1. Silver-filled conductive epoxy films: These are commercially avail- able, and some examples are Henkel CF3350 and Ablefilm 5025E, and                       Rogers COOLSPAN®. The PCB and metal are bonded using temperature and pressure with a sheet film adhesive.

     2. Silver-filled conductive silicone films: These are patented ASC materials and have some differences from the commercially available materials.               The PCB and metal are bonded using temperature and pressure with a sheet film adhesive.

Figure 4-6 shows how a sheet film bonded assembly is put together.

Critical Design Factors

Critical design factors for sweat solder include the following:

• Adhesive selection: There are several different options that are avail- able, including ASC’s patented material Electrasil-2. Table 4-3 compares Electrasil-2 to commercially available sheet film adhesives from Rogers (COOLSPAN TECA) and Henkel (CF3350 and Ablefilm 5025E) and sweat solder as a reference

• Bonding fixture design: This is also custom-designed by the PCB fabricator to help ensure that there is good adhesion between the PCB and metal and good registration between the PCB and the metal. It is important to work with a fabricator that has a lot of experience with custom designing bonding fixtures. Figure 4-7 shows a typical setup of how a PCB is bonded to metal using a sheet film adhesive

• Metal selection: Typical choices are Aluminum 6061-T6, Copper C110, or occasionally brass
• Metal surface finish: Table 4-4 illustrates the various surface finish options
• PCB surface finish: Gold and silver tend to be preferred surface finishes. HASL or lead-free HASL are not acceptable surface finishes. It is also important to review the datasheets of the sheet film adhesive or discuss with the PCB fabricator any specifics related to surface finish choices
• Hot vias on ground layers: If there are either hot vias or circuit lines on the bottom layer, they should be covered with solder mask. In addition, the metal should either be partially milled out or cut out completely