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Tin/Silver Solder Paste in Die Attach (Sn/Ag)

A customer at Semicon West this year asked about Pb-free solder usage in die-attach applications. Although many smaller discrete components are attached using high melting, high reliability, and high lead (Pb) solders, the die-attach method of choice for many IGBT manufacturers is the tin-silver eutectic (96.5Sn/3.5Ag), which has the known advantages of:
  • High thermal conductivity (33W/mK)
  • Higher melting point than SAC alloys (221C)
  • Low tensile stress, so suitable for large die (5800psi)
  • Excellent thermal cycling properties (-55 to 125C)

The solder can be applied in a number of different ways onto the substrate in Power Semiconductor applications:
  1.  Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
  2.  Solder paste, which holds the die in place with no extra materials added 
  3.  Soft solder die-attach wire, a fluxless type of solder wire, which is melted onto the substrate metallization under an inert cover gas, and the die directly mounted onto the molten solder pool, then allowed to cool.

Heat transfer through the baseplate and direct-bonded copper (DBC) makes 1/ and 2/ (above) the preferred method of attachment for IGBT modules. By using a vacuum reflow process, it is also possible to make even solder paste (which always seems to generate some voids, even in standard processes) almost void-free, which was demonstrated in our recent paper.

Cheers!  Andy