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What Are Flex PCB Stiffeners - The Easiest Guide [2021]

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Sometimes it is necessary to enforce some parts of the flexible circuit board or FPC with stiffeners. PCB stiffeners are used to make a certain part of the board rigid so it can be easier to add/solder interconnects or components to the stiffer part.

The PCB stiffener is not an electrical piece of the PCB. It is simply applied with the purpose to provide mechanical support to the printed circuit board during the assembly process. Furthermore, the PCB stiffeners include an increase in abrasion resistance, reinforcing of solder joints, and better handling of the board for the automated process of pick-and-place.

flexible-pcb-with-stiffeners.jpg

Note
You might note that the square area of the already small panel gets wasted, but on the design side, connector choice forces you to do this as connectors can suck.

Part 1: PI Stiffener vs. FR4 Stiffener vs. Stainless Steel Stiffener

There are 3 main types of PCB stiffeners - Polyamide (PI) stiffener, FR4 stiffener, and aluminum/stainless steel stiffener. These materials are most commonly used on projects requiring increased stiffness and heat dissipation.

The PI stiffeners are a cheaper alternative to FR4. On the other hand, if manufacturers are looking for heat sinking properties and better rigidity, they use stainless steel or aluminum stiffeners.

  • Polyamide (PI) Stiffener

PI stiffeners are commonly used to increase the thickness of the ZIF connector, where finers contact and provide full support. This way it allows tolerances in bending thickness and contours the component at the contact. The finger area bend thickness tolerance is typical +/- 0.002" and the width profile is +/- 0.003". Also, PI stiffeners are used to limit the bending ability of specific PCB areas to prepare an installation in the final assembly. The most common area to enhance would be the hole locations to provide additional wear resistance whenever wear problems occur.

Polyamide stiffener is an excellent option for solder resistance and high bond strength. The most preferred locations for stiffeners application are on the backside of the insertion gold fingers. Sometimes you can simply pick the part of the PI stiffener that is not laminated or attached to the FPC body and easily insert the flexible printed circuit connector fingers into the ZIF socket or connector.

pi stiffener

  • FR-4 Stiffener

FR-4 can be the CCL of the rigid printed circuit board and is the most common material used for the PCB stiffeners application in and flex circuits. The main purpose of adopting an FR4 stiffener includes:

1. Providing flat enough support when the flexible PCB is in the pick-and-place and reflow processes.

2. Plated-through hole (PTH) components, it is important for the stiffeners to be placed on the same side of the flex PCB in which the components are inserted to allow direct access to solder pads. In some cases, the customer wants to save money in the turnkey process of the project assembly, and in order for manufacturers to do so, they add an FR-4 frame on the border of the panelized PCB to successfully replace the SMT carrier.

FR4 stiffener

In some flexible printed circuit board designs, the diameters of the hole of FR-4 stiffeners are the same as the FPC. The holes will be extremely difficult to be attached, and it could result in dis-alignment. As a leading flex circuit manufacturer, PCBONLINE always ensures that the holes on FR-4 stiffener up to 0.3mm bigger than those related holes on FPC to ensure good alignment.

  • Stainless Steel/Aluminum Stiffener

Stainless steel/aluminum stiffeners are usually used for flexible circuit designs requiring anti-corrosion resistance, capability, robustness, and formability. The stainless steel/aluminum stiffener in flexible circuits is always added to the connecting fingers to stiffen the flexible printed board to facilitate assembly.

stainless steel stiffener

  • Multiple PCB Stiffener

Whenever applying PCB stiffeners, manufacturers ensure that the stiffener overlaps the bored overlay by .030” to relieve stress. In this case, the overlay acts as a solder resist for the flexible printed circuit board. Also, the same exact stiffener thickness should be maintained whenever using multiple stiffeners. After the process is completed, the stiffener strengthens the solder joints due to the increased abrasion resistance.

On the image below you can see a multi-layer flexible printed circuit board with FR-4 stiffener on 3 different areas of the board.

multiple-pcb-stiffener.jpg

Part 2: PCB Manufacturer Good at Dealing with Stiffeners

As you can see above, the flexible circuit board with two types of stiffeners is manufactured by PCBONLINE, an advanced custom PCB manufacturer. In fact, PCBONLINE can easily do with  any PCB boards requiring all the above stiffeners. Our PCB company fully understands the difficulties of the designing side to get the board to stay in spec, so we offer  free tech support to customers.

Why choose PCBONLINE for PCB Requiring Stifferenrs:

  • We provide high-quality flexible PCB that can apply stiffeners.
  • We offer free tech support to help customers deal with issues in stiffeners.
  • Free & professional Gerber check and BOM check ensure the success of PCB and end production.
  • PCBONLINE is an advanced circuit expert providing one-stop PCB solutions for engineers and makers.

