Like everyone, I sometimes have trouble aligning the top and bottom films onto the PCB when exposing to UV. What I've noticed is that if I line up (for example) a pair of pins in the bottom left and top right hand corners of the design, then the pins in the bottom right and top left don't exactly match up. We're not talking about a big off-set, but enough so that if I drill in the center of a pad on the top side, it doesn't always come out in the center on the other side.
I think there are two possible reasons for this.
a) The printer can't print very accurately, or
b) The printer is printing accurately, but the heat of the printing process is distorting the film.
Has anyone else experienced this, and is there a simple cure?
I doubt it's the printer since a printer is capable of placing a dot of ink on a previously printed dot in exactly the same dot location. Try printing on paper and check alignment. - Scotty
old printers trick. print two copies, hold them up to the window/light then move them to align them.
old home brew trick is to use pads with holes in the center ( etch center) that gives the drill a place to start. the etch should not be a lot larger than the web of the drill (the solid flat portion.
when I was still doing home etching, I would lay the two sheets and align them at the window and then once I was satisfied, I would tape them together. then slip the board in-between. tape one to the glass, them move the other over that until you like the alignment.
I used to use the laser transfer for letters and such as my home-brew silk screen.
hold them up to the window/light then move them to align them
The problem is that it's almost impossible to align all parts of the film at the same time, because of fractional differences in the size of each print (typically about 0.3 - 06 mm across an arduino-mega sized board.
When you print both sides do you print in the same direction.
Iv noticed that printing one on x direction and one on y direction can do this as can changing the resolution from 300 to 600 dpi.
Absolute positioning gets affected by both.
A long board can be affected by roller slippage while the printer head should be constant.
A standard eurocard is the max for me before slippage becomes a problem, although at that size i get the off centre hole problem.
I have a fairly old epson i keep purely for this use.
Expensive on ink but nice dense black i dont need to double up.
I find printing onto coated ohp inkjet film to give better results than the mega stuff.
I use the one by rymans by preference although. Nobo is good if you can find it these days.
If you have two prints that align using the light method, then dave-in-nj 's method will give you 100% results.
If things are a bit out of alignment, use the largest pads possible to make sure you capture the two sides.
Yep totally agree.
Tried several printers before i got a free hp photosmart printer.
Origional hp black ink is dense even if expensive a does not need doubling up.
Tried other printers previously that did , also found geometry errors the op was complaining about.
I think it about finding the right printer.