Flip Chip Attach of Silicon and GaAs Fine Pitch Devices as well as Inner Lead TAB Attach Using Ball‐bump Technology

Abstract

Ball‐bumping is a flexible low cost bumping technology based on the conventional wire bonding procedure. It is applicable to single chips or whole wafers as well as to substrates. As established wire‐bonding machines can be used, expensive bumping‐process equipment for phototooling and plating is not necessary. Flip‐chip bonding is the most advantageous attach method of high frequency applications. Compared with wire‐bonding and TAB it allows the highest contact density, the shortest signal paths and lowest interconnection parasitics. The reduced pad sizes and pitches, not only of GaAs devices, demand a well controlled bump deformation during flip‐chip bonding. This work develops process parameters for the flip‐chip bonding of silicon and GaAs devices with respect to the best interconnection result by lowest bonding force and ball‐bump deformation. Ball‐bumps with diameters of 50 and 80 urn (2.0 and 3.2 mils) were created 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 achieved. A special preparation allowed the shear test investigation of each bump/pad interface after flip‐chip attach. Bonding forces of 20 and 25 cN/bump respectively lead to a good welding in the bump/substrate interface due to the special shape of ball‐bumps. For silicon devices which have a pad metallisation of aluminium, the shear forces of the bump/pad 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 a Fujitsu FLR 016 GaAs‐FET which has pad sizes of 35 urn is demonstrated. In this case, substrate bumping is the more advantageous bumping method. The feasibility of fine‐pitch TAB attach using ball‐bumps is introduced. 100 µm (3.9 mils) pitch silicon devices with 328 pads were ball‐bumped for both solder and thermal‐compression TAB. Bond forces were in the range of 9–11 cN/bump and 15–21 cN/bump respectively. Pull forces of approximately 30 cN/lead show good results of the bump/lead interconnection after TAB.

Citation

Eldring, J., Zakel, E. and Reichl, H. (1994), "Flip Chip Attach of Silicon and GaAs Fine Pitch Devices as well as Inner Lead TAB Attach Using Ball‐bump Technology", Microelectronics International, Vol. 11 No. 2, pp. 20-24. https://doi.org/10.1108/eb044528