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Vintage threads from the first ten years

PCB Drill Bit Size

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5/12/1999 5:54 AM
JD Sleep	PCB Drill Bit Size Anyone know what size drill bit you use for drilling board holes for resistors/cap leads? I've been using .043 bits, but the holes are pretty big. I'm thinking of ordering some .034 bits from Hosfelt, will that be too small? Sorry I don't have callipers to measure the leads, so I'm guessing here. JD

5/12/1999 6:18 AM
Dave Chun	I use a 1mm drill bit, which fits the leads from small 1/4 watt resistors up to slightly bigger than a 1n4007 diode (in other words, most components.) 1mm is ~ .0393". These steel bits were $0.75 each at the local electronic supply store.

5/12/1999 7:49 AM

The pros use 0.028" for normal 1/4 W resistors, caps, transistors and IC leads.

0.032" is a good compromise, 0.034" would also be OK.

There is an optimal hole diameter for every lead size, usually about 0.004" larger than the lead diameter or largest dimension. Likewise, there is an optimum pad size, usually 0.020" to 0.040" larger than the hole size for small holes, getting proportionately bigger for the larger holes.

5/12/1999 8:21 AM
JD Sleep	R.G. Thanks for the information on bit sizes. Can you tell me what are the disadvantages of having pad sizes larger than the optimum? Are there also disadvantages to having larger trace widths (other than the obvious - "saving space")? Thanks, JD

5/12/1999 2:08 PM

quote:
"Can you tell me what are the disadvantages of having pad sizes larger than the optimum?"

Solder wicking. The solder wants to spread out over the available pad, and the main mass of solder is not concentrated at the lead wire. With optimum sized pads, the solder wicks in to a neat little cone around the lead wire.

Too-big pads either use a lot of solder (bad primarily for big manufacturing operations, granted) or invite only partial solder filleting of the lead wire.

quote:
"Are there also disadvantages to having larger trace widths (other than the obvious - "saving space")?"

Solder wicking away from pads, thermal wicking of heat from pads which promotes cold joints. Very minor problems in DIY stuff.

5/13/1999 11:05 AM

Hello, JD. I wouldn't want to disagree with R.G., but I would like to put in my $0.02 worth here.

There is an optimum pad size for leads and it varies depending on the lead diameter. However, in practice 0.020" larger than the lead is the IPC minimum for manufactured boards and is a little small. R.G.'s guidelines are pretty good on this, and there are some info sites on the web that can give recommended pad sizes. Another point to consider is that the main support for a standard through hole part on a single sided PCB is the adhesion of the copper pad to the PCB (unless glue or physical mounting is used). In this case, a larger pad can be better. All large components should be secured to the PCB so that the pads do not come loose.

Regarding trace width, I have found that larger is always better unless you get the traces too close (arcing, crosstalk or etching problems) or are trying to control impedances (maybe on a 400mHz PC motherboard but not in a fuzzface!) In production PCB's, solder mask (the coating over the traces and not the pads) is used to prevent the solder from wicking up the trace. A commercial PCB is pretty much unmanufacturable without solder mask. I cannot recall seeing a PCB in a commercial product without solder mask. However, solder mask is out of the reach of most DIY projects. Hope this helps!

5/13/1999 12:20 PM

No disagreement here. As leads and components scale, the pads also have to get proportionately larger as well. My answer was predicated on the nominal resistor/capacitor/IC lead. There is a big range of thngs to consider in doing PCB artwork right, and we're fortunate that we can subset it for most parts.

Even more than that you're dead on, 100% accurate about supporting bigger components and solder mask.

Many of the supposed failings of PCB's in tube amps come from people simply putting large, heavy, or hot parts on a board and then being suprised when it doesn't work out well.

I'm trying to find a source of sheets of positive acting positive photoresist and dry film solder mask. I think these could in theory be ironed onto copper clad at home with reasonable results. They're just about impossible to find in less than 100foot by 2 foot rolls, though.

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