The roles of plasma science towards plasma-activated reflow soldering on Cu substrate with organic solderability preservatives surface finish

The issue of inherent surface oxidation on IC chips is decreasing the solderability in packaging industries. In this study, a direct current-atmospheric pressure plasma (DC-APP) with air gas is applied to activate the surface wettability of the Cu substrate for further soldering process. The DC-APP possesses a fast plasma initiation time (0.5 s) and produces reactive oxygen species (ROS), i.e. OH, O*, etc., which acts as the major oxidative agent for the formation of CuOx layer. The plasma-treated Cu substrate with an organic solderability preservatives (OSP) surface finish changes into a hydrophilic property with the increased polar component, which is contributed to form the formation of a uniform copper oxide layer. Compared with the pristine substrate, the plasma-treated substrate shows a decrease of C-C/C-H. The oxygen containing polar functional group C = O is increased after the 0.5 s plasma treatment. Quantitatively, the contact angle of the pristine substrate tends to be hydrophobic (63.7 +/- 4.7 degrees), and becomes hydrophilic after the 0.5 s plasma treatment (28.4 +/- 2.6 degrees), due to the increase of surface roughness. The results are: the solder spreading ratio after the 0.5 s plasma treatment is better than the pristine substrate. After 500 to 1000 h ageing time, the IMC thickness of the treated-sample slowly increases from 6.63 to 6.82 mu m, the IMC growth rate is relatively stable (from 1.32 x 10-5 to 1.46 x 10-5 mu m/s), and the ductile breaking mode is majorly observed. Furthermore, the wettability of SAC305 on the OSP substrate is observably improved when the DC-APP is applied.

Automated summary

In this study, a DC-APP with air gas was used to activate the surface wettability of Cu substrate in order to improve the solderability issue caused by inherent surface oxidation on IC chips. The plasma treatment resulted in a hydrophilic surface with increased polar component and reduced C-C/C-H content. The solder spreading ratio was improved on the plasma-treated substrate compared to the pristine substrate. After long-term aging, the IMC thickness of the treated sample increased steadily with a ductile breaking mode observed. Additionally, the wettability of SAC305 on the OSP substrate was significantly improved with the application of DC-APP.