Effect of Reflow Process on Glass Transition Temperature of Printed Circuit Board Laminates and Model for Prediction of Package-on-Package (PoP) Warpage and the Effect of Process and Material Parameters
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The effect of temperature exposure encountered both during assembly and in fielded products, has a known influence on glass transition temperature of printed-circuit board (PCB) laminate materials. Printed circuit board laminates such as FR4 are composites of epoxy resin with woven fiberglass reinforcement. Interaction between manufacturing process variables that impact the changes in glass transition temperature (Tg) has been studied. The laminates studied have been broadly classified into high-Tg, and mid-Tg laminates. Different sets of reflow profiles were created by varying the process variables including, time above liquidus, peak temperature, ramp rate and cooling rate. The effect of multiple reflows encountered in normal assembly or board rework has been studied by exposing the assemblies to multiple reflows between 2x-6x. Changes to the glass transition temperature have been classified by measurement of the glass transition temperature were measured via Thermo Mechanical Analysis (TMA). Statistical analysis of the variables has been used to determine the statistical significance of the measured changes for large populations. Package-on-Package (PoP) assemblies may experience warpage during package fabrication and later during surface mount assembly. Excessive warpage may result in loss-of-coplanarity, open connections, mis-shaped joints, and reduction in package board-level reliability (BLR) under environmental stresses of thermal cycling, shock and vibration. Previous researchers have shown that warpage may be influenced by a number of design and process factors including underfill properties, mold properties, package geometry, package architecture, board configuration, underfill and mold dispense and cure parameters, package location in the molding panel. A comprehensive inverse model incorporating a full set of design and process parameters and their effect on PoP package and PoP assembly warpage is presently beyond the state of art. In this paper, data has been gathered on multiple package-on-package assemblies under a variety of assembly parameters. The packages have been speckle coated. The warpage of the PoP assemblies have been measured using a glass-top reflow oven using multiple cameras. Warpage measurements have been taken at various temperature of the reflow profile between room temperature and the peak reflow temperature. Finite element models have been created and the package-on-package warpage predictions have been correlated with the experimental data. The experimental data set has been augmented with the simulation data to evaluate configurations and parameter-variations, which were not available in the experimental dataset.