Estimated reading time: 15 minutes

SMART Group Seminar: Harsh Environments & Electronics

How were the suppliers of conformal coatings responding to these new challenges? New coatings were being developed and Greenman quoted examples of UV curable coatings that looked and behaved like silicones, but were not silicone, synthetic rubber coatings with extreme flexibility and high-temperature performance, and new UV cure silicones that overcame the high slump ratio of thermal cure silicones. And these coatings were being tested to the extremes of all of the specifications currently published or proposed.

The seminar had provided, in logical succession, the expert views of the laminate supplier, the PCB fabricator, the researcher into novel joining methods, the solder supplier and the conformal coating supplier. It remained for Ian Fox from Rolls Royce Controls and Data Services to tie it all together from the point of view of an avionics OEM whose safety-critical products were required to work in close proximity to aero engines. His wrap-up presentation was 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. At high soldering temperatures, flux degradation was very rapid and inert or reducing atmospheres were recommended. Ian considered that vapour-phase soldering was preferable to convection reflow.

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