Saturday, April 19, 2014

Minima - CPU Module Progress

After making modification to my experimental Minima CPU board layout, I built a board via the Toner Transfer Method, then coated it with solder, and then started installing the via's and a few components. At this stage, the board looks a little rough as the thin layer of rosin coats everything. When complete, the board will be washed with Alcohol and a very thin coat of high tension oil (Corrosion Block) will be applied.

The 11 mil Via holes are Stitched
With Stripped 8 mil Wire Wrap Wire
The new layout is larger than had been previously posted, because I decided to include small push button switches for each of the normally external controls. By including switches on this board other modules or wired connectors are not necessary for full operations, although edge headers (as shown along the near side) are supplied for eventual use of the external controls.

The current board is slightly wider than a 16x2 Character LCD module and other dimension (height) allows the switches and header to be exposed below the mounted LCD. Note: the LCD plugs in via a header across the top of the board, and in normal use, the LCD would typically be mounted remote.

Stitched Loops of Wire Wrap Wire
Installing small via's is NOT a lot of fun, but can be done with the aid of a Microscope. My method starts with drilling the hole in the via that is just big enough for the intended wire used to make the via connection. If the hole just fits the wire, the wire will stand straight through the hole. I stitch all via's together with a single wire, leaving very short loops on each side.

The very thin stripped Wire Wrap Wire is 8 mils in diameter, and the via holes are 11 mils (a tight fit), the via pads are 40 mils. Note: 8 mils are less than the thickness of 3 sheets of typical bond paper, but I really enjoy working on microscopic projects.

Once all holes are stitched together, the loops are bent tight to the board and then cut just out side of the edge of the pad with an X-Acto Knife. Wire Wrap Wire cuts very easily with a gentle wiggle of the knife. The loops on the backside are left in place to hold the wires in place.

After each via is soldered from the first side, the "cut and solder" procedure is repeated on the other side. Care must be taken to avoid heat transfer to the opposite side solder joint, even as small as the Wire Wrap Wire is, it can transfer enough heat to melt the backside solder (if to much heat is applied).

A very-very fine tip soldering iron is used with very fine 15 mil solder.

Because the stitching process is somewhat random, the Wire Wrap Wire is typically bent in to a "Z" shape, with a short flat tail on each side of the board.

I make a Microscope video with a USB camera of the process, but it did NOT turn out. I need to find an adapter for my good low light camera.

About one half of the parts are installed now, but a few (i.e., the 16 MHz Crystal) has not been ordered yet (maybe tonight).


1 comment: