PCB layout Basic guidelines:
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 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. |
| • | Use the DRC (design rule checker). |
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. |
| • | 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. |
