Page 63 - Troubleshooting Analog Circuits

5. Preventing Material and
Assembly Problems

PC Boards and Connectors,

Relays and Switches

In addition to your choice of components, the materials you use to assemble your
circuit will have an impact on how well it performs. This chapter covers what you
need to know to solve the occasional problems caused by PC boards, solder, connec-
tors, wire, and cable. Also covered is PC-board layout-a poor layout can cause more
than occasional problems; it can determine completely how well your circuit works.
Some of the topics discussed in this troubleshooting series so far may have seemed
obvious. But far too often it is this “obvious” information that engineers overlook,
and it is this information that can make troubleshooting so much easier. So, be
careful not to overlook the obvious. Don’t assume that your PC-board materials or
layout don’t matter or that wire characteristics don’t differ; you’ll find that PC
boards, connectors, wire, and cable can cause problems when you least expect it.
First of all, the use of the term “printed-circuit board” is a misnomer; these days,
almost every board is an etched-circuit board. But I’ll continue to use the abbrevia-
tion “PC board” because it’s a familiar term. There are six basic troubles you can get
into with PC boards:
The board is made of the wrong material.
The quality of the vendor’s board is so bad that there are opens or shorts in the board
or, worse yet, intermittent connections in the plated-through holes.
The foil starts to peel off the board because of mistreatment.
You were so concerned with cost that you neglected to specify a layer of solder
mask; you ended up with a board full of solder shorts.
The surface of the board is leaky or contaminated.
Your circuit layout is such that signals leak and crosstalk to each other, or controlled-
impedance lines are interfering, thus causing reflections and ringing.

Avoid PC-Board Problems at the Outset

The fixes for these problems, and ways to avoid them in the first place, are fairly
straightforward.
These days, the G10 and G11 fiberglass-epoxy materials for PC boards are quite
good and reasonably priced. Trying to use cheaper phenolic or “fishplate” is not
economical in most cases. Conversely, a special high-temperature material or an
exotic material or flexible substrate may be justifiable. If you don’t have an expert on
these materials, the PC-board maker or the manufacturer of the substrate material can