Ideally the Z-direction expansion should match that of the copper in the plated through holes (about 17 ppm/ºC) to avoid damaging the plated copper during thermal excursions resulting from process steps such as solder reflow. Most standard materials have CTE’s of 50-60 ppm/ºC below the Tg, and roughly 4x higher above the Tg. High Tg materials such as polyimide have less overall Z-direction expansion (about 1.1% from 50ºC to 250ºC) than typical epoxy systems (with about 3 to 4% from 50ºC to 250ºC) due to their higher Tg.