Study on the board level reliability test of package on package (PoP) with 2nd level underfill

In spite of a great success of stacked package (PoP) in the market, some reliability issues have been raised. One of them is a board level reliability evaluation of PoP with 2nd level underfill. Some customers have issued the application of 2(nd) level underfill for PoP to improve the reliability for drop test. Unfortunately, using the 2(nd) level underfill for PoP package causes reducing the reliability for thermal cycling test. Under temperature cycling test, PoP with 2(nd) level underfill shows early failures before 300 cycles whereas most of samples without 2(nd) level underfill survived over 1000 cycles. This paper discusses the optimal underfill material for PoP to achieve reliable board level performances. Various factors such as filler content, CTE, Tg and underfill dispensing pattern are considered. As a result, it is found that lower CTE & higher Tg underfill are better than other factors for temperature cycle performance. Non filler type underfill was passed 500 cycles for PoP but filler type underfill was passed 2500 cycles under the JEDEC JESD22-104C Condition G (40'C similar to 125'C, 1cycle/hour). Especially one of non filler type underfill material with different dispensing pattern passed 1730 cycles without any failure. We assumed that this results caused by specific dispensing patterns & damping effect of non filler type underfill. In the drop test, most of underfill materials have better performances than no underfill case. Especially, several no filler type underfills have no failure until 400 drops.