The fourth installment in our PCB through hole plating series - plating the PCB to get plated through holes.
In the previous three articles, we've built the tank, made the solution, and activated the through holes. We even made a constant current source. Now, it's time to plate the through holes with copper.
Here are a couple shots of the actual setup:
|A closeup of the anode banks and cathode bar
||A shot of the entire plating setup plating the test PCB.
Calculating Plating Current
Before turning on the current and dunking the PCB in, there are a few things to figure out. based on some experimentation, we saw that a good plating current was between 10 and 15 amps per square foot (ASF) of PCB surface area. This current provided the cleanest, most even plating (keep in mind that our physical setup is far from optimal), if we were using all of the additives and a different plating tank setup, we may be able to plate with up to 30 ASF - which would reduce plating time considerably. The down side to the higher current density is that there is less "throwing power" which means that smaller holes could potentially plate poorly, as well as a rougher finish. Currently, our plating solution has a leveler (poly ethylene glycol - PEG), but lacks brighteners, which would help with better plating at higher current densities.
So, assuming 10ASF as the plating current density, we'll first calculate the total area of the PCB to be plated. Make sure to include both sides of the board here:
Area = Len * Wid * sides
1.57" x 2.9" x 2 sides = 9.1 in2
9.1 (in2)/ 144 (in2/ft2) = 0.0632 ft2
Now, plug the area into the following equation to get the actual current that needs to be applied during plating:
AmpsPerSqFt(current desnity) * Area (sq ft) = plating current needed (A)
10 * 0.0632 = 632mA plating current
Next, calculate the required plating time. The plating solution should deposit around 0.0011" (1.1 mils) of copper per hour at 20ASF. One oz/inch copper clad has copper thickness of 1.3 mils. We're going to be aiming for 0.5 oz/in - 0.65 mils.
For our test PCB, we'll be plating at 0.55 mils/hr (because of the targeted 10ASF vs. 20), so we'll need to plate for:
Plating Time = desired thickness (mils) / plating rate (mils/hr) * 60 min/hr If this is too long for you, try experimenting with a higher current to reduce the time.
= (0.65mils /0.55 mils/hr) * 60 min
Plating time ~= 71 minutes
Plating the PCB
The PCB is going to be placed into the tank and "hung" from the cathode bar. In order to do this, some sort of hanger needs to be made. A first attempt at a simple hanger was a couple of pieces of bent copper wire. This turned out to be fairly awkward to handle and the PCB fell into the bath, needing to be fished out.
The second attempt at a hanger was some alligator clips crimped and soldered onto the end of solid copper wire, which was much more secure:
|alligator clips for hangers
||Test PCB secured with alligator clip hangers
Place the PCB into the Tank
To plate the board, simply dunk it into the plating solution. Ideally, it should be parallel to the anode banks to maintain even resistance between all portions of the PCB and the anode banks. It should also be flipped on two axis (top to bottom, left to right) half way into the plating time to promote even plating to all parts of the PCB.
There might be a layer of copper sulfate on the cathode hanger, so to get a good electrical connection some rubbing might be necessary. Ideally, the board will be parallel to the banks (it is NOT in the pictures).
Before turning on the power supply, set the current to the minimum. Turn on the power supply and then slowly adjust the current to the level previously calculated. If you've implemented bubble sparging, turn this on now. Now, simply star the stop-watch and set it for half of the time calculated previously, turn and flip the PCB, then plate for the remaining time.
After the board has plated for the required time, pull it out of the solution and rinse it off. Here's a shot right out of the plating solution and after rinsing.
Finally, some shots of the board with plated through holes:
|A shot of the majority of the PCB
||Closeup of the 40 mil hole grid. Plating can be seen on the inside of the holes
For electrical testing, a really rough grid was milled into the PCB with a bit that was too old. The hole grid is misaligned with the holes because of all the inconsistent re-fixturing. This is also a perfect example of why it is important to use sharp tooling (the milling quality is really poor here). But, for purposes of creating some pads that are isolated from one another, this will work just fine.
The PTH's work! The resistance between top and bottom layers isn't anything noticeable with a cheap hand-held DMM. Here are some of the advantages of using PTH's over soldering individual wires into the holes:
- They flow solder from top to bottom quite well - this adds a significant amount of re-enforcement to the solder joint.
- If a large number of vias is on the PCB, PTH's are much faster, less labor intensive, and less error prone than hand soldering small diameter wire through the vias.
- They can be placed under components - try doing that with a hand soldered via or a TH electrolytic capacitor! This includes thermal vias placed directly under the components - while this may be possible using other methods, it would be extremely time consuming.
- When mechanical riveted vias are too large - DIY PTH's can be made the same dimensions at home as it is in a "standard spec" professional PCB fab- we've successfully tested them down to 15 mils diameter, which is the same via size we use on PCB's sent to the pro's for fabrication.
Some of the times not to use PTH's
- You are planning on chemically etching the boards and do not want to setup an additional process tank for masking the PTH's before etching the traces
- You only have a few through holes and they haven't been placed in any inconvenient locations. In this case, the (minimal) overhead of the PTH process may be higher than the time to just solder the couple of wires on the board.