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58 DESIGN007 MAGAZINE I JANUARY 2019 example, companies using solder, a conductive polymer, or a plated-microvia interface will require an additional metallization process to provide a copper termination at each bond site. Facedown Semiconductor Termination Several processes have evolved for joining the semiconductor die to the embedded substrate surface. Smaller outline die with wire-bond sites greater than 400 microns may be furnished with solder bump or conductive- polymer bump features for termination. However, when the terminal spacing on the die is less than 400 microns, companies may consider implementing a selective plating process that furnishes miniature copper pillar-like terminals. Examples of common termination features are illustrated in Figure 2. More complex, high I/O semiconductors are commonly processed further to redistribute the wire-bond terminals to a uniform array contact pattern as described in Part 6. The array format enables larger termination features for interconnect and affords greater flexibility for circuit routing. The most common materials utilized for flip-chip termination are tin-alloy based solders and conductive polymers. Flip-chip Assembly Process Reflow solder processing is a common technique for flip-chip assembly and a substantial infrastructure already exists for supplying systems for both high- and low- volume manufacturing. To ensure long- term reliability, the solder alloy composition selected must be compatible with the contact alloy and surface finish supplied on the mounting structure. As expanded in Part 5, tin-silver-copper (SAC) alloy compositions become liquidus and enable flux activation at a temperature range between 220°C and 230°C. The temperature required to complete the joining of the component to the land patterns, however, will reach 245–260°C. A question one may ask is, "What will be the impact of the solder interface on the buried components during when the same solder processing temperature required to attach components on the outer surface(s) of the finished multilayer substrate or PCB?" To avoid the potential for disturbing the buried termination sites assembly process, specialists may consider adopting one of the lower temperature alloy composition for attaching the surface-mounted components. Solder alloys are available requiring no more than 185°C peak temperature to complete the joining process (compared to 245°C+ for SAC alloys). Alternative Conductive Polymer Assembly Polymer-based joining materials containing fine particles of a silver alloy are commonly dispensed directly onto the land-pattern sites provided for the semiconductor die prepared for facedown mounting. In preparation for conductive polymer termination, the land patterns on the mounting surface must be clean and free of oxidation to ensure conductivity with the fine silver particles embedded in the polymer compound. The cure temperatures required may range from room temperature for two-part epoxy systems to a range of 80–175 °C for other single-component materials (far less than solder processing). Although several of the materials referenced in Part 5 require elevated temperatures to complete the joining process, a number of anisotropic materials are available for room temperature joining using a single component UV-curable polymer. Figure 2: Comparing semiconductor terminal sites for facedown attachment.