PCB Stiffener: A Mechanical Support for Flexible and Rigid-Flex PCBs

Flex and rigid-flex PCBs come in handy when making compact devices, but they have one main disadvantage: the flexible part is not very sturdy. Therefore, the circuits can get damaged when adding or soldering electronic components. However, if you introduce a supporting mechanism to keep the flexible sections stable, they will remain in one piece as you solder the components. PCB manufacturers often use stiffeners to hold and support these parts during the assembly process, ensuring durability and precision.

So what is a flex PCB stiffener? If your project involves using flex boards, read on to find out how this piece of hardware can make your work easier.

What is a PCB Stiffener/ Busbar?

A PCB stiffener is not a part of the circuit board. Instead, it is an external structure that provides mechanical support to the board. Doing so holds the flexible section of flex or rigid-flex circuits to enhance durability and reliability.

A PCB stiffener supporting a white connector for a flexible circuit

However, busbars are slightly different. They are dielectrically coated stiffeners that can carry ground and power distribution up to 128 amps. They eliminate the need to have heavy traces on the PCB, so they offer support and electrical conductivity.

When and Why Use a PCB Stiffener?

To understand the when and why of using a PCB stiffener, you need to know its purposes. They include:

• Component/ Connector area support
• Meeting ZIF connector requirements (Zero Insertion Force)
• Reducing component stress
• Heat dissipation using metal stiffeners (stainless steel or aluminum stiffener)
• Forming a flat surface for SMT component or pad placement
• Localized bend constraint(s)
• Increasing bond strength
• Making the panel rigid enough to pass SMT assembly for SMT carrier replacement
• Increasing overall thickness

A flexible circuit with a stiffener

PCB Stiffener Materials

PCB stiffeners usually have polyimide, FR-4, stainless steel, or aluminum as the construction material. As explained below, each has different characteristics and benefits that suit various applications.

Polyimide Stiffeners

Polyimide stiffeners are the perfect choice for increasing the ZIF connector thickness to give full support where fingers contact. In the process, they contour the component at the contact and provide tolerances in bending thickness.

These stiffeners also come in handy when you want to limit the bending ability of certain PCB areas to prepare for final assembly installation. Hole locations are perfect for polyimide stiffener enhancement because it provides additional wear resistance when issues arise.

Also, PI stiffeners provide high bond strength and solder resistance, and the ideal location for stiffening is on the backside of insertion gold fingers.

It is possible to pick the stiffener part that is not laminated or attached to the flex board. After selecting it, insert the flex board connector fingers to the ZIF connector/ socket.

ZIF connector and FFC

Source: Wikimedia Commons.

FR-4 Stiffeners

The primary purpose of using an FR4 stiffener is to:

• Provide flat support when the flex circuit board is in the reflow and pick-and-place processes
• Support areas with connectors attached
• Hold the parts where plated-through hole components get inserted to direct access to the solder pad.

In some instances, the holes on both the stiffener and flex circuit at the same positions might have the same diameters. If this occurs, enlarge the access holes (stiffener holes).

Flex circuit

You can enlarge them by at least 0.15mm on each side to prevent misalignment by manual bonding. In this case, the stiffener bonding process will require drilling, registration, and pre-tack of prepreg or adhesive.

Aluminum and Stainless Steel Stiffeners

The two main metal stiffeners (aluminum and stainless steel) are ideal for flex circuits requiring anti-corrosion resistance, robustness, formability, and capability. Metal usually stiffens the board around the connecting fingers for easy assembly.

However, the two are expensive and have a longer turnaround than polyimide and FR4. That said, aluminum is perfect for heat dissipation, while stainless steel is ideal when there is very little space for a stiffener.

The bonding process for aluminum requires drilling, milling, registration, and pre-stacking of the prepreg. On the other hand, stainless steel requires drilling, laser cutting, registration, and prepreg pre-stacking.

CMOS sensor on flexible printed circuit board

PCB Stiffener Thickness

Each stiffener material has various thickness choices, as shown below.

Advantages of PCB Stiffeners and Busbars

Strengthening

A busbar or stiffener gives structural support to a circuit board to correct sagging and warping on PCBs and backplanes. The license also helps prevent electrical shorts within the chassis of one panel or from board to board when stacked in complex circuits.

Rigidity

As the name suggests, a stiffener makes the board more rigid to prevent unwanted vibrations and bend.

Conductivity

In addition to providing structural integrity, a busbar can act as a conducting bus or multi-buses, especially if made using copper.

Power Distribution

Busbars deliver high electrical power distribution for ground circuits (on boards and power supply circuits).

Saves Cost

If you use a stiffener, you don't have to mount installation screws or rivets during the PCB assembly process.

How are PCB Stiffeners Attached to Flexible PCBs?

A flexible printed circuit board

There are two main ways of attaching a PCB stiffener to a flexible circuit board: Using Pressure Sensitive Adhesives (PSA) or Thermal Bonding. The two have the following differences.

The Difference Between Rigid-Flex and Rigidized Flex PCBs

A rigid-flex printed circuit board

A rigidized PCB features a stiffener to enable the easy assembly to strengthen certain flexible surface areas. Also, it has no traces on the rigid segment or pads, and the rigidized section only reinforces its specific location.

On the other hand, a rigid-flex PCB production is a hybrid circuit that combines a flexible and rigid board into one. Both parts have traces that manufacturers connect using through holes. Therefore, the aim is not to provide mechanical support but rather for electrical connectivity.

How to Pick the Best Stiffener PCB Manufacturing Company

Selecting the best PCB stiffener manufacturer will help you determine the cost and quality of the circuit board. Thus, when assessing the manufacturers, you should consider these criteria to pick the most qualified company.

• Check the company's profile. Do they offer high-quality, diverse PCB stiffener products? These are vital indicators to show you what to expect.
• Consider the manufacturer's quality management system certifications. They should have the resources, pedigree, and capacity to source and manufacture quality stiffeners using any of the materials described above to suit your needs.
• Some companies are very rigid when adopting new technology, so check whether the manufacturers are flexible to these advances. If they have state-of-the-art equipment, it means they adopt technology quickly and will produce quality stiffeners to suit your needs.
• Cost is always a significant factor to consider. Typically, manufacturers should have in-house sourcing systems for all their products to keep production costs as low as possible. As the client, you should not incur extra expenses associated with sourcing stiffeners from other providers.

If you are looking for the most reliable PCB stiffener manufacturer, OurPCB should be your go-to option. We provide high-quality manufacturing and excellent quality-of-service at the most affordable prices. Contact us today for more information.