Homebrew PCBs

In the trade what you have what is called "undercutting." If the resist in
firmly applied, undercutting is usually caused by exhausted etchant and
therefore overlong etch times. It can also occur if too much agitation or
heat is applied. I don't know about the "tank" you are using, but a simple
photo tray (or Pyrex backing dish) and a gentle rocking agitation is all
one needs for one-off PCBs. Etching in a simple apparatus such as this
makes it easier to monitor the etch process and to stop it once the
unwanted copper is clear.

One other thing: do not use 2oz copper clad. Resist on 2oz copper will
often be undercut before all the unwanted copper is clear. 1oz is plenty
for every application. There are online tables that show the sectional area
of a 1oz trace versus temperature and current load. If needed, a trace is
made wider to handle more current. They're usually designed wide anyway. To
keep grounds at the same potential, they're almost always made a big as
possible--at least for analog and RF applications. Here is a nifty online
calculator for trace width vs. current calculations:
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

Best,

Todd
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K7TFC / Medford, Oregon, USA / CN82ni / UTC-8
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QRP (CW & SSB) / EmComm / SOTA / Homebrew / Design


On Tue, Feb 19, 2013 at 7:27 AM, phil.quinton@... <
phil.quinton@...> wrote:

> **
>
>
> Hi,
>
> First post, hopefully one of many..
>
> To cut a long
> story short, I'm having problems *after* etching with Sodium
> Persulphate in my very old RS etch tank ( purchased from ebay many years
> ago ). The etch tank has a working agitator which I've been
> using.
>
> The press-n-peel blue transfer ( Laserjet 4 ) to the 1.6mm
> single sided copper board, via the laminator, appears to be pretty close
> to spot on ( sometimes the lines are a little blurred, which I'm
> guessing is due to too many passes ).
>
> Issues:
>
> 1.
> Etching is taking a bit longer than I expect ( ~15 minutes ). The very
> first couple of etches I did took < 5 minutes. Since then it's been
> getting progressively worse. Assumption was that the etchant was getting
> worn ( used 5 or 6 times ). So yesterday I cleared out the etchant,
> mixed some fresh 2.5 litres with 500g of etchant and had another go.
> Same, about 15 minutes.
>
> 2. The result looks pretty good ( post
> etch ) with the toner still visible. If you check under a magnifying
> glass you can see all the tracks in tact, nice clear and strong. I clear
> the toner off with a little thinners and the end result is just plain
> awful. A lot of copper tracks are etched straight through, most are very
> thin thin indeed and pads look massacred, even though the toner above it
> looked spot on. In short, it appears to be etching *under* the
> toner somehow.
>
>
> My guess is that the etch tank is no
> longer getting to the right temperature ( ~ 45 degrees ) and I'm etching
> too long with the agitator on. I'm going to get myself a temperature
> probe to confirm.
>
> Has anyone had a similar experience ( Etch
> looking fine until the toner is removed )?
>
> Thanks in
> advance,
> Phil.
>
>
>  
>


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