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NOVEMBER 2018 I DESIGN007 MAGAZINE 49 SnBi alloy families have a competitive advan- tage over SnIn or SnGa variations due to their lower cost and broader source of supply. Conductive Polymer Termination The polymer-based materials used for com- ponent termination are a "thermoset" compo- sition furnished with a catalyst to accelerate curing and a large percentage of silver alloy particles for electrical conductivity. "Thermo- set" refers to the characteristic of the material to remain solid once cured even if subjected further to high temperatures. To enable even dispensing of the polymer- based material, a small amount of solvent is added to the polymer/tin mixture that is even- tually dispersed when exposed to the elevated temperature required for activating the cata- lyst. The conductivity of these materials will vary with different crystallographic orienta- tion. When conductivity flows in one direc- tion (bidirectional), the material is defined as anisotropic. Alternatively, when the conductiv- ity flows in all directions (unidirectional), the material is classified as isotropic (Figure 1). Similar to the solder paste materials previ- ously noted, manufacturers offer conductive polymer materials in a paste form for screen and stencil printing and a more fluidic form for dispensing. Dispensing of the conductive polymers is preferred when in-cavity compo- nent placement is required. Following component placement, the joining material will thermally cure to a solid state. The cure temperatures required may range from room temperature for two-part epoxy sys- tems to 80–175°C for other single-component materials. In particular, thermoset materials will cure more quickly in an elevated tempera- ture environment. Process variations for SAC solder material and the conductive polymer joining families noted above are compared in Table 4. Even though the materials referenced in Table 4 require elevated temperatures to com- plete the joining process, some anisotropic materials are available for room temperature joining using a single component ultraviolet (UV) curable polymer. To prepare for com- ponent termination with solder or tin-based conductive polymers, the copper land pattern surface must be clean and free of oxidation to ensure conductivity with the SnAg terminal plating commonly furnished on commercial passive components. Microvia Termination Microvia is term identifying a small plated via hole formed between layers of the circuit structure (either blind or buried) that is less than 150 μm in diameter. These copper-plated via holes may exist between one or more adja- cent board layers or extend through all layers of the circuit board. Some companies have found that rather than relying on solder or a Figure 1: Conductive polymer characteristics. Table 4: Comparing solder and conductive polymer process variations.