Interfacial Reaction and Failure Mechanism of SAC/Co-P Solder Joint under Rapid Thermal Fatigue

In this paper, the interfacial reaction and failure mechanism of SAC305/Co-P solder joint under rapid thermal fatigue loading was studied. The Co-P UBM with P (phosphorus) concentrations of 3% and 6% were prepared on BGA substrate by ultrasonic assisted electro-deposition process. Sn3.0wt.%Ag-0.5%Cu (SAC305) BGA solder ball were reflowed on Co-P finishes to form SAC/Co-P solder joints. This solder joints were used to conduct rapid thermal shock test, With the application of electromagnetic induced heating and cooling circulation system, the solder joints were rapidly heated and cooled. After 4500 cycles of rapid thermal shock, the average thickness of the interfacial intermetallic compound (IMC) of SAC/Co-3% P increased from 2.012μm to 2.953μm. While the IMC of SAC/Co-6% P increases from 2.075μm to 2.377μm. The thickness of IMC increased slightly. The morphology of IMC changes due to the difference of P concentrations in Co-P UBM. As the number of shock cycles increases, the thickness of interfacal IMCs with higher P concentration is relatively stable. Through the comparative analysis of SAC/Co-6% P solder joints at different cycles, it was found that the cracks began to emerge at the outermost root of the solder joints. With the rapid thermal shock, the crack propagates between solder and interface. and the main direction of the propagation is horizontal with the direction of pad. The ultimate failure form is fatigue toughness failure. Experiments show that the solder joints still show regular fatigue failure mechanism under rapid thermal fatigue loading. Therefore, this establishes an important theoretical date for the failure mechanism of the new Co-P UBM interface under the rapid thermal fatigue system.