Multiayer Tips
Balancing Copper

When designing multilayer PCBs it is recommended to balance the copper area of internal layer pairs. This is needed to ensure the boards do not bow after pressing. The bow is due to one side having more copper than the other side and pulling unevenly when the board cools down after pressing. This can some times be fixed by hating the board and letting it cool down within some heavy steel plates, however once the board is heated again during soldering it will return to its bowed or twisted state. The copper weight of the layers should be the same on the matching layers to reduce this problem. For example, on a 6 Layer board, Layer 1 and Layer 4 should have similar copper area and copper weight. Layer 2 and Layer 3 should follow the same rules.


Clearance

Specifying tight track width/spacing simply decreases the yield from a panel and raises costs. We recommend that you use 8 thou (or higher) track/space but in general the larger the track/space, the higher the yield and the lower the cost. On Multilayer boards, a clearance from internal pad edge to the internal copper plane of 10 thou is required. This will almost certainly avoid the chance of internal shorting or breakout.


Blind and Burried vias

There are several ways to make blind vias. One method uses two cores (for a 4 Layer board) which are drilled and plated as if it was a double sided board with the track pattern of the inner layer on one side and full copper on the other side.
This method has the advantage of not using blind holes during the plating process. This method of making blind vias is considerable easier to control and allows for deep blind vias. The main limitation of this process is that it is not possible to have burried vias that connect to the inner layers that contain the blind vias.
Using this process it is possible to make a 6 layer board with blind vias between the top layer and inner 1 and inner 2 and bottom layer and inner 3 and inner 4. However using multiple pressing cycles will add cost to the job. When making multilayer boards it is possible to define constructions that are either impossible to manufacture or very difficult an expensive to make. The other method of making blind vias is to drill part way thru the board to the desired layer. This method has the advantage that you can make blind and burried vias to the same layer but has the limitation on the depth of the hole that can be reliably plated.
This last method is more expensive and is generally not capable of making deep blind vias reliably. If you need to make a board with a complex build please contact us before spending time drawing some thing that can not be made.

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