Monday, September 3, 2012

PCB Fabrication, part deux

Today is a national holiday in the USA, but in Europe it's just another work day. PCB-Pool has continued the fabrication of my PCB, and I have two new sets of pictures to share.

The first set shows the board after the application of the green solder mask and the white silkscreening. These days the printing is applied by something resembling an ink-jet printer instead of a silkscreen, but the name lives on.


This step is labeled as "UV Curing", which I guess refers to the use of Ultraviolet light to harden the coatings. You'll note there is no silkscreening on the bottom of the board; I've only placed two components there -- a pair of bypass capacitors directly under the push-pull row select drivers -- and I didn't seen the benefit.

One of the things I wanted to do with the silkscreening is make it easy to identify the groups of components that make individual bits in the DRAM array. If you look closely at the top of the board you'll see dashed lines running horizontally between the rows of the array. What you won't see are the dashed lines running vertically between the columns of the array; they were too close to the component pads, and only little bits of the lines survived the clipping process. If I'd made them solid instead of dashed more of them would have survived. Oh well, something to fix on the next board -- this is intended as a learning process.

At this point the pads where the surface-mount components will be mounted are still bare copper, which would rapidly begin to oxidize and become difficult to solder. To prevent that, the exposed copper areas get a thin coating of tin.

Hey, it looks like a real PC board, doesn't it? In fact, the only thing left is cutting the board from the panel. Somewhere along the line they have, or will, perform an electrical test to make sure everything is properly connected. With a multi-layer board, this is an important step.

I'm really hoping to get the board in time to work on it this weekend. The original estimated shipping date is Thursday, but given that the board looks to be complete I'm hoping they'll ship it a day or two early.

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