A novel pre-applied underfill is presented. In this approach, a B-staged thermoset is applied to a chip-scale package (CSP) as a thin layer over the solder bump array. During reflow, the coating expands to fill the standoff gap between the PCB and package, providing reinforcement to the solder joints. The underfill material is stable at typical drying and burn-in conditions, allowing it to be applied early in the manufacturing process. The expanded underfill was found to improve drop impact resistance significantly relative to a non-underfilled CSP.

This paper explores an alternate process for the application of underfill. The ideal process for an efficient SMT line is to apply the underfill to the package prior to shipping, thus moving all underfill handling and processing steps to the package manufacturer. The process creates a component that can be loaded into tape and reel and handled as any other surface mount part. Standard pick-and-place equipment can then be used to pick the component and place it on the PWB. The underfill cures during reflow, and may be reworkable. This process is called pre-applied chip scale package underfill and must handle the moisture coming off the PWB during reflow to create a near void-free underfill layer, minimizing any potential for solder bridging. The current investigation provides results of a study aimed at enhancing mechanical shock reliability. Reliability results of four different pre-applied underfill options are presented and development and qualification of a pre-applied CSP underfill process is described. Issues surrounding development of effective materials, processes and equipment for reinforcing board-mounted CSPs in a high volume production environment are also discussed in this paper.