Via-in-Mold (ViM) process for embedded wafer level package (eWLP)

Abstract

In this paper, a different type of through mold interconnects, known as Via-in-Mold (ViM) was developed for embedded wafer level package (EMWLP) to enable 3D stacking application. Via in Mold (ViM) interconnects are blind vias laser drilled into the mold compound with Cu plated sidewall metallization forming the electrical connection between top and bottom sides. The two main challenges faced in the development of the ViM are: laser formation of the blind vias and Cu plating along via sidewall. Mold compound is a composite material made up of inorganic fillers and epoxy resin. In order to improve the packing density of the mold composite, a wide distribution of filler particle size is often used. This resulted in a non-uniform material which is very difficult to be laser drilled. ANd:YAG laser of a spot size of 20 um was used to drill blind vias and the laser drilling process was optimized to achieve consistent blind vias of ~100 um top diameter and 200 um depth stopping on the front side Cu pads. The design of the front side Cu pads plays a very important aspect during the laser drilling. Fabrication result shows that smaller Cu pads structures tend to delaminated during the laser drilling as they are unable to dissipate the heat generated by the laser source during the via formation process. Electroless Cu plating process was used to deposit the Cu seed layer along the blind via sidewall. The electroless process was optimized to achieve better sidewall coverage in the blind via. From the fabricated test vehicle, the via-chains electrical resistance was measured to extract the resistance of a single ViM. From the electrical resistance measurement, the resistance for a single ViM chain is ~0.095 Ω.