Authors: Jim Hisert, Jeff Schake, and Paul Flynn Company: Indium Corporation, DEK International, and FRT of America, LLC Date Published: 10/13/2008
IWLPC (Wafer-Level Packaging)
Abstract: The wafer-level microsphere process is an accessible system of bumping a wafer with solder, which focuses on achieving high output at a low cost. This process begins with a wafer that has undergone front and back end-of-line procedures and is ready to accept solder as means of later interconnection. Flux is printed on the wafer UBMs (under bump metallizations) in a standard wafer paste type printing operation. This operation employs either a mesh screen or stencil to align flux deposits directly over the UBM. It is very common to use solder to act as an interconnect, while the UBM provides an attachment point for the solder, as well as a barrier to unwanted diffusion. The UBM also controls intermetallic formation. One common under bump metallization stack is titanium/nickel/gold. Each material has a purpose. In this example, titanium is used as an adhesion layer, nickel limits diffusion, and gold passivates the nickel to limit oxidation. After the flux is deposited, spheres of the correct size (typically 60 – 300µm) are placed into the flux deposits and sent through reflow. The temperature for wafer reflow depends on the alloy, which is selected for the application. Tin/silver/copper alloys are very popular, although many people still use tin/lead and other low melting point alloys. The main consideration for choosing a certain alloy is often driven by processing restrictions during packaging or assembly. Sometimes a particular alloy is needed to endure life cycle testing or in-use conditions. Other lower temperature alloys are, at times, needed to allow the joining of die, which can not endure standard processing temperatures. The resulting solder formations should be spherical, with minimal height variations and maximum metallurgical attachment to the UBM. The flux can then be cleaned from the wafer surface if desired.