Influence of Additives added to Pd-catalyst Treatment Solutions on Electroless Palladium/Gold Plating on Copper Fine Patterns

To improve the solder joint reliability of Pd/Au layer formed on Cu substrates, the effects of additives added to Pd-catalyst treatment solutions on the solder joint reliability were examined. As additives, ammonia, ethylenediamine-N,N,N',N'-tetraacetic acid dipotassium salt（EDTA･2K）, glycine, ammonium molybdate tetrahydrate （AMTH）, and sodium hypophosphite （SHP） were used. Surface and cross-sectional scanning electron microscopic observations were made of the Cu substrates after treatment with Pd-catalyst treatment solutions. Furthermore, Pd layers formed on the Cu substrates and electrochemical measurements of the Pd/Cu were examined to verify the degree of surface treatment of the Cu surface with the Pd-catalyst treatment solutions and the formation of voids and pinholes in the Cu/Pd interfaces and Pd layers. After a Au layer was deposited on the Pd layers, the Cu/Pd/Au solder joint reliability was evaluated by measuring the shear strength of the solder formed on the Au layers. The solder joint reliability test results showed that the SHP additive can improve the Pd/Au layer solder joint reliability. The Cu and Pd surfaces treated with the Pd-catalyst treatment solutions containing SHP were flat among those prepared with other additives. The number of voids around the interfaces between Cu and Pd layers in the case of SHP was the lowest among the additives examined. It can be inferred that the suppressed void formation contributed to improvement of Pd/Au layer solder joint reliability.