The criteria for using an underfill depends 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 due to each material’s Coefficient of Thermal Expansion or CTE. 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 the reliability of the device.

Figure 1. Effect of Underfill on Flip Chip Fatigue Life

For Flip Chip components, the requirement for underfill is clear. In the EMMA (Electronics Miniaturization for Missile Applications) Program, ACI along with Raytheon, Rockwell Collins, NSWCCrane, Marquette University, and Georgia Tech Research Institute, evaluated commercial electronic 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 CTE of the underfill material and the solder were similar minimizing the solder joint strain in the vertical direction. In addition, underfill materials also improved the vibration reliability for Flip Chip components by increasing the mechanical connection between the component and the board.

For BGA components, the criteria for underfill is not as clear. The larger the BGA ball and solder joint, the higher the stand-off, and the lower the strain at the connection. This allows a higher number of thermal cycles before failure occurs. Several BGA packages without underfill were also tested in the EMMA program. As shown in Figure 2, many pass the Commercial Goal in number of thermal cycles to failure and some even pass the High Reliability Goal.

Figure 2. Component Thermal Cycling Results (Click the image above for higher resolution graph)

To determine if the reliability of your BGA device would be significantly improved by underfill, testing with the device (or a similar “dummy” BGA) in a similar environment must be performed. This is a service that ACI can provide. Manufacturer’s specifications may also provide reliability data on their components from thermal cycle and vibration testing that they performed.