Copper Electroplating part 4 (plating the PCB)

testPCB grid PTH closeup

The fourth installment in our PCB through hole plating series – plating the PCB to get plated through holes.

{gallery id=electro4}electroplating4{/gallery}

 

The Setup

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:

{gallery link=electro4 index=2}anodeBanks{/gallery} {gallery link=electro4 index=10}testPCB plating{/gallery}
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:

{snippet code|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:

{snippet code| 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:

{snippet code|Plating Time = desired thickness (mils) / plating rate (mils/hr) * 60 min/hr
= (0.65mils /0.55 mils/hr) * 60 min
Plating time ~= 71 minutes} If this is too long for you, try experimenting with a higher current to reduce the time.

Plating the PCB

Making Hangers

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.

{gallery link=electro4 index=4}hangers poorSolution{/gallery}

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:

{gallery link=electro4 index=5}hangers aligatorClips{/gallery} {gallery link=electro4 index=7}testPCB readyForPlating2{/gallery}
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.

{gallery link=electro4 index=8}testPCB inSolution{/gallery}

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).

 

Apply Current

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.

{gallery link=electro4 index=13}testPCB freshOutOfBathGridCloseup{/gallery}

 

Finally, some shots of the board with plated through holes:

{gallery link=electro4 index=14}testPCB PTH closeup{/gallery} {gallery link=electro4 index=16}

testPCB grid PTH closeup{/gallery}

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.

{gallery link=electro4 index=17}testPCB PTH milled{/gallery}

 

The Verdict

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.
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Posted in Prototyping and Fabrication
  • tha_kreature

    Awsome! But I think we can do even better!
    I have been working on a way to combine etching and pth for a while now and I think I have something that may work.
    (I used the chemical info from these pages to learn about plating.)

    Steps I try for 1-pass home production:
    It uses boards pre-sensitized with UV-resist on both sides and covered in protective plastic film.
    1. Drill board
    2. Activate holes, Dry thoroughly
    4. Remove film and UV expose board with inverse images.
    5. Develop board.  (The board will now have all areas, to remain copper, exposed.)
    6. Plate copper to make pth!
    7. Plate tinn/bright tin or any other finished surface you want that will not react with your etchant.
    8. Etch board!

    My current issue is the adhesion between the conductive ink and the upper/lower copper layer due to the epoxy in the pcb and the glue in the protective sheeting preventing contact.
    Sometimes it works other times I lack contact on one or both sides. A cleaning step to get rid of this residue on the inside of the holes may be needed and I am looking into that.
    The cool thing with this setup is you can do 6/6 mil width/iso is you are carefull. 10/10 is very reliable. It also does not require iso routing and you don’t have to laminate the board with uv resist as it’s already professionally coated.

    Hope you guys try this out.  Maby iron out that last kink for me ?

    • bamos

      Very nice!  I’m glad this was helpful.  Your method definitely sounds like it should work well. 

      A few suggestions:
      - drill the holes with carbide bits – these remain sharp far longer than HSS (high speed stainless steel).  The carbide bits will reduce burring and cut the FR4 very cleanly (it sounds like it may be burning a little bit right now).  Using pre-sensitized copper clad could also be causing some of the headache
      - use backing material when drilling – you may also consider putting some backing material on both sides of the board when drilling – this will also help prevent burring (although it doesn’t seem to be completely necessary with nice sharp bits).

      A good source for (resharpened) bits is http://drillbitcity.com/catalogue/25_piece.asp – their 25 piece sets are < $1 per bit and they're willing to put together custom sets if you ask (so if you always use 15 mil vias, 32 an 40 mil through holes, you can ask for a set with only those diameters.  They also sell new carbide tooling as well at a different site: http://drillcity.stores.yahoo.net/  If you're into milling at all it's worth taking a look at.

      Hope all goes well; feel free to start a thread in the forum to track this, or post back here with results/questions – whatever!

    • robonut

      How do you prevent the through hole coating etching away during the etching step? The acid resist from the exposed / developed surface wouldn’t cover the holes would it?

      • bamos

        Step 7 above would take care of protecting the through holes. you would need a negative “resist” to make that method work.