What is the recommended number of passes when underfilling flip chips?
Single pass is typical, however, multiple passes may be necessary for larger die sizes (12mm+). Multiple passes (equal split of the material amount dispensed in single pass) help reduce the tongue size at the dispense edge, prevent underfill overflow on top of the die, and increase fillet height. The delay time between each pass should be minimized in order to prevent air entrapment between passes, which may cause voids.
