Last quarters Gearphoria article is a look into gooping of effects pedals and it’s more legitimate parent, conformal coating. You can read the latest edition of Workbench Confidential at Gearphoria
I don’t read music gear forums much, but recently I did a little weekend surfing and saw fair bit of comment about the ‘gooping’ of components in guitar effects pedals. Indeed there was even an article on the subject in the last issue of Gearphoria. It got me thinking about what this stuff really is, and how it’s used in various electronics.
The opaque, glue like substance used to cover up the components in certain effects pedals is in most cases almost certainly potting compound. This is commonly used in components with coils such as transformers, to secure parts in place and deal with vibration issues. Guitar players maybe most familiar with potting compound in pickups, which potted partly for protection, and also to reduce the susceptibility to microphonics, where noise and feedback can be caused by the wires in the coil itself minutely vibrating. Certain pickup manufacturers prefer using wax vs. epoxy for this. I’m not a pickup expert, but I believe the lower viscosity liquid wax can penetrate better into the coils of fine wire. It’s also easier to remove if the pickup has to be repaired. For larger coils, epoxy is a more common choice.
So, are their sound engineering reasons for using this to cover components on a PCB? Some other components can have issues with vibration. Large electrolytic capacitors for example, can vibrate causing mechanical problems such as noise or even a weakening of the solder joints. However, these can normally be dealt with at the specific component level. If you look inside an AC power supply for example (with the power disconnected of course!) you might see a glue-like substance around the base of some of the large caps. Silicone elastomers such as Dow Corning’s Silastic® are commonly used for this. You only need to apply it around the component in question, though, no need to cover the whole PCB.
Many years ago I was working for a communications electronics company on a network interface PCB for use by the Navy. The system would be installed on warships and be subjected to hostile environments such as sea air, salt-water ingress, and potentially battle conditions. We covered a large section of the board with epoxy. It weighed a ton, and was impossible to repair if anything went wrong, but at the time with the availability of materials and the unusual environment, it made sense.
Fortunately, these days we have a much better choice for hostile environments with conformal coatings. Conformal coatings are commonly used now on electronics products that will be installed outside. Think traffic signals, or roof mounted solar panels. The coatings are available in several different base materials, and can be sprayed on either by machine or by hand. They can provide IPC level protection at thicknesses of only around 1000th of an inch. They are also flexible for improved reliability, relatively easy to remove to allow repairs, and often transparent for easier quality control and troubleshooting.
And therein lies the moral of this tale, if you really do want to go to the extent of protecting the PCB on your effects pedal from the hostile environment of beer spillage and maybe worse at the local dive bar, a transparent conformal coating is the way to go. Many board assembly shops will have conformal coating facilities or can send the boards on to a specialty coating service. Coating can be done with a spray head that covers an entire panel, or with a selective coating machine that dispenses the coating to specific areas. For small run, hand assembled products, you can pick up spray cans from a chemical or electronics supplier.
Selective conformal coating machines are kinda hypnotic to watch. Well I think so anyway. I’m not sure what that says about me …
If, on the other hand, you wish to hide your work from prying eyes, then gooping is just way too old school. The technique du jour is to laser etch chip ID’s from your integrated circuit packages.