Sensorless Closed-Loop and Selective Heating for SiP MEMS Flip Chip

System-in-package (SiP) devices are increasingly becoming popular. When they are packaged using controlled collapse chip connection or derived processes, precise substrate heating at the land location is necessary. Currently, this is commonly achieved by heating the chip-handling probe. This approach restrains free motion of the chip and inhibits self-alignment of the chip to the substrate. Alternatively, the system is heated in a furnace. We present a method that heats the substrate locally at the desired spot while precisely controlling the temperature. This is achieved by embedding a Joule heater into the substrate. Additional sensors are not required because the change of the resistance of the heating, which reflects the change in temperature, is closely monitored. Because the temperature is determined inside the substrate, it provides accurate feedback of the solder´s temperature. The method has been implemented and tested in an SiP microelectromechanical systems assembly process. It has proven to be a viable alternative to current methods, and features fast, localized, and tightly controlled heating and does not restrict the chip´s free motion, thereby allowing chip self-alignment. Since the heating is integrated into the substrate, it also does not interfere with other components and requires no housing to protect the operators against optical radiation. Our method can be implemented at virtually no variable cost and with very low equipment costs.