Some non-underfilled parts pass high-rel thermal cycle tests.

Criteria for using an underfill depend on the environment and stress the BGA device will experience. When exposed to thermal cycling, a BGA or flip chip will expand or contract differently than the substrate on which it is mounted because of each material’s coefficient of thermal expansion. This differential length creates a stress on the device solder connection. Underfill is used as a “stress reliever,” spreading the expansion and contraction effects and increasing device reliability.


For flip-chip components, the requirement for underfill is clear. Under the EMMA (Electronics Miniaturization for Missile Applications) Program, a group of major OEMs and academic institutions, including Raytheon, Rockwell Collins, NSWC Crane, Marquette University, Georgia Tech and ACI, evaluated commercial electronics packaging technologies. The results, published in the Technical Applications Guideline (TAG) Handbook, show a statistically significant and dramatic improvement in solder joint thermal cycle reliability for flip-chip components when an underfill is used (Figure 1). The respective CTE of the underfill material and solder were similar, minimizing solder joint strain in the vertical direction. In addition, underfill materials also improved the flip chip components’ vibration reliability by increasing the mechanical connection between component and board.

For BGA components, the underfill criteria are not as clear. The larger the BGA ball and solder joint, the higher the standoff and lower the strain at the connection. This permits a higher number of thermal cycles before failure occurs. The EMMA Program also tested several BGA packages without underfill. Many pass the commercial goal for number of thermal cycles to failure, and some even pass the high-reliability goal.

To determine whether underfill would significantly improve a BGA device’s reliability, the device (or a similar “dummy” BGA) must be tested in a similar environment. Component manufacturer specifications may also provide reliability data from thermal cycle and vibration testing.


The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.