Underfilled BGAs for a variety of plastic BGA package types and the impact on board-level reliability

Abstract

There are continuous efforts in the electronics industry to reduce electronic package size. The main force pushing for smaller package size is the desire by original equipment manufacturers to reduce their mother board size and thereby their own costs. Reducing the size of electronic packages can be achieved by a variety of means and for ball grid array (BGA) packages an effective method is to decrease the pitch between the individual balls. Reducing the pitch between balls decreases the ball size and therefore the standoff between the packages and motherboards is reduced. The reduced standoff has the negative effect of reducing board-level reliability. An experiment to understand board-level reliability improvement by underfilling the BGA will be presented for several finepitch plastic BGA types (fleXBGA and MAP (mold array process)). Four different (snap-cure) underfill epoxies with varied coefficients of thermal expansion and elastic moduli were used so that the preferred set of epoxy properties could be determined. A full factorial numerical experiment was used to identify the optimum combination of CTE and elastic modulus. Results indicate that a signficant increase in boardlevel reliability can be achieved by underfilling the BGA. I. IntroductionMany end customer have a continuous desire to reduce the size of their final assembled boards. This is coupled with end customer needs to constantly squeeze costs out of their final product. A simple method for accomplishing both is for an end customer to push their own suppliers in supplying smaller packages. The smaller package takes less materials and therefore nominally might cost less. With a smaller package, the end customer can begin to shrink the final board design.One consequence of pushing suppliers into smaller packages is that the board-level reliability can decrease. For a BGA type package, a primary driver for board-level reliability is the standoff height [1], the distance between the board and the package. As this decreases, the life of the solder-joint connecting the board and package can decrease.Past efforts have shown that underfilling the BGA for ceramic substrate packages can significantly increase the board-level reliability for these parts [2,3]. Using the knowledge from [3] the present board-level reliability experiment and numerical modeling of underfilled plastic BGA packages were started.

Thermomechanical Fatigue Performance of Lead-Free Chip Scale Package Assemblies with Fast Cure and Reworkable Capillary Flow Underfills

“…Finally, indeed, the CTE is also very critical to the thermomechanical fatigue performance; generally speaking, the underfill with a CTE close to that of a typical mold compound and PCB laminate can minimize the in-plane shear loads applied to the solder joint. [24][25][26][27][28][29] However, its effect is not so obvious in this study owing to the similar CTEs of the underfills used.…”

“…This is even more true for lead-free assemblies than for eutectic-SnPb soldered assemblies. [24][25][26][27][28][29] In this paper, we discuss the thermomechanical fatigue performance of chip scale packages (CSPs) with FCFUs and PCFUs used in conjunction with lead-free solder (solder paste and solder balls on the CSP). The assemblies without underfills were also tested for comparison.…”

Board-Level Solder Joint Reliability of Edge- and Corner-Bonded Lead-Free Chip Scale Package Assemblies Subjected to Thermal Cycling

“…Wang et al 32) systematically analyzed the effects of underfill thermomechanical properties on the thermal cycling fatigue reliability of flip chip BGA using a steady-state creep model, they found by calculation that an underfill with a high modulus, a high T g , and a low CTE is best for thermal fatigue performance, while an underfill with a high modulus, a low T g , and a low CTE is worst. Burnette et al 33) evaluated the board-level reliability of a variety of plastic BGAs with underfills. Their parametric study based on a valid finite element model showed that the optimum underfill material would have a CTE of approximately 15 ppm/ C, but that its modulus could range between 2 and 10 GPa without obvious effects on the thermal cycling reliablity.…”