Underfill

Introduction

Embedded systems are marked by a need for small components, unmatched reliability, and the ability to withstand harsh conditions. Transcend Information offers underfill as a customization option for its embedded products to increase reliability under high thermal stress, high gravitational acceleration, and high fatigue cycle applications.

Main functions

Underfill provides a strong mechanical bond between key components and the underlying printed circuit board (PCB). By spreading stresses throughout the chip and PCB interface with a mechanical bond, less stress is concentrated on the solder joints, increasing device reliability. 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 and increasing device reliability.

How Underfill works

Underfill is typically a polymer or liquid epoxy that is applied to the perimeter of key components on a PCB after it has passed through a reflow oven. The PCB is then heated so that the underfill is absorbed underneath key components via capillary action. See Figures 2 and 3.

Transcend recommends underfill for embedded flash and DRAM products used in handheld devices, automotive electronics, and military applications that require stringent thermal cycling performance and shock resistance.

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