Solder Joint Reliability Simulation of Fan-out Wafer Level Package (FOWLP) Considering Viscoelastic Material Properties

In this paper, the solder joint reliability under $-40^{\circ} \mathrm{C} -125^{\circ}\mathrm{C}$ thermal cycling loading of different package sizes of mold-first FOWLP and redistribution layer first (RDL-first) FOWLP was studied by finite element simulation considering the viscoelastic material property of epoxy molding compound (EMC), dielectric and underfill. The critical solder joint is located at the die corner for the designed mold-first and RDL-first FOWLP. Volume average creep strain energy density range of critical solder joint increases with the package size from 12 mm $\times$12 mm $\times$0.2 mm to 18 mm $\times$ 18 mm $\times$0.2 mm for mold-first FOWLP. The distance to neutral point (DNP) becomes invalid when the RDL-first FOWLP package size increases to 18 mm $\times$18 mm $\times$0.2 mm. Volume average creep strain energy density range of package corner solder joint is overestimated without considering the viscoelastic material properties. However, the volume average creep strain energy density range of die corner solder joint is underestimated without considering the viscoelastic material properties. Low CTE PCB can help to improve the reliability of the critical solder joint at die corner of the designed mold-first FOWLP. The effects of the low CTE PCB for improving solder joint reliability the designed RDL-first FOWLP is not significant. Thinner PCB can help to improve the reliability of the critical solder joint at die corner of both mold-first and RDL-first FOWLP.