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Ulvac crafts new direct solder deposition processes

Posted: 15 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:solder deposition? Si devices? power devices?

Ulvac Inc. has developed new solder deposition processes for the manufacture of Si devices, including power devices.

The process crafted by Ulvac use sputtering to deposit solder, unlike conventional solder deposition which is done by evaporation or printing.

The market for power devices such as IGBTs and MOSFETs is rapidly expanding in accordance with the growth of the market for electric vehicles (EVs) and energy-saving home appliances that require inverters and switching power sources. Electrodes on the back of these power devices have two purposes: to make ohmic contact with Si substrates and to provide solder joints with heat sink substrates. A typical deposition composition is formed from the layer closest to the Si substrate by: (1) an Al or silicide ohmic contact layer; (2) a Ti barrier metal layer; (3) an Ni bonding film layer; and finally (4) an Au 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 in order to deposit a solder layer with an appropriate thickness by evaporation or printing using another device; and finally, they are joined to a heat sink substrate by reflow soldering. However, as prices of precious metals increase, efforts are being made to reduce the thickness of the Au deposition layer on the electrode surface as well as to use alternative materials.

Ulvac's process provides the same or higher joining strength as the conventional process with reduced cost. It performs sputter deposition of the solder in a vacuum immediately after depositing the Ni bonding film layer, without Au film deposition on the surface of Si device electrodes. Two processes were actually developed, both make it possible to reduce material costs compared to the conventional deposition method using Au.

The first process involves solder pasting/solder sputtering/Ni/Ti/Al/Si wafer without the use of AU. This process deposits a 0.5?m Sn-Ag-Cu 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 percent compared to conventional electrodes with Au layers.

The second process technology uses solder pasting/solder sputtering/Ti/Al/Si wafer without Au or Ni. This process eliminates the use of Au and Ni in order to further reduce electrode film material costs. Ni films, which form an alloy with solder, are generally used as bonding films. The newly developed process uses a Ti film as an alternative to the Ni film. Ti, which forms an alloy with Sn at a reflow temperature of about 230Celsius, makes it possible to create solder joints similar to before. By using this method of depositing electrode film layers, we have developed a new process that provides the same joining strength as the conventional process and further reduces materials costs.

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