Process considerations of TC-NCP fine-pitch copper pillar FC bonding

In the presence of pre-applied NCP (non conductive paste) as underfill for thermo-compression (TC) bonding of fine-pitch copper pillar flip-chip (FC), careful optimization of the process parameters is needed to ensure reasonable bonding yield. Otherwise, many NCP-related failures such as NCP voiding, filler entrapment at soldering interfaces, no solder wetting on bond pads and etc., will affect the quality of the TC bonding. Satisfactory results can be obtained by operating in a narrow process window. We have investigated the effects of some major parameters and their process considerations for improving bond quality of TC-NCP bonding. These include the studies on: (i) NCP dispensing, (ii) bond tool setting such as its descending speed and initial temperature (iii) the applied compressive force profile for deforming the solder and enlarging its area for bonding, (iv) the bonding temperature profile that defines the heating and cooling rate, the peak bonding temperature, the curing temperature, the time for solder wetting and NCP curing. Proper setting for force and temperature profiles can promote solder wetting and minimize NCP voiding and filler entrapment. Other considerations affecting the solder joint integrity are: the planarization of the bond tool with respect to the bonding substrate, the bond pad design and its metallization as well as the selection of appropriate NCP with the consideration of solder wettability on the given metallization.