DuPont Microcircuit Materials recently exhibited its broad and growing portfolio of specialized thick film compositions for a wide variety of electronic applications in the photovoltaic, display, automotive, biomedical, industrial, military, and telecommunications markets, at the 43rd International Symposium on Microelectronics Conference and Exhibition, November 2-4, 2010 at the Raleigh Convention Center in Raleigh, North Carolina, USA, at Booth #623.
David B. Miller, President, DuPont Electronics & Communications, addressed “The Critical Role of Materials in the Photovoltaic Industry” as a featured speaker at the IMAPS 2010 Global Business Council Marketing Forum on November 3, 2010.
Mr. Miller provided an overview of the PV market, noted industry investment, and presented on the relevance of materials in achieving grid parity. He focused on energy conversion efficiency, system lifetime, and overall system cost. David also touched on both manufacturing and public policy factors that impact PV as a sustainable energy solution.
DuPont featured two key technical presentations:
Investigation of Silver Migration Impacts on Microwave Systems Fabricated on LTCC Substrate Under High-Power RF Drive and High Temperature and Humidity Conditions. Authors: Deepukumar Nair, K. M. Nair, Ken Souders, Michael Smith, Mark McCombs, James Parisi, Tim Mobley, and Bradley Thrasher.
The demand for a cost effective LTCC system has led to increasing use of silver based thick film conductors for inner layers of RF modules. If silver is used in a properly designed LTCC system on inner layers, the LTCC dielectric forms a hermetic seal around the silver circuit pattern, which leads to a highly reliable circuit and a low cost solution. A perception in the industry is that silver cannot be reliable to use due to a migration of silver through the glass-ceramic matrix. If silver is exposed to certain environmental conditions, dendrites can grow and cause shorting between two traces, if placed close enough, and also cause impedance changes in RF lines or shorting of interconnects. This paper describes the results of investigation to characterize and quantify the impact of silver migration on real-life microwave systems fabricated on LTCC. A buried filter was chosen from an actual production design in a high-reliability application, and was tested at 15W of continuous RF power at 1.5GHz under 85°C/85% relative humidity. Previous work1 has been done at DC, which shows that silver can be reliable for DC solutions, and has been used in high volume automotive applications for the past 10 years. Designers were not convinced that the successful work performed at DC would be indicative for high power RF applications, so a test method needed to be development and designs needed to be tested to prove that silver is a viable and cost effect solution. The test conditions chosen were meant to represent actual circuit usage, plus some amount of margin. Also, a difference from this paper from previous papers, which are used by proponents of silver2, is that we are not forcing the silver to migrate, but rather elevating that actual circuit conditions and holding for 1000 hrs. Forced silver migration failure on a typical non-LTCC thick film circuit is used for comparison purposes.
New Mixed Metal Transition Via-Fill Conductors for Cost Effective DuPont GreenTape™ 951 & 9K7 LTCC Circuits Authors: K. M. Nair, M.F. McCombs, K.E. Souders, S. E. Gordon
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.