Homebrew PCBs

derekhawkins wrote:

> The etched board looks like it was from laser artwork. The undercut
> is no greater than the width of the jaggies. Why would the undercut
> be any greater with a 5 mil track?

Because from past experience with bubble etching there is anywhere up to 
a 2:1 ratio between fastest and slowest etch rates across the board. 
This _does_ effect reliability of fine traces. A 6 mil line is 150um. If 
you etch 35um down, and 35um laterally, then you already reduced your 
line to 50% of its original width (150um - 35*2 = 80 um). If your bubble 
etcher has uniformity problems then thats even more undercut. Even if 
lateral etching is slower than downward, its still is a problem and its 
becomes critical you don't over etch, period.

> 6/6 rules artwork snippet;
> 
  > Etched 6/6 snippet from the above;
> 

If thats 6/6 mils, then those laser print lines edges look a lot cleaner 
than what I've ever seen from any inkjet at its perpendicular edge. I've 
looked in detail at many inkjet printer for photomasks, and all of them 
suffer this same problem. There is no way of slowing down the print head 
necessary for elimination of perpendicular jaggedness. Its also 
impossible to see any of this this from the naked eye.

The fuzziness of the edges are not as bad as the seem. The photoresist 
has ability to average out the edges because its polymerization reaction 
is quite abrupt. When exposed to sufficient energy it suddenly 
polymerizes, meaning the tapered light intensity due to the dispersed 
inkjet droplets at the vicinity of line edges will in effect produce a 
straighter polymerized resist pattern.


> 
> Bottom line, you shouldn't have to try 6/6 or 5/5 rules in order to
> determine whether it can be done. You should be able to determine
> that by inspecting your 10/10 rules results. If the printer is the
> limitation then the artwork is going to tell you that before having
> to go any further.

True. But experimental 5/5 lines are good for testing reliability of 
10/10 lines. I've never needed to go below 8/8 lines to date.

Adam