Nuts & Volts - July 2004

In The Trenches

PCB board houses always want some identification of top and bottom layers. Otherwise, things really get fouled up. In the past, optical “targets” were added to aid in registration between sides and to provide focusing for the optics. More recently, this has been omitted. Round pads are standard. However, often pin 1 of an IC is identified with a square pad.

Trace Basics

The wider the traces and farther apart they are, the easier the board is to manufacture, the more reliable it will be, and the better its performance will be. Most PCB manufacturers can work with 0.007” traces and spaces. I use 0.020” traces and 0.015” traces when I can. This allows one trace between two DIP IC pins, which have a 0.050” pad. Trying to put two thin traces between 0.100” DIP pins always seems to lead to problems.

Thin traces are more fragile and have higher resistance than thick wires. They also handle less current. Here’s how to estimate how much resistance it has and how much current a trace can handle: Remember that traces are about 0.003” thick for a standard PCB. So, you can calculate the cross-section area by multiplying by the trace width. A 0.020” wide trace has about a 0.060 square inch cross section area. Look this up in a wire-table and you will find that this is close to #32 gauge wire, which has a 0.063 square inch cross section. The table says that #32 wire has a resistance of 0.17 Ω per foot and can carry 90 mA ( The Radio Amateur’s Handbook, 1966).

These values are reasonably linear for different trace cross section areas. (Twice the area is half the resistance and twice the current, etc.) So, a 0.007” wide trace that has a 0.021 square inch cross section area can handle 30 mA and has 0.51 Ω/foot resistance. (Note: These current-carrying values are very conservative, which results in negligible heat generation. A 0.020” trace carrying about 2,000 mA causes a 10° C or 18° F increase in temperature.)

Traces that are close together are easier to short out and can electrically couple to each other. The traces act as antennas and capacitors. High speed digital lines next to high impedance analog lines are going to cause problems every time. Long traces are just as bad as long wires. They pick up noise and have appreciable impedance. Remember, even though your digital design only runs at 1 MHz, the state changes can cause transients into the GHz range.

Don’t use acute angles in traces (more than 90°). This leaves a point that causes problems. First, it nearly acts like a one-turn inductor. More importantly, the point is not well supported and can easily lift. Obviously, it’s not good to have your traces peeling off the PCB.

Using two 45° angles instead of one 90° angle will save space. This is especially true when you have a lot of traces running parallel to each other. The more space you can save, the smaller and less expensive the PCB will be. Don’t forget — you pay for the PCB by the square inch (mostly).

JULY 2004

ELECTRONIC TECHNICIAN

Learn at home in your spare time.

Earn up to $100 an hour

No previous experience needed!

You can earn more money if you get an FCC License!

Not satisfied with your present income? Add prestige and earning power to your electronics career by getting your FCC Government License.

The Original Home-Study course prepares you for the “FCC Commercial Radiotelephone License” at home in your spare time.

This valuable license is your professional “ticket” to thousands of exciting jobs in: Communications, Radio-TV, Microwave, Maritime, Radar, Avionics & more…you can even start your own business!