Wednesday, March 2, 2011

More PCB Work

Practice has not made perfect, yet.

I thought the new found targets for DipTrace PCB was going help me do the Double Sided Alignment trick, see previous post. The concentric circles looked good on paper, but in practice they are too thin to stick to the PCB for Toner Transfer. The solution is simple make the circles lines thicker - ah, no-can-do with DipTrace. The targets were made from software built-in "Mounting Hole" art (for reasons given before) and the image thickness for circles can not be altered. Too bad, it was a good idea. Other software PCB packages may have more flexibility for creating Targets.

So, back to the drawing board.

I created PAD art for a target. Pads and holes are the only items that DipTrace prints on all layers (Component and Circuit Side of the PCB). My new Target (pad) is 500 mils dia, with a hole 475 mils, which creates a nice circle with a line thickness of about 12 mils. This photos shows the results after Toner Transfer and Etch.

A major part of the lower center Target (circle) was lost in the process. The rest of the circuit looks great.

Now, for the new (maybe hard) part, aligning the second side art with the the first. The Transfer Paper is shown behind the PCB material with the Targets (circles) in alignment. So far, everything has worked as I expected or hoped. Note: the shiny surface on the PCB is clear tape, used to avoid addition etching while the second side is being etched. I inadvertently cut the Transfer paper too short, note the missing targets on the right side, with multiple redundant targets recovery was easy.

The second side did not Transfer or turn out a well as the first, some of the Toner came off in the water soak process. I think when I changed the water in the bath, the water was TOO hot, which "shocked" the PCB material and it expanded faster than of the paper (or vise versa).

A little of the Board Outline Trace and most of the Targets came off the PCB (maybe popped off) near one end of the image. Other than that, the alignment as inspected on the light table looks very good, as shown by light through the center of the pads and VIAs (click on the next image for a good look).

Note: the 60Hz hum bars on the photo were created by the flickering florescent lights of the light Table.

Good alignment can be check by inspecting the small 10 mil holes in the VIAs, five VIAs can be found on the board.

I will not use this PCB to build a project, it will be kept as an example. Due to some of my experimental last minute PCB design edits, the holes on the major PADs had reverted to the default size (35 mils), which is too large for my project (I actually wanted 12 mils holes). I will have to re-make the board.

My next experiment will include more attention to the thermal environment of all processes.


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