Flatness plays a key roll in assisting with the automated process of assembly. Co-planarity falling out of specification can cause components to be placed improperly, can cause tomb stoning, or get damaged, and to make matters worse, potentially damage the nozzles on the assembly line.
Stack ups that are symmetrical will help avoid mechanical stress in the boards during thermal cycling. Designing boards with an even number of layers will minimize the potential for bow and twist. The most cost effective process is foil lamination.
Bow & Twist: The typical acceptance of deviation from flatness is 0.0075 – 0.015 in. /in.
STANDARD FOIL CONSTRUCTION SHOWN BELOW: