Flip-chip attachment of fine-pitch GaAs devices using ball-bumping technology

Reduced pad sizes and pitches of devices demand a well-controlled ball-bump deformation during flip-chip bonding. This paper develops process parameters for the flip-chip bonding by the use of gold ball bumps for the best interconnection result, the lowest bonding forces, and a controlled bump deformation. Different initial ball-bump diameters of 50 μm (2.0 mils) and 80 μm (3.2 mils) were created by using 98% AuPd bump wire with diameters of 18 μm (0.7 mil) and 25 μm (1.0 mil), respectively. Ball bumping with a minimal pitch of 70 μm (2.8 mils) has been reached. A special preparation allowed the shear test investigation of each bump/pad interface after flip-chip attach. Bonding forces of just 20 and 25-50 cN/bump (1 cN = 10 N), respectively, lead to an excellent welding in the bump/substrate interface due to the special shape of ball bumps. For silicon devices that have a pad metallization of aluminum, the shear forces of the bump/chip interface increase after flip-chip bonding, too. No cratering of GaAs and silicon occurs after flip-chip bonding due to a low bonding force ramp of 5 cN/s and 10 cN/s, respectively. The flip-chip attach of small GaAs (FETs) with minimal pad sizes of 35 μm (1.4 mils) as well as minimal pitches of 70 μm (2.4 mils) are demonstrated. In this case, substrate bumping is the more advantageous bumping method. The well-controlled bump deformation during flip-chip attachment leads to a good adaptation to the reduced pad sizes.