pcb design patterns

8/8/2019 PCB Design Patterns

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PCB Design Patterns

http://www.robotroom.com/PCB-Layout-Tips.html

I've been laying out custom printed circuit boards for over ten years.There are a few tricks that I use repeatedly from prototype to productionto provide flexibility on parts, easier assembly, and faster circuit testing.Most of these techniques are applicable regardless of the circuit boardCAD program and manufacturer that you use.

Flexible Voltage Regulator Layout

Linear voltage regulators are popular for their simplicity. They usuallyconsist of three leads:

Unregulated voltage inputRegulated voltage output (5V, 3.3V, etc)Ground (GND)

The order of the leads and the type of package (TO-220 or TO-92) variesfrom part to part. In the picture below, notice that none of the pinoutsmatch.

Voltage regulator pinouts: 78L05, MCP1826, MCP1702-5002E






Sometime after the board has been made, you may find that the circuitneeds a bit more current, a higher input voltage, a lower dropout voltage,or maybe you just ran out of stock of a particular voltage regulator. Witha little bit of forethought, you can design your circuit board to accept avariety of regulators.

Flexible voltage regulator PCB layout.

Adding a fourth hole (in this case duplicating the output pin) permits thefollowing pinouts:

+5, +UN, GND (top three holes)GND, +UN, +5 (installed backwards in top three holes)

+UN, GND, +5 (bottom three holes)+5, GND, +UN (installed backwards in bottom three holes)

In fact, if you add a fifth hole (duplicating +UN) then you have covered all6 permutations (3 factorial = 3! = 6) of pins. And, if you make sure all of the holes are 0.040" diameter, then you have covered both TO-220 andTO-92 packages.

Choice of Trimmer Potentiometers

Small potentiometers (often called trimmers or trimpots) are alsoavailable in a variety of packages. The most common trimpots have 0.1"spaced leads, but the leads are attached in slightly different places withrespect to the component body.



Trimpot potentiometer pinouts.

The leftmost trimpot is the least expense and doesn't tend to twist off theboard due to the staggered pins. The next trimpot is more compact, butmore expensive. The next trimpot is a multiturn that can be mountedvertically or laid flat. The rightmost trimpot has a finger-friendly dial, withthe non-staggered pins located at the front of the package.

Potentiometer multi-compatible PCB layout.

To be compatible with all of those pinouts, you can lay out a 3×3 gridfrom the front of the package up through the center. All of the trimpots inthe previous photograph can be dropped into that same location on theboard.



On almost all through-hole potentiometers, the center pin tends to be thewiper. However, you could extend the hole pattern grid to the top of thelayout in case you need to orient the potentiometer "upside down" if turning the potentiometer in a certain direction produces the oppositeeffect desired on the circuit.

Error-Free Part Insertion

Tactile pushbuttons are commonplace on electronic circuits due to theircompact size, high reliability, and low cost. The most popular tactilepushbutton package contains four pins (for stability when pressed) andappears to be square.

Tactile pushbutton.

In reality, the package is rectangular (slightly longer in one dimension),but you can incorrectly install it 90° out of orientation if you force the pinsinto the PCB holes. Normally, when pressed, the button connects one pairof pins to the other pair of pins. But, if the tactile button is installedincorrectly, it will seem as though the button is always pressed.

Before installing, you can carefully measure the continuity of the pinsusing a multimeter. Or, you can lay out the PCB such that the button canbe installed in any orientation and still work correctly.

Pushbutton corner connections PCB layout.

By wiring the circuit only to the pins on opposite corners, you are sure toget only one pin from one side of the switch, and another pin from theopposite side, electrically speaking. You can demonstrate this to yourself

by measuring the continuity of opposite pins on a switch using amultimeter. No matter which two corner leads you select, a standard



tactile pushbutton will always show continuity when pressed and nocontinuity when released.

Faster and More Reliable Soldering of Through-Hole Parts

By default, most PCB layout programs use 0.056" diameter pads aroundthe common 0.029" diameter holes. This works well for DIP chips andother closely spaced holes as there is still enough room to pass a tracebetween the two holes without contacting the pads.

However, in many circuit boards, most of the holes don't have tracespassing very closely -- at least not on the side of the board that the holewill be soldered. For those holes with free space around them, I alwaysadd a wider circular pad around the hole to provide a larger contact areaduring soldering.

Hole with larger solder pad.

In the above example, you can see the wide shiny area around the hole toensure good solder contact between the part pin and the circuit boardtrace. Soldering goes faster as well, since it is easier to target the largerpad with the soldering iron tip.

Solder Mask Issue

A nice feature of professional circuit boards is a solder mask. A solder

mask is an electrically and chemically insulating coating that protects themetal traces against the environment and unintended electricalconnections.



Extended area accidentally covered by solder mask.

If you do not choose the correct element in your PCB layout program,then any extended soldering pad area that you add will get covered bysolder mask during PCB production. That would eliminate the benefit of anincreased surface area to solder.

Hole with BGA pad PCB layout.

In ExpressPCB, make the extended soldering pad by choosing a BGA (ballgrid array) pad from the pad tool, rather than a circle from circle/arc tool.ExpressPCB will not coat pads, because those are intended to be soldered.Simply drop the 0.080" diameter BGA pad on top of the 0.029" diameterhole to make a hole with an extended solder area.

Copper Fill to Reduce Heat and Increase Efficiency

Some parts, such as voltage regulators and motor drivers, can get hotduring usage. A simple way to wick away heat is to place copper fills

(large squares) under the chips. In my thermal chip tests, I demonstratethat these copper plains reduce heat and increase performance even if they are not electrically connected to the chip or the power supply.

http://www.robotroom.com/Thermal-PCB-Motor-Driver-Tests-4.html






Motor driver IC PCB layout.

Of course, sometimes you'll need the area underneath the chip to runtraces, and thus cannot add a copper plain. Instead, you'll see somebenefits if you use thick enough traces for the chip's power supply, and

connect those traces to even small copper areas nearby.

Wire Loop for Test Probe Hook

When measuring DC voltages, it is necessary for the ground probe to beconnected to the same ground as the circuit. One of the first voltagesyou'll measure will be the regulated power supply.

You can hunt around the board for a resistor or connector with a barelocation to attach the multimeter probe. But, it can be difficult to attach

probes to those locations, or that area of the board may be too crowdedto access the pins.

A better solution is to add a couple of test points when you design thecircuit board.



Wire loop hook test points.

Place the test points at the edge of the board to avoid wires hanging overthe circuit on which you're trying to work.

Left: Looped wire test points PCB layout.

For the loop itself, I tend to use wire cut off from component leads. Useslightly larger holes (0.035-inch diameter) so that you can use thickerwire to make the hook loops. The weight of the test probes tends to bend

thinner wire.

If you place one of the positive voltage holes (+V) within 0.1" of one of the ground holes, and you use the larger diameter holes (0.035"), thenyou can install a Molex KK header or other similar connector as a sparepower connector. That brings up the other use for these test points: sparepower holes that you can wire up to correct mistakes in a circuit.



Simple Changes

As you can see, layouts featuring a few extra holes and a little bit morecopper can increase flexibility, speed installation, and improve reliability of your printed circuit boards.

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