Designing Electronics for Harsh Conditions


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How to Interpret Ingress Protection (IP) Ratings

This chart provides a visual of how IP ratings are applied and read. (Source: superbrightled.com)

EWMfg_IP_Ratings_Chart-1-2.jpg 

What are Conformal Coatings?

Most electronics include printed circuit board assemblies (PCBA), fiberglass boards with components attached that are then sprayed or 'painted' with conformal coatings to keep the circuits dry and dust free. Conformal coating is an inexpensive way to keep electronic components functioning in less than ideal environments. There are five main types of conformal coatings, each with their advantages and disadvantages. The coating you choose is based on the application of your electronic product and the functionality requirements within the application. 

Urethane Resin (UR)

Urethane resins can be either single- or double-part substances.

Advantages

  • Great chemical resistance
  • Good humidity resistance
  • Resistant to mechanical wear

Disadvantages

  • Difficult to remove
  • Risk of peeling
  • Long cure times common
  • Use of soldering iron for rework could leave brown-colored tint

Acrylic Resin (AR)

Acrylic resins are preformed acrylic polymers that have been dissolved in a solvent (Source: ACI Technologies). They are typically one-part substances.

Advantages

  • Easily applied and removed
  • Makes for easy rework or repair
  • No shrinkage during curing process
  • Affordable option

Disadvantages

  • Low chemical and solvent resistance
  • Low abrasion resistance
  • Not ideal for harsh environments
  • Not ideal for high-temp applications

Epoxy Resin (ER)

Urethane coatings are usually two-part compounds, although one-part varieties are also available.

Advantages

  • Great abrasion and moisture resistance
  • Great chemical resistance
  • Good humidity resistance
  • Excellent performance in harsh environments

Disadvantages

  • Difficult to remove
  • Shrinkage during curing process
  • Soldering iron required for rework or repair

Silicone Resin (SR) 

Silicone resin coatings are single-component compounds that are often chosen for electronics that will be subjected to extreme temperature ranges. 

Advantages

  • Performs well in extreme temperatures
  • Excellent humidity and corrosion resistance
  • Good chemical resistance
  • Adheres well to most PCB components/materials

Disadvantages

  • Most difficult to remove
  • Removal requires strong chemical strippers and harsh abrasion
  • Localized repairs only

Parylene (XY)

Parylene coatings are applied through a process called chemical vapor deposition. Parylene becomes a gas when heated. After cooling, it is put into a vacuum chamber, where it polymerizes and turns into a film. The film is placed over the electronics. 

Advantages

  • Best solvent and extreme temperature resistance of all coatings
  • High dielectric strength
  • Coating forms at room temperature; no curing time required
  • Transparent and colorless

Disadvantages

  • Difficult to remove, must be abraded off
  • Specialized equipment needed for chemical vapor deposition
  • Not ideal for long-term exposure outdoors

Designing electronics to survive the abuse we humans can create is no easy task, but you're not on your own. Guidelines like IP ratings, MIL STD 810g and understanding the pros and cons of the various conformal coatings can help direct your engineering team as they work to create your ideal product—able to withstand whatever gets thrown at it.

This article originally appeared on the East West Manufacturing blog, which can be found here.

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