LTCC (Low Temperature Co-fired Ceramic) technology provides an option for circuit designers that combines the benefits of HTCC and thick film technologies and is the technology of choice for many complex automotive, consumer, and military applications. For demanding and high reliability applications, LTCC circuits containing gold conductors for all 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 all gold LTCC structures. The use of all silver conductors raises questions of reliability, especially for the external silver layers, and wire-bonding to silver is not possible.
The traditional method employed to address cost and reliability has been the use of mixed metallurgies, rather than using gold 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.
