Impact of moisture absorption on warpage of large BGA packages during a lead-free reflow process

During a solder reflow process, IC components deform under a rather excessive temperature loading. A too high warpage at temperatures above solder melting, can cause either that the joints in the corner are not soldered to the PCB pads, or that the solder joints are shorted due to compressing of the corner joints. In this work, the warpage of a 35by35 mm² large PBGA package has been measured during a temperature profile which is similar to a lead-free soldering process. It was found that the warpage becomes very high when the applied temperature is above the glass transition temperature of the overmould material. At that moment, there exists a very large CTE mismatch between the overmould and the BT laminate. The warpage measurements have been successfully verified by finite element modelling adapting the right material properties. This proves that modeling can be used as an estimator of warpage for packages. Also the impact of initial moisture uptake has been experimentally investigated and it was shown that it has a dominant effect on its warpage behaviour.