IMPROVED LEAD DESIGN FOR SURFACE MOUNT PACKAGES
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2002-Mar-13
Solder joint failure is a potential hazard for all surface mount assemblies. Thermal fatigue and creep rupture failures have occurred in products under both accelerated test and field use conditions. The relia- bility issue becomes more serious as die size increases (which increases expansion mismatch) and lead length decreases (which increases stiffiess). A good example of a package with this combination is the Thin Small Outline Package (TSOP) which has short, stiff gull wing leads. Both testing and finite element analysis indicate that reliability can be enhanced by a process which results in good solder fillets. In fact the toe fillet is more important than the heel fillet. However, it is nearly impossible to guarantee good toe fillets with current assembly processes because the lead frame is plated before form and trim, so the end of the lead has no protective plating. Hence, the end of the lead oxidizes, and does not readily wet during solder assembly. The present invention describes an improved lead shape which guarantees good toe fillets, and thus improves solder joint relia- bility. With the optimized lead design the lead is formed up at the toe, so a solderable surface is always available to form a large toe fillet. This simple design change is completely transparent because it does not change the overall package foot print or height. A well correlated fatigue model indicates that the optimized lead will offer a 2x reliability improve- ment over the current lead design.