PCBONLINE's PCB manufacturing capability:

  • Layer: 1 ~ 42
  • Laminate: normal Tg/high-Tg/lead-free/halogen-free
  • Surface finishing: OSP/HASL/LF HASL/immersion ENIG/immersion tin/immersion silver
  • Board thickness: 0.15 ~ 3.2 mm
  • Max board size: 500 × 580 mm
  • Copper thickness: (inner finish copper) 1-4oz, (outer finish copper) 1-7oz
  • Min. line width/spacing: 0.0635 mm/0.0635 mm
  • Min. CNC drilling size: 0.15 mm
  • Min. laser drilling size: 0.075 mm
  • HDI stack up: 1+N+1, 2+N+2, 3+N+3

Part 3: PCB Stiffener Thickness

For designers, the thickness of PCB stiffeners is an important factor to consider because it affects the PCB specs, especially mil-specs.

Polyamide stiffeners are available in several thicknesses: 0.001" (25μm), 0.002" (50μm), 0.003" (75μm) and 0.005" (125μm). The thickness of FR-4 stiffeners varies from 0.003"(0.08 mm) to 0.125" (3.18 mm). The common thickness of stainless steel/aluminum stiffeners varies from 0.1mm (4mil) to 0.45mm (18mil).

Stiffener Material
Custom Thickness
Usual Thickness
PCB Stiffener material FR-4
0.2mm(8mil), 0.3mm(12mil), 0.4mm(16mil), 0.5mm(20mil), 0.6mm(24mil), 0.7mm(28mil), 0.8mm(32mil), 1.0mm(39mil), 1.2mm(47mil), 1.5mm(59mil)
0.1mm(4mil), 0.9mm(35mil), 1.1mm(43mil), 1.3mm(51mil), 1.4mm(55mil), 1.6mm(63mil)
PCB Stiffener material Polyamide (PI)
0.1mm(4mil), 0.15mm(6mil), 0.2mm(8mil), 0.25mm(10mil)
0.075mm(3mil), 0.125mm(5mil), 0.175mm(7mil), 0.225mm(9mil), 0.275mm(11mil)
PCB Stiffener material Stainless Steel
0.1mm(4mil), 0.15mm(6mil), 0.2mm(8mil), 0.25mm(10mil), 0.3mm(12mil), 0.35mm(mil), 0.4mm(16mil), 0.5mm(20mil), 1.5mm(59mil)
0.45mm(18mil)

Part 4: How are PCB Stiffeners Applied?

Manufacturers ensure to attach the PCB stiffeners with plated-through hole components and that the stiffener is on the same side of the board where the components are inserted. This way they can access the solder pads and easily attach stiffeners to both sides of a part, but the assembly later will require a wide range of configuration to make sure that the printed board will not run in any complications down the road.

There are two main approaches to applying PCB stiffeners to a flexible printed circuit board - Thermal Bonding& Pressure and Sensitive Adhesives (PSA). In the table below you can follow how each of both methods is executed:

Pressure-Sensitive Adhesives (PSA)
Thermal Bonding
Stiffeners are attached with the flex PCB by only using pressure
Stiffeners are attached to the flex PCB by using heat and pressure
Bond not as strong
Stronger Bond
Used in bonding in IPC class 2 products (TV, laptops, consumer electronics, etc.)
Used in bonding in IPC class 3 product (military application, avionics, etc.)
More affordable
More expensive
The process takes less time
The process takes more time
Usually removed without damaging the circuit if care is taken while removing.
Will cause significant damage to the circuit while removed from the flex circuit.

PCB Stiffeners Common Uses

To illustrate better the applications of PCB stiffeners, the most common uses in the industry are listed:

  • Maintaining proper thickness in the flexible PCB.
  • Support for PCB components and connectors.
  • Enforcing any specific area of the printed circuit board directors.
  • Restricting flexible parts to the area they should be.
  • Keep a certain area of the flexible printed board stable and flat.
  • Fulfilling the ZIF connector requirements.
  • Better board handling.
  • Increasing the radius of bending at the inspection of rigid and curved portions (this will avoid stress to the flexible part during the multiple bending operations during the assembly).
  • Reducing stress and pressure on the curved parts of the PCB.
  • Connectors that require assembly or repeated insertion need PCB stiffeners to reveal pressure.

Part 5: The Difference Between Rigid-Flex and Rigidized Flex PCBs

The rigidized flex PCB is bonded with a PCB stiffener to strengthen a certain area of the flexible surface to make the assembly process easier. As far as the common flexible circuit, it is laminated into a single flexible structure.

There are no traces on the rigid party of rigidized flex even on the pads. This rigid part only strengthens the rigidity of the location on which is applied and has nothing to do with functionality.

However, for the common rigid-flex printed circuit boards, both the rigid and the flexible parts are designed with traces that manufacturers connect via holes. In this case, this is not considered mechanical support, instead, it is an electrical connection for the