During CAM preparation of a 2-layer flexible circuit measuring 15.66 mm by 154.1 mm, our team determined that the customer-supplied stackup could not be fabricated within our process window. The board uses rolled copper, is listed at 0.13 mm finished thickness with 1 oz and 1/3 oz copper, carries yellow solder mask, requires 50 ohm impedance control together with a report, and is to be laser-formed as single pieces. Two PI stiffeners of 0.1 mm are specified, immersion gold is called out at 3.00 µm, and strong adhesive is indicated. These parameters describe a very thin FPC in which dielectric construction, copper type, and local reinforcement must be reconciled with both impedance targets and assembly constraints.
The main engineering proposal was to replace the original stackup with a factory construction that still satisfies board thickness and impedance requirements. The suggested build uses adhesive-free electrolytic copper, controls the FPC itself to 0.23 mm ±0.03 mm, and targets a combined FPC-plus-FR4 thickness of 0.45 mm ±0.05 mm. In parallel, the 50 ohm impedance traces were adjusted from 0.17 mm to 0.075 mm to match the selected materials. Additional notes addressed two customer-raised concerns: stiffener placement over vias was optimized to avoid open circuits, and the circuit is produced with a coverlay-plus-local-ink process.
2-Layer FPC Stackup Optimization for Manufacturing Compatibility
The stackup supplied with the order ( #FPC-20260418-0008 ) could not be realized with available materials and process parameters while still meeting the thickness and impedance requirements. We therefore proposed an alternative construction and requested confirmation. On a 2-layer FPC the dielectric options are limited; rolled copper and adhesive systems must be paired with standard polyimide gauges that can be laminated, laser-formed, and finished at the target thickness. When the customer drawing falls outside those combinations, the factory must substitute a buildable stack that preserves the electrical and mechanical intent.
The proposed stackup holds the FPC total thickness at 0.23 mm ±0.03 mm. This value is thicker than the 0.13 mm listed on the order form yet remains within a range that can be achieved with adhesive-free constructions and still supports 50 ohm impedance. The combined thickness with FR4 stiffening is given as 0.45 mm ±0.05 mm, indicating that local rigid reinforcement is contemplated even though the order form lists PI stiffeners. Confirmation of the drawing allows the material set to be fixed before impedance compensation and laser-forming tools are generated.
Figure 1: To meet both board thickness and impedance requirements, we recommend using adhesiveless electrolytic copper and follow
Adhesiveless FPC Construction with Electrolytic Copper
The suggested construction specifies adhesive-free electrolytic copper as the base material. Adhesive-free (adhesiveless) laminates eliminate the bondply or adhesive layer that can introduce thickness variation, resin flow, or stringing during lamination and laser cutting. Electrolytic copper, while different in grain structure from the rolled copper listed on the order, is commonly available in the thin gauges needed for a 0.23 mm 2-layer flexible PCB and provides a consistent surface for imaging fine impedance traces.
The change addresses process compatibility while retaining the ability to meet the impedance and thickness windows. Because the order already indicates strong adhesive as a separate category, the adhesive-free designation applies to the base laminate rather than to any pressure-sensitive adhesive that may still be applied for final assembly. Acceptance of the material substitution locks the dielectric constant and thickness used for the 50 ohm calculation and removes the adhesive-related variables that rendered the original stackup unmanufacturable.
50 Ohm FPC Impedance Control: Trace Width Adjustment from 0.17 mm to 0.075 mm
The FPC requires 50 ohm characteristic-impedance control. With the materials selected for the proposed stackup, the impedance trace width was reduced from 0.17 mm to 0.075 mm. We submitted the adjustment for confirmation. In a thin adhesiveless construction the dielectric spacing between the signal copper and its reference is small; a narrower trace is therefore required to keep the impedance at 50 ohms. The original 0.17 mm width corresponded to a thicker or higher-dielectric-constant stack that is no longer used.
Figure 2: We adjust the trace width from 0.17mm to 0.075mm based on the selected material.
The 0.075 mm geometry remains within the 4 mil general trace/space capability listed on the order and is compatible with the etch characteristics of the electrolytic copper. Because an impedance report is required, the revised width becomes the controlled dimension against which the finished circuits will be measured. Confirmation of the change ensures that the artwork generated from the new stackup will meet the 50 ohm target within the agreed tolerance once the board is fabricated at 0.23 mm ±0.03 mm.
FPC Via Placement Optimization to Prevent Open Circuits Under Stiffeners
One customer-raised concern was that stiffener bonding over vias could produce open circuits. We optimized the via locations so that conductivity is preserved and noted the change for confirmation. When a stiffener—whether PI or FR4—is laminated or bonded directly above a via, the pressure and heat can deform the via barrel or crack the plating, especially on a thin 2-layer FPC. Relocating the vias slightly outside the stiffener footprint, or ensuring adequate annular ring and fill before bonding, removes the mechanical risk without altering the net connectivity.
The order lists two PI stiffeners of 0.1 mm; the engineering note also references an FR4 combined thickness, indicating that the reinforcement scheme may include both materials. In either case the optimization keeps the vias electrically continuous after stiffener attachment. The adjustment is local and does not affect the impedance traces or the overall stackup thickness. Acceptance of the relocated vias allows the drill and stiffener artwork to be aligned before bonding fixtures are prepared.
FPC Coverlay and Local Solder Mask Ink Process
The second customer-noted point was that the FPC is fabricated with a coverlay-plus-local-ink process. Coverlay (polyimide film) provides durable outer insulation and precise openings, while local ink (solder mask) can be applied in selected regions for finer pad definition or additional protection. The order form specifies yellow solder mask; the hybrid process uses coverlay as the primary cover and adds ink only where the design or assembly requires it.
On a 0.23 mm adhesiveless FPC this combination maintains flexibility in the unreinforced areas while giving controlled openings at connectors or impedance launch points. The local ink does not alter the base stackup thickness outside its application zone. Confirmation of the process simply records that the customer accepts coverlay as the main cover layer with supplementary ink, allowing the corresponding artwork and lamination sequence to be finalized together with the revised stackup and impedance geometry.
2-Layer FPC CAM Review: Stackup, Impedance, and Manufacturing Adjustments
The elements of the main engineering communication form a single manufacturing proposal for this 2-layer FPC. The original stackup is replaced by an adhesive-free electrolytic-copper construction that can be produced at 0.23 mm ±0.03 mm (FPC) and 0.45 mm ±0.05 mm (with FR4), thereby satisfying process limits while preserving thickness and impedance capability. The dielectric change requires the 50 ohm traces to narrow from 0.17 mm to 0.075 mm. Independently, via positions are optimized so that stiffener bonding does not open circuits, and the circuit is documented as using coverlay plus local ink.
Each item requires customer confirmation because it modifies material type, finished thickness, trace width, via placement, or cover method relative to the supplied data. Once accepted, the working stackup, impedance artwork, drill locations, and coverlay/ink films can be frozen. The board can then proceed through imaging, lamination to the proposed thickness, laser forming, stiffener attachment, and immersion-gold finishing, with the impedance report generated against the 50 ohm target achieved at 0.075 mm line width. The case shows how CAM review of a very thin 2-layer FPC commonly resolves an unmanufacturable customer stackup by substituting a process-compatible adhesive-free build, adjusting the impedance geometry to match, and incorporating local via and cover optimizations that protect connectivity and assembly reliability.
