Through hole solder pad dimension rules?

Question

Is there an electronic rule or just a rule of thumb regarding through hole solder pad dimensions?

That is, if I have a 0.025 inch diameter drill hole (suitable for 24AWG wire), what should be the total diameter of the solder pad? This is to have an appropriate surface of pad to solder on for both good electrical and mechanical properties.

What ratio should I use?

For example, x2 or x3, would give a total diameter of 0.05 and 0.075 inches respectively. Or I can try to have the same surface between the drilled hole and the soldering pad. Doing some math, that would mean taking a sqrt(2) ratio. However, that looks (at least visually) a very small surface.

Any suggestion? Or even pointer to material for good PCB design practices where that can be explained? Thank you.

Best regards,

Answer 1

You can calculate a minimum annular ring from the hole size. If you use a 10 mil annular ring then a 25 mil hole would lead to a 45 mil pad size.

You'd want to make it larger than that if possible if there is mechanical stress on the leads and much larger if there is mechanical stress and the PCB does not have plated-through holes.

Generally it's a bad idea to just solder a stripped loose stranded wire into a hole in the PCB, the wire ends up breaking off just above the PCB just from a bit of handling.

Answer 2

Bare PCB manufacturer will have their minimum value, due to drilling precision and PCBA manufacturer (who populate the PCB with components) might want to optimize it, if it comes to bigger volumes. Ask the manufacturer their recommendation, if they don't have it on their web page: http://support.seeedstudio.com/knowledgebase/articles/447362-fusion-pcb-specification

Copper area around the hole is called annular ring. Seeed is recommending > 0.152 mm for plated through hole components and > 0.1 mm for vias. So for 1 mm hole, the copper area diameter should be at least 1.304.

If you were to optimize, there would be other factors to take into account. In general for automated soldering you don't want the area to be too large to make sure the solder actually sticks to the pin. For manual "hobby" population of components 0.152 mm is usually not enough and I'd rather use 0.5 mm or so, depending on your soldering equipment.

For soldering wires, it's important to check there are no loose strands, the wire is clean (cleaned with flux), solder has wetted the wire thoroughly and you have strain relief, that prevents the wire bending at the solder joint or causing stress that might crack the joint.

Answer 3

I use a pad nearly double the size of the hole. For example 40 mil drill diameter, 80 mil pad diameter. This makes pads that can be soldered easily by hand and provide good mechanical strength. But you shouldn't go below about 40 mil pad diameter in any case if you want to solder by hand.