4 Package and Mold Compound Stress

In plastic package assembly, the die is mounted on the die pad using an epoxy and the pins are bonded out to the leadframe as shown in Figure 4-1. Afterwards, a liquid mold compound material fills the package mold to create the package. The mold compound cools down to from a liquid to a solid state and then packaged device is cut down from the base. Due to the curing process, the mold compound settles over time and hence total stress on the die also changes over the time which causes drift in output voltage. The package compound, assembly, and any component that can cause stress on the die will affect the Long-Term Drift.

A lower LTD is observed when a larger package is used for the same die. The stress on the device changes in lesser proportion in larger die package compare to small die resulting in lower LTD. For this example, the REF34 in VSSOP is a larger package and it has better LTD than the REF34 in SOT23-6 due to the VSSOP adding less stress to the REF34 die.

Ceramic packages have different compounds and assembly than plastic package. In a ceramic packaged device, the die is hermetically sealed (airtight) between two ceramic shields. The ceramic shield packaging does not need to cure and the filler material has a minimal effect on stress. So the ceramic (hermetically shielded) packages usually have the best LTD performance. Figure 4-4 shows the effect of LTD on the REF70 package.