The effects of heat treatment on matte pure tin-plated Cu leadframes at high temperature and humidity conditions were investigated. After 1800 hrs of storage at $55^{circ}C/85%$ RH, approximately 14.5 ${mu}m$ long striation-shaped whiskers were observed on the surface of the without postbake treatment (WOPB) samples, while no whiskers were found in with postbake treatment (WPB) samples. The preferred orientations of Sn grains in WOPB and WPB sample did not change after the postbake treatment at $125^{circ}C$ for 1 hr. However, both changed from (112) to (321) and (101), respectively, after 1800 hrs of storage at $55^{circ}C/85%$ RH. The tensile stress of 8 MPa generated in as-plated sample was changed to a compression stress of 17 MPa after 2 days in room temperature storage. Due to the grain growth during postbake treatment, the WPB samples have more regular grains than the WOPB samples. In the as-plated sample, 0.32 ${mu}m$ thickness of planar intermetallic compound (IMC) was observed. The IMCs in the WOPB and WPB samples had two distinct layers with large grains of $Cu_6Sn_5$ and with small grains of ${eta}-Cu_{6.26}Sn_5$.