In production we never run stiffener holes at the same finished diameter as the FPC drill holes. CAM always applies a positive compensation so the flexible PCB reinforcement hole ends up larger. The usual working range is +0.10 mm to +0.30 mm on diameter, chosen according to stiffener material and the bonding method that will be used on the floor. That clearance is the practical FPC stiffener hole tolerance we live with every day.
The stiffener is attached after the flexible circuit has already been drilled, etched and covered. Placement is done either by hand or with simple mechanical fixtures. Even the better fixtures still leave a registration window of roughly ±0.10 mm to ±0.15 mm. Add the dimensional change that occurs while the adhesive cures and the different expansion rates between polyimide and FR4 or aluminium, and a 1:1 hole pair has zero margin left. The stiffener edge will sit over part of the FPC hole on a percentage of the panels.
When the compensation is omitted, the first thing we see on the assembly line is that screws or connectors will not drop through cleanly. Operators start forcing the hardware, which tears the polyimide or lifts copper around the hole. In tighter connector areas the partial blockage can also leave insufficient annular ring on the FPC pad, so the joint fails electrical test or becomes a reliability risk later. Scrap rises, and the panels that can be salvaged need hand rework with a file or a secondary drill—both of which slow the shipment and introduce further dimensional variation.
The offset is not only lateral. During the bonding press the adhesive flows and the stiffener can shift a few tens of microns in any direction. Manual placement of small stiffeners is especially sensitive; an operator's finger pressure or slight rotation of the piece is enough to move the hole edge across the FPC copper. That is why a pure 1:1 design almost always generates DFM flags during panel review.
How the compensation number is chosen on the CAM side
We start from the finished FPC hole size that the customer specified and then open the stiffener hole. For FR4 stiffeners that will be CNC-drilled after bonding, the common rule is +0.15 mm to +0.20 mm. If the stiffener is pre-drilled or punched before attachment, we push the compensation toward +0.20 mm to +0.30 mm because the placement tolerance is larger. Aluminium stiffeners are usually punched or laser-cut; their compensation sits in the same band, but we also watch the punch clearance so the hole wall stays clean.
The exact value is not arbitrary. It is tied to the process capability of the bonding station that will run the job. When the factory uses optical alignment fixtures and controlled temperature presses, the lower end of the range is enough. When the job is high-mix, low-volume and the stiffeners are placed by hand, we stay at the upper end. CAM also checks that the enlarged stiffener hole does not cut into adjacent copper features or reduce the mechanical support the stiffener is supposed to provide.
FR4 stiffeners are most often drilled on a CNC after the adhesive has cured and the panel has been routed to size. The drill program uses the compensated diameter so that even if the stiffener sat slightly off-centre, the finished hole still clears the FPC hole. Aluminium pieces are frequently punched in a progressive die or cut by laser; both methods accept the same oversized diameter without extra cost. Polyimide stiffeners can be laser-cut with tighter accuracy, so the compensation can sometimes be reduced, but we still keep a minimum +0.10 mm to cover adhesive squeeze-out.
What the floor actually does when the data arrives
Once the compensation is locked in CAM, the stiffener artwork is released with the enlarged holes. During bonding the operators use simple pin fixtures or edge stops; they do not try to achieve optical-centre alignment on every piece. After cure, a sample of panels is checked with a pin gauge or by visual overlay under a microscope. If the remaining clearance falls below 0.05 mm on any side, the lot is flagged and the next bonding setup is adjusted. That closed-loop check is how we keep the FPC stiffener hole tolerance under control without slowing the line.
For aluminium stiffeners the punch tooling is ordered to the compensated size from the start. Changing a punch later is expensive, so the diameter decision has to be correct before the first tool is made. FR4 is more forgiving because CNC drilling can be reprogrammed if a mid-lot measurement shows the clearance is too tight.
When the factory will accept a tighter or 1:1 condition
A few situations allow us to relax the rule. If the customer's design uses large mounting holes (above 3 mm) and the stiffener is attached with a high-precision optical bonder, the compensation can drop to +0.05 mm to +0.10 mm. In rare cases where the stiffener is bonded first and the entire stack is then drilled in one pass, the holes can be the same size because registration is controlled by the drill machine itself. Those jobs are the exception; they require explicit process notes and usually a higher unit price because the bonding and drilling sequence changes.
For most day-to-day flexible PCB reinforcement hole work we stay with the positive compensation. It costs nothing in material, keeps the bonding station running at normal speed, and removes the scrap and rework that appear the moment the holes are forced to match 1:1. That is the practical manufacturing answer we apply on every panel that carries a stiffener.
