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, th e fluxing capability of no-flow underfill material on tin-lead coated and g old 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-flo w underfill as flux agent. By inspecting the wetting angle, it was found th at 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 suscept ible 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 cl eaning could remove tin-oxides on the coated surface substantially, thus en hancing the no-flow underfill fluxing capacity soundly. For comparison, the fluxing capacity of a no-clean flux paste was also investigated in this wo rk the results showed that the tin-oxides had a negligible effect on the fl uxing capacity of the no-clean flux paste. The catalyzing effect of metal s alt to the epoxy curing process was used to explain the negative effect of tin oxides to the fluxing capacity of the no-flow underfill. (C) 2001 Elsev ier Science B.V. All rights reserved.