The copper PCB board came in, too. I'm pretty sure the 12" x 12" board represents a life-time supply. I made a sacrificial tooling plate from 3/4" MDF. I faced the top with the fly cutter. I swept the top surface with my dial test indicator to check it for level. It was quite flat but bumpy. The next step was to mount a test piece of the PCB. Flat. That's the hard part, actually.
I don't have a decent clamping mechanism for holding down the PCB. I read how some others have done it. First I drilled holes around the perimeter and screwed it in place with the screw heads overlapping the edge of the PCB. Surprisingly that didn't work. I drilled a couple holes in the board and put in more screws. Still no go. It was fairly easy to push down the corners and see the board move.
Next I tried double-sided cellophane tape. That didn't work. The PCB board appears to have a curve in it, probably just one axis. It's tough stuff, too. It appears it's going to need a thorough, strong clamping or other hold-down method. I read about one person's approach where he uses an aluminum tooling plate and glues the PCB down with CA/Krazy Glue/Super Glue, Cyanoacrylate. Then he uses acetone to soften the CA and remove it from the plate. I'm not real clear how he did the drilling. Maybe he drilled into the aluminum, I don't know.
I haven't had much luck with CA myself, at least not the hardware store variety. Maybe the industrial kind is better, but I never find it "bonds instantly" with much strength. I can't get anything to hold together with it, actually. I went to Lowe's (home center) and looked over the adhesives. I figured the tape didn't work so I didn't want to buy something that wouldn't be otherwise useable. I suppose I could have tried some 5-minute epoxy I already had... Anyway, I decided to give Gorilla Glue a try. Not the "super" variety, just regular GG.
I applied the GG to the MDF tooling plate, sprayed it lightly with water and put the PCB on. I clamped it really well and left it over night. Next day, I swept the surface and found it was much flatter but still had about 5 mils variation. I was pretty bummed at this point. I really didn't expect to have such a problem holding it flat. I decided to go ahead with a test cut anyway.
I set the system up to cut at "zero" depth. It's tough to set the Z height just right, so this was playing is safe. It did cut lightly into the copper on one half and through the copper on the other. I lowered it 2 mils and ran it again. This resulted in a complete cut over the whole path which is really a pretty small area. Here's the result:
The isolation is complete, the edges are clean. I had done a test cut on the red resin material. I don't have a photo because the cut was so clean you really can't see where the lines are. The cuts I did with the cheap conical bit are white on the red because the edges are actually fuzzy.
The Gorilla Glue is pretty good stuff. I thought GG was at least partially CA but it may be that the standard formula is polyurethane only. I put so much on it was clear I wasn't going to soften it with anything. I tried some Goof Off anyway (Xylene formula). Finally I just ripped it off with a chisel. Quite a lot of the MDF came off with the board. This was not going to be a solution for the actual production.
Then I glued a piece of PCB to a piece of plexiglass using much less glue. I clamped it real well and waited overnight again. The 6 or so small drops spread out to cover almost the whole surface. To my surprise a little easy flexing of the plexiglass separated the PCB. There was no glue on the plexi. I tried some nail polish remover (with a large acetone component) to remove the GG from the PCB. I don't think that route is going anywhere.
The glue is on the component side, of course. It makes a nice shiny surface if the coverage is complete. It might not be a problem at all to leave it on there. Maybe there's something I don't know that will lead to trouble. We'll see. Maybe I'll find someone who knows a little more about it.
After taking the above picture I drilled a 1/32" hole in the small pad in the middle and soldered a scrap lead to it. The solder went on nice, so all the questions about isolation width, pad area around drill holes and solderability have all been answered favorably.
No comments:
Post a Comment