Improving board assembly yield through PBGA warpage reduction

Since its introduction in the early 1990s, plastic ball grid array (PBGA) package had become the ldquopackage of choicerdquo due to its good electrical performance, lower cost, high assembly yield and self-alignment during board assembly process. Thermo-mechanical behavior of PBGA is highly dependent on the properties of the constituent components. The relative mechanical compliances and thermal expansion mismatch between the silicon chip, the mold compound material and the organic laminate substrate are particularly important to the design and performance the package. Strong coupling between the chip and the packaging materials can cause thermal deformation and deviation from an ideal state of uniform planar flatness, i.e., package warpage. If it is not well controlled, the temperature dependent package warpage can result in open or bridge BGA solder connections when mounting the device to a printed circuit board (PCB) using the surface mount (SMT) solder reflow process. The problem can be more severe as we migrate to lead free SMT soldering process. In this study attention has been focused on improving PBGA SMT process yield through package warpage reduction. Combined experimental and modeling methods were used to investigate the thermo-mechanical behavior and the mechanisms controlling PBGA package warpages through reflow temperatures. Materials effect of mold compound and die encapsulation was first studied for minimizing the chip-package thermo-mechanical coupling over temperatures. Packaging process factors such as encapsulation curing time and temperatures were also investigated. Fully assembled PBGA packages with two different mold compound materials were evaluated. Thermo-mechanical response of the package was measured and analyzed using thermal shadow moireacute and numerical modeling technique. The experiments and modeling were correlated with a well controlled manufacturing build with over 10,000 boards built. The combined experimental and numerical anal- ysis confirmed our selection of the packaging materials and demonstrated that significantly improved board assembly yield can be achieved by controlling the PBGA warpage during board mount assembly process. It is also concluded the importance of a package warpage and the shape of the warpage at not only room temperature but also throughout reflow temperatures.