How underfill works

Underfill is typically a polymer or liquid epoxy that is applied just underneath the perimeter of the silicon chip on a PCB after it has passed through a reflow oven. The PCB is then heated so that the underfill is absorbed underneath the chip via capillary action.

Main functions

Underfill is commonly used for ball grid array (BGA) based storage for applications such as handheld devices, which must pass drop or tumble tests.

When a BGA device is exposed to repeated heating and cooling cycles, a BGA chip will expand or contract at a different rate than that of the underlying substrate, due to the difference in each material’s coefficient of thermal expansion. This differential creates mechanical stress on the device’s solder joints.

Underfill is used as a stress relieving agent, evenly distributing the expansion and contraction effects. By spreading stresses throughout the chip and PCB interface with a mechanical bond, less stress is concentrated on the solder joints, increasing device reliability.