Is preheating the substrate recommended for underfill applications?
Underfill viscosity and the contact angle of the underfill on the substrate surface decrease with temperature. The recommended temperature is actual temperature of the substrate, not the set point temperature of the hotplate. The preheating time will depend on factors such as thermal mass and heating method, but 30 – 90 seconds is typical. Using a thermocouple and a timer, the minimum preheating time may be measured as the time to heat the substrate from ambient temperature to the recommended temperature and stabilize at the recommended temperature. Insufficient preheating may cause irregular flow or voiding. Typically higher temperature will allow for faster flow, however the temperature is limited by gellation, which is the start of underfill cure. Please refer to the product’s technical data sheet for the recommended substrate temperature for underfilling. Typically temperatures of 90 – 110°C are used.
