Evaluation of secondary wire bond integrity on silver plated and nickel/palladium based lead frame plating finishes

As the legislatures demand the use of lead (Pb) free plating finishes in lead frame manufacturing, the industries have witnessed different plating finishes from the lead frame makers. The development of a new lead frame finish involves a wide variety of issues which must be addressed and thus mandates further evaluation of these new structures.;Using the common Pb free lead frame plating finish of selectively plated silver (Ag) as a basis, a comparative approach was used to evaluate the secondary wire bond integrity of a 25 micron (1 mil) gold wire on Ni/Pd based lead frame plating finishes. The integrity of the secondary wire bonds for four different plating finishes was investigated at various assembly thermal exposure stages using the wire pull strength test as the arbiter. Reliability tests, such as high temperature storage (HTS) and unbiased highly accelerated stress test (UHAST), were also conducted. Finally, failure analysis was conducted with the help of metallographic cross sectioning, SEM/EDX analysis and statistical analysis of the wire pull strength test results.;Failure analysis showed that there were silver and copper migrations over the silver (Ag) plated lead frame when exposed to the high temperature storage test, but this did not affect the bond integrity. However, the Ni/Pd based lead frames did not show any metal migration since nickel acts as a barrier against the base metal diffusion. The wire pull results for all cases showed only a heel break at the secondary bond with no pad lift on any of the lead frame types.