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How are or were vias made commercially?

Wikipedia (http://en.wikipedia.org/wiki/Via_(electronics)) mentions "The hole is made conductive by electroplating, or is lined with a tube or a rivet"

Can anyone provide more details on these processes, with an eye towards replicating the process? (I realize the standard DIY way is to thread some single-core through and solder it. That seems relatively slow and not amenable to automation).

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I don't know the answer, but here are a few decent links to read up on (I'm also interested in this topic): en.wikipedia.org/wiki/Electroplating and hackaday.com/2012/10/03/… –  Shamtam Apr 9 at 19:42
    
@orangenarwhals You may find this video interesting: PCB Copper Panel Plating Through-Hole / Via Tutorial –  Nick Alexeev Apr 9 at 20:04
    
Why would anyone downvote a legitimate question like this? –  Chris Laplante Apr 9 at 21:11
    
This U.S. outfit: thinktink.com offers the chemicals required to do activation and plating. I've never dealt with them, but I think it's rare to find such a supplier not aimed at mass production, so worthy of a comment. –  Spehro Pefhany Apr 10 at 0:35
    
I have heard of a few people using a rear window defogger repair kit for "plating" vias. Need to be careful about resistance of it and might have to do it after any reflow. –  Joe Apr 10 at 16:00

2 Answers 2

up vote 18 down vote accepted

PCB production after stack up cure:

  1. Drill the hole. This is through the solid copper (un-etched) outer layers and feature etched internal layers (for a 4+ layer board).

  2. Copper burrs are removed in the deburring process.

  3. Melted epoxy resin is removed by a chemical desmear process. (Without this, you cannot get good plating coverage to the internal copper.)
    Clarification: This step is only on 4+ layer boards. The plating around the via top and bottom annular rings will get good conduction on a 2 layer board, even if the edges are epoxy insulated.

  4. Sometimes (but less seen due to nasty organic chemicals needed) the resin and glass fiber is etched back to expose more of the copper layers. (Again: only on 4+ layer boards)

  5. Around 50 microns of electro-less copper is deposited within the hole to allow electroplating.

  6. Polymer resist is added to the board to cover everything that will be etched away (all but via pads, normal pads, traces, etc).

  7. Around 1 mil of electroplated copper is deposited into the barrel and on every surface of the PCB not covered with resist.

  8. Metallic resist is plated over the electroplated copper.

  9. Polymer resist is removed.

  10. The Etching process removes all copper not covered by metallic resist.

  11. Metallic resist is removed.

  12. Solder mask is applied.

  13. Surface finish is applied (HASL, ENIG, etc.)

Some things to consider about vias and DIY via replacements. Thermal expansion is the death of PCB boards, and vias are the most abused portion.

An FR4 material is resin impregnated glass fibers. So you have a weave of fibers in the X and Y direction, covered with "Jello". The glass fibers have little CTE (Coefficient of Thermal Expansion). So the board will have maybe 12-18 ppm\C in the X and Y direction. There are no glass fibers restricting motion in the Z direction (the board thickness). So it might expand 70-80 ppm\C. Copper is only a fraction of that amount. So as the board heats up, it is tugging on the via barrel. This is where cracks will form between inner layers and the via barrel, severing the electrical connection and killing the circuit.

For a home made via, you are most likely going to have issue with the plating being thinnest in the middle of the barrel, and this area failing with temperature expansion.

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Is step three (removing excess epoxy resin) only necessary in 4+ layer boards? Am I right in thinking this resin is introduced when gluing the layers together? –  Calrion Apr 10 at 5:07
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Resin exists in a 2 layer board, but you are correct in desmear being only a 4+ board process (it has been a while since I've been in only 2 layers). An FR4 material is just glass fibers with resin. The difference between 2 layer and higher layers is the use of prepreg (a partially cured version of the same material making up the precured 2 layer board.) So the resin is always there and drilling melts it and smears it around. –  Joe Apr 10 at 12:13
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It is worth knowing that the copper in vias is a lot thinner than the copper on the layers of the board. –  Will Apr 10 at 12:36
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Absolutely and that unless the board house controls things well or the via size is large with respect to minimums the board house can fab, the thickness of the via can vary along the barrel. Sometimes to the point that the board will prematurely fail with thermal expansion. –  Joe Apr 10 at 12:39

One commercial alternative to plating for 2-layer prototype boards is to use rivets, such as this machine. A DIYer could either solder the rivets instead of pressing them, or fabricate suitable dies for a cheaper press if they have access to a lathe.

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