Wafer Solder Bumping

Advacam uses electro-chemical plating processes for depositing the solder bumps with UBM on 6” - 8” wafers. The solder bumping process is only be offered for complete wafers (not chips). The diameter and pitch of the deposited solder bumps begins from 20 µm and 40 µm respectively. The wafer bumping process requires one mask layer. The process is compatible with the standard 8” CMOS wafers (with a notch) and also with the 6” and 8" silicon sensor wafers.

High temperature solder bumps

The typical solder bump alloys available at Advacam are eutectic SnPb (63:37) and InSn (52:48). Also AgSn solder can be deposited in case of customer requests. The high temperature solder bumps are used for flip chip bonding of Si or GaAs sensors.

Fine pitch solder bumps.

Low temperature solder bumps

InSn solder is used for low temperature soldering of compound semiconductor sensors. These sensors, such as CdTe ad CdZnTe, are typically sensitive to temperature and their coefficient of thermal expansion is significantly larger than that of silicon.

Low temperature solder bumps deposited to every second pixel of the readout ASIC.

Solder bumping technology

Electroplating has been widely used in deposition of flip chip bumps because rather high deposit rate, well understood chemistry and good deposition uniformity. Both the UBM and the solder bump will be deposited in sequence using the same photoresist mask. Electroplating process requires typically one mask layer and one lithography round. UBM/solder is deposited galvanically at the photoresist openings and after the removal of the photoresist, the deposited metal layers act as a mask for etching the conductive seed layers from the wafers. Despite the simplicity of the electroplating, the process is very sensitive for chemical compatibility of different materials.

Picture depicts one pixel in different steps of electroplating process: 1) cleaning of wafers, 2) field metal deposition (adhesion/seed layer), 3) thick photoresist lithography, 4) electroplating of UBM, 5) electroplating of solder, 6) stripping of photoresist, 7) wet etching of seed layer, 8) wet etching of adhesion layer & 9) solder reflow.

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