Hi PK,
I've done scratch etching before, and it does work well, at least in theory. :-) There are refinements to my process that need to be made before it works as well as I'd like.
I use a 7475A plotter. I've replaced the felt pen part of one of the pens with a carbide tipped machinists scribe. I clean the copper board real well and then apply a coating of Dykem blue (machinists marking fluid). I do all my layout in Eagle, then I use a custom ULP program in Eagle to generate HPGL that traces around all of the nets multiple times in increasingly larger distances from the net. Then I use a Liberty Basic program I wrote to take the resulting HPGL file and apply the finishing touches, like the drill file holes, centering the image on the page (for double sided registration), etc. Then my Liberty Basic program spools the HPGL file out to the plotter. Then I flip the board over and scratch the other side. Then etch and clean off the bluing.
The good news from this (from my experimentation) is that the line that the scratch tip (if honed well) is less than .003 wide in the Dykem, and it is very consistent. No flaking of the bluing or anything. It generates very sharp lines in the copper when etched. Also, you run the plotter at it's fastest speed - it's not like milling where you have to go slow.
The down sides of the process are that you have to go over, and over, and over since you move the scratch tip out from the net only .001 each pass (to make sure you catch everything). So if you want .020 then you have to make 20 passes around EACH net. It does take awhile. But it's something you can leave run on it's own. The other downside of the process as I'm running it right now is that there is a physical 'drag' or 'lag' between the tip of the scratcher and the center of the carriage that moves the scratcher. This manifests itself as a small 'kink' each time you change direction. It has not affected the quality of my boards yet (electrically) but it doesn't look quite as good. I believe that I can modify the HPGL file in software to compensate for the tip drag, but I haven't gotten far enough along yet to try that out.
I've got the source code for the ULP that I use along with the Liberty Basic program too at
http://greta.dhs.org/EagleScratch/index.html also, there are some examples of the scratched boards at the EagleScratch photo album at
http://photos.groups.yahoo.com/group/Homebrew_PCBs/lst If you have any questions, let me know.
The bottom line is that it works, and it can work pretty well, but it takes more equipment, time, and putzing that TT.
∗Brian
-----Original Message-----
From: onenastyviper [mailto:
oneNastyViper@...]
Sent: Thursday, January 27, 2005 9:07 AM
To:
Homebrew_PCBs@yahoogroups.comSubject: [Homebrew_PCBs] RE: Scratch etching
Hi, I have recently rescued a HP 7475A plotter and I am looking at
converting it to handle pcb scratch etching (I think the mechanism is
nowhere near strong for milling).
My question is do any members have any good links or files on scratch
etching?
I am thinking either direct copper-scratching (styluses??) or
scratching though an etch-resist and etching the board as normal.
Has anybody tried this?
What type of results did they get on track resolution??
regards, PK
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