Reliability of Printed Microwave Electronics

Abstract

In recent years, the focus of research and industry in the field of printed electronics has been primarily on challenges relating to process improvements like resolution and process stability or material improvements. In contrast, environmental simulation on-tests such as temperature shock tests or humidity-heat tests and their effects on electrical and mechanical properties as well as the high frequency (HF) properties of printed structures have hardly been considered so far. However, such environmental requirements for electronic components are particularly important for reliable use in all areas of printed electronics. In this paper, environmental simulation tests on printed conductive structures were therefore carried out and their effects on the electrical conductivity and microwave frequency properties were measured, analyzed and evaluated. The common environmental simulation test methods as well as their purpose and implementation variants are examined in detail for this purpose. Based on these fundamentals, first of all the selected substrate material RO4350B is printed with a conductive silver paste according to microwave frequency technical specifications using a dispensing printing process and sintered according to the manufacturers specifications. The substrate material has a relative permittivity $\varepsilon_{r}=3.48$ on which the geometry of the additively produced structures depends. To achieve the required characteristic impedance $Z_{L}\approx 50\ \Omega$, a width of $1080\ \mu\mathrm{m}$ must be reached. The printed samples are then subjected to various environmental simulation tests and examined using various measurement procedures. For the long-term reliability tests, the temperature shock test between −40°C and 140 °C for 1000 cycles, the humidity-heat test with 85°C and 85% relative humidity for 1000 h and the vibration test were selected according to DIN EN 60068. The evaluation methods are to focus on the effects of the environmental simulation tests on electrical and mechanical properties as well as the influence on the high-frequency properties. The conductivity is measured by means of four-wire measurement. A comparison was made of the electrical conductivity in the sintered state, during the reliability tests and at the end of the tests. The samples in the thermal shock test were examined after 250 cycles, 500 cycles, 750 cycles and 1000 cycles to make premature failures of the samples visible. In the moisture-heat test the samples were taken and examined after 500 h and 1000 h. The detection of defects and cracks is carried out using optical control. To determine the high-frequency characteristics, a 2-port measurement of the S-parameters up to 12 GHz was performed. The insertion loss without impact from the transitions was determined using a multi-line method. It can be summarized that the reliability tests have no significant influence on the insertion loss of the printed samples compared to the sintered references. While, especially in the temperature shock test, a change in electrical conductivity and isolated crack formations can be measured. This change regarding conductivity is due to the post-sintering effect caused by temperature exposure during reliability studies and suggests that the sintering time and method recommended by the manufacturer must be adjusted.