324 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 10, NO. 3, SEPTEMBER 2010 Ball Grid Array Solder Joint Reliability Under System-Level Compressive Load Tz-Cheng Chiu, Darvin Edwards, and Mudasir Ahmad Abstract—Heatsinks have been widely used in the electronics industry as a thermal solution for high-performance and high- power-density devices. The thermal efficiency of heatsink solutions may be improved by increasing the compressive load applied on the interface between the electronic package and heatsink. Typical approaches for heatsink retention, however, would also lead to high levels of compressive load on the package ball grid array (BGA) solder joints. In this paper, the effect of compressive load on SnPbAg solder joint reliability is investigated by using both experimental and numerical approaches. Accelerated system-level solder joint reliability tests under temperature cycling and isother- mal aging conditions, with the presence of compressive loads, are first performed to identify and characterize the critical reliability failure mode. Creep constitutive behavior under compression is then characterized and implemented in numerical finite-element simulations for developing a phenomenological model of the BGA solder joint failure under compressive loading. A life prediction formula for SnPbAg solder joint subject to constant compressive load is also proposed. Index Terms—Bridging failure, compression, creep, heatsink, solder joint reliability, viscoplasticity. I. I NTRODUCTION T HE IMPLEMENTATION of dedicated heat dissipation solutions in microelectronic systems by using heatsinks with fans or heat pipes has been increasing due to the con- tinuous rise of power density in microelectronic components and systems. Aside from the usual applications, such as mi- croprocessor, optoelectronic, and power devices, heatsinks are also used for other high-performance applications, including network processors and transceivers. In these applications, the heatsink is attached to the exposed surfaces of electronic packages with a thin layer of thermal interface material be- tween them. Typically, the thermal interface material consists of particles of high thermal conductivity suspended in polymer matrix. When external compression is applied to the interface material, the bulk thermal conductivity increases as a result of layer thinning and compression of the particles in the gap. In addition, the applied compression results in closure of micro- gaps at the interfaces of the package– and heatsink–interface Manuscript received February 1, 2010; revised March 20, 2010; accepted April 19, 2010. Date of publication May 24, 2010; date of current version September 9, 2010. T.-C. Chiu is with the Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan (e-mail: tcchiu@mail.ncku. edu.tw). D. Edwards is with Semiconductor Packaging, Texas Instruments Incorpo- rated, Dallas, TX 75243 USA (e-mail: rvin@ti.com). M. Ahmad is with the Technology and Quality Organization, Cisco Systems, Inc., San Jose, CA 95134 USA (e-mail: mudasir.ahmad@cisco.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2010.2049848 Fig. 1. Schematic of the board-level assembly with heatsink and backing plate. materials, reducing the thermal contact resistance. Therefore, from the perspective of thermal dissipation, higher compressive force is desired for heatsink attachment. A typical approach for achieving high compressive load is to connect the heatsink to a backing plate placed on the opposing side of the printed circuit board (PCB), as shown in Fig. 1. Springs, as shown in Fig. 1, are typically used for controlling the bolt tension and the compressive force. In certain motherboard layouts where components are placed very close to each other, a single ganged heatsink is used for multiple components. While this assembly approach has been used extensively for socketing pin grid array or land grid array microprocessors, it is also being used more frequently for attaching heatsinks to surface-mount ball grid array (BGA) packages. Due to the compliant nature of solder, the BGA joints are susceptible to excessive creep deformation and may fail under the increased compressive load. It is therefore important to characterize the reliability failure mode and associated mechanisms for BGA packages under the presence of heatsink-induced compressive load. The effects of heatsink compression on board-level solder joint reliability for BGA packages were highlighted by several researchers in recent years. Garner et al. [1] investigated board- level BGA solder joint reliability under either temperature cycling fatigue or constant temperature creep conditions, with the presence of compressive load for the test board assembly. Bhatti et al. [2] studied the change of critical solder joint failure locations for both SnPb and SnAgCu flip-chip BGA packages under board-level temperature cycling and compressive pre- load. The effects of heatsink type and PCB thickness on BGA solder joint reliability were investigated by Beh et al. [3]. Zhang [4] studied the board-level temperature cycling reliability for Pb-free high-CTE ceramic flip-chip BGA packages with the presence of heatsink compression. The influence of heatsink 1530-4388/$26.00 © 2010 IEEE