Define the installed shape and motion before asking for a flex quote. Philifast’s flexible PCB service page<\/a> covers the product category; this guide focuses on the radius decision that must travel from mechanical design to board fabrication and assembly.<\/p>\n A static flex<\/strong> is bent during assembly or installation and then generally stays in that shape. It may see handling or occasional service movement, so \u201cstatic\u201d is not permission to crease it. A dynamic flex<\/strong> bends repeatedly during normal operation, such as in a moving hinge or actuator. Its design needs an expected cycle count, motion range, bend direction and support arrangement. Altium’s static-versus-dynamic discussion<\/a> makes the mechanical distinction; JLCPCB’s radius guide<\/a> illustrates why dynamic designs call for a different margin.<\/p>\n The classification applies to each bend zone, not necessarily to the whole circuit. A product may have one tail formed once around a housing and another section moving with a hinge. Give these zones separate radii and duty cycles. Also distinguish operating motion<\/em> from installation: forcing a dynamic section through a smaller temporary radius during assembly can damage it before cycle testing begins.<\/p>\n Illustration: Radius is measured to the inside of the installed curve; the drawing should state exactly which bend and thickness are referenced.<\/em><\/p>\n The outer surface of a bent circuit stretches while the inner surface compresses. A tighter radius raises strain. But the stress at a copper trace also depends on where that trace lies within the stack, copper type and thickness, dielectric and adhesive system, layer count, coverlay, plated features, and the actual bend cycle. Two flex boards of the same nominal thickness can therefore have different useful radii or fatigue lives.<\/p>\n The concept behind this is the neutral bend axis<\/strong>: a region through the stack that experiences less tensile or compressive strain than the outer and inner surfaces. Moving copper closer to it can reduce conductor strain, while adding copper layers or a local stiffener changes both thickness and the axis location. Epec’s dynamic-flex design discussion<\/a> connects layer count, copper construction and fatigue. It is a useful design explanation, but its published radius multiplier reflects its stated construction assumptions; it is not a guaranteed limit for another supplier’s stack.<\/p>\n Likewise, compare guidance before copying a number. Pica Manufacturing’s flex discussion<\/a> gives different indicative multipliers by construction and application. The spread itself is evidence that \u201cminimum flex radius\u201d is underspecified without a stackup and duty cycle. Use a preliminary multiplier to discover that a concept may be too tight, then have the proposed construction reviewed and tested where life matters.<\/p>\n Published supplier multipliers assume particular constructions and use cases. They are useful for early feasibility checks, but the released value should be reviewed against the proposed stackup and life requirement. Ask the fabricator to confirm the minimum inside<\/strong> radius and any restrictions on bend direction, copper layers or repeated motion for the specific design. If the product cycles, agree on how life will be validated; a static bend rule cannot establish dynamic fatigue life.<\/p>\n For a moving hinge, specify the smallest radius anywhere in its travel, not only the nominal radius at the midpoint. The circuit’s effective length, anchor points and path through the enclosure determine where curvature concentrates. A generous bend radius on a drawing is ineffective if an adhesive, cable clamp or stiffener edge makes a smaller local bend in the actual product. A motion study or prototype fixture should represent the installed path.<\/p>\n Components, solder joints, vias, pads and stiffener edges can concentrate strain when placed in the active bend. Define a bend zone and component keepout in the mechanical and assembly drawing. Route conductors through the zone with geometry suited to the bend direction, and avoid abrupt width transitions or features that force a sharp crease. Keep the rigid-to-flex transition and the start of a stiffener away from the point of maximum curvature where possible; review the exact location with the supplier. Philifast’s stiffener process guide<\/a> explains why local reinforcement changes handling and geometry.<\/p>\n Trace routing needs a bend-direction review. Conductors running through the bending length experience repeated strain; abrupt corners, notches, sudden copper-width changes and aligned features can concentrate it. In multilayer flex, traces directly stacked above one another may behave as a stiffer beam, so ask the fabricator whether staggering is appropriate for the proposed construction. Do not add a solid copper plane or shielding layer across the bend simply because there is space in the layout: it changes bending stiffness. If shielding is necessary, include it in the stack review rather than treating it as a late artwork change.<\/p>\n Illustration: The keepout and transition positions are conceptual and must be dimensioned for the actual product.<\/em><\/p>\n Send a formed-state sketch or 3D model that identifies every bend, the inside radius, angle, bend direction, and installation sequence. State whether each bend is static or dynamic; for dynamic use, give expected cycles, travel, speed and operating environment. Provide the proposed stackup, material and copper details, component\/stiffener locations, and tolerances that govern the assembled envelope. Epec’s flex data checklist<\/a> likewise calls for the bend radius and application requirements.<\/p>\n The assembler also needs to know how the circuit is supported during paste printing, placement, soldering and test, and when the final forming step occurs. A component may be safe on the flat flex but strained by the forming fixture or installed shape. Philifast’s flex assembly overview<\/a> provides the broader process context. Ask fabrication and assembly engineers to review the same bend-zone drawing, rather than approving the board outline in isolation.<\/p>\n For validation, agree on what failure means and when to measure it. A dynamic qualification plan can record the prototype stack revision, bend fixture, minimum radius across the motion, cycle rate and count, environment, and electrical continuity checks during and after movement. Inspect representative circuits for conductor cracks or coverlay damage as agreed with the manufacturer. A successful flat-board electrical test before forming demonstrates connectivity at that point; it does not establish fatigue life. If the stackup, copper, coverlay, stiffener or installed path changes, reassess whether the earlier result still applies.<\/p>\n The radius shown on a flat fabrication drawing may not be the tightest radius in the product. A housing, adhesive, clamp, connector, coverlay transition or nearby stiffener can force a local bend after assembly. Review the flex in its installed state and over the full movement envelope. Identify where the active bend begins and ends, how the flex is anchored, whether it twists as it moves, and which surfaces can contact the circuit. The smallest local radius and the bend duty should be tied to a dimensioned product interface, not inferred from a nominal envelope.<\/p>\n The stackup also changes the mechanical problem. Copper layer count and placement, copper type and thickness, coverlay, adhesive, stiffener and plated features all contribute to the finished construction. Do not substitute a web multiplier for the fabricator’s review of the released stack. IPC-2223E is the flexible printed-board design document listed in the IPC revision table<\/a>; IPC-6013E is the corresponding flex and rigid-flex performance specification. Those references help identify the design and fabrication framework, but the product drawing still has to state the actual route and acceptance need.<\/p>\nStatic and dynamic are different duty cycles<\/h2>\n
![\"Cross-section](\"https:\/\/flj-pcb.com\/wp-content\/uploads\/2026\/09\/bend-geometry.png\")
<\/p>\nWhy a thickness multiplier cannot be copied blindly<\/h2>\n
Keep rigid features outside the active bend<\/h2>\n
![\"Plan-view](\"https:\/\/flj-pcb.com\/wp-content\/uploads\/2026\/09\/flex-pcb-bend-zone-keepout.png\")
<\/p>\nWhat to put on the drawing and in the RFQ<\/h2>\n
Measure the installed strain path, not only the free flex<\/h2>\n