Page 67 - Troubleshooting Analog Circuits

54 5. Preventing Material and Assembly Problems

out to be nor the LM317 or the resistors or the capacitors, but the flux build-up where
the board hadn’t ever been washed after soldering. In this case, the impedance of the
scorched flux was as low as 500 R when measured across a 0.1 X 1-in. area of the
PC board. The leakage was from the +Vh to the output, and it pulled the output
voltage up out of regulation! So, even if you are not trying to achieve 10l2 of
leakage resistance, you should still observe rudimentary standards of cleanliness, or
even your simplest circuits won’t work right.
Similarly, one of our PC boards designed for a S/H circuit was yielding 10” R of
leakage resistance, which was unacceptable. We tried cleaning the board with every
organic solvent but had no luck. Finally, I took a few boards home and set them in the
dishwasher along with the normal charge of Calgonite. After a full wash-and-rinse
cycle, I pulled out the boards, banged them to shed most of the water beads, and set
them in my oven to dry at 160 OF. The next day, they checked out at the more accept-
able value of 1013 0. I have used this technique several times on leaky PC boards
and sockets, and it works surprisingly well. It can work when alcohol, TCE, and
organic solvents are not helping at all.
After you get your board clean and dry, you’ll want to keep it that way. For this
purpose, you may want to use a coating such as the urethane, acrylic, or epoxy-types,
sprays or dips. Humiseal’ is the pioneering name, and they have a broad catalog of
different types for various production needs. In a similar vein, the guys up at Essex
Junction, VT told me about some varnish made by John Armitage CO.~ which is a
rather thick, heavy high-impedance coating. It takes a while to dry, but it’s pretty
durable and I like it. When I was building some little 1/3-ounce modules that some
scientists were going to carry up to the top of Mt. Everest, I chose a couple well-
baked coats of the “Armitage” to keep the modules clean and dry; it’s much lighter
than potting in epoxy, which is important when a guy has to carry a scientific
package on his back up to 29,000 feet.
Of course, with any of these coatings, it is not trivial to cut in and repair the circuit
or change Components. So your choice of a durable coating should be tempered by the
awareness of how much fun it is to go in and remove the coating and do your repairs.
When I was at Philbrick, we potted most of our products in epoxy, and it gave
good reliability and security. If you get a good circuit in there, well potted in epoxy,
it has an excellent chance to survive forever, with no moisture getting in, and with
everything held isothermal, protected from shock and physical abuse. Of course, if
somebody abuses the circuit electrically and damages it, it’s substantially impossible
to get inside to repair it. You may have to drill down to the PC board, just to do some
troubleshooting. It’s fun and a challenge to have to delve in and troubleshoot a potted
circuit. Sometimes the potting material adds extra stresses to components: Squeezing
the resistors and capacitors can change their values, and pouring epoxy around a cir-
cuit can add significant capacitance, too. If you pot a circuit that hasn’t been baked
and dried out thoroughly, the moisture may get sealed inside the potted module. Epoxy
can cover up a multitude of sins, but there is no substitute for good workmanship and
good engineering. Potting a piece of junk usually leads only to well-potted junk.
Make sure the designers who lay out your PC board keep a list of rules to avoid
troubles. For example, if your circuit has a high-impedance point and you suspect
leakage might be a problem, don’t run that high-impedance trace beside a power-

1. Humiseal Div. of Chase Corp, 26-60 Brooklyn-Queens Expressway, Woodside, NY 11377. (718)
932-0800.
2. Armitage MM-00941 Clear Brushing Alkyd Vanish John Armitage & Company, 1259 Route 46,
Parsippany, NJ 07054. (201) 402-9000.