Printed Circuit Boards and Parylene
Posted by Sean Horn
Friday, June 26, 2015 8:00
@ 8:00 AM
While Parylene can coat just about anything, one of its most common uses is for protecting printed circuit boards. Product engineers specify Parylene because it offers a unique blend of five capabilities.
Thin and Complete
Parylene is the only commonly used conformal coating that gets deposited in a vapor state. While vapor deposition is a relatively complicated process, it offers two fundamental changes over the more typical liquid deposition processes used for other conformal coating materials.
First, because it is a vapor, it goes everywhere that air can. This means that the coating is truly conformal. Thick liquids have trouble coating in the small gaps between parts and the PC board substrate, for instance. Parylene has no problem covering these small gaps, since it is just a vapor.
Second, because of the combination of the vapor deposition and Parylene’s unique chemical make up, it works even when applied in an extremely thin coating. It can be so thin as to have almost no impact on the total thickness or weight of a coated item. This has the obvious benefit of working well when products need to meet tight tolerances.
Tin Whisker Mitigation
Tin whiskers are small crystals of tin that come out from soldered joints. Given enough time, the whiskers can become long enough to touch other connections, creating short circuits. Since PC boards usually have a significant number of solder points, tin whiskers are extremely likely to form. Typically, there are two ways to mitigate tin whiskers. One is to use solder containing lead. Unfortunately, lead solder is a major environmental and health hazard and is almost always unsuitable for applications where the item will be used in a consumer or health care system. The other solution is to conformally coat the PC board with Parylene or another compound which prevents the whiskers from crossing with other connections.
Parylene as a Dielectric
While electric signals are what make printed circuit boards work, interference from outside signals can also make them fail. Luckily, Parylene is an excellent dielectric. Coated boards are better able to resist outside electrical and radio frequency signals, leading to better-operating and longer-lasting electronic devices.
Parylene doesn’t just insulate PC boards from the outside. It also insulates PC boards from themselves. Every printed circuit board has traces that carry signals from place to place — those traces are the circuits. However, because those traces are uninsulated, they leak RF or EMI signals into the air and into each other. Parylene coating doesn’t just protect the board from the outside world. It also protects it from itself, letting designers build smaller boards with closer-together traces since they are able to resist each others’ interference.
Resisting (Almost) Everything
Parylene doesn’t just resist electric signals, though. It also protects printed circuit boards from just about everything else that nature can throw at it. Moisture, acids, bases, solvents, corrosive materials and most other chemicals and liquids cannot permeate it. This makes it an excellent choice for protecting PC boards that will be used in the real world — or in outer space. It is such a good coating for chemical and moisture protection that is also approved for use in one of the wettest and most chemical-laden places on earth — inside the human body.
Workable and Reworkable
Parylene provides a comprehensive and strong coating, which is what most PC boards need. However, just because it can cover just about everything and can resist just about anything doesn’t mean that you can’t also work with the coating both when it is applied and rework it if you need to make changes or do repair work.
Masking allows you to control where the Parylene deposits. For instance, you might want to coat a PC board, but leave its contacts uncoated so that data or power signals can flow through them. Putting a masking down before coating the board will allow you to do this.
Once the item has been coated, Parylene can also be removed through mechanical means. Microabrasive tools gradually remove Parylene in small areas, letting you access exactly what you want to work on without compromising the rest of the printed circuit board’s coating.
Given all of Parylene’s unique benefits for coating printed circuit boards, it is not surprising that more and more manufacturers are choosing it as their preferred PC board protectant. Contact us to learn how to use Parylene in your next printed circuit project.