Multilayer PCB DFM Tips

1. Avoid using Blind and Buried vias

When making multilayer boards it is possible to define constructions that are either impossible to manufacture or very difficult an expensive to make.
 
There are several ways to make blind vias. One method uses two cores(only applicable for 4layer PCB), 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.
 
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, only to adjacent layer. The other method of making blind vias is to drill partly through the board to the desired layer. This method has the advantage that you can make blind to the other layers but has the limitation on the depth of the hole that can be reliably plated.

2. Mind Clearance

Specifying tight track width/spacing simply decreases the yield from a panel but raises costs. We recommend that you use 5mil (or higher) track/space but in general the larger the track/space, the higher the reliability and the lower the cost. On Multilayer boards, at least 3mil clearance from internal pad edge to the internal copper plane is required, ideal is 8mil or above. This will almost certainly avoid the chance of internal shorting or breakout.
 
 

3. Balancing Copper area and copper weight at each layer

In general, copper weight of the layers should be the same on the matching layers to reduce this problem. For example, on a 8 Layer board, Layer 1 and Layer 8 should have similar copper area and copper weight. Layer 2 and Layer 7 should follow the same rules.
 
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.