For capacitor pads we have found the outermost dots should be larger than the inner dots to give a flat pad upon fusing. For dot dispensing, care must be taken to keep the needle from contacting other dispensed dots. The substrates with deposited thermoplastic are heated in an oven at 160 C to remove the solvent and fuse the adhesive. Though this can be accomplished in as little as 15 minutes, we use a one-hour cycle to be standardized with other cycles in the ovens. Figure 3 shows the relationship of these three parameters with respect to acceptable bond adhesion.
Temperature is the variable that produces the most effects on the bonding process.
The uniformity of the pressure application is also a critical parameter, especially for large dice. Without uniform pressure, a void-free bond cannot be produced. To get this uniform pressure, a pickup tool that provides full face contact is required. Acceptable adhesion requires a suitable combination of time, temperature and pressure. Once the thermoplastic and the two surfaces to be joined reach a minimum temperature permitting the flow of the material, pressure becomes the dominant factor.