Reliability of Pb-free preplated leadframe under atmosphere and accelerated aging test

摘要

Atmospheric corrosion of Pb-free preplated leadframe (PPF) finish, which causes unacceptable appearance due to discoloration, has been meticulously investigated. The discoloration appears particularly on the shoulder, tip, and sidewall of the leadframe, where bare Cu is directly exposed to the given environments. Such Cu exposure is attributed to the forming and trimming process of the PPF. It is found that the geometrical feature of the ragged shear edge on the sidewall of the PPF is dependent on the condition of a cutting tool. There is a porous Ni layer on the Cu substrate. It is also found that the tarnish products consist of cuprous oxide (Cu/sub 2/O) and cuprous sulfide (Cu/sub 2/S). The depth profile reveals a thick Cu oxidation layer over cuprous sulfide, which implies that the integrity of a dual inline package assembled with the PPF is susceptible when exposed to a relatively higher humidity and sulfur-involved environments. To reproduce the atmospheric corrosion, a variety of laboratory aging tests that include the mixed flowing gas (MFG) test was used. Except the MFG test, the laboratory corrosion that yields Cu oxidation only is not identical to the atmospheric corrosion. As results, electronic devices assembled with the PPF must be sealed into a moisture-barrier bag to extend the shelf life during storage, if particularly they are exposed to relatively harsh circumstances, higher humidity in H/sub 2/S atmosphere.