I got a few questions around the strange look of the board. Why did I do that? I mentioned it briefly in A looow cost expansion.
Low Cost PCB
I usually minimum do 4 layer boards. That way I can have a complete ground plane and a complete power plane. It simplifies routing a lot. Any connection is just a via away. It also simplifies any reasoning around signal integrity and EMC. The cost addition is reasonable low. However, here we try to do the lowest lowest possible cost of a cartridge. That means every penny need to go.
Furthermore, the cost of a PCB is similar to the cost of silicon. It’s the area you pay for. So the game is to get as much circuits as possible in as a small area as possible.
One of the limitations or constraints here is the fixed size of the production panel. This is the panel that the PCB manufacturer uses in his or hers process. The usual production panel for this kinds of boards is 24″ by 18″. So we need to get as many multiples of that as possible into that space.
The utilisation is measured as your boards area divided by the panels area. Some area are always lost due to margins, coupons and other features needed for the production. Anything below 70% is piss-poor though.
Shipping / Customer Panel
On the production panel your boards are placed. Either single items, but more often in a customer or shipping panel. Usually it is more efficient in the following processes to have a number of single PCB clustered on such a panel. It simplifies handling during SMA, or test and programming. Again, You pay for the people handling your boards per hour. The less manual operations with as many boards in one go, the better and cheaper.
GCart Jr II
The determining feature of the board is the width of the edge connector and the distance PCB edge connector to the back of the cartridge that holds the mic SD, buttons and external connector.
Those two distances is nothing we can do much about. On the other hand I do not need the area that a “rectangular” shape would give.
So I choose this shape because I could then rotate one PCB and make more use of the space, and put them on a panel. This way I get one extra board with 25% extra area.
Mousebites and V-cut
For efficient depanelization after SMT you usually use either mouse bites or v-cut.
The V-shape feature is generally provided on both sides of the PCB and only in a straight line. A V-Groove depth that will provide a sturdy work-piece and still separate with light to moderate pressure after assembly is an important element in manufacturing.
Tab Routing uses perforated breakaway tabs (sometimes referred to as “Mouse Bites”). It is preferred to have at least one tab per side. If the PCB placement is too dense for a tooling holes and fiducial, then they should be placed on the breakaway material.
The size I use is 5 or 7 drilled holes at 0.75mm pitch and with 0.5mm drill bits. They are placed 0.25mm inwards in respect to the PCB edge. Copper is removed around the hols on all layers. As are solder mask.