Processability and reliability of commercial palladium plated structures

This paper summaries the processability and board level reliability of five commercial plated leadframe structures. Extensive surface analysis has been performed on these Pd plated structures using XPS and AES. The surface chemistry changes at different temperature exposures and the associated Ni diffusion have been studied. The presence of stoichiometric palladium oxide was found to match closely with the solderability performance of the 2-layered structures. While the Au flash a in 3-layered structure enhanced dissolution rate in solderability, its thinner Pd layer was found to be susceptible to Ni diffusion at higher temperatures. The extend of Ni diffusion is dependent upon the Pd layer thickness and the process temperature. This paper also addresses the processability related issues such as solderability, adhesion, wire bonding, lead pull and susceptibility to bending cracks. In the adhesion studies all the Pd plating structures were found to have lower adhesion to mold compound when compared with Cu. Surface oxide was found to have little impact on the adhesion strength. For lead bend test analysis, bend radius of 0.22 mm and above were found to minimise the risk of cracking in the plated layer. The study also revealed the impact of leadframe rolling direction on cracking surface. For board reliability assessment, 2 types of volume production leadframes (TQFP-100 and QFP-160) were evaluated. In the present study, temperature cycling, thermal shock and thermal aging tests were performed. Samples at different reliability test intervals were subjected to lead pull test and microstructure analysis on cross-sections. This paper discuss the reliability results with respect to the surface mount quality, solderability and fillet formation of the various Pd plating structures. The intermetallic growth of the various structures with their impact on mechanical integrity of the solder joint was also examined.