Reliability design and experiment of low cost WLP-QFN modules

Wafer level packages (WLP) and Quad flat no-lead (QFN) packages are both widely used in electronics assemblies. This is because they both provide good electrical and thermal performance and especially low cost as they are simple in the construction. In this paper, two designs of experiment (DoEs) have been constructed. The first DoE is employed to characterize the impact of the module reliability from die size, underfill, shield-can and substrate selection. The second DoE covers the center pad area and board thickness impact. The study indicates that underfill selection and curing strategy impact both the stress and strain of the solder joint and the redistribution layer (RDL) at the WLP. The thermal pad design and PCB board thickness will also impact those strain levels. Reliability tests of both WLP and QFN show that the WLP solder joints and RDL are the weakest link in the WLP-QFN assembly. Design optimization shall be considered at the system level to be effective.