The effect of metal oxide on the no-flow underfill fluxing capacity

In order to investigate the effect of solder pad surface composition on the wetting behavior of solder bumps during reflow using no-flow underfill, the fluxing capability of no-flow underfill material on tin–lead coated and gold plated Cu solder pads were studied. Before reflow, the solder pads were cleaned with dilute HCl and acetone. The X-ray photoelectron spectroscopy (XPS) was then used to examine the cleaned solder pad surface composition. The eutectic solder balls were reflowed to the solder pads using the no-flow underfill as flux agent. By inspecting the wetting angle, it was found that the fluxing capacity of the no-flow underfill on the tin–lead coated pad is much better than that on the gold plated solder pad, but it was susceptible to tin-oxides on the coated surface. Relatively high concentration of tin oxides also gave the wetting angles a wide range of values. The acid cleaning could remove tin-oxides on the coated surface substantially, thus enhancing the no-flow underfill fluxing capacity soundly. For comparison, the fluxing capacity of a no-clean flux paste was also investigated in this work the results showed that the tin-oxides had a negligible effect on the fluxing capacity of the no-clean flux paste. The catalyzing effect of metal salt to the epoxy curing process was used to explain the negative effect of tin oxides to the fluxing capacity of the no-flow underfill.