The effect of power cycling on the reliability of lead-free surface mount assemblies

A power cycling in-test monitoring system has been constructed to test the reliability of eight different lead-free surface mount assemblies. The asse mblies included SnAg3.8Cu0.7 and SnAg3.8Cu0.7X solder joints on OSP-Cu (org anic solderability preservative on Cu), electroless NiAu, immersion Ag and immersion Sn board metallizations, One Pb containing assembly, Sn62PbAg2 on OSP-Cu board metallization, was included for comparison. The components on the assemblies were 1206 resistors (Sn100 metallization on the end termina ls) and 100 lead QFP with gullwing leads (SnPb15 metallization), All assemb lies experienced up to 5000 power cycles of ambient to 100 OC,vith a 15 min dwell at each temperature. Solder joint reliability was evaluated by monit oring electrical resistance after each power cycle and examining mechanical strength and microstructure with number of power cycles. The 1206 resistor s on four of the assemblies (one of which was the Pb containing assembly) e xhibited electrical resistance increases after 4000 power cycles. All resis tor samples decreased in strength by more than 70% at 5000 cycles and crack s appeared after 1000 power cycles. All gullwing lead solder joints exhibit ed good reliability over 5000 power cycles, with no resistance increase and no strength reduction. Small cracks appeared after 3000 power cycles.