Liquid Bonding Interface for Thermocompression or Solder Joined TABs

Liquid Bonding Interface for Thermocompression or Solder Joined TABs

      As is commonly practiced, thermocompression bonding of gold
bumps to TAB tape has required the application of high temperature
and pressure.  This is necessary so that the gold on the tape fuses
to the gold bump on the chip. Additional problems are experienced if
the TAB tape leads are not exactly in the same plane.  The high
pressure has also resulted in chip cracking, commonly referred to as
"cratering".  Solder attaching chips to the TAB tape can be
accomplished with lower temperatures and almost no pressure, but one
is limited to the solder technology with its limited strength.

      To greatly reduce the temperature and pressure on the
thermocompression tool, and to increase the strength of soldered
chips, the following method is proposed:
   1)  Bare copper TAB tape is plated with approximately 20 - 40
microinches of bright tin.
   2)  This tin is then plated with the 25 microinches of gold.  (The
parts may have to be immersed "live" to prevent the Au bath chemicals
from attacking the tin.)

      When localized heat is applied, the energetically favorable
state for gold and tin is the eutectic phase which consists of 80%
gold and 20% tin.  This is a gold-tin brazing alloy which is used in
many applications.  As this liquid eutectic is formed, minor
deviations in planarity can be taken up with this liquid phase.

      With the thermocompression bonder (TC bonder),...