1.3. Glass Transition Temperature
In addition to material loss contributions, the manufacturability of the PCB must also be considered. One parameter that can impact PCB manufacturability is the glass transition temperature (Tg).
All materials are subject to expansion and contraction as a result of temperature changes. The rate of expansion and contraction is determined by the material’s coefficient of thermal expansion (CTE). The glass transition temperature (Tg) is the knee point temperature at which the resin in the material begins to expand much faster than the expansion of the surrounding glass weave and copper. Ideally, the resin, glass, and copper in the PCB all have similar temperature coefficients so all three materials expand and contract together at a similar rate. However, at temperatures beyond Tg, the faster expansion of the resin increases the mechanical stress on the PCB structure. Because the copper and glass-weave are laminated together, they reinforce the PCB in the lateral X and Y direction. The resulting expansion is forced to take place mostly in the vertical Z direction. This rapid vertical expansion can over-stress via structures resulting in via fractures and even PCB delamination in severe cases if the expansion is excessive.
When choosing a material, ensure the Tg is high enough to withstand the expected manufacturing temperature cycles for both assembly and rework to ensure PCB manufacturing reliability. For restriction of hazardous substance (RoHS) compliant processes, reflow temperatures can sustain 240° C and even peak at 260° C. Work with the PCB manufacturer to ensure the selected material is appropriate for the targeted manufacturing process.