Hi Mike,

I have two suggestions for you. The first concerns registration (alignment) of the artwork. You are absolutely correct that tape, even clear tape, will interfere with the exposure. This is why you should ensure that it is applied only to areas that are outside the area occupied by your PWB. If your PWB occupies the whole blank board, obviously this is not possible.

In order to accomplish the registration, I used to put cross-hair registration data directly on my transparencies. Ideally this would involve putting a cross-hair datum on the artwork corresponding to the relative positions of each of two opposite corners of the board. Unfortunately, it wasn't really feasible to define the relative positions of opposite corners of the physical board with sufficient accuracy, usually because the edges weren't precisely square. I found that putting the cross-hair data beyond the corners of the board and aligning them to each other often yeilded workable results, but the consistency left much to be desired.

Ultimately I found that the best way to align the artwork was to place a single cross-hair datum to correspond to one corner of the board, and then extend two of its lines along the full length of the two adjacent edges of the board. Then registration was accomplished by aligning the intersection of the lines to the corresponding corner of the board, and rotating the transparency until one of the edges of the board was aligned to one of the cross-hair lines too (making certain to use the same edge for rotational alignment on both sides of course). If I had room for tape I could align and fix the artwork to both sides at once. And if I didn't have room for tape this produced sufficiently repeatable results that I could align and expose each side independently. The results were more than sufficient and impressively repeatable.

Now I have another suggestion. You likely have the ability to "reverse" or "mirror" the artwork when you print it out. You should do this to the artwork for one side of the board. Here's why. When you print your artwork, the actual toner/ink of the image is on one side of the transparency film. If you print both sides normally, then on one side of the board the toner/ink of the image will be physically adjacent to the bare copper, and on the other side it will be separated from the copper by the thickness of the film. You will want to "reverse" or "mirror" print that side so that the toner/ink of the image is physically adjacent to the bare copper on both sides of the board.

This is why. Light refracts around corners. If the light being masked by the toner/ink of the image still has a distance to travel after it's blocked by the image (the thickness of the film), it will refract around the edges of the image and cause a "fuzzing" of the trace edges. The net result will be that you will greatly diminish your ability to produce small scale features. By "reversing" or "mirroring" the appropriate side of your artwork, you will be able to produce nice, clean copper features, and when needed at a much smaller scale.

So there are my two suggestions. Good luck,

Joe