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I've recently found, in some old stuff, a circuit board from an toy, specifically a target for a light gun (if you light up the photoresistor in the middle of the target with the light from the "gun", it generates a sequence of pretty lights).

What baffled me is that the traces look like this:

Front of the CB

As you can see, a large portion of the traces is covered in solder. Since I wasn't able to deduce a sensible reason (only several nonsensible ones relating to either incompetence or improvisation), my question is:

What would be the rationale for covering the traces with solder in the manner shown?

For reference, here's the other side of the board: other side

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  • \$\begingroup\$ Could it be a really crappy wave soldering job? \$\endgroup\$ Mar 23, 2013 at 16:19

3 Answers 3

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This is a classic example of the early days of wave soldering w/o solder mask. It looks like a phenolic pierce and blank style board too so a very low cost. Obviously solder was not considered a cost item nor something to be minimized. It did have the additional effect of reducing trace resistance. Do keep in mind that the copper needs to be covered to prevent oxidation and corrosion so this solder cover was necessary too.

You used to get PCB's (or PWB's however you want to call them) made with solder coating on the traces, (see picture in question? - probably) and when solder masks first came out these were applied over the solder coated traces, and then subsequently wave soldered. It resulted in the solder reflowing under the mask and getting all ripply and even causing tin whiskers between traces. The next step was Nickel plating to prevent copper oxidation and NOT having the ripples.

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  • \$\begingroup\$ "Classic example", not classical, perhaps? Or, Classic example of classical techniques ;-) \$\endgroup\$ Mar 23, 2013 at 19:47
  • \$\begingroup\$ Accepted due to slightly more detail than in Olin's answer. \$\endgroup\$
    – mikołak
    Mar 23, 2013 at 20:40
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This is due to the process used to make the board. No soldermask was used, so the solder stuck to all parts of all copper traces. There is really nothing wrong with that. It all comes down to cost. Apparently for this board, at that time, with the available processes to that manufacturer, at that volume, this was the most cost effective way to produce these boards. Note that these are cheap boards, being single layer and probably punched instead of drilled and routed. At high volumes the few cents saved can add up.

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    \$\begingroup\$ With boards like this, a common failure mode was that pads would crack off the board, due to the weight of the component acting through the lever of its lead (with through-plated holes, the solder joint forms a metal 'rivet' anchoring the lead into the hole). So, having solder buiildup on all the copper would tend to reduce this kind of failure. But I agree cost is the main issue. \$\endgroup\$
    – greggo
    Mar 23, 2013 at 17:24
  • \$\begingroup\$ "probably punched instead of drilled and routed" not sure what you mean by "routed" here. Can you elaborate a bit please? \$\endgroup\$
    – jippie
    Mar 23, 2013 at 21:47
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    \$\begingroup\$ @jippie: More advanced boards are cut out of the panel they were made in with a router. Small holes in these boards are drilled, and large ones routed or drilled and routed. High volume single sided boards are made from phenolic and the whole board is punched from the blank in one operation with a special tool for just that board. This is cheaper per board, but much more expensive to set up, which is why it is only done for very high volumes. The board in the dash panel of my last car was like that, for example. \$\endgroup\$ Mar 23, 2013 at 22:35
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I would agree with a conclusion that this bakelite based boards tend to quickly degrade over time and that etched traces because of LQ gluing to bakelite tend to crack off the board if there's slightly heating present. Todays impregnated epoxy based resin pcbs are way better thermal conductors.

And soldering traces is cheapest way to protect copper traces from oxidation so these could retain designed properties over time and in various operating environment (as no solder mask is used). And cheap way to achieve better electrical and thermal conductivity over a whole board.

So there's nothing wrong with those pcbs. They could only "look ugly from todays pov" if you prefer using modern CAD tools and solder mask.

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