PCB Design Guidelines

Here, I summarized some general PCB design guidelines/considerations:

(1)   When placing component footprint, remember that a good placement almost always lead to good routing. Good routing usually consist of short conductor length of connection, not too many bent in the trace, and not using a lot of via to reach the target connection.

(2)   It’s USUALLY easier to place big component/main component first (e.g. microcontroller, microprocessor, PLD, FPGA, connectors) whose position is fixed on the board, then place all the little ones that connect to them (e.g. passive components: res, cap, etc) around them.

(3)   Place components while imagine how the finished board will look like. 

(4)   Place decoupling capacitor as close as possible to IC power pin. 

(5)   Place port, jack, header, jumper, plug, etc by taking into consideration user experience on usage (e.g. easy to reach, easy to plug-unplug, orientation, etc)

(6)   When routing, critical timing component should use as short as possible route.

(7)   Avoid sharp covers and acute angle bends in conductors. If possible do not route the trace that turn more than 45 degree ( should be maximum turn angle ).

(8)   Consider the relation between trace width and current carrying capacity of the trace. On a long trace, resistance of copper trace can introduce a voltage drop. For example, (this is a rough assumption) a copper of 35um thickness will need a minimum of 0.25mm width to carry a maximum of 1 A current.

(9)   Use ground plane (copper pour). Put a single copper area the entire size of the board on both side (top and bottom) to provide shielding from interference and noise, also reduce cross coupling. Ground plane solve the problem of ground routing on the layout. It also provides a good safe path for static electricity (ESD). Ground plane safe more etchant solution because less copper need to etch away. But remember, isolated “floating” plane/island (unconnected to any signal) should be removed as it will act as a large antenna that will cause noise and interference problems to other circuits on the board.

(10)  If ground plane is not used, then trace width should be:

Wgnd > Wsupply > Wsignal

( as rule of thumb, for TTL circuit:

Wground >= 2Wsupply >= 2Wsignal )

(11)  Connect a pad (that will be soldered) to ground plane using thermal connection. Thermal provide easy soldering, otherwise the board assembler need to heat the entire ground fill in order to solder the grounded pad. It also avoid the risk of creating a cold solder joint.

(12)  Try to minimize the number of via used, as the board house will charge extra cost for it. 

(13)  Connect port shield (e.g USB connector) to ground. Then tie the ground to the mounting hole (screw, nut), then to the metal case of the equipment. Its to prevent ESD problem. 

(14)   Usually digital circuitry like a microcontroller involves the use of crystal + capacitor (15pF-22pF). To ensure it will work properly, place the crystal first, right after the microcontroller pin with the shortest route possible (try to maintain it symmetrical), then after it place the capacitor with a very short connection to ground. Surrounding the oscillator circuitry with ground ring is also good practice.

(15)   Run separate ground plane for digital and analog circuitry. Both plane usually being connected with a 0 Ohm resistor or a ferrite bead.

(16)  Avoid routing a trace which contain a changing width, it will cause a problem with hi-freq signal ( cause reflection, mismatch, etc ). Also note that for example a 0.4mm trace that shrink to 0.25mm will only capable of carrying current of 0.25mm trace.

(17)  Solder mask swell commonly 2-5 mil. But I like to choose 0.05mm as value.

 

(18) Run DRC (Design Rule Check) before concluding the layout is finish, and preview all layer before exporting the gerber.