PCB layout guidelines
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Usually an electronics or electrical engineer designs the circuit, and a layout specialist designs the PCB. PCB design is a specialized skill. There are numerous techniques and standards used to design a PCB that is easy to manufacture and yet small and inexpensive.
Most PCBs have between one and twenty conductive layers laminated (glued) together in a sandwich with insulating plastic. PCBs with more than two layers help construct complex or dense circuits. They are not always used because they are more expensive, and the inner layers are more difficult to inspect and repair.
In more complex PCBs, two or more of the layers are dedicated to providing ground and power. These ground planes and power planes distribute power well. They also prevent radio waves from antennas unintentionally formed by tracks. These planes are rectangular sheets of foil that occupy entire layers (except for small holes to avoid unwanted connection to vias and through-hole components). They distribute electrical power and heat better than narrow traces. Sometimes solid metal PCBs with thin layers of insulation are used. The power electronic substrate carries away waste heat when air cooling is impossible.
Four-layer PCBs with a ground and power plane are often used in high-quality, but cost-conscious audio, avionics and medical electronics. Most consumer products have one or two layers.
The width and spacing of conductors (or "traces") on a PCB is very important. If the traces are too close, solder can short adjacent traces, and the PCB will be difficult to construct or repair. If too far apart, the PCB may be too large and expensive. When a PCB carries high frequencies, traces may need to be exact widths and lengths to control the characteristic impedance of the trace.
Some designs cut the ground plane or the entire PCB in strategic locations to control the return paths of currents. The usual desire is to keep high voltages or frequencies away from sensitive portions of a circuit. The actual properties of the design are critical, because in some cases, cutting the ground plane makes the PCB into an antenna that radiates radio noise into nearby equipment.
Removing large areas of copper wastes etchant and increases pollution. Also, a PCB etches more consistently and tends to resist warping if all regions have the same average ratio of copper to bare board. Therefore, designers may widen connectors, leave unconnected copper in place, or cover large areas of what would otherwise be bare board with arrays of small, electrically isolated copper diamonds or squares.
Most PCBs have alignment marks (called fiducials) and tooling holes to align layers. These permit the PCB to be mounted in equipment that automatically places and solders components. Some designs also have quality control patterns to measure soldering and etching processes. In some cases, the test patterns are on break-off tabs that can be removed before the PCB is installed.
Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither, but these are expensive to build and difficult or impossible to inspect after manufacture. Good designers minimize the number of vias to reduce the cost of drilling. On older, two-layer PCBs, it was common to solder a wire through the hole.
A solder mask is a plastic layer that resists wetting by solder (the solder is said to "bead up"), and keeps islands of solder from running together. It also protects the outside conductors layers from abrasion and corrosion. Without the solder mask, the fiberglass-reinforced epoxy appears a translucent off-white. Solder masks are usually green, but they may be found in other colors.
A silkscreen legend on the top or bottom surface of the board provides readable information about component part numbers and placement. This aids in manufacturing and repair. To aid manual construction and repair, diodes, capacitors and integrated circuits are sometimes oriented in the same direction.
New technology allows for the component designators to be printed directly onto the board surface, saving time and money by doing away with silkscreens. This is sometimes done by a special inkjet printer. A similar process has experimentally produced solder masks.
Radio transmitters and radio receivers are difficult to design. PCB designers working on them must minimize parasitic effects due to layout of components, or take them into account with a general model and use simulation software such as SPICE.
Fortunately, many practical circuits can be laid out using a much simpler lumped element model.
PCB layout Basic guidelines:
- it is often a good idea to have made a prototype circuit using point-to-point construction or wire wrap, as you will have solved certain basic issues to do with component selection: (eg: should I use a 1/4 watt resistor here, or do I need 1/2 watt? etc.)
- consider physical constraints on the assembled board's size and heat dissipation requirements; choose your heat sinks if needed.
- consider carefully the physical size of the components you are laying out; the circuit schematic doesn't tell you this. Equivalent components often have different packages.
- How do the components attach to the board? Are they surface mount components? or do they require holes, screws, washers, etc?
- are there mechanical parts directly mounted to the board? eg: switches or variable resistors?
- How will the board mount in its container? What stresses (shock, strain, shear) will there be upon it and upon components?
- How will the board connect to its power source? What other connectors will be required (e.g: signal inputs, outputs)?
- use construction paper and a pencil and sketch the board in its actual size; or use component layout software that includes information about the component outlines.
- decide appropriate widths for each of the signal traces; this depends on the current each trace is expected to carry.
- decide whether you will have a single-layer board, 2-layer, or multi-layer based on the circuit complexity and fabrication costs.
- begin by placing component outlines, then by placing signal traces; leave a little room around each for tolerances.
- for a single layer board, spend more effort to avoid having traces cross each other; play with component placement or run traces underneath components; sometimes a jumper wire is needed.
- in 2-layer and multilayer boards simply run the traces on different layers, and use plated-through holes to jump from one layer to another.
- try to predict and avoid assembly errors: where there are multiple components of the same kind, or where pins have a polarity (eg: electrolytic capacitors), try to place them in parallel and orient the positive pin in the same direction.
- If your PWB design software has a DRC (design rule check), use it.
PCB layout guidelines for RF circuits on a 2-layer or multilayer board:
- identify the critical parts of the circuit and lay them out first
- have one of the layers act as a continuous ground plane (usually the 'bottom' side).
- if signal traces are constant width and height above the ground plane, and are properly terminated, then their characteristic impedance is more well-behaved and may be calculated.
- avoid sharp corners.
- keep signal traces and component leads as short as possible.
- inputs and outputs should be far apart, so that RF energy will not leak back from output to input. stages should line up, rather than snake around.
- decouple the RF parts of the circuit from the DC parts of the circuit.
- shield AF and IF components from RF components.
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[edit] See also
[edit] External links
[edit] Manufacturer design tips
Generally every PCB manufacturer gives some design for manufacturing tips on how to design things to fit their particular manufacturing process, for example
- A comprehensive PCB Design Tutorial
- Olimex DFM
- Advanced Circuits design tips
- Tips for making PC Boards
- Dealing with Parts Shortage Nightmares by Danny Simpson
- Tips for marking SMT LEDs and diodes for accurate PCB assembly by Screaming Circuits
- QFN solder paste stencil guidelines
