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Advances in the use of AuSn solder for Die Attach
Keywords: AuSn Solder, Die Attach, semiconductor devices
Gold tin eutectic solder has been used for the die attach of high power III-V semiconductor devices for many years. Conventionally, AuSn has been used as a discrete preform that is positioned between the gold-coated backside of the semiconductor chip and the gold-plated flange or chip carrier. AuSn has many advantages for this application, including no need for a flux, a good thermal conductivity (57 W/m-K), and a low eutectic temperature (280C) which prevents damage to delicate GaAs or InP devices. Recent developments in wideband semiconductors has expanded the use of AuSn solder to high power GaN devices, where the AuSn solders to the substrate beneath the GaN active layers. Typical substrates include SiC, sapphire and bulk GaN. There has been a strong demand to reduce the thickness of AuSn layers used for die attach for two reasons: (1) the escalating cost of gold, and (2) a desire to minimize the thermal resistance between the high power chip and its underlying flange. Years of engineering development have reduced the thickness of freestanding AuSn ribbon to 15 µm (0.6 mils). Ribbon this thin can be stamped into preforms and then mechanically positioned beneath the die. However, thin AuSn is brittle and is easy to fracture. To avoid problems with handling, Materion has developed a process to tack weld AuSn preforms onto specified positions on the surface of a flange or chip carrier. AuSn can also be clad onto both faces of Cu, Ni or Mo to create pedestals or carriers for the semiconductor die. By repeated rolling, the thickness of the AuSn layer can be reduced to 5 µm on both faces of Cu or Ni 45 µm thick, followed by stamping to final dimensions. For the ultimate reduction of AuSn solder thickness, AuSn solder can be deposited onto the backside of a wafer by sputtering. AuSn sputtering targets are available with Sn-rich compositions to promote the formation of eutectic AuSn. To minimize cost, AuSn can be recovered and reclaimed from the spent sputtering target and shield kit so that the net consumption of AuSn is limited to the material deposited onto the wafer.
Richard Koba, Product Market Manager
Materion Corporation
Buffalo, NY

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