Introduction
Flexible printed circuits (FPCs) are widely used to interconnect components in products like mobile phones, wearables, displays, medical devices etc. where flexibility is required. FPCs use thin and flexible polymer materials as the base substrate unlike rigid printed circuit boards (PCBs). However, this flexibility also makes FPCs prone to bending and twisting stresses during handling, assembly and use. Stiffeners can be attached to FPCs to provide localized reinforcement and protect certain areas from these stresses.
This article will provide a detailed overview of FPC stiffeners – their role, materials used, design considerations, attachment methods and applications.
What is a FPC Stiffener?
An FPC stiffener is a relatively thick, rigid structure that is fixed to the FPC using adhesives at strategic locations. It helps prevent excessive flexing, twisting or bending of the FPC in those areas during manufacturing, assembly and usage. Stiffeners protect delicate components as well as the FPC substrate itself from damage due to handling and mechanical stresses. They also provide stiffness for connectors and interface points to external devices.
FPC stiffeners are commonly made using materials like stainless steel, FR4, polyimide and acrylics. Adhesives such as epoxies and acrylics are used to bond the stiffener securely to the FPC. The stiffener often includes holes and cutouts to allow mounting of ICs, connectors, buttons and other components.
Need for Stiffeners in FPCs
While the thin, flexible construction of FPCs is advantageous for many applications, it also creates some unique challenges which FPC stiffeners help overcome:
- The thin polyimide substrate can get damaged by repeated flexing and twisting stresses during assembly and usage. Stiffeners reinforce areas prone to such stresses.
- Components on FPCs are also susceptible to failures and cracked joints due to excessive vibration or shock loads without support. Stiffeners protect fragile components.
- Flexible sections of FPCs by nature are prone to frequent handling and manipulation during fabrication, component placement and testing. Stiffeners prevent damage to copper traces and joints.
- Display driver ICs are sensitive to bending stresses due to the brittle silicon. Stiffeners support these components on display driver FPCs.
- Connectors and interface points to other PCBs or cables undergo mechanical stresses which can crack solder joints. Stiffeners prevent flexing damage.
- Stiffness is required during handling, clamping and assembly of FPCs using automatic equipment. Targeted stiffeners facilitate this.
- Stiffened areas can be grabbed or held firmly during manual assembly and repair procedures without causing damage due to bending.
- Components mounted on stiffened islands remain stable for inspection and rework. Flexible FPCs can distort from flatness.
- Stiffeners allow mounting of components on both sides of FPCs without short circuits due to touch. Open areas can be kept flexible.
FPC Stiffener Materials
Stiffeners designed for FPCs need to meet requirements like dimensional stability, mechanical stiffness, easy machinability or stamping, moisture resistance and adhesion compatibility with the FPC substrate and components. Here are some common materials used:
Plastics
Polyimide sheets offer a balance of stiffness, temperature resistance, dimensional stability and cost-effectiveness. Popular grades include Vespel, Duratron and Kapton. However, polyimide absorbs some moisture which may be unsuitable for high humidity environments. It has limited chemical resistance against solvents.
Acrylics like PMMA provide good moisture and chemical resistance along with stiffness and stability. PMMA stiffeners retain flexibility and do not become brittle over time. They adhere well to FPC substrates using acrylic adhesives. However, acrylics have relatively lower temperature ratings.
Polycarbonate offers high tensile strength for mechanical rigidity combined with temperature resistance exceeding 155°C. It also offers flame retardant properties required in some applications. Polycarbonate is prone to environmental stress cracking however.
LCP (liquid crystal polymer) grades like Zenite offer the best temperature resistance (>240°C) and stiffness among plastic options. LCP has excellent chemical resistance and ultra-low moisture absorption. But it is an expensive material choice.
Metals
Stainless steel offers extremely high stiffness and environmental resistance. It can withstand repeated flexing without cracking. Steel can be easily stamped or machined to desired shapes. It needs nickel/gold plating for soldering compatibility. Stainless steel substantially increases the weight of the FPC assembly.
Aluminum provides good stiffness-to-weight ratio and corrosion resistance while being lighter than steel. It is also low cost and easy to fabricate, but not as rigid as steel for the same thickness. Plating may be needed for soldering.
Copper has excellent thermal conductivity to dissipate heat from components. This lightweight metal stiffener can be readily soldered to the FPC copper traces without plating. But copper is prone to corrosion and needs protective coatings.
Kovar alloy has matching coefficient of thermal expansion (CTE) as silicon chips, minimizing solder joint stresses. However, kovar is not easily soldered and is mainly used as leadframes for ICs rather than stiffeners.
Brass offers good stiffness and manufacturability though not as strong as steel. It is prone to corrosion and needs protective nickel-tin plating typically. Soldering compatibility is also excellent.
Composites
FR-4 combines good stiffness, dimensional stability and cost-effectiveness. It can withstand processing temperatures over 150°C. Being fiberglass reinforced epoxy, it bonds well with the FPC substrate. However, FR-4 absorbs moisture and is not suitable for all environments.
CEM-1 laminates consisting of woven glass fabric in a cyanate ester resin provide the highest thermal capabilities as stiffeners, withstanding over 280oC. They have very low outgassing and moisture absorption. But CEM-1 material cost is also high.
Carbon fiber composites offer extremely high rigidity for their weight, along with corrosion resistance and ambient temperature tolerance exceeding 200°C. The high cost of carbon fiber is the main deterrent.
FPC Stiffener Design Considerations
Some key factors to consider during design of FPC stiffeners are:
Stiffness requirements – The amount of flex that needs to be prevented guides required stiffness. This helps select the material and thickness.
Area coverage – Partial patches or strips may suffice for localized reinforcement versus covering larger areas. Stiffener footprint impacts weight, cost and assembly considerations.
Component space – Cutouts, holes or voids need accommodation in the stiffener for mounting components, connectors, displays etc.
Routing provisions – Stiffener location should avoid blocking interconnection paths between components located on both sides of the FPC.
FPC material and layers – More layers and thicker dielectric require higher rigidity for equivalent stiffening. Stiffener adhesive selections depend on the FPC substrate.
Assembly and inspection – Stiffener should not hamper component placement, clamping, machine handling or manual assembly. Visual inspections and rework access need consideration.
Weight and thickness – Overall thickness after stiffener installation should meet product specifications. Weight limits steer material selection for wearable and portable devices.
Environment – Temperature range, chemical exposure, moisture levels and outgassing drive stiffener material selection.
Thermal dissipation – Heat dissipation may require thermally conductive stiffeners under high power components. Electrical and thermal insulation is needed for some applications.
Reworkability – Strippable stiffener adhesives may be preferred to allow rework of underlying components after stiffener attachment.
FPC Stiffener Attachment Methods
The stiffener must be securely fixed to the FPC substrate to prevent separation or movement during use. The common joining methods include:
Adhesive bonding – Acrylic, epoxy or polyimide adhesives are commonly used to bond the stiffener. Adhesive selection depends on materials and environmental needs. Die-cutting can create custom bondline shapes.
Soldering – For metal stiffeners, solder paste can attach and electrically ground it. Reflow attaches stiffener but risks heat damage to FPC substrate. Selective hand soldering is preferred.
Press-in – Interlocking press-fit features on metal stiffeners physically lock into the FPC substrate when compressed together. No adhesives are needed. Allows stiffener removal for repairs.
Clips – Metal spring clips or fasteners screwed into the stiffener mechanically clamp it to the FPC substrate firmly. Allows easy installation and removal.
Screws – Screws passing through clearance holes in the FPC substrate can thread into tapped holes in the stiffener to clamp it together. Supports reworkability.
Riveting – Rivets installed in the stiffener are peened over onto the FPC substrate to capture it. Allows stiffener removal by drilling out rivets.
Typical Applications of FPC Stiffeners
Some common applications where FPC stiffeners provide important benefits include:
Display driver FPCs – Stiffeners reinforce areas under driver ICs and connectors to prevent damage due to handling and vibration in devices like mobile phones, tablets, televisions etc.
Battery FPC interconnects – Mechanical robustness to withstand vibration and shocks over long device lifetimes make stiffeners essential for battery connector FPCs.
Medical products – For patient monitoring patches, implants and wearable devices, stiffeners allow robust interface connectors while keeping sensors flexible.
Automotive FPCs – Stiffeners used in cabling and sensors prevent damage due to continuous vibration and thermal cycles over vehicle lifetime.
Robotics – Frequent robotic arm and gripper movements require local stiffening on flex cables interconnecting movable sections for extended mechanical endurance.
Consumer Electronics – Stiffeners reinforce FPC connections to PCBs in smartphones, laptops and portable electronics that undergo drops and shocks.
Industrial – FPCs used in motion control systems, pneumatic valves, instrumentation etc. use stiffeners to create stable component mounting interfaces and robust cabling.
Benefits of FPC Stiffeners
Some key benefits provided by FPC stiffeners include:
- Reinforce connector areas for increased mechanical robustness and reliability
- Support fragile components like driver ICs and prevent failures
- Allow clamping and assembly handling of FPC without damage
- Improve stability for component placement, inspection and rework
- Prevent copper trace damage due to repeated bending stresses
- Absorb vibration and shock loads instead of transmitting them to FPC
- Facilitate manual and automated assembly and fabrication processes
- Allow double-sided component mounting for increased density without shorts
- Enable reliable performance over expected product lifetime
- Protect flexible cable segments from complex motion stresses
FAQs
Do FPC stiffeners help in heat dissipation?
Yes, choosing a thermally conductive stiffener under high power components allows heat spreading into the stiffener effectively. This reduces local hot spots.
Can multiple small stiffeners be used instead of one large one?
Yes, multiple discrete stiffener patches can be used for localized reinforcement and component protection instead of a single large stiffener.
Is the stiffener always in direct contact with FPC substrate?
Not always – standoffs can create an airgap between stiffener and FPC to facilitate inspection while still providing mechanical protection.
What tolerance is essential in FPC stiffener fabrication?
As stiffeners align FPC substrates for assembly, typical machining tolerance held is around ±0.10 mm. For placement near connectors or components, ±0.05mm tolerance or better is recommended.
How thick is a typical FPC stiffener?
Stiffener thickness depends on rigidity required but is typically in 0.3mm to 1mm range. Stainless steel can be as thin as 0.1mm but plastics need to be thicker for equivalent strength.
The importance of FPC stiffener
The Flex PCB stiffener refers to a stiffener attached to the back of the FPC when a widget such as a connector is attached to the FPC.
From the point of view of reinforcing materials, various materials, including metals and resins, can be provided depending on the application.
FPC（Flexible Printed Circuit）it is a soft and flexible structure. This is the biggest feature of FPC, but due to its soft structure, when connecting a small part such as a connector to an FPC, it is necessary to place a reinforcing plate on the back side of the electronic component to which the connector is attached.
But why do you want to put a stiffener? There are three main reasons for this. 1 solder resist off measure 2 solder pad drop countermeasure As shown in the right side of Figure 1, when the connector mounted on the FPC is inserted and removed, the FPC is soft, so external force is applied to the terminal portions of some connectors. As a result, the welded portion may fall off directly. Similarly, since the copper foil portion (pad) of the FPC may also easily fall off from the underlying structure, the base portion. This is where the mat falls off.
In either case, as shown in the left part of Fig. 1, as long as a sufficiently stiff reinforcing plate is placed on the FPC, it is possible to prevent the force from being concentrated on the terminal portion, and the solder/pad can effectively prevent the falling off.
3 Anti-substrate warpage countermeasures during reflow soldering
If the reinforcing plate is not attached during reflow soldering, the FPC will bend and the correct soldering may not be possible.
At the same time, even if it is difficult to weld, the mounting portion sometimes bends, which causes the mounting failure. (parts fall off)
In order to keep the mounting surface flat and securely mounted, the reinforcing plate is essential.
