Characterization of Compressive Die Stresses in CBGA Microprocessor Packaging Due to Component Assembly and Heat Sink Clamping

Abstract

Microprocessor packaging in modern workstations and servers often consists of one or more large flip chip die that are mounted to a high performance ceramic chip carrier. The final assembly configuration features a complex stack up of flip chip area array solder interconnects, underfill, ceramic substrate, lid, heat sink, thermal interface materials (TIMs), second level ceramic ball grid array (CBGA) solder joints, organic printed circuit board, etc., so that a very complicated set of loads is transmitted to the microprocessor chip. Several trends in the evolution of this packaging architecture have exacerbated die stress levels including the transition to larger die, high coefficient of thermal expansion (CTE) ceramic substrates, lead free solder joints, higher levels of power generation, and larger heat sinks with increased clamping forces. Die stress effects are of concern due to several reasons including degradation of silicon device performance (mobility/speed), damage that can occur to the copper/low-k top level interconnect layers, and potential mechanical failure of the silicon in extreme cases. In this work, test chips containing piezoresistive stress sensors have been used to measure the buildup of mechanical stresses in a microprocessor die after various steps of the flip chip CBGA assembly process. The utilized (111) silicon test chips were able to measure the complete three-dimensional stress state at each sensor site being monitored by the data acquisition hardware. Special test fixtures were developed to eliminate any additional stresses due to clamping effects. The developed normal stresses are compressive (triaxial compression) across the die surface, with significant in-plane and out-of-plane (interfacial) shear stresses also present at the die corners. The compressive stresses increase with each assembly step (flip chip solder joint reflow, underfill dispense and cure, and lid attachment). The experimental observations from this study show clearly that large area array flip chips are subjected to relatively large compressive in-plane normal stresses after solder reflow. We also observed that the majority of the die compressive stress is accumulated during the underfilling assembly step. Typical increases in the stress magnitude were on the order of 300% (relative to the stresses due to solder joint reflow only). As a general “rule of thumb,” approximately two-thirds (∼66%) of the final die stress magnitudes were observed to be developed during the underfill dispense and cure, with the second largest contribution coming from the die attachment, and the smallest contribution coming from lid attachment. The experimental test chip stress measurements were correlated with finite element simulations of the packaging process. A sequential modeling approach has been utilized to predict the build-up of compressive stress. The utilized method incorporates precise thermal histories of the packaging process, element creation, and nonlinear temperature and time dependent material properties. With suitable detail in the models, excellent correlation has been obtained with the sensor data throughout all packaging processes. Finally, CBGAs with the stress sensing chips were soldered to organic printed circuit board (PCB) test boards. A simulated heat sink loading applied, and the stresses were measured as a function of the clamping force. Compressive stress changes of up to − 60 MPa were observed for a 1000 N applied clamping force. The experimental test chip stress measurements were correlated with finite element simulations of the clamping process. With suitable detail in the models, excellent correlation has been obtained for the stress changes occurring during simulated heat sink clamping.