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Investigation of Silver Migration Impacts on Microwave Systems Fabricated on LTCC Substrate Under High-Power RF Drive and High Temperature and Humidity Conditions
Keywords: Silver Migration, High power, high frequency testing, LTCC packaging
Demand for cost effective LTCC systems 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 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 due to 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 an investigation to characterize and quantify the impact of silver migration on real-life microwave systems fabricated with DuPont GreenTape™ 951 LTCC materials system. 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 showing that silver can be reliable for DC solutions, and used in high volume automotive applications for many 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 developed, and designs needed to be tested, to prove that silver is a viable and cost effect solution. The chosen test conditions were meant to simulate actual circuit conditions. Also and unlike previous papers used by proponents of silver, this paper is 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.
Deepukumar Nair, Applications Development Manager
Research Triangle Park, NC

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