As printed circuit boards (PCBs) evolve, they are gradually changing over from rigid boards to flexible types. With increasing demand from medical, military, aerospace, and industrial markets, manufacturers are gearing up to produce flexible circuits board along with rigid ones. Rush PCB has the technology and expertise in place for producing various types of flexible boards to meet the market demands.
The major advantage flexible circuits offer designers is the capability to outfit products with a suitable circuit when the design of the product will not admit using a rigid circuit board. That makes flexible circuits invaluable to several applications in the electronics industry.
Although flexibility may be a desirable feature in most applications, some parts of the circuit may need to be stiff as well. For instance, if the area where a connector needs soldering on the PCB is not stiff, there is a potential risk of damage to the PCB with the insertion and removal of the connector. As the board is soft and flexible, the force of insertion and removal of the connector falls on the soldered pads, causing them to lift off the board, thereby damaging it. With a stiff board, the force distributes itself over the entire area and does not damage the solder joints on the circuit.
The necessity for PCB Stiffeners
Rush PCB UK adds stiffeners in flexible circuits to reinforce areas containing components and where rigidity is necessary, but not in areas where the board will flex. There are various instances where a flexible circuit needs to have rigid areas. These include:
- Presence of components near a flex zone
- Increasing the abrasion resistance of the flex circuit
- Specific areas of the board need mechanical strengthening
- Maintaining the required thickness in the flex circuit
- Supporting components and connectors on the PCB
- Constraining flexibility to parts of the circuit that require it
- Enabling better handling of the flexible board
- Keeping specific areas of the flexible circuit stable and flat
- Improving heat dissipation and providing strain relief
- Meeting Zero Insertion Force (ZIF) connector specifications
- Increasing and constraining the bend radius at the intersection of flex and rigid parts of the circuit — for avoiding stress on the flex part during repeated bending operations.
The basic reason for the use of stiffeners is when the flexible circuit requires a rigid area as protection against damage maybe from connectors or components attached to it. As the stiffener does not allow the circuit to bend, it protects the integrity of the solder joints in the area.
One may not need to use stiffeners if the flex circuit only has small SMD components that do not exert too much strain on the flexible part. Also, if there are no components mounted on the PCB in the flex region, the designer may decide not to use any stiffener.
Materials for Stiffeners
Although FR4 is the most common material Rush PCB uses for making stiffeners, they also use Polyimide, Aluminum, and Stainless Steel as the application demands. FR4 and Polyimide stiffeners may also have additional features such as pads, plated through holes, along with mounted components.
Although stiffener thickness may range from 8 mil to 59 mil (0.008”-0.059”), manufacturers prefer to use them in standard thicknesses of 20 mil (0.020”), 31 mil (0.031”), 39 mil (0.039”), and 59 mil (0.059”). The requirement for a specific thickness of the stiffener depends on the application, but in general, thicker the stiffener, better the support it offers the flexible PCB.
For instance, a ZIF connector requires the area of the flex circuit that plugs into it to be stiff, and manufacturers achieve this by using Polyimide stiffeners at the contact fingers. Typically, the PCB thickness requirement for ZIF connectors is between 0.2 and 0.3 mm. Allowing for the thickness of the actual flex PCB material, the stiffener thickness at the contact fingers can vary between 2 mil and 8 mil (0.002” and 0.008”).
Certain applications demand higher stiffness and manufacturers use Aluminum or Stainless-Steel stiffeners for the purpose. Metal stiffeners have the added advantage of acting as a heat sink if necessary. However, adding metal stiffeners increases the cost of the flexible circuit considerably.
Read Also: New Configurations for Rigid-Flex PCBs
Application of Stiffeners
It is possible to use stiffeners for components requiring plated through holes (PTH). Rush PCB recommends using the stiffener on the side of the flex board on which the components will mount, as this will allow access to solder pads on the flex circuit, making assembly easier. Designers should always enlarge holes on the FR4 stiffener to about 0.3 mm larger than those on the flex circuit, as this will ensure good alignment.
Rush PCB also recommends making flex circuits in the form of arrays, while including FR4 stiffeners all along the array border. This helps to make the array rigid enough to allow it to run through the automated assembly processes without the need for any additional tooling plates.
Although manufacturers typically attach stiffeners to the circuit using heat and pressure, Rush PCB recommends attaching stiffeners using pressure sensitive adhesives. Among the several varieties of such adhesives available to meet specific performance requirements, two of the more popular types are thermally bonded adhesives and pressure sensitive adhesives.
Manufacturers select the type of adhesive to use depending on the location and configuration of the stiffener. For instance, if the stiffener size does not extend beyond the flexible board outline, pressure sensitive adhesive will be the best method for attaching it to the circuit. The manufacturer decides the specific pressure sensitive adhesive to use based on whether the flex PCB will undergo automated an automated reflow cycle, and or to what material it will finally adhere. Stiffeners often require their own prepreg lamination cycle, with certain stiffener materials requiring additional lamination cycles as well.
Bonding Between Rigid and Flex Circuits
Manufacturers prefer to use no-flow prepreg as the typical material for bonding flex and rigid material, and is most common in standard FR4 or Polyimide material. Depending on the application, a manufacturer may attach stiffeners to only one side or to both sides of the flex PCB. When attaching stiffeners to both sides, Rush PCB recommends a review by the designer to avoid complications at PCB assembly stages.
A single flex circuit design may require multiple stiffener thicknesses. This type of design has some limitations, and may need a thorough review of the PCB supplier for their capabilities.
Strain Relief for Stiffeners
Although designers add stiffeners to certain areas of a flex circuit to relieve the strain on the flexing part, the stiffener itself may need strain relieving, especially where the circuit extends beyond the stiffener. Rush PCB recommends filleting the transient edge with a resilient adhesive or epoxy for relieving such strains. Providing strain relief at the edge of a stiffener helps to prevent stress risers from occurring at transition areas such as from the flex to the rigid.
Stiffening some part of a highly flexible circuit may seem contrary to its functionality. However, the ability to do so helps the durability and reliability of the fragile circuit in many ways. Rush PCB recommends that designers use a stiffener mainly to relieve the strain on the flex circuit at a specific area and thereby extend its working life.