ULVAC cuts gold from package-level solder deposition process with sputtering methods

March 13, 2012 — ULVAC Inc. developed solder deposition processes for silicon device packaging, including power devices, that sputters solder to deposit it rather than printing or evaporating the materials. The 2 processes eliminate gold, or gold and nickel, from the step.

Electrodes on the back of power devices (IGBTs, MOSFETs) make ohmic contact with silicon substrates and provide heat sinks for solder joints. A typical deposition composition is formed from the layer closest to the Si substrate by an aluminum or silicide ohmic contact layer; a titanium barrier metal layer; a nickel bonding film layer; and a gold deposition layer, which prevents surface oxidation and improves soldering.

In current device manufacturing processes, these electrode layers are deposited by sputtering or evaporation; then, electrodes are taken out of the vacuum for solder deposition at a given thickness. Finally, they are joined to a heat sink substrate by reflow soldering. Efforts are being made to reduce the thickness of the Au deposition layer on the electrode surface as well as to use alternative materials.

The ULVAC solder sputtering method claims the same or higher joining strength than the conventional process, with reduced cost. Sputtering deposition occurs in the vacuum immediately after Ni film deposition, without Au film deposition on the surface of Si device electrodes.

Process 1: Solder pasting/solder sputtering/Ni/Ti/Al/Si wafer (no use of Au)
This process deposits a 0.5um tin/silver/copper (Sn-Ag-Cu, SAC) lead-free solder layer by vacuum sputtering immediately after depositing the Ni film layer. The Ni film layer serves the solder layer is used as the joining layer on the electrode surface. Sputter deposition of Ni and solder not only provides soldering with solder paste and the same joining strength, but also makes it possible to reduce material costs by approximately 50% compared with conventional electrodes with Au layers.

Process 2: Solder pasting/solder sputtering/Ti/Al/Si wafer (no use of Au or Ni)
This process eliminates the use of Au and Ni, further reducing electrode film material costs. A Ti film is an alternative to Ni, forming an alloy with Sn at a reflow temperature of about 230C for similar solder joints as those made conventionally.

ULVAC’s SRH series sputtering deposition systems are used in these new processes, and form backside electrodes for power devices, electroplating seed layers in wafer-level chipscale packages (WLCSP), barrier metals for under-bump metallization (UBM), and other devices.

Demonstrations of the newly developed processes will be performed at ULVAC’s Chigasaki Plant, Japan, starting in April.

The details of the newly developed technology will be presented at the 59th Spring Conference of the Japan Society of Applied Physics (6.4 Novel materials for thin films: 16a-F2-12) to be held at Waseda University’s Waseda Campus in Shinjuku-ku, Tokyo, March 15 to 17.

ULVAC, Inc., is a vacuum equipment manufacturer for flat panel displays, solar cells, semiconductors, electronic components, and general industrial equipment manufacturing. For more information, visit www.ulvac.co.jp/eng/

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