DIY Copper Riveted Vias
I created this method when I needed to make thermally conductive vias for a stepper motor controller that has a thermal pad under the chip. These riveted vias probably carry 4x the heat of a commercial via. Regular vias on a commercial board would not work well under this kind of stepper chip because the solder wicks down the via hole, and they don’t carry much heat.
In order to create the copper rivets I needed a cutter that would accurately cut bare wire to length, with the ends cut square. I also needed a method of riveting them. The following method is easy, and inexpensive too. Since it’s so convenient I use this method for all vias now.
This method can be used for:
- Vias that sit under chips or other components
- Plugged vias in pads.
- Vias for through-hole parts – just use a larger diameter wire rivet, and drill a hole through it after riveting. I haven’t tried this but it should work.
I was informed by reader L L Fudd (comment below) who used to work on TV motherboards with rivets that “heat and age caused expansion/contraction to break the joint”. I do agree this could potentially be an issue, so I have posted it prominently. Thanks L L.
I have done a lot of research, and there really isn’t much information available about this issue, except for issues with current via manufacturing: the PCB and the via could expand at different rates when reflowing, causing the via to stretch and crack in the center, and the more cycles the worse the failure rate.
That being said, the methods and materials used long ago were different in may significant ways, and I believe the method shown here has merit for thermal vias, and I will continue to use it with other vias for my own boards, with solder – I think that would overcome this potential issue, and there may be no issue if the board is not thermally cycled.
It would be possible to create a test board with say a hundred vias in series, and thermally cycle it. It’s an interesting issue I’ll be thinking about. A heater could be controlled by an Arduino with a solid state relay, with a temp sensor, and fan to cool the board between cycles.
I am building a Thermal Cycling Test Rig and test board with 100 riveted vias. There is some information there about what I ran into when making the 100 rivet test board.
Making Copper Via Rivets
Required Tools and Supplies:
A Drill Press with a steel table, or a metal plate for the anvil.
Some good Tweezers
Xuron 2193F .060″ Music Wire Shear This shear is perfect for this because it cuts the copper wire off square without mashing it. It has some springy metal clamps at the top that hold the wire strait for a good square cut. A DIY spacer made out of wood can be conveniently placed inside one of these metal clamps to allow setting the rivet length – kind of like a stop block.
Calipers to measure the rivets.
20AWG Bare Tinned Copper Bus Wire (.032″) The 20AWG wire is a bit larger than most vias need – I want the thermal capacity, and better electrical characteristics. You can also use smaller wire like 22 or 24 gauge, and smaller holes.
A Carbide Drill Bit for the Vias. I used a #63 (.0370″) drill bit. The .005″ clearance is fine. I use resharpened carbide drill bits – the quality is very good, and they are very sharp. Just make sure to use a drill press and hold the work down securely or you will break them.
A Tapered steel rod (see below). I made this from some scrap round steel rod from an old printer that I tapered to about 3/16 or 1/8 inch on one end. Make sure the end is ground smooth.
A spacer made out of hardwood (poplar). This fits into the top of the Zuron cutter inside one of the spring metal clamps and allows the rivets to be cut the same size. I used a bench grinder to shape and fine tune the spacer to just the right size – it should fit snugly. You could also use a file.
Before making your spacer you will want to determine the target rivet length:
- Drill multiple holes in a piece of scrap PCB of the same kind you will be using.
- Measure the thickness of the PCB with calipers, and add about .045 inch or more.
- Cut several rivets to that approximate size, measuring as you go. You will want some smaller, some that size, some larger (5 thousandths intervals).
- Test riveting each size and determine the optimum size. See below for the riveting method. You will determine from this what the acceptable tolerance will be. Update: A slightly over-sized rivet is what you want if your Vias will not be under a chip. If your hole is not perfect you will want that extra to compensate so you do not have margins and a bad electrical connection.
After determining the proper rivet length: place your partially completed spacer in the cutter and test cutting a rivet to length – the wire goes through the cutter when it is partially closed, and against the spacer like a stop block. If the rivet is too short then remove the spacer and either file or carefully grind the “stop block face”, reinsert and test cut another rivet, measure… until they are the right length. If you insert the wire strait-in each time then you should be able to cut rivets to within a few thousandths of an inch of your target size.
I recommend removing the stop block and adding a few drops of super glue on the face where the wire will be touching in order to add more durability to the wood.
Cutting Copper Wire Rivets
You will push the wire in until it buts up against the spacer when the cutter is slightly closed, and then finish the cut. Use tweezers to retrieve the rivet if it gets stuck between the jaws and the spacer after a cut. If the cut rivet will fall out more easily you do not push the wire tightly against the spacer.
- When I cut a bunch of rivets I measure them with a digital caliper, sorting them into three piles: a bit too big, perfect, a bit small (in 5 mil ranges). If your spacer is the right size then most will be perfect. Rivets that are a bit large are no problem as long as you are not placing them under a chip.
Placing Copper Via Rivets
- Chuck the tapered steel rod firmly into your drill press. Make sure you have a flat place on the metal drill press table to rivet against, or use a piece of flat metal for an anvil.
- Insert a rivet in a via hole in your PCB using tweezers. Place several if they are close together.
- Lift the PCB slightly so that the rivet is about centered – this is very important– you want the rivet to evenly fill the hole and mash flush on both sides. If you simply drop the rivet in the hole and mash it – then the bottom side will not mash as much and will not have as good connection, and there will be too much rivet on the top – it will not be flush. I did this in the center of the board above before I remembered…
- Update: Based on my 100 rivet test board I am evaluating what may be an improved method of setting the rivets to eliminate the guesswork of centering them. Will post an update soon.
- Pull the drill press handle to firmly mash the rivet flush. This does not take a lot of pressure. If the rivet is the right length it will fill the hole and be nearly (or even perfectly) flush on both sides without margins. Check this with a jewelers loop. If you see any margins then make the rivets longer.
- When riveting multiple vias close together (as under a chip with thermal connection) you will want to drop all the nearby rivets in the holes before riveting. The tolerances are tight and a slight deformation from a nearby hole will make it harder to get the next rivet in. Remember to lift the board slightly when riveting, and always rivet each one individually – the metal rod should have a tapered end so you can rivet one at a time.
- When laying out vias in Eagle, I make the pads larger (say .065), but with a much smaller hole than usual. Then when I drill and rivet I have no gaps and good connections even if my alignment is not perfect. Of course I could make everything smaller if I used smaller wire for the rivets.
- Now is the time to plate the board with tin (to protect it from corrosion and make it easier to solder) using Liquid Tin, if you are going to do so – after riveting all the vias. Clean the board very carefully (perhaps with alcohol & green scrubby pad) first. It must be shiny with no fingerprints or oxidization whatsoever. Then just place the board in a shallow plastic pan, pour in the Liquid Tin, and just wait a few minutes. Remove and rinse. Do not use that pan for anything else. You will want a funnel to help get the Liquid Tin back into the bottle.
- In most cases you will have a good electrical connection without soldering, however for safety (see the update above) you should place a small drop of Kester EP256 Lead Solder Paste on each via before reflowing, or just use a soldering iron to apply some solder. The previous link also has syringes and tips available separately. FYI: another cheap source of solder paste is Deal Extreme but it takes a couple weeks for it to arrive.
Also see my articles on Perfect single or double sided PCBs with the Toner Transfer Method. and DIY Dextrin Coated Paper for the Toner Transfer Method.