Static Circuit Board Effect
Leakage resistance is the dominant static circuit board effect. Contamination of the PCB surface by flux residues, deposited salts, and other debris can create leakage paths between circuit nodes. Even on well-cleaned boards, it is not unusual to find 10 nA or
more of leakage to nearby nodes from 15-volt supply rails.
Nano-amperes of leakage current into the wrong nodes often cause volts of error at a circuit’s output; for example, 10 nA into a 10 MΩ resistance causes 0.1 V of error. Unfortunately, the standard op amp pinout places the −VS supply pin next to the + input, which is often hoped to be at high impedance!
To help identify nodes sensitive to the effects of leakage currents ask the simple question: If a spurious current of a few nano-amperes or more were injected into this node, would it matter?
If the circuit is already built, you can localize moisture sensitivity to a suspect node with a classic test. While observing circuit operation, blow on potential trouble spots through a simple soda straw. The straw focuses the breath’s moisture, which, with the board’s salt content in susceptible portions of the design, disrupts circuit operation upon contact.
There are several means of eliminating simple surface leakage problems. Thorough washing of circuit boards to remove residues helps considerably. A simple procedure includes vigorously brushing the boards with isopropyl alcohol, followed by thorough
washing with deionized water and an 85C bake out for a few hours.
Be careful when selecting board-washing solvents, though. When cleaned with certain solvents, some water-soluble fluxes create salt deposits, exacerbating the leakage problem.
