PCB Tech - What are the cost drivers of Flex PCB?

Cost optimization is one of the main goals when designing a PCB. In order to achieve this goal, every designer should understand the main cost drivers of the circuit board. The main cost drivers of flexible PCB include board material, number of layers, effective panel utilization, rib type and surface finish.

What is driving the cost of flexible PCB?

The total cost of the circuit board mainly depends on the type of circuit board (rigid or flexible), the materials used, the number of layers in the stack, and the combination of HDI or ELIC (interconnect per layer) structure. The following are some of the parameters that affect the overall price of flexible boards:

Circuit board material

The PCB material used for manufacturing is one of the main cost drivers of flexible boards. Standard rigid boards are laminated using FR-4 substrate. Polyimide substrates are the most commonly used materials for manufacturing flexible cores and cover layers. Compared with standard FR4 laminates, flexible substrates have better thermal and electrical properties. The thickness of the flexible material is uniform across the substrate. These substrates also provide improved D k values between 3.2 and 3.4. The lack of braided glass reinforcement reduces the change in D k. Generally, the thickness of the flexible layer is between 1 and 5 mils. The cost of flexible laminates may be 2 to 3 times that of standard rigid materials.

Adhesive-free and adhesive-based flexible core

The adhesive-based flexible core uses a flexible adhesive layer to connect each copper layer to the polyimide core under heat and pressure. Adhesives are usually based on epoxy or acrylic and range in thickness from 0.0005" to 0.001". These materials are cheaper than binder-free materials.

The copper is directly bonded to the polyimide core, eliminating the need to use adhesive in the adhesive-free flexible core. Although adhesive-free materials are expensive, they have many advantages, such as reduced deflection thickness (due to the absence of an adhesive layer), increased temperature ratings, and excellent plated hole reliability.

Factors affecting the choice of flexible PCB materials

Thermal reliability: Always choose materials that meet the application temperature expectations. If the circuit board is intended to operate in a high-temperature environment, the material should be able to withstand high-intensity heat without transferring the heat to adjacent components.

Mechanical properties: This factor determines the material's ability to withstand physical stress during assembly or operation. The bending radius is one of the important physical parameters of the flexible PCB.

Signal performance: It is the ability of a material to promote uninterrupted signal propagation during the entire operating cycle. This is essential in high-speed and controlled impedance boards.

If cost is the main consideration, avoid over-specifying material requirements.

Number of circuit layers

The number of layers is another important cost driver for flexible PCBs. As the number of circuit layers increases, the total cost of the circuit board increases. As more layers are added to the board, the lamination process becomes complicated. This will take longer and require more material supplies. The processing problems that occur due to the high number of layers are:

Layer-to-layer alignment

Plated through hole integrity

Thermal expansion on the z axis

Lamination defect

The shape and size of the circuit board (circuit board outline) are determined by the PCB designer. The larger the surface area, the higher the price. When multiple panels are manufactured in one panel, the surface of the panel should be effectively used. The flexible board is manufactured into different shapes, such as square, rectangle, circle and many random shapes. Manufacturing random-shaped PCBs may increase costs because the panel utilization rate may decrease.

Trace width and spacing

Signals need proper trace width to propagate through traces without the risk of overheating. The smaller the pitch, the more difficult it is to reliably etch traces and pads. This increases the total cost of the circuit board.

Copper foil thickness

The cost of the circuit board increases as the thickness of the copper layer increases. When a thick copper layer is implemented on the inner layer, a larger amount of prepreg is required during the lamination process. These prepregs avoid insufficient resin by filling the gaps between the copper parts. When the inner layer has more than ½ ounce of copper and the outer layer has more than 1 ounce of finished copper, the price of PCB will increase.

Another disadvantage of using thicker copper is that sufficient spacing must be maintained between the traces. Thicker copper requires wider trace width. However, due to additional processing costs, the use of very thin copper (less than 1/4 ounce) can cause costs to skyrocket.

The diameter of the standard bit is 8 mils, and the diameter of the advanced bit is 5 mils. The smaller the drilling size, the longer the drilling duration. This leads to higher total costs.

Drilling to copper in flexible PCB

Drilling to copper is the distance from the edge of the drilled hole to the nearest copper feature (pad, pouring, trace, etc.) on the layer. Typically, the hole to copper gap is 8 mils. The smaller the copper hole, the more expensive the PCB manufacturing process.

Surface treatment

A PCB surface finish is the metal-to-metal connection between the printed circuit board and the bare copper of the solderable area of the component. The surface finish should be selected in a way that improves product performance, while considering cost.

Types of PCB surface treatment

ENIG (electroless nickel/immersion gold): This is the most common type of circuit board surface treatment. It is very popular because it does not change color and is reliable. ENIG is more expensive than other surface treatments, but it provides excellent PCB solderability. This surface finish can sometimes lead to the formation of black mats. This is where phosphorus accumulates between the nickel layer and the gold layer. Broken and incorrect circuit board connections are possible results.

Immersion Silver: Silver is a strong finish material that can be used in a variety of applications. Due to its growing popularity, the material has become more free, thereby reducing and stabilizing its price.

Immersion tin: This surface treatment involves depositing a thin layer of tin on the circuit board. Immersion tin provides a consistent finish. The disadvantage is that the life span is short. However, it provides excellent performance at an optimal cost.

Reinforcing rib type

The total cost of a flexible PCB also depends on the type of stiffener used. In flexible design, stiffeners are used for the following goals:

Modify thickness to meet ZIF connector specifications

Component/connector area support

Promote heat dissipation

FR4 and polyimide are the two most common reinforcement materials. Although aluminum and stainless steel are used in some designs, they are more expensive than FR4 and polyimide. Aluminum is a popular material for heat dissipation applications.

The ribs are bonded by thermal bonding adhesive or pressure sensitive adhesive (PSA). Thermal bonding adhesives are preferred, although design constraints may require the use of PSA. Thermal adhesives are flexible epoxy or acrylic adhesives that are used to connect the cover layer to the flexible circuit and provide a permanent bond. They are cheaper than PSA.

How to reduce the overall cost of flexible and rigid-flex PCB

Keep as few layers as possible

As the number of design layers decreases, the number of prepreg layers required also decreases. Limiting the number of layers of flexible circuits will reduce the overall cost. At the same time, fewer layers improve the manufacturer's ability to increase manufacturing output.

Use rigid substrates to achieve overall thickness in rigid-flexible design

If you need to achieve a specific total thickness, use rigid board laminates instead of no-flow prepregs or flexible laminates. Flexible laminates are more expensive than rigid laminates.

Efficient panel utilization

Efficient flexible panelization to reduce overall costs

A panel or circuit board array is a large piece of material that contains multiple independent PCBs. The overall manufacturing cost can be reduced by effectively using the surface area of the panel.

Compared with through holes, blind vias and buried vias require more manufacturing steps, which increases processing time and reduces yield. Reducing the number of vias on the flexible board can reduce the total cost.

Cost optimization should be considered in the initial stage of circuit board design. Determining the cheapest flexible PCB cost drivers requires accurate design solutions and clear engineering strategies. Understanding the pros and cons of using specific materials, drilling techniques, stiffener types, trace width and spacing, etc. can help prevent future production failures. Planning ahead and having a consistent discussion with your PCB manufacturer will help you optimize time and cost.