Layout
mil
One mil is a thousands of an inch.
$$ 1 \text{mil} = 25.4 \mu m $$
Clearance Distance
IPC2221 suggests a minimal spacing of \(\approx 4 \text{mil}\) for external uncoated conductors from sea level to 3050m altitude for voltages up to 30V. For internal conductors the clearance distance is lower ($\approx 2 \text{mil}).
Therefore a spacing of \(8 \text{mil}\) can be selected for all conductors to ease the production and lower cost.
Track Width
For increased currents the required track width to avoid excessive heating of the track is increased. IPC2221 gives an approximation formula, which relates the track width \(w\), the temperature rise above ambient \(\Delta T\), the track thickness \(h\) and the current trough the track. The approximation should be used up to approximately \(I<35A \land \Delta T < 100K \land w < 400 \text{mil}\) for approximating outer layer tracks:
Calculations are for \(\Delta T = 10K\), \(h = 35 \mu m \widehat{=} 1 \frac{\text{oz}_{cu}}{\text{ft}^2}\) unless otherwise noted.
To ease manufacturing and lower costs the minimal track width is selected to equal \(8 \text{mil}\). A maximal current is therefore obtained: $$ I_{8mil} \approx 750mil $$
A class of tracks carrying up to \(I = 3A\) is desired and the minimal track width is approximately: $$ w_{3A} \approx 50mil \implies I_{50mil} \approx 3A $$
For currents up to \(I = 12.5A\) the calculated track width is equal to: $$ w_{12.5A} \approx 400mil \implies I_{400mil} \approx 12.5A $$ The track can also be split up on the top and bottom layer for THT mounted connectors to allow for connection at terminal blocks with a pitch of \(5.08 mm = 200mil\). A copper pour is preferred for the tracks of this class.
For currents up to \(I = 25A\) the calculated track width is equal to: $$ w_{25A} \approx 800mil \implies I_{800mil} \approx 25A $$ The track can also be split up on the top and bottom layer for THT mounted connectors to allow for connection at terminal blocks with a pitch of \(5.08 mm = 200mil\). A copper pour is preferred for the tracks of this class. A track width of \(200mil\) on top and bottom layer will give an approximate temperature rise of \(30 K\) and is acceptable for connection to terminal blocks, but copper pours should be preferred when possible.
Terminal Block Connection
A copper fill under terminal blocks may reduce the track width from \(200mil\) to \(192mil\) to allow for adequate clearance spacing to adjacent tracks.
Vias
Keeping PCB manufacturing costs low can be achieved by using standard sized vias as per manufacturer requirement. A standard size acceptable for most manufacturers "simple"/"economical" pcb quotes is: $$ d_{via,drill} = 0.3mm \land d_{via,copper} = 0.5mm $$ Micro-Vias should be avoided to keep manufacturing costs low.
Using KiCads Via Calculator the via with \(d_{via,drill} = 0.3mm\), \(d_{via,copper} = 0.5mm\), plating thickness \(T = 18 \mu m\) and via length \(h = 1.6mm\) has the following properties: $$ C_{via} \approx 0.4 pF$$ $$ L_{via} \approx 1.3 nH$$ $$ R_{via} \approx 1.5 m \Omega$$ $$ R_{th,via} \approx 220 \frac{K}{W} $$ $$ I_{via} \approx 1.5A \bigg|_{\Delta T = 10K} $$
Plated Through Hole (PTH)
PTH should be in the range of \(0.5mm < d_{pth,drill} < 5.6mm\). The annular copper ring should be a minimum of \(\Delta d_{pth,copper} = 0.6mm\) larger than \(d_{pth,drill}\) to avoid increased cost for precision manufacturing.
Board Outline
To allow for mounting and handling of the pcb a distance of \(2.5mm\) should be kept free of copper and parts measured from the board outline.
PCB Dimensions
PCB dimensions should be a height of \(h = 100mm\) for mounting in 3U sub-rack and a maximal length of \(l_{max} = 220mm\) for mounting in the sub-rack. Shorter pcbs are possible to mount and preferably comply to the standard eurocard sizes.