Builds that use adhesive systems rely on adhesive to bond the copper to the flex core. As such this adhesive layer along with full coverlay, penetrates into the rigid construction and directly into the hole. The greater expansion coefficient of the adhesive increases the risk of through hole problems, such as cracks and via hole failures as a result of the expansion, while having this material within the hole itself can lead to hole wall copper formation issues resulting from poor desmear of adhesive.
The solution to improving via / through hole reliability was to move towards an adhesiveless copper clad system (copper bonded directly to the polyimide) and limit the overlap of the coverlay into the rigid part of the board. This ensures a much more reliable through hole structure without adhesive penetrating into the hole. This is by far the most common approach to rigid flex.
