Ph.D. Thesis Defense by Yarom Polsky
Wednesday, April 22, 1998

(Dr. I. Charles Ume, advisor)

"Improved Prediction Modeling with Validation for Thermally-Induced PWB Warpage"


Modeling efforts to accurately predict the thermally induced warpage of Printed Wiring Boards (PWBs) have been hampered by a variety of difficulties in the past. Some of these difficulties include: a high degree of variability in the mechanical behavior of available board core materials; a lack of temperature-dependent property information in the literature; inadequate representation of the complex geometric features of PWBs; a lack of definition of parameters essential to prediction of board warpage; and perhaps most significantly, a lack of correlation of modeled results to experimental results. Although much effort has been expended in recent years in the creation of tools and techniques for modeling the thermally induced deformation of PWBs, because of a lack of material property information and a limited ability to validate models experimentally, the fundamental mechanisms and parameters essential to warpage prediction are still not thoroughly understood.

This research developed a methodology to systematically address this problem. Simple PWB configurations were constructed for modeling purposes and the board core materials were obtained from the vendor. An automated material characterization system, capable of measuring the in-plane properties of the board core materials as a function of temperature, was developed and utilized to measure the thermomechanical properties of the acquired board materials. The measured property information was inserted into analytical and finite element models of thermally induced board warpage. An experimental system, capable of measuring the out-of-plane deformation of real PWBs in a simulated infrared reflow environment, was utilized to measure actual board warpage during temperature cycling for comparison to modeling results.

This study more clearly assessed the roles of support conditions, material property change with temperature, transverse shear strains and through board thermal gradients in the warpage process. The ability of an elastic theory to accurately model the deformation of an actual temperature cycled PWB configuration was also evaluated. Additionally, the use of a micromechanics approach for effectively representing trace layers was examined.