Local mismatches in SM solder joint FE analysis

The relative effects of material properties on the attachment reliability of 68 I/O (input/output) 50-mil pitch PLCC (plastic leaded chip carriers) were assessed using FEA (finite-element analysis). Initially, the intent was to rank the local lead/solder and solder/PWB (printed wiring board) CTE (coefficient of thermal expansion) mismatches against the global device/PWB CTE mismatch as the driving cause for crack initiation. The experimental design changed only the CTE and elastic modulus of the PLCC lead frames. Under conditions of pure thermal expansion and in the absence of additional induced mechanical forces, such as with board flexure, the localized interfacial CTE mismatches are shown to be the primary cause of plastic strains in surface-mount solder joints. The belief that lead compliancy is a valid parameter for insuring attachment reliability is addressed and confirmed, but the differential strains caused by loading conditions equivalent to reducing the lead spring constants by an order of magnitude yield only a factor of three increase in the maximum plastic strain values on FR-4 PWBs. In contrast, altering the interfacial CTE differences by changing from an FR-4 to a Kevlar substrate or from a copper to Kovar lead frame, induces order-of-magnitude differences in the solder-joint plastic strains