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New Mixed Metal Transition Via-Fill Conductors for Cost Effective DuPont GreenTape™ 951 & 9K7 LTCC Circuits
Keywords: LTCC, Packaging, Mixed Metal
LTCC (Low Temperature Co-fired Ceramic) technology provides an attractive option for circuit designers by combining the benefits of both HTCC and thick film technologies for many complex telecom, automotive, consumer, and military applications. For demanding and high reliability applications, LTCC circuits containing gold conductors for internal and external ground planes, signal traces, and via fills are common. With the rise in gold cost from $400/TO in 2005 to a recent peak of over $1,200/TO in 2009, OEMs and circuit designers are forced to seek lower cost alternatives to these all-gold LTCC structures. The use of all lower cost silver conductors raises the questions of long term reliability, plus wire-bonding to silver is not possible. The primary method employed to reduce cost and maintain reliability has been the use of mixed metallurgies throughout the LTCC module. Silver based conductors for signal and ground are used internally, with gold conductors for wire bonding, brazing, or soldering on the external surfaces. Today there are two primary options to achieve this type of structure: 1) Ni/Au plating the top silver surface; or 2) Use of precious metal based transition via fill compositions that interconnect the silver to the gold conductors. Traditional LTCC transition via fills compositions have consisted of Pd/Ag or Pd/Pt/Ag mixtures. However, such systems may be disposed to certain phenomena at the surface interface(s) between dissimilar metals. For example, the Kirkendall Effect has been found in various alloy systems and can impact the bonding between different materials. In particular, it has been studied and is used to describe voids that are produced in the boundary region at a bonding interface especially during high temperature processes such as metallic powder sintering. This paper introduces two new mixed metal via-fill conductors that are compatible with the DuPont GreenTape™ 951 and 9K7 LTCC systems. Reliability and refire stability are described in detail, and LTCC module costs will be compared.
K. M. Nair, Research Scientist
DuPont Microcircuit Materials
Research Triangle Park, NC

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