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We purchased a number of stainless steel kiosk keyboards from a Chinese manufacturer a while back and have been seeing extraordinary failure rates (around 30%). Groups of keys just stop working. I've pulled several of them apart and traced the problem to plated through-holes / vias that have failed. I've been repairing them by passing a small wire through the hole and soldering it directly to the trace or pad on either side. Photos show what they look like when I pull them apart:

Odd through-hole 1 Odd through-hole 2

Given this information I have two questions:

  1. What's up with these? Any theories on why they're failing? Anything we can do to prevent it?

  2. Is the repair method I mentioned earlier appropriate? Is there a better way or anything specific I should be taking into consideration?

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That's interesting. They use a hatched ground plane; is the board hand soldered? – Thomas O May 12 '11 at 6:44
I don't know, but I'd hope not since we're generally purchasing these 30-40 at a time. They're keyboards without a lot of components (couple dozen resistors, a couple of ICs and other misc parts), so I suppose it's possible. They seem to have other oddities as well such as the blackened traces and black splotches on some of them (both visible in the second photo above). The problems with the plated holes is driving us nuts. – Justin Scott May 12 '11 at 6:53
Which vias are failing? The soldermask covered ones, or the little square 4-via arrangements? – Connor Wolf May 12 '11 at 9:52
@Thomas: Is anything in China not hand-soldered? – endolith May 12 '11 at 13:58
@endolith, For small production runs this is probably hand soldered, but I'm working with a manufacturer for a run of 100 boards, and that will be done by pick and place with only the final through hole stuff being hand soldered. – Thomas O May 12 '11 at 14:07
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3 Answers

up vote 5 down vote accepted

What's up?

First, I assume your board has two layers only (top and bottom, no internal layers).

It may be the case that there is not enough copper in the vias. Copper is applied to vias in a galvanic or electro-chemical process. Maybe the solution used in the process was old, maybe the boards were not put into the process long enough, or maybe the board was not prepared or cleanded properly before they were put into the process.

It also happens that during soldering, the board expands more than the vias can handle and the copper layer cracks (almost) open. This is especially tricky with boards containing a polyimide layer for rigid-flexible-combinations. After a while, the crack goes from almost open to open. These failures - like many other issues you can have with PCBs - are nasty, because they are hard to find while the boards are still tested. Instead, they occur after a while in the field...

My personal experience is that, however well your production tests are done, boards remain a case of trust between the manufacturer and the customer. Money spent on a good board manufacturer is money well spent.

Repair:

For home brew devices, this would be the way to go. For mass production, you would have to rework all vias, because any via could be weak and almost fail open. This is likely not a practical solution, and also very likely not a repair method recommended by IPC guidelines...

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Yes, it's a two-layer board, fairly basic in design from what I can tell as an amateur. Unfortunately we purchase the assembled keyboards, so I'm not sure whether the manufacturer we get the keyboards from actually manufactures the boards themselves or if they outsource to someone else. – Justin Scott May 13 '11 at 0:50
The manufacturer has a problem and they need to take care of it. It seems like a combination of poor pcb manufacturing and poor design to me. They can get a more expensive, better quality manufacturer, or they redesign to use larger vias and annular rings. Really, there is plenty of room on that board, so why the tiny vias? – markrages♦ May 13 '11 at 3:26
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Could it be that the PCB is flexing when keys are pressed, leading to gradual cracking of the via plating as the copper work-hardens? If so, a solution might be to improve how the PCB is supported, e.g. with high density foam and a metal plate.

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The board are mounted on brushed aluminum plates with sort sort of laminate over it. The full assembly gets bolted into a steel kiosk enclosure bolted to a wall, so there shouldn't be much movement for the board once it's in place. It's a possibility though. We'd considered that possibly the frame was distorted or bent during installation, but even the hand-installed units have been failing from time to time. – Justin Scott May 13 '11 at 0:46
up vote 1 down vote

I don't have anything to add to the cause explanations; I think all the bases are covered in the earlier posts.

Your repair method will work as long as there aren't any interconencts to the repaired vias on internal PCB layers. If your keyboard is a two-layer PCB, your fix is just fine.

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