Improving the Solder Wettability Via Atmospheric Plasma Technology

Solder pad oxidation is a well-known issue in the electronic packaging industries where the oxide layer degrades the solder wettability and consequently posts solderability and reliability risks. One common approach on solving this problem is to include an add-on plasma cleaning process. Conventionally, this process has to be operated with a vacuum system, therefore, it is costly and less-uniform. With the recent well development on atmospheric pressure plasma (AP-plasma) technology, the cleaning process is now allowed to be proceeded in the ambient environment. In this work, AP-plasma will be used in the cleaning process. Solder wettability after cleaning and joint reliability after high temperature storage (HTS) will be studied. Experimentally, AP-plasma with compressed dry air and H2O vapour gas sources are used for cleaning PCB substrate with common surface finishes, i.e. OSP, ENIG. The sessile method is then carried out where the SAC305 solder ball is mounted on the substrate. The wetting angle and spreading ratio are then measured to perform the wettability performance. The ball shear test is applied in an atmospheric temperature after HTS at 150 degrees C up to 1,000 hrs. Results show that the SAC305 wettability is improved when AP-plasma is applied. The improvement can last for 1 - 4 hours which provides sufficient staging time of the process. It is also worth mentioning that the wettability of the plasma-cleaned-bare Cu without surface finish is better than that of all surface finishes without plasma cleaning, which suggests that it is a dominant factor on the wettability improvement. The ball shear results at post HTS will also be presented and discussed in this paper.