What is (chip) warpage?
Warpage is the result of stresses from bonding two adherends with mismatched CTE’s (coefficient of thermal expansion). As parts are heated for curing the adhesive the adherends expand to a certain degree, depending on their CTE. During cure the adhesive hardens, with the parts in an expanded state. As the adherends cool they shrink different amounts and this causes the package to warp slightly. Therefore, warpage is a measure of the amount of stress in a package. For example, a silicon die (CTE of 3 ppm/C) attached to a Cu lead frame (CTE of 17 ppm/C) will shrink less than the lead frame upon cooling after cure. This will cause the assembly to warp concave/downward. Using a low modulus die attach adhesive can help absorb some of the stress generated and reduce warpage. Other factors that may reduce warpage include thicker adhesive bond line and the adhesive cure profile (lower ultimate cure temperature and a slower cooling rate).
