Reliability Research On Package On Package (PoP)

Package on Package (PoP) is a state-of-the-art packaging approach, which offers flexibility, expansibility, and faster time to market. PoP also allows for testing prior to assembly and has higher yield. PoP is widely used in the area of portable electronics, such as cell phone, digital camera. Although PoP has many merits, the reliability problems are needed to deeply study. So far, the reliability issues mainly focus on the warpage analysis and drop test, but seldom on the reliability subjected to thermal cycling or damp & hot environment. Therefore, this work researches the reliability of PoP under the thermal cycling and damp & hot environment using finite element analysis (FEA) method.In this work, the coefficient of thermal expansion of epoxy molding compound in PoP was measured by three dimensional moiréinterferometry systems, which used to support the finite element calculation. Afterwards, the three dimensional finite element model was established using ABAQUS to inspect the solder ball reliability under the thermal cycling load. Subjected to thermal cycling load, results showed that the maximum stress solder ball was concentrated at package corner and the stress distribution presented dumbbell-shaped in the individual solder ball. The maximum cumulative equivalent creep strain was located in inner solder ball and at the edge of silicon chip. The thermal fatigue life of top and bottom solder balls were 776cycles and 847cycles respectively. The optimization design was done for top module; increasing the chip thickness and coefficient of thermal expansion of epoxy molding compound and reducing the thickness of substrate were helpful to improve the thermal reliability under certain conditions. Among the four kinds of substrate materials, AlN, SiC, BeO, and Al2O3, BeO substrate had the highest thermal fatigue life of 747cycles.In this project, 192 hours moisture absorption under the 30℃, 60%RH conditions and reflow bake process were simulated for PoP. After moisture absorption, substrate as well as most epoxy molding compound and bottom die attachment had reached saturated humidity in the top module. For bottom module, the epoxy molding compound, die attachment and substrate all reached saturated humidity. Epoxy molding compound, die attachment and substrate existed moisture gradient. The maximum vapor pressure of 5.8Mpa occurred in die attachment for both top and bottom modules in the process of reflow bake. Die attachment/substrate interface delamination tended to occur with high risk in reflow step.Finally, the reliability analysis was simulated for PoP under the reflow load. During reflow load, the maximum stress appeared in the four corners of bottom silicon chip for both top and bottom modules, the values of stress were 59.35Mpa and 15.6Mpa respectively. At peak temperature, the warpage for both top and bottom modules ware 10μand-58μm respectively, the warpage difference value was 68μm. The optimization design was carried out for bottom module using Taguchi method and finite element analysis simulation, the results showed that increase in the coefficient of thermal expansion of epoxy molding compound and the thickness of substrate and decrease in silicon chip size could improve the warpage of bottom module.