C-4 BLM Terminal Which Minimizes Kirkendall Void Damage
Original Publication Date: 1985-Oct-01
Included in the Prior Art Database: 2005-Feb-19
Fig. 1 shows a C-4 terminal in which the solder is attached to the TSM (top surface metal) at the substrate and the BLM (ball limiting metal) at the chip. The BLM is composed of layers of Cr, Cr+Cu, Cu and Au and is evaporated as shown in Fig. 2. During solder reflow the Au forms Au-Sn intermetallics which float off into the solder, while a layer of Cu3Sn intermetallic forms from the Cu, as shown in Fig. 3. The figure also shows a sheet of Kirkendall voids along the Cr/Cu - Cu3Sn interface. The voids are formed because Cu diffuses more rapidly through the reaction product (Cu3Sn) than does Sn. Therefore, there are no Sn atoms available to fill vacancies created when the Cu atoms leave the unreacted Cu layer to diffuse through the Cu3Sn to react with Sn at the intermetallic-solder interface.