Electroplated solder joints for optoelectronic applications

Flip-chip solder joint technology has been widely used in the integrated circuit (IC) packaging and optoelectronics packaging. Several solder deposition techniques have been developed. In this paper, we will report on an electroplating technique to form Pb/Sn (both eutectic and 95/5% Pb and Sn) solder joints in any shape. For some optoelectronic applications, we successfully plated the Pb/Sn solder pads larger than the under bump metallurgy (UBM). In addition to solder shape and size, solder pad profile (cross section) is one of the most important issues for passive alignment flip chip bonding of optoelectronic devices where the vertical clearance is sometimes limited to less than one micron. Our experiments show that the solder pads surface profile is highly dependent on the process conditions. We found that in addition to an electric field effect, electrolyte diffusion played a very important role in forming the solder pad profile. Based on our model, we optimized plating conditions and improved the thickness uniformity to 0.1 μm (standard deviation) over 2-inch wafers for an average solder pad thickness of 4.6 μm. This process reduces the cost of solder deposition as well as providing a means for controlling airborne Pb toxicity which might be found for pads deposited by vacuum deposition methods