Tuesday, March 1, 2011

Targets for PCBs

It is amazing, how simple ideas can make a world of difference.

As stated in the previous post, adding Targets to the edge of PCB Layout Art will allow me to accurately align both sides of the PCB images for the Toner Transfer Method of building double sided boards.

Use of targets is not new, but my use of them with Toner Transfer is new to me.

I have struggled with image alignment while building double sided boards in the past, Toner Transfer make the task much more difficult compared to Optic Film Exposer Methods. With the optic film method it is easy to see through the art work for alignment and both images can be taped together and/or exposed together in a double side exposer fixture.

I have search the web for ideas regarding Double Side Toner Transfer methods and found nothing that I liked. Some suggest holes and pins to provide alignment, but transferring the resulting sandwich for lamination is problematic.

The Toner Transfer process normally uses non-transparent transfer paper, and therefore alignment of the two images is difficult. But, alignment can be done by adding target around the layout image (similarly as done with optical method) and partly extending them beyond the edge of the intended PCB material.

The etch process becomes a two step process, one for each side. The first etch creates the first side of the intended PCB layout pattern and copper targets that extend beyond the edge of the board. The etched targets on the edge of the board are aligned with the second side target images that extend beyond the edge of the PCB material.  Both target sets are created from the same computer art work.

Circles are used for targets, as your eye is much more sensitive, to slight phase shift (or displacement) of concentric overlapping circles, than as can be seen with just straight lines.

With DipTrace (the PCB software that I use), targets can be created using Mounting Hole images, multiple holes are stacked one on another, and then saved as a library part.

What this means to me

My abilitiy to build much more complex Homebrew PCB has greatly improved.

I have now experimentally used this method to build a PCB with 8 mil traces, 12 mil clearance, and 40 mil "VIA's" pads with 8 mil (or maybe smaller) holes. Solder and tight fitting clipped wires will be used in the VIA's  to provide double sided connectivity.

Note: This is a great tip that I found on the Web; use a single long wire, stitched between all VIA's with loops between, solder the wire at the VIAs and then clip the loops away.

I have found installing (looping) VIAs after all other parts to be helpful, it avoids adjacent part installation dislodging the very sort clipped wire that form the VIA.

Component Side Targets                Circuit Side Targets

The targets are randomly placed around the Layout, the odd number of targets and placement help avoids confusion when attempting alignment. In theory, only two targets are needed, but more targets ensure enough are properly etched and are usable in practice.

For use with PCB Toner Transfer Method, the Component Side image is must be Mirrored via the laser print process.

The example shown here are very small boards (1x2 inch), alignment for larger boards with larger traces should be even easier.

It is interesting, once a long time ago, I was involved with the production of Technical Manuals and their Art Layup, many of the techniques that I used almost 40 years ago find their way into my projects of today.


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