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In an effort to address the impact of rising gold costs on the fabrication expense for high frequency circuits, and maintain circuit quality and reliability, the DuPont GreenTape™ 9K7 LTCC with top silver conductors have been evaluated using electroless nickel immersion gold (ENIG) platting. Results comparable to the quality of efficiency and reliability of traditional gold conductor systems were achieved at reduced costs.LTCC coupons produced using the ENIG process were tested and compared to standard specifications for coupons with gold printed conductors. The test methods used include wire bond and adhesion pull tests on eight layer coupons, and MIL-STD-883 [1] temperature cycle testing on a layered high frequency test pattern. The reliability of the high frequency characteristics including insertion loss and return loss measurements were evaluated periodically throughout 1090 temperature cycles. Adequate results for most applications were obtained using the ENIG platted system compared to gold printed coupons.In an attempt to better understand the boundaries of the ENIG process on 9K7 LTCC and its use as an alternative for gold conductors systems; future testing will explore the effects of additional environmental testing. The added tests will include thermal shock, and temperature and humidity cycling on 9K7 LTCC test coupons. The testing will be conducted in accordance with MIL-STD-883 as a benchmark, and also evaluate wire bond and adhesion properties. This paper will report the reliability test results on plated GreenTape™9K7 LTCC systems after exposure to the stated environmental conditions.
Low Temperature Co-fired Ceramic (LTCC) material systems offer a highly versatile microwave and millimeter wave packaging platform. Extremely low microwave loss, excellent control of dielectric constant, uniform dielectric thickness, non-existent water absorption leading to very high hermeticity, ability to support multilayer structure leading to 3-dimensional packaging, ability to embed passive functions within the tape layers, availability of a wide range of metallizations, etc. are some of the key advantages of LTCC for microwave packaging. One of the important parameters which needs to be determined at the very early stages of circuit designs are the dielectric properties - dielectric constant and loss tangent, both of which are functions of frequency. These properties need to be known accurately over the entire frequency range of operation for the circuit. For LTCC based designs, the use of dielectric constant of bulk material can lead to deviations between the performance expected at the design stage and for the fabricated circuit. Such deviations are a significant concern for broadband circuits as well as for circuits with sharp resonant behavior such as filters. One of the significant sources of deviation between bulk LTCC and “in-circuit” dielectric constant is the nature of the thick film metallizations used in LTCC technology. Work described here is a comprehensive characterization of three DuPont™ GreenTape™ LTCC systems 951, 943, and 9K7 - in the frequency range 10 to 70 GHz. Both bulk and “in-circuit” dielectric properties with silver and gold metallizations are studied to quantify the deviations in dielectric properties. A Fabry-Perot open resonator technique is used for the bulk characterization while printed ring resonators are used for the in-circuit characterization. This comprehensive characterization will provide key design data for LTCC designers in the 10 – 70 GHz frequency range.
The demand for cost effective LTCC systems has led to increasing use of silver based thick film conductors for inner layers of RF modules. There exists some level of concern about the reliability of silver as a conducting material for electronic circuit packaging. This arises from the tendency of silver to migrate under the influence of electric fields when subjected to certain favorable environment conditions. However, previous research has shown that 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. 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 material system with silver metallization. A buried multilayer filter was chosen from an actual production design in a high-reliability application, and was tested at 15W of continuous RF power at 1.5 GHz under 85°C/85% temperature and relative humidity. The chosen test conditions were meant to simulate actual operating conditions encountered by the circuit in real-world, high reliability applications. The focus of this work is to collect useful operational information under environmental stresses encountered in typical applications. To this effect, unlike previously reported work [1], [2], [3] by proponents of silver, the work described in this paper is not forcing silver to migrate, but rather tries to evaluate the performance of an actual application circuit fabricated with silver under high-reliability test conditions and high power, high frequency RF excitation. Material characterization data as well as RF performance data-in terms of S parameters and power deliveredindicates no evidence of detrimental effects or performance impairments due to silver migration or other mechanisms.
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