DFM for Multilayer

Multilayer circuit boards require some considerations that do not apply to, or are different than those for double sided work.

Board Outlines - It is recommended that all the (inner & outer) layers have complete outlines.

Copper areas - on inner layers, all copper should be kept at least 0.010" from the outer edge of the board.

Relieving vias on inner layers (antipads) - allow for clearance around pads unconnected to the layer. This clearance should be at least 0.020" greater than the hole.

Thermal pads the tie should be a minimum of 0.012" while 0.015" is preferable. The greater clearance allowed in these circumstances, the higher the yield for production orders will be.

Routing the inner layers- allow for greater clearance between pads and traces.

This is to take into account the manufacturing tolerance buildup - ie the maximum allowable misregistration combined with the max. allowable etchback.

- Thermal and antipad clearances on the power &/or ground plane layers of multilayer boards.

If layer to layer registration, tolerances for hole size, location ,film expansion and registration have not been taken into account then manufacturing will experience high reject rates and increased costs.

Antipads should be 0.023 to 0.030" larger than outer layer pads. For thermal pads the moat should be a minimum of 0.012 ( preferable 0.015) and pads should be equal to or larger than outer layer pads.

- Restrictive tolerances for multilayer board thicknesses.

Specifying tolerances that are very tight can result in higher costs, increased time and lower yields. It is suggested to request restrictive tolerances only when they are critical.