Die cracking in flip-chip-on-board assembly

The die cracking in a flip-chip-on-board (FCOB) assembly occurs very often during the solder reflow process and the fracture path follows a straight downward crack extension and then kinks at a certain depth. Such a fracture path selection in FCOB structures is determined here based on a mode-I domination criterion. This is done by solving the corresponding problem of a kinked crack in a FCOB structure for a series of kinking angles. The numerical results indicate that the energy release rate maximum assumes initially at a zero-degree angle, which points to the straight downward extension in the thickness direction, and then shifts to a large angle, which corresponds to kinking. The kinking angle and the depth at which the kink starts have been predicted and compared agreeably with cross sectioning of failed samples from a manufacturing floor. The maximum allowable manufacturing defect size, below which the die cracking can be prevented, is then determined by a beam theory. The analysis suggests that in a typical FCOB structure a defect size of more than 35 micron will result in die cracking failure after solder reflow. The numerical results also reveal that a good tolerance of defects can be achieved by limiting the board thickness to be less than twice of the die.