SMART Group Webinar: Electronics in Harsh Environments
SMART Group presented a webinar June 2, 2014 to introduce issues to be explored in depth at a seminar on July 2, 2014 at the National Physical Laboratory (NPL). Technical Director Bob Willis moderated a programme of four presentations from Technical Committee specialists Sue Knight, Richard Boyle, Chris Hunt, and Ian Fox.
Sue Knight from STI gave an introductory overview: Why are electronics needed in harsh environments? "I’ll outline the problems; the other presenters will explain how to solve them!” said Knight. She listed applications in satellite communications and base stations, oil exploration and power generation, and aerospace, automotive, and industrial instrumentation, sensors, and controls.
Against this background, she summarised the main environmental factors affecting the performance and reliability of electronic assemblies. Temperature was the biggest issue: Standard industrial components were typically rated for 85°C maximum operating temperature, and mil-aero components to 125°C. Standard high temperature laminate was only usable in applications up to about 175°C. And solder joints were unreliable when equipment operating temperature approached the melting point of standard solder alloys, particularly if accompanied by mechanical shock and vibration.
Other environmental concerns were the effects of oil, water, mud, and chemicals. How did the harsh environment considerations typically associated with military and aerospace applications relate to everyday electronics? She explained how research and development generated from those areas was often subsequently applied to benefit the functionality and reliability of commercially available equipment.
RoHS had prohibited the use of lead in solders for all but certain specific applications where no practicable or proven alternative existed. High-lead alloys had for many years been used for die-attach and had given reliable results as solders for high-temperature environments. Under current RoHS and ELV legislation, they could continue to be used in selected electronics and automotive applications, but if and when suitable alternatives to high-lead solders were found then it was likely that the legislation would be amended to prohibit all use of lead-containing materials.
Solder specialist Richard Boyle from Henkel discussed the EMEA Solder Replacement Umbrella Programme, which had the initial aims of finding suitable replacements in terms of electrical, thermal, and mechanical performance, developing products in compliance with 2014 RoHS and ELV standards, and ideally producing drop-in alternatives to existing paste, wire and preforms. The preliminary focus was on paste, potentially the simplest option. The programme was taking a multi-faceted approach, looking at different solutions for sub-segments of the market, in respect of application, cost and performance, and was considering organic-based, metallurgic-based, and combination organic-metallurgic approaches including high silver-content organic die attach, epoxy-solder paste based on tin-antimony alloys, transient liquid phase sintering, and silver sintering.
Boyle’s reference to silver sintering was expanded upon by Dr. Chris Hunt, who discussed current research at NPL on sintered silver interconnect technology with different component types and PCB finishes, and NPL’s participation in the TSB-funded Electronic Component Sintered Interconnections (ELCOSINT) project, aimed at development of novel polymeric, sintered interconnection materials to replace high-lead solders and further increase the operating temperature of electronic assemblies. These materials, based on nano-silver, would be suitable for components subjected to operating temperatures of 250° or more, and the technology was designed to be compatible with standard microelectronics manufacturing processes. Several challenges remained to be addressed and mechanical performance and reliability had not yet been fully characterised.
The webinar was brought to a close by Ian Fox from Aero Engine Controls, with a wrap-up presentation entitled Harsh Environment Electronics: Materials and Making it Work. Typical examples of harsh environment conditions were high or extremely low temperatures, high humidity, and corrosive atmospheres.
He first considered the effects of high temperatures on components: The maximum storage temperature for conventional semiconductors was 150ºC, although many would operate up to 175ºC with some possible degradation in performance and reduction in lifetime. To achieve useful life above 200ºC, devices based on silicon-on-insulator were required, and for temperatures above 300ºC there were very few options. One was to use devices based on silicon carbide but these were very expensive and of limited availability. For soldered assemblies, at operating temperatures above 150ºC, the properties of eutectic tin-lead were significantly degraded, the solder was extremely plastic and its fatigue resistance was low. Changing the alloy to tin-silver (SnAg4) or tin-antimony (SnSb5) gave satisfactory performance at temperatures up to 175ºC. Modern phenolic based FR4 printed circuit laminates with 180ºC glass transition temperature were acceptable for operation at 150ºC, but higher temperatures required the use of polyimide. For operating temperatures in the range 175-225ºC, high-lead solder alloys Pb95Sn5 and Pb93.5Sn5Ag1.5 were commonly used, although their wettability was limited, and 225ºC was considered the upper temperature limit for laminate-based PCB designs, and ceramic substrates offered a better option, with hermetic microelectronics approach considered the most robust solution.
This webinar gave a broad overview of issues associated with design and manufacture of electronics for operation in harsh environments, particularly at high temperatures. Anyone working in high-reliability electronics and interested in learning more can hear the full story and participate in interactive discussion at the SMART Group seminar at NPL July 2, which will also feature detailed presentations on laminates from Alun Morgan of Isola and conformal coatings from David Greenman of Humiseal. Full details can be found here.