During CAM preparation of a 4-layer flexible circuit measuring 475 mm by 78.5 mm, our team documented several process parameters and artwork adjustments that required customer confirmation. The board uses rolled copper, is listed at 0.35 mm finished thickness with 1/2 oz and 1 oz copper, carries yellow coverlay on both sides, and is to be laser-formed. No stiffeners are specified, via cover with solder mask is indicated, hole size is 0.3 mm, and the surface finish is immersion gold. Delivery contemplates panelization with 4 mm process edges and routing separation. These parameters describe a relatively large FPC in which copper thickness, total build, via filling, legend placement, and panel format must be aligned before release.
The main engineering items centered on confirmation of inner-layer copper at 1 oz (35 µm ±8 µm), outer-layer finished copper plated to 1 oz (35 µm ±8 µm), double-sided yellow coverlay, an FPC total thickness of 0.4 mm ±0.1 mm, and full electroplated filling of all 0.3 mm drilled holes including via-in-pad. Additional confirmations covered optimization of characters that originally landed on pads and production of the panel at 487.90 mm by 90.50 mm for one piece.
4-Layer FPC Copper Weight and Finished Thickness Confirmation
We proposed inner-layer copper of 1 oz (35 µm ±8 µm) and outer-layer finished copper plated to 1 oz (35 µm ±8 µm), together with double-sided yellow coverlay and an overall FPC ( #FPC-20260420-0032 ) thickness of 0.4 mm ±0.1 mm. The order form lists a nominal thickness of 0.35 mm and copper weights of 1/2 oz and 1 oz; the engineering note therefore refines both the copper call-out and the finished thickness into a controlled window. On a 4-layer rolled-copper FPC the inner layers are typically formed from the base foil, while the outer layers receive additional plating to reach the final thickness. Specifying both at 1 oz with a ±8 µm tolerance establishes a uniform copper weight that supports plating uniformity and subsequent coverlay lamination.
Figure 1: Final FPC thickness is 0.4+/-0.1MM.
The 0.4 mm ±0.1 mm total thickness accommodates the two copper layers, the polyimide dielectrics, and the double-sided yellow coverlay while remaining compatible with laser forming. The wider tolerance reflects normal variation in adhesive or coverlay thickness on a large-format flex. Because no impedance requirement is listed, the copper weights and thickness primarily affect mechanical stiffness, current-carrying capacity, and the flatness needed for immersion-gold finishing. Confirmation of these values locks the material stack and plating target before inner-layer imaging and outer-layer plating are released.
Full Copper Filling of 0.3 mm Vias and Via-in-Pad
All 0.3 mm drilled holes, including those located in pads (via-in-pad), were specified to be fully electroplated and filled. We requested confirmation of this requirement. On a flexible PCB, via fill by electroplating produces a solid copper plug that eliminates voids, improves reliability under flexing, and provides a flat surface when the via sits inside a pad that will receive solder or a connector. The 0.3 mm diameter is within the range that can be filled by controlled plating without excessive dimpling or over-plating on 1 oz copper.
Figure 2: All 0.3mm holes and via in pad should be electroplated and filled
Because the order already indicates via cover with solder mask, the filled vias can be covered or left exposed according to the final mask artwork; the fill itself ensures electrical continuity and mechanical integrity regardless of the cover method. Full fill also prevents chemical entrapment during subsequent immersion-gold processing. Confirmation of the fill requirement allows the plating cycle and via-fill inspection criteria to be set before the outer layers are processed. The large 475 mm length of the circuit makes uniform plating current distribution important; the specified copper weights and fill process are therefore linked in the same confirmation.
FPC Silkscreen Optimization for Pad Clearance
Original silkscreen characters, shown in magenta, landed on pads. We optimized the legend by deleting some characters and moving others so that no ink remains on the pads; the resulting production artwork is shown in yellow. Confirmation was requested. White silkscreen on the top side must remain clear of all exposed copper that will receive immersion gold or solder. Characters that overlap pads can contaminate the surface finish, interfere with solder wetting, or create cosmetic defects after assembly.
The optimization removes or relocates only those elements that conflict with pads while retaining as much reference information as space allows. Because the bottom side has no silkscreen and the circuit is delivered with process edges, all retained characters must lie inside the final outline and clear of the 4 mm process-edge zones if they are to survive routing. Acceptance of the yellow production artwork finalizes the silkscreen data and prevents later requests to restore characters that would again overlap pads.
Figure 3: We deleted and moved some silkscreen to avoid silkscreen on pad, yellow part are adjusted silkscreen
FPC Panelization Design: 487.90 mm × 90.50 mm
Panelization was proposed at 487.90 mm by 90.50 mm for a single piece. The order indicates panel delivery with 4 mm process edges on all sides and routing separation. The suggested dimensions accommodate the 475 mm by 78.5 mm circuit plus the process edges and any additional margin required for tooling holes or handling. A single-piece panel of this size simplifies registration for a large-format FPC while still providing the process edge needed for plating and laser or routing operations.
Because the delivery type already contemplates factory panelization, the proposal refines the exact outline rather than introducing a new format. Confirmation of the 487.90 mm by 90.50 mm panel locks the working array, process-edge width, and routing paths before the job is released. The same panel size also supports uniform current distribution for the 1 oz plating and via-fill process described above.
Key CAM Confirmations for 4-Layer FPC Manufacturing
The items in the main engineering query form a coherent set of process definitions for this 4-layer rolled-copper FPC. The copper weights of 1 oz on inner layers and plated 1 oz on outer layers, together with the 0.4 mm ±0.1 mm total thickness and double-sided yellow coverlay, establish the material stack. Full electroplated fill of all 0.3 mm holes, including via-in-pad, ensures via integrity within that stack. Character optimization removes legend from pads so that the immersion-gold surface remains uncontaminated. The 487.90 mm by 90.50 mm panel provides the format in which these processes are executed.
Each element requires explicit customer confirmation because it either refines the thickness and copper call-outs beyond the order-form values, adds a via-fill requirement, modifies silkscreen artwork, or sets the panel outline. Once accepted, the stackup, plating and fill specifications, silkscreen data, and panel dimensions can be frozen. The circuit can then proceed through inner-layer imaging, lamination, outer-layer plating with via fill, coverlay application, silkscreen, laser forming or routing, and immersion-gold finishing. The case illustrates how CAM review of a large-format 4-layer FPC commonly consolidates copper weight, thickness, via fill, legend clearance, and panel size into a single confirmation set that aligns the design with process capability before fabrication begins.
