Study on the relationship between the surface composition of copper pads and no-flow underfill fluxing capability

The purpose of this paper is to investigate the effect of copper pad surface composition on the wetting of solder bumps during reflow process for a certain no-flow underfill material. A purchased copper foil which is laminated on FR4 board is used as a control surface. Six different procedures are followed to prepare the surface of the copper foil with six different compositions. XPS is then used to analyze the surface compositions of the six surfaces and the control surface. An in-house developed G25 no-flow underfill encapsulant is used to examine the wetting status of eutectic solder balls on these copper surfaces. The correlation of the copper surface compositions with the solder wetting is then established. It is verified that the compositions of the copper foil surfaces strongly depend on the cleaning procedures. For G25 no-flow underfill material, copper oxide (CuO) is the main composition that prevents the solder ball from wetting the copper foils while the observed organic contamination does not have noticeable effect on the solder wetting.