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I'm designing an intermittent wiper motor controller to be fitted on a classic car.

As I'm selecting components, I came around this relay that seems to be fitting my needs (PCB mount, automotive, high current).

When looking at the PCB layout, I find pin 3 and 5 way too close to each other to allow for drilling plus some copper around the holes for soldering.

enter image description here

Power Relay Mounting Hole Layout

This makes me think I can't use this relay as I need wide copper path for the current demand (20A). I'm not familiar with relays, am I missing something ? I find it strange that it is designed that way.

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    \$\begingroup\$ Suggestion? Don't patent it. Even the producers of this movie lost money... \$\endgroup\$ – Harper - Reinstate Monica Jun 14 at 18:24
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    \$\begingroup\$ Just in case it's enough for you note there exists the SPST version of the same relay which lacks of pin 3 \$\endgroup\$ – carloc Jun 14 at 18:25
  • \$\begingroup\$ Do you need both pin 3 and 5? Or is one superfluous to your need? \$\endgroup\$ – Harper - Reinstate Monica Jun 14 at 18:32
  • \$\begingroup\$ Actually for my need pin 3 is not needed. I can do with a form A relay. \$\endgroup\$ – Fredovsky Jun 14 at 19:48
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If you use the recommended 2.1mm holes (in my experience the recommended sizes tend to be pretty sloppy and designed for ease of automated assembly rather than being optimal in other ways) then you can have pads with annular ring 0.36mm (14 mils, quite acceptable) and space 0.36mm. Then you can apply traces as so:

enter image description here

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Even if you need a wide trace to carry 20 A over long distances, you can still use a narrower trace for a short distance to connect to the relay pins.

The trace width is limited by the resistive self heating of the traces, and the values are usually calculated assuming the trace is infinitely long, so each segment of track must dissipate all the heat it produces itself, without any heat flow axially along the track. But in the case of a short narrow segment, connected to a wider track, or to a big bulky relay, axial heat flow becomes significant and the track can be narrowed without over-heating.

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    \$\begingroup\$ Very useful. I always thought a track could only do as good as its weakeast (ie. most restrictive) part. \$\endgroup\$ – Fredovsky Jun 14 at 19:49
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The pins are quite close together, but given the voltage rating of the relay is pretty low, it is relatively simple to route.

Here is an example using 2.2mm holes, 0.3mm annulus on the pin, giving a clearance of 0.3mm. You could also use 0.2mm annulus and get 0.5mm clearance.

PCB example

There is plenty of room to make the traces entering the pins actually very large, by placing the pins in the corner of the entering traces rather than in the centre. This allows for trace widths of nearly 10mm at the pins. You can make this wider still by using multiple layers if needed, and connecting to planes.

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You can drill a non plated hole with no pad for pin 3. Or no hole and cut pin 3 off of the relay.

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