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 processing for a certain no-flow underfill material. A purchased uncleaned copper foil which was laminated on FR4 board and an in-house developed G25 no-flow underfill encapsulant were used in this study. Six different procedures were followed to clean the copper foil to obtain six different surfaces. XPS was then used to examine the surface composition of the six surfaces and the uncleaned surface (as a control). The actual wetting test for each surface was performed using small eutectic solder balls. The correlation of the surface composition of copper foil (equivalent to the copper pads of substrates) with the solder wetting was then established. The composition of the copper foil surface was verified to depend on the cleaning procedure. 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 a noticeable effect on the solder wetting.