analysis was con- ducted in multiple locations on each solderable termination, usually at egress from the package, a midpoint, and adjacent to the board contact pad. The materials analysis was primarily per- formed using X-ray fluorescence, and the valid- ity of the X-ray fluorescence measurements was checked on a sample basis with cross-sectional analysis and Scanning Electron Microscope En- ergy Dispersive Spectroscopy. The results have suggested that a rule based upon a sum of the horizontal and vertical dimensions of a termi- Tin Whisker Self-Mitigation in Surface Mount Components Attached with Leaded Solder Alloys

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