Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Dave:

The T-Tech is used to make prototype boards to prove the circuit or for my use. I do not attempt to plate the through-holes. I solder on the both sides where it can be easily done. If the top solder pad is not available for solder, then a via is made adjacent to that pad so that the top-bot connection can be made with a piece of wire-wrap wire. The vias are small with 0.017 holes. I have tried filling the through-holes with conductive paste of several types, but I have been unable to get 100% confidence. One bad via will generate more problems than soldering both sides using the small wire. The via connections are made first; the vias are easily seen because of their small size. The added vias are removed for the production board. I normally operate with a 0.025 path-to-path spacing. Surface-mount devices give no problem, but I prefer the devices with 0.050 or 0.025 pad spacing to match with the path-to-path spacing. The surface-mount devices are soldered with the aid of a 20x microscope.

I normally use only two tools plus the drills. The holes are drilled first. A drill is seldom broken by the machine. The drills have a long life. The circuit is milled in two steps; the first step is done with the 90-degree tool and the second with a 0.039 end mill. I use the 90-degree tool rather than the 60-degree tool only because I get longer tool life, not because it gives better results. Also, I use the tool longer than I should since this tool and the end mill are the only real expense in making the boards. As the tool wears, I cut a little deeper and thereby maintain a relatively clean edge on the cut. With this procedure I can cut several 4x4 in boards with one tool. As the end mill wears, the performance of the end mill drops rapidly; it is difficult to extend the life of the end mill by making a deeper cut. The text is milled with the 90-degree tool. The PCB is cut-out by making two cuts half the thickness of the board; one cut from each side. The cut-out is made with old 90-degree tools. The router tools can be used for the cut-out. However, because of its wider cut, the spacing between the PCB outline and the circuit will have to be increased. Manual clean-up of the PCB is required after the milling is completed. There is always some uncut copper that needs to be removed.

From the circuit s/w I normally generate 5 files: Top, Bottom, Holes, Text, Top-outline, Bottom-outline. The Text file contains any text, date, logo, etc that is to be milled into the board. The top or bottom side must be selected for the text; two files are required here if both sides are used.

I use 0.020 organic core solder so that the completed board can be cleaned with brush and soap/water. This brushing cleans the milling cuts as well. After the PCB is completed and initially tested, I brush "paint" a heavy coat of green water-based polyurethane onto the PCB only for circuit protection. Under a light this paint dries in minutes and the board is available for use. I am sure there is a better coating available. Although the performance of this paint is acceptable and the final appearance is similar to a mask-coated PCB, I would like a tougher coating.

If there are comments or suggestions on the procedures outlined here, they are definitely welcome. I will try to add further details, if needed.

Al Daniel


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