Influence of Prepreg Material Properties on Printed Circuit Board (PCB) Stack-up

Abstract

As the complexity of electronics devices increases, one critical component is the printed circuit board (PCB), which must balance both thermomechanical and electrical requirements. In this paper we evaluatehow properties of individual constituent layers affect the stack-up properties of the PCB. Five new FR-4 materials in the form of cured prepreg were investigated and thermomechanical properties such as glass transition temperature (Tg), coefficient of thermal expansion (CTE) in X/Y/Z-plane and Young’s Modulus were measured. Results shows that X/Y CTE decreases, and Young’s Modulus increases as the glass fiber density of prepreg increases and these values can vary up to 50%. Higher Tg is desirable to match reflow temperature of 250-260°C lead-free solder to reduce warpage and CTE mismatch during processing and reliability. Promising FR-4 material candidates with higher Tg, lower CTE and optimized Young’s Modulus were chosen and build into PCBs with different glass fiber styles, form factor and dimensions. Regardless of PCB design, improvement in prepreg properties enhances final PCB properties and warpage performance. PCB Tg and Z-CTE are highly dependent on the intrinsic properties of resin material, while in contrast PCB X/Y CTE is more dependent by the choice of stack-up design. PCB modulus is affected by both prepreg properties and stack-up design choices, as copper content and distribution has an influence on modulus. It is understood that this correlation between prepreg properties and PCB stack-up can enhance current knowledge on PCB design for better reliability.