Karthik Thambidurai, Viren Khandekar, Tiao Zhou Ph.D. and Kaysar Rahim Ph.D.
Dallas, TX, USA
Wafer level packages (WLP) are widely accepted in portable electronics and wearable electronics due to their small form factor and low cost. As the trend of high integration continues, increases in both die size and WLP pin counts are required to meet demand. Improvement in solder joint reliability is the key to enable larger, higher pin count WLP. In this work reliability improvement at low cost is achieved by using plastic core balls at selected ball locations.
An 8x8 mm 16x16 array 0.4 mm pitch daisy chain WLP is used as the test vehicle. A manufacturing process to incorporate mixed solder ball types is developed. In addition, board assembly reworkability is evaluated. Board reliability tests are performed to confirm the improvements. It is found that that use of plastic core balls improves the solder joint reliability by 20% which allows a larger WLP size. Plastic core balls also improve assembly yield which enable finer ball pitch WLP.
In this paper, a large WLP is developed that meets board level reliability requirements through the use of plastic core balls at selected ball locations. A process is developed to place plastic core balls at selected ball locations. Manufacturability and reliabilities are assessed and the following conclusions are made:
(1) Large WLP with plastic core ball at selected locations is developed to achieve solder joint reliability improvement with low cost. WLP ball placement process is developed to accomplish mixed ball types.
(2) Up to 20% solder joint fatigue life improvement is achieved. Impeding crack propagation is one of the key contributors for the improvement.
(3) Standard assembly process can be used for WLP with plastic core balls. Plastic core balls reduce the risk of solder bridging and improve process margin since there is minimum solder collapsing. Based on the units considered in this study, there is no special requirement for board rework.
(4) Use of plastic core balls provides more SMT process margin which allows finner ball pitches.
Initially Published in the SMTA Proceedings