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I'm designing a new PCB where I have a whack of connectors that have to line up with the metalwork. These are a two-piece Neutrik XLR connector where the insert is soldered to the PCB and then mates to the shell already mounted on the chassis. There is NO wiggle-room once the insert has been soldered to the PCB - the insert is a sliding fit onto the shell with almost no perceptible side play.

One series of connectors is easy - the solder tab is a thin rectangle. The Plated-Through Hole is perhaps 5 mils larger than the width of the tab and I get great solder adhesion to the flat sides of the tab.

The other series of connectors is more problematic. All 4 pins are round. Thus, I want to have the holes as small as possible while allowing for good solder adhesion over the full length of the pin.

If the exact insert position on the PCB wasn't so important, I'd simply have my usual 10 - 15 mils of clearance. In this case, though, I have 12 to 24 connectors on a single PCB and need to ensure that all of them are as close to the proper location as possible.

I've spent some time with Google and have already looked at some of the guidelines here. Specifically, In pin-through-hole, how much bigger should the hole be than the pin? and In pin-through-hole, how much bigger should the hole be than the pin? and What pad hole (drill) size is appropriate for a given through-hole lead diameter?.

Unfortunately, none of those address the issue of solder wetting and adhesion of component leads in Plated Through Holes in a PCB.

I should mention that we are using 63/37 Tin/Lead solder and that these connectors will be wave-soldered using AZ2331 Water-Soluble Flux.

Our PCB fab house is pretty good when it comes to delivering boards with PTH holes having the correct finished diameter as specified in the drill file.

I'm looking for guidance on how much clearance around the component lead I should have in order to get good solder wetting and adhesion.

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Dwayne Reid
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2 Answers2

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I don't think you should depend on the holes to locate the part when tolerances are important. I agree with your 10-15 mils as I said earlier here (in metric 0.25 to 0.4mm).

Instead, make up a jig to hold the parts in the proper location while they are being soldered. It can be fancy for wave soldering or something simple that holds the parts while being hand soldered (perhaps fashioned from the mating part such as the punched chassis).

Round pins are a particular hassle if you call it too close- at least with square or rectangular pins you can skive off some plating inside the holes at the corners of the pin and still force the pin in. If you insist on the using the holes... um, maybe make the peg round and the hole square.. (call out slot-drilled square holes in the PCB).

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Spehro Pefhany
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    When I read the question above I was going to suggest the same thing you mentioned about making a jig. Only way to fly with this one!!. Also worth mentioning is that the mechanical design that arranged for the approach needed by the OP should really be considered sub-optimal. There are better ways. – Michael Karas Feb 04 '15 at 04:24
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    +1 for building a gig. For a small-scale job with a male and a female Neutrik XLR, I converted a spare enclosure into a gig. – Christian Lescuyer Feb 04 '15 at 12:35
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I appreciate the suggestions and guidance.

Although I've decided how to tackle this problem, I'm still curious as to exactly what the minimum clearance between component lead and plated-hole diameter should be. I have spent significant time researching this but haven't yet found an authoritative reference.

In terms of how I'm going to handle my particular situation, Spehro's suggestion of using a jig is almost exactly how I'm going to build these units.

My original plan was to wave-solder the connector inserts but hand-solder the other connectors on the boards: Phoenix Spring-Cage connectors and shrouded double-row ribbon headers. Then mount the assembled boards into the metal panel containing the connector shells.

But Spehro's suggestion made me step back and have another look at that manufacturing flow.

Our metalwork supplier produces punched and bent metal that is darned accurate. Way more accurate than anything that I can fabricate in-house. So: why not use the punched metal that these boards are going to mount to as the mounting jig for the connectors?

So now I'm going to reverse the soldering flow.

Mount all the connector shells in the metalwork ahead of time. Now we have racks of metal panels with connector shells waiting for their inserts.

Wave-solder the Phoenix connector blocks and the ribbon connectors to the PCB. This is done using water-soluble flux, so next step is to wash and dry the PC boards. Now we have racks of PC boards waiting for their connector inserts.

Place all of the connector inserts on the PCB. Hold the metalwork horizontally with the connector shell openings facing Down. Then simply slide the PCB with connector inserts up into the connector shells and ensure that they all seat fully into the shells. Flip the whole assembly over so that the PCB is now at the top, wiggle things a bit more to ensure that everything is seated, then hand-solder the connector inserts to the PCB.

Here's the cool part. We can remove the soldered board if necessary for cleaning, then remount back to the connector shells on the metal panel. Because the metalwork has been punched so accurately, we don't have to worry about which PCB goes with a particular panel. They should all be completely interchangeable, which is what I was striving for.

This technique allows me to have lots of pin clearance on the connector inserts but still gets those inserts in the exact positions required on the circuit board.

I'd like to leave this question open for a little while longer to see if anyone can provide guidance regarding the minimum required clearance for a component lead through a PCB to get adequate solder wetting. That is what this question was initially asking about.

However, I think that I have solved my manufacturing problem.

Dwayne Reid
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