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How to specify stiffener requirements in flexible PCB design
Before selecting stiffener material for flexible-rigid combined PCB, please try to follow common laminate thicknesses, such as 0.010 inch, 0.047 inch, and 0.062 inch. Please note that each manufacturer's product inventory varies. This is why it is important to determine the stiffener thickness before ordering.
Reinforcements are widely used in various flexible circuit designs. A common use of reinforcements is to provide mechanical support for flexible PCBs, rather than being part of the circuit itself.
Adding reinforcements to flexible circuits may require several lamination cycles, which increases the overall cost of the design.
Although reinforcements do not play any electrical role in flexible circuits, they have unique design and material requirements.
The main purpose of using flexible PCB reinforcements
Reinforcements can perform a range of functions, including:
Stiffening flexible circuits;
Meeting ZIF (Zero Insertion Force) requirements;
Creating localized bending constraints;
Creating a robust surface for the mounting of electronic components and SMT pads;
Reducing stress on component placement;
Aiding in heat dissipation (these reinforcements are typically made of steel or aluminum);
Making the circuit rigid enough to complete the assembly process without manual input;
Polyimide Reinforcements
Polyimide reinforcements are used in circuits that connect ZIF connectors. Certain connector specifications require flexible circuits to have very specific thicknesses to work with the connector.
Common ZIF connector thicknesses are 0.2 mm and 0.3 mm. Additional polyimide reinforcements are used to increase the thickness of a localized area, rather than increasing the thickness of the entire board. Thickening the entire board can be very expensive and can compromise the reliability of the circuit.
Polyimide is particularly useful because it meets the tight tolerance specifications of ZIF connectors. Few materials compare to polyimide in meeting all the tolerances required for the desired connector.
FR4 Reinforcements
FR4 reinforcements form rigid parts in flexible circuits. They provide support for components and any connected connectors.
Adding FR4 reinforcements prevents the flexible circuit from bending. Note that adding reinforcements will increase the thickness of the board, up to 0.059 inches.
Aluminum and Stainless Steel
Aluminum and stainless steel reinforcements are primarily used to improve the heat dissipation capabilities of the circuit. Both materials are significantly more expensive than polyimide and FR4. Aluminum is the preferred material for heat dissipation. Stainless steel can also be used if there is not much space on the circuit board.
Adding Reinforcements
Reinforcements can be manually adhered to the flexible PCB with PSA (pressure-sensitive adhesive) or laminated under heat-press conditions (discussed below). Designers prefer lamination under heat-press conditions, but depending on circuit limitations, they may have to use PSA.
Hot melts create a permanent bond between the reinforcement and the circuit. They are more cost-effective than pressure-sensitive adhesives for designers.
Specifying Reinforcement Requirements in Flexible PCB Design
Let's look at various scenarios where reinforcements may be needed and the methods.
Scenario 1:
Applying rigidity across the entire circuit board where components are mounted
Protecting solder joints by increasing the rigidity of the component area
Methods and Materials
Depending on the budget, materials such as polyimide, aluminum, stainless steel, and FR4 can be used.
Reinforcement thickness is determined by the designer.
Reinforcements can be adhered with pressure-sensitive adhesive or thermally bonded with flexible adhesive
Scenario 2:
Locally increasing board thickness using ZIF connectors to meet ZIF connector specifications.
Methods and Materials
The only material choice for ZIF connectors is polyimide.
Designers can choose from a variety of thicknesses to meet design requirements.
Scenario 3:
Creating rigid parts in flexible circuits to meet specific bending requirements, aid in board assembly, or other requirements.
Methods and Materials
Two popular choices are polyimide or FR4
Designers can choose from a variety of thicknesses
Two application methods are pressure-sensitive adhesive (PSA) and thermal bonding with flexible adhesive
Scenario 4:
Heat dissipation for flexible PCBs, ensuring component integrity and performance
Methods and Materials
Aluminum and stainless steel are two good choices for heat dissipation of electronic components and flexible circuits. Stainless steel can be used for smaller circuits due to design limitations.
Designers can choose from a variety of available thicknesses depending on the budget.
Points to Consider When Using PCB Reinforcements
A useful design tip is to overlap reinforcements by at least 0.030 inches to reduce handling time during the manufacturing process and avoid stress thickness.
Before selecting reinforcement materials for flexible and rigid-flex PCBs, try to follow common laminate thicknesses such as 0.010 inches, 0.047 inches, and 0.062 inches. Note that each manufacturer's inventory varies. This is why it is important to determine the reinforcement thickness before ordering.
Summary
The major advantage of PCB reinforcements is the protection of components and connectors. This protects your circuit integrity and joints.
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