NEXT GENERATION BOARD LEVEL UNDERFILL (BLUF) FOR FINE PITCH BGA AND POP
Authors: Siang Miang Yeo, Cheng Siew Tay, Ching Ching Chong, and Jin Siang Beh Company: Intel Corporation Date Published: 10/24/2010
Abstract: BGA ball pitch trend is getting tighter over time due to increase of input and output (I/O) count in new artificial technology packages. Solder joint mechanical reliability against drop/shock and temperature cycling, deteriorates as the result of solder joint size shrinkage by pitch reducing. Next generation BLUF needs to provide temperature cycling protection on top of mechanical shock/drop protection as in current BLUF. The new BLUF material requires reliability performance as good as flip chip underfill (FCUF) while manufacturability as good as current generation BLUF with fast flow, fast cure and reworkable. The objective of this study was to define the optimum material properties in meeting these two requirements that always in contradiction. In this paper, 15 underfill materials from multiple suppliers with distinct compositions were studied. Results showed that Coefficient of Thermal Expansion 1 (CTE1) lower than 35ppm/°C and Transition Glass (Tg) greater than stress temperature are critical in providing thermal cycling improvement. It also demonstrates that next generation BLUF material provides an additional 3X shock performance improvement over packages with no BLUF. It has proved the feasibility of BLUF technology in meeting solder joint reliability and board level manufacturability requirement with 5minutes cure time at 150°C, fast flow and reworkable. A flow void is common in Package-on-Package (PoP) with multiple gap heights within a package. Optimizing pre-heat temperature, dispensing pattern and number of dispensing passes are critical in reducing underfill flow void. The outcome of the study was targeted to serve as reference guide in selecting the right underfill technology direction.
Key words: underfill, BGA, PoP, shock, temperature cycle.