Interfacial stability of solder joints under multiple reflow cycles: A comparison of intense pulsed light, laser, and reflow soldering

The effects of different soldering processes on the joint properties of Sn-3.0Ag-0.5Cu (wt.%, SAC305) solder on electroless-nickel electroless-palladium immersion-gold (ENEPIG)-finished and organic solderability preservative (OSP)-finished substrates after multiple reflow cycles were systematically investigated. The standard thermal reflow profile exhibited a long high-temperature dwell which provided sufficient time and thermal driving force for continuous Cu–Sn interdiffusion and resulted in the formation of the thickest intermetallic compound (IMC) layer. The shorter time profiles of the rapid-heating laser and intense pulsed light (IPL) methods limited atomic diffusion and resulted in significantly thinner IMCs. Due to its longer thermal exposure, the IPL process produced a thicker IMC and slightly increased surface roughness compared to laser soldering. Continuous IMC growth was observed on OSP-finished substrates due to direct Cu–Sn reactions, whereas the Ni layer in ENEPIG effectively suppressed excessive IMC formation by functioning as a diffusion barrier with a significantly lower diffusion coefficient than Cu. To reproduce the conditions encountered in package assembly and rework processes, up to five reflow cycles were applied after the initial application of each soldering method. Although IMC thickness of the ENEPIG interface increased with repeated cycles owing to enhanced Cu diffusion, pronounced IMC spalling which became more severe with reflow cycling was observed. IPL maintained the most stable interface with minimal IMC spalling, whereas laser soldering showed more severe and frequent IMC spalling behavior. Mechanical reliability evaluations indicated that IPL provided a favorable balance between interfacial stability and mechanical performance.