“Underfill” is a process and adhesive originally designed for use with flip-chip devices to enhance the mechanical strength of their solder joints and improve their reliability.
Flip-chips made of silicon material have a much lower coefficient of thermal expansion (CTE) than typical PCB materials. As a result, temperature cycling tests often cause relative displacement, leading to mechanical fatigue and problems such as solder joint detachment or cracking. Later, this Underfill technology was also widely used under some BGA packages to improve their reliability when soldered onto the PCB and to prevent them from falling or dropping.
