Development of convective reflow-projection moire warpage measurement system and prediction of solder bump reliability on board assemblies affected by warpage

Out-of-plane displacement (warpage) is one of the major thermomechanical reliability concerns for board-level electronic packaging. Printed wiring board (PWB) and component warpage results from coefficient of thermal expansion (CTE) mismatch among the materials that make up the PWB assembly (PWBA). Warpage occurring during surface-mount assembly reflow processes and normal operations may cause serious reliability problems, such as severe solder bump failure, die cracking, and delamination of the solder bumps between electronic components and the PWB.; In this research, a projection moire warpage measurement system capable of measuring the warpage of PWBs, PWBAs, and electronic packages during the convective reflow process was developed. This real-time, non-contact, and full-field measurement system was used to investigate the warpage of PWBAs assembled with chip packages during various thermal reflow processes. Based on the projection moire system and finite element modeling (FEM), the effect of PWB warpage on solder bump fatigue reliability on board assemblies was studied. In order to accurately simulate the reflow soldering process and to achieve the ideal heating rate of 2°C/second, a convective heating system was developed and integrated with the projection moire system. An advanced feedback controller was implemented to obtain the optimum heating responses using the heating system. The developed heating system has the advantages of simulating different types of reflow processes, and reducing the temperature gradients through the PWBA thickness to ensure that the projection moire system can provide more accurate measurement results. To simultaneously measure the warpage across the PWB and omnidirectional chip packages in a PWBA, automatic package detection and segmentation algorithms based on mask image model and active contour model (snake) were developed. They were used to detect and segment assembled packages from the PWB in a PWBA displacement image generated by the projection moire system. The warpage of the PWB and chip packages can be determined separately irrespective of the package locations and orientations. Real-time continuous and composite Hermite surface models were constructed to estimate the PWB warpage values underneath the electronic packages.; The convective reflow-projection moire measurement system was used to study PWBA warpage behavior during thermal reflow soldering processes. Different PWBA test vehicles were considered. The effects of package size, location, number, and different temperature profiles on PWBA warpage change were discussed in details. In addition, a repeatability study was performed to demonstrate the suitability, reliability, and repeatability of the projection moire system for online experimental measurement. The classical laminated plate theory was applied to study the warpage behavior of PWBs and PWBAs during the thermal reflow process. The rule of mixtures was used to estimate the effective material properties of PWBA composites. Closed form solutions of the differential equations for the PWB and PWBA deformation were generated to evaluate their warpage. The calculated warpage results were compared with experimental and FEM simulation results obtained under the same thermal loading and boundary constraints.; The effect of PWB warpage on the low cycle fatigue of the solder bumps on board assemblies was investigated using FEM and experimental study. The three-dimensional (3-D) models of PWBAs with varying board warpage were used to estimate the solder bump fatigue life for different types of plastic ball grid array (PBGA) packages mounted on PWBs. In order to improve the accuracy of FE results, the projection moire technique was used to measure the initial warpage of PWBs, and this warpage was used as a geometric input to the FEM. Both Sn-Pb and lead-free solder materials were used in this study. The simulation results were validated and correlated with the experimental resul...