Practical Electronics/PCB Layout

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The efficient laying out of traces on is a complex skill, and requires much patience. This task has been made vastly easier with the advent of readily available PCB layout software, but it is still challenging.

Copper Thickness

The vast majority of PCBs are manufactured with "1 ounce copper" on the outer layers. (If there are inner layers, they are almost always manufactured with "1/2 ounce copper"Template:Fact).

The thickness of the copper layer on the PCB affects the behaviour of the circuit. PCB copper thickness is usually measured in ounces per square foot, or frequently, just ounces. It can also be given in micrometres, inches or mils. The measurements for common thicknesses are given below.

oz/ft² μm in mil
0.5 17.5 0.0007 0.7
0.75 25.5 0.0010 1.0
1 35 0.0014 1.4
2 70 0.0028 2.8
3 105 0.0042 4.2

As always, the thickness of a thin slab of metal with a given top surface area is always exactly

thickness=massarea×density

The area is "1 square foot" (144 square inches), the density of copper is 8.96 mg/mm^3 = 5.18 ounce/(inch^3), and usually the mass is "1 ounce of copper",

Trace Width

Different widths of traces have different properties that could affect the operation of the circuit. For, example, a thin trace has a higher resistance than a thick one, and can therefore carry less current or will heat up more for the same current.

Due to the large number of tables and charts, this information is presented on different pages:

Most manufacturers can manufacture a minimum trace width of 0.010 inch. (Many can manufacture traces 0.008 inch wide). Such minimum-size traces are more than adequate for most digital and analog signals.

footprints

The manufacturer of each part recommends a "footprint", a copper pattern for the part to be soldered to the PCB.

holes

Most PCBs have many holes drilled in them.

Most PCBs with more than 1 layer have every hole "plated-through-hole", even holes that don't need to be plated through-hole (tooling holes and some through-holes). They do that because it takes extra effort to plug "non-plated" holes before throwing the PCB into the plating bath.

There are 3 general types of holes:

A via -- literally, a "way" to get from one layer of copper to another layer of copper. The vias on a particular PCB should all be the same size. Some people[1] recommend 0.025" (0.6mm) diameter via holes, surrounded by a 0.042" (1.0mm) diameter via copper pad Template:Fact, if at all possible. Some very dense SMT boards require smaller vias. Some manufacturers can handle 0.012" diameter via holes, surrounded by 0.024" diameter via pad Template:Fact.

Since there is no actual component put into a via, many PCBs are manufactured with "plugged" vias (vias completely filled with metal) and "tented" vias (vias completely covered with solder mask).

A through-hole -- many components (called "through-hole components") require a hole (a "through hole") for each pin. The part manufacturer should specify a "footprint" including the location and size of each hole. If there is no recommendation, common practice is

  • round leads: add 6 mils to the nominal round lead diameter to get the recommended PCB hole diameter. Rectangular leads: find the lead diagonal (sqrt(x^2 + y^2)). Then add 6 mils to get the recommended PCB hole diameter. ( 6 mils ~= 0.15 mm )[2]
  • "The component lead - hole clearance should be 0,4 mm" [3]
  • "The optimum pad diameter for a through-hole component is twice its finished hole diameter."[4]

A tooling hole -- ... preferably 0.125" ...Template:Fact ... mounting holes ... A "tooling hole" -- used to temporarily attach the board to test fixtures, and later bolt the board into the final product case. Needs to big enough for the mounting bolt. If the board is wave-soldered (rather than reflow soldered), there is a risk that these holes may get plugged with solder. So these holes are temporarily plugged with a stopper -- either plugged when the PCB is manufactured (becoming non-plated-through-holes, so solder won't stick), or plugged during the wave-solder process.

layers

The vast majority of PCBs have an overall thickness of 1/16 inch (1.58 mm). Some very dense SMT boards have an overall thickness of 1/32 inch ( 0.79 mm ), which allows smaller via holes to be drilled, allowing denser packing. Occasionally boards are made with an overall thickness of 3/32 inch (2.3 mm), which makes it more rigid (but requires bigger via holes).

Often some or all layers are covered with a "copper pour" ("ground plane" or "power plane"). Such pours typically have a signal-to-pour clearance of 0.010 inch Template:Fact and clearance from the cut edge (perimeter of the board edge) to pour of 0.020 inch[5].

(more layer stackup tips)

Reference Handbook

A PDF file is being prepared which will contain many of the diagrams and tables in a downloadable, printable format:

Further reading

Template:Wikipediapar

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