Vias are at greater risk for peeling on flex designs. To reduce this risk:
• Make annular rings as large as possible.
• Vias should be tear-dropped.
• Adding tabs or anchors to vias, as shown, will also help prevent peeling.
• Vias are not reliable in areas that will be flexed.
• In a dynamic application, flexed vias can crack very quickly.
• Vias are okay over a stiffener (reinforcements), but vias just off the edge of a stiffener (reinforcements) are at risk for cracking.
• For rigid-flex designs, hole to flex distance is important. That is, the distance between vias and the rigid-flex transition area.
• Avoid going below 50 mils for high reliability applications.
• Keep in mind the rule most broken in rigid-flex designs: most manufacturers will not allow less than 30 mils for commercial applications.