Pick and the placing a brass tube

[Simon]

I'm doing a design which will carry 70 A. Having decided that putting wires in would be a waste of time as the thing need potting meaning the wires would need to be longer than necessary and then cut down to size and of course you only have one shot at it. I was going to use 6 16 gauge wires but have instead decided to use a threaded stud which should make assembly easier and quicker and be more flexible for the end user.

So I have created pads on my board with a 4 mm diameter hole and an 8 mm diameter exposed copper. The plan would be to have a screw through from the bottom of the board a brass tube from the board up to the height of the potting compound and then a nut and washer so that the screw clamps across the board and the brass tube. Obviously it would make electrical sense that the brass tube is soldered down to the board. But the best way of doing this would be to have it soldered in the reflow oven along with every other component. But as far as I know to do that it would be necessary to "pick and place" the brass tubes. Is this something feasible or was it be quite the manual operation?

I'm hoping this solution would be a lot easier and quicker and cost-effective than trying to solder in many wires as with a large copper planes this would be a hard job to do manually.

[DerekG]

But as far as I know to do that it would be necessary to "pick and place" the brass tubes. Is this something feasible or was it be quite the manual operation?

Greetings Simon,

Whilst everything is possible, it is unlikely that a feeder can be found for the SMD PnP robot for the brass tubes.

If the tube is a tight fit in the hole, then pushing them in by hand before loading the rest of the SMD components would probably be the best method. BUT the PnP robot will need programming to ensure its head does not collide with the brass tubes.

An alternative might be to have the tubes exit through the bottom side of the board. You can then make a holding tool with female holes in it to allow the male brass tubes to "sit inside the holes". This ensures the PnP placing head cannot collide with the tubes.

But the best way of doing this would be to have it soldered in the reflow oven along with every other component.

Yes, this would certainly save time. I would suggest making a thicker (at least 8 thousands-of-an-inch) solder paste stencil to ensure you lay down enough paste for a good joint.

You will need plenty of heat in the reflow oven. Consider pure copper tubes instead of brass. These conduct both electricity & heat much better than brass & will expand & contract at the same rate as the copper on the board.

If the copper/brass tube is malleable enough, you could make up a peening tool to actually peen each end/one end of the tube over the pad.

Depending on the diameter of the tube, wave soldering may mainly fill the tube with solder if the SMD side is left open to the air. If the solder paste covers the top of the tube, the resultant air-lock will likely prevent the tube filling with solder.

Just a few pointers that might help you.

[daqq]

Is this something feasible or was it be quite the manual operation?

Bit of both - depending on the assembly process of your manufacturer you can have a manual manipulation step - for instance we had some obscure connectors that had to be placed by hand and there was no problem with that. They just first put on the solder paste, then put it through the pick and place machine, then hand placed the nasty parts and then put it into the oven. It didn't really increase the cost that much.

Your manufacturer should have something similar.

Also, if you are out for cost, you might want to consider sheet metal cutouts rather than tubes and similar - cutting out a "line" from sheet metal via waterjet costs very little and you can create pretty much any shape you want.

[Simon]

My board is designed so that there are holes for an M4 screw to pass through with an 8mm diameter pad around it, the other option is to just screw and M4 screw and nut over the hole and rely on that but i think the brass tube would solder more easily and be a better conductor. The tube could be presoldered if it won't re-flow properly.

[Corporate666]

Can you just use an off-the-shelf copper standoff along with an appropriate nut/screw?

http://i00.i.aliimg.com/wsphoto/v1/1434435770_1/10pcs-Variouse-Size-Copper-M3-Male-Female-Brass-font-b-Standoff-b-font-Space.jpg

Custom nozzles can be made to pick and place most parts, but insertions are a different matter and use different machines.  Unless your screw was pre-loaded, I'm not sure how well the brass tube would stay in place, and the PnP machine certainly wouldn't be able to press it down into a hole.

There are also board-to-board peg type devices I've seen that are made to be soldered as high-current interconnects between boards.  No idea who makes them but I've seen them between boards in power supplies and such.

[Simon]

Well I've ordered boards now and will be assembling the first ones myself anyway so labour is less of a concern. I'll have to speak to my subby to see what he prefers.

[DerekG]

http://i00.i.aliimg.com/wsphoto/v1/1434435770_1/10pcs-Variouse-Size-Copper-M3-Male-Female-Brass-font-b-Standoff-b-font-Space.jpg

Yes, excellent idea.

Having used these brass risers before (for earthing between two pcbs) in a potting mix, we found the most reliable method was to manually apply solder paste to the thread & pad.

Also use a compression star washer under the brass screw. The solder paste helps to hold the potting mix away from the riser's connection to the pcb. Do the screws up very tight.

The solder paste was not cured in a reflow oven & we found that we were not able to "solder to the brass risers".

[Simon]

The other option is to just paste the ring and let it coat in solder and then put the trews through after and screw the tube down hard so that it can dig into the soft solder and make contact, although soldering still sounds more robust electrically. I've see studs before that can be pushed into pcbs and soldered, they had a broad base with lots of small square pins.

[T3sl4co1l]

Usually, potted, high-amp assemblies use bent copper tabs.  These can be made relatively cheaply by waterjet cutting or stamping, bending and using press-fit (e.g. PEM) nuts to retain them.  (There are CNC stamp and bend machines out there, so don't wince at the description.)

Tim

[Simon]

Yes I understand what you mean I work for a company that does a lot of punching and unfolding of sheet metal.

[SeanB]

Just want to add that if you are going for a brass tube or shaped tab make sure you make it large enough in cross sectional area ( thick plate or wide thin plate) to handle the expected current. If potting it the required cross section will be surprisingly large, as the potting compound will have appreciable thermal resistance. Not good to have a module fail from the terminals catching fire inside the housing. You probably would want a brass tab ( shaped and formed however you desire) with at least a 15mm width, and with the termination on top either being a tapped hole in the brass or a captive  square nut under the brass sheet. If you can go for a 3mm thick plate then you can tap the M4 hole in the sheet, and to mount it on the board you simply use a pop rivet ( copper one, available probably as a 3.2mm or 2.5mm one with a steel mandrel) to hold it to the solder pasted board, then reflow the lot as this will ensure both a good mechanical attachment, and the solder will form a good electrical join between the board and plate.

You might need to do the terminals as a separate operation after reflow, simply riveting them and soldering with a 150W iron afterwards. Board spot will just need to be bare copper with the regular ENIG or tinned surface, and with the hole as a square with a through plated hole for the rivet, square for the terminal and with a few in pad untented via's to provide extra electrical connection aside from the single hole, so that the compression applied to the hole via the rivet can safely break the via with the other via's still providing a good electrical connection to the bottom and top copper pad.