Study of critical load force towards thin plating on pre-plated leadframe

Abstract

Pre-plated leadframe (PPF) is an evolved technology to plate Nickel (Ni) — Palladium (Pd)-Gold (Au) or Gold Alloy on copper, with a typical plating thickness <2 μm. The introduction of PPF eliminates the need of tin plating process and whisker growth. Latest PPF technologies focus on exploring new combinations of plating materials, roughening technique to enhance the adhesion between mold compound to leadframe interface, and also moving towards thinner plating layer for further cost reduction. Integrated Circuit (IC) assembly process and test contact may exert excessive mechanical force onto the thin plating material, and thus deformation of this protective layer may lead to expose copper (Cu), which subsequently created solderability problem on Printed Circuit Board (PCB). Constant load scratch test is introduced to determine the maximum allowable load on thin plating layer before copper exposure occurred. The critical load (Lc), which corresponds to copper exposure, is determined, together with the coefficient of friction (CoF), which is ratio of normal load over scratch friction force. Nano-indention test, Field Emission Scanning Electron Microscope (FESEM), Non-contact 3D Optical Profiler, micro-X-ray Fluorescence (μXRF) are employed to characterize plating layer hardness, surface morphology, surface roughness, and plating layer thickness respectively. This enables to understand plating layer mechanical property to Lc. In general, result reveals that both Lc and CoF are able to characterize various PPF leadframe technologies. Results can serve as baseline for new PPF qualification and as test contact allowable insertion force guideline to prevent potential exposed Cu in the downstream process.