Processing and reliability of CSPs with underfill

Abstract

The use of chip scale packages (CSPs) is rapidly expanding, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. This paper examines the assembly process with capillary and fluxing underfills. Issues of solder paste versus flux only, solder flux residue cleaning and reworkability are investigated with the capillary flow underfills. Fluxing underfills eliminate the issues of flux-underfill compatibility, but require placement into a predispensed underfill. Voiding during placement is discussed.To evaluate the relative performance of the underfills, a drop test was performed and the results are presented. All of the underfills significantly (5-6x) improved the reliability in the drop test compared to nonunderfilled parts. Test vehicles were also subjected to liquid-to-liquid thermal shock testing. The use of underfill improved the thermal shock performance by 5x.Index Terms-Chip scale packages, drop test, underfill.

Electronic Components for High-g Hardened Packaging

“…Both flux-only and solder-paste assembly processes are used for the capillary type underfill. The results (8) indicate that any underfill material choice improves the probability of the CSP surviving a high acceleration event.…”

“…Underfill has also been successfully used on chip scale packages (CSPs) to increase their reliability (8).…”

“…Until recently, underfilling components also precluded their rework. Recently, Loctite 2 developed a reworkable underfill (8). When heated in a reflow oven, this underfill breaks down, allowing the component to be removed and replaced.…”

“…The performance of capillary and fluxing underfills was examined (8). Most production processes currently use capillary underfill.…”

“…The advantage of this underfill is that rework of the component is enabled. Upon heating of the component to solder reflow temperatures, the underfill breaks down and loses its adhesive properties (8). The component can then be lifted from the board and cleaned.…”