Fungi. They decimate Dutch elm trees and disfigure toe-nails, blacken bread and rot logs. And that’s not all. A group of researchers at MIT and Harvard University have now shown that the same culprits can corrode the protective polymer coatings that package and insulate complex integrated circuits. If this work holds up, it may suggest an explanation for the unexpected failures that sometimes plague electronic systems.
Complex electrical circuits resemble a club sandwich, with chips and wires smothered between insulating layers of polymers called polyimides. The aerospace and electronics industries rely on polyimides because they are strong and lightweight, repel moisture, stand up to high temperatures and flames, and resist ultraviolet rays and other damaging atmospheric radiation.
Makers and users of integrated circuits have also assumed that polyimides fend off attacks by microbes such as fungi. But in 1990 Bryce Mitton and Ronald Latanision, materials scientists in MIT’s Department of Materials Science and Engineering, noticed “white stuff” growing on polyimides used in their laboratory. The matter looked fuzzy under a microscope. They teamed up with Ralph Mitchell and ji-Dong Gu, microbiologists at Harvard’s Laboratory of Microbial Ecology in the Division of Engineering and Applied Sciences, who identified it as a mixture of fungi, especially one called Aspergillus versicolor that is commonly found in air and water.
The MIT/Harvard group didn’t know whether the fungi growing on polyimides were pernicious. To find out, they constructed simple test circuits coated with polyimide insulation and deliberately contaminated some with fungi, keeping the rest sterile. The scientists then measured the degradation of the polymer by monitoring the loss of electrical resistance. Polymer coatings with high resistance are good insulators. Compared with sterile samples, the insulating ability of the contaminated samples decreased rapidly. In fact, half the contaminated samples failed in one month, whereas the sterile samples took six months to fail. Such degradation in an electronic chip “could be bad news because you might get a short circuit,” says Mitton.
The extensive fungal networks the MIT/Harvard group found may be destructive in three ways, the researchers suggest. First, the fungi use some chemical in polyimide as a nutrient, which the researchers hope to identify. That substance causes the coating to break down while enabling the organisms to grow filaments-hyphae-which branch and tangle in masses called mycelia that look like disorganized spider webs.
“The mycelia grow into small spaces beautifully. If you have even one hypha running down through an insulator carrying water, you have a wonderful short circuit,” says Mitchell. Moreover, the fungi also release highly corrosive waste products such as hydrochloric and sulfuric acid.
The MIT/Harvard researchers stress that all their findings have come from laboratory work and need reproducing to verify that they’re not the product of some quirk. But should the findings hold up, they might explain mysterious failures in electrical systems. “Often electronics fail and no one knows why,” says Mitchell. A theoretical worst-case scenario could involve dysfunctional circuits controlling military aircraft.
While high-risk systems generally have back-up electronics, the concern remains real=especially for circuits exposed to high humidity. “Fungi love moisture,” explains Mitchell. System designers often leave electronics unsealed to make changing chips and components easy, he notes.
Producers of electronics may have recently added to any problem by a change in their cleaning procedures. In the past, cleaning agents used in the manufacturing process contained chemicals such as chlorofluorohydrocarbons, which damage the ozone layer surrounding the earth. To become environmentally friendly, in recent years manufacturers have switched to water-based cleaning products. Unfortunately, water is rarely completely free of contaminants. Mitton has consistently found contamination in supposedly pure water, coming from a variety of labs, that has undergone stringent cleaning to eliminate microbes. “I don’t think it’s possible to get rid of everything,” says Mitton. “Just leaving water exposed to air will probably contaminate it,” since fungi are everywhere.
Should future scenarios confirm that electronics can fail from fungal corrosion, Mitchell suggests that circuit designers could minimize moisture by sealing systems or lowering humidity.
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