Warpage Control of Thermally Enhanced PBGA Package with Internal Heat Spreader during Reliability Test

Thermally enhanced PBGA (TEPBGA) package with internal heat spreader is becoming more and more popular due to the ever growing power dissipation requirement of the advanced CMOS logic devices. The additional stiffness provided by the Cu heat spreader also helps the package to remain flat during SMT reflow process. However, it has been observed that, in some cases, TEPBGA package would show large warpage in accelerated stress test such as air temperature cycling. The excessive warpage may increase yield loss during test.

In this work, room temperature package warpage of TEPBGA package after temperature cycling were examined with three types of Cu alloys together with factors like stress condition and bill of material. In-situ monitoring with strain gages attached on the heat spreader was performed to understand the deformation behavior of package during temperature cycling. Hardness test on Cu heat spreaders was done at different stages of the stress. The correlation between the warpage and Cu material properties is discussed. A finite element model was also created to assess the impact of cycling condition on the thermal stress and driving force for package warpage. The results of these analyses will be used to provide a robust solution to ensure the flatness of TEPBGA package in temperature cycling. Finally, the packages were assembled to PCB board and solder joint reliability tests were performed to understand the impact of package warpage on solder joint reliability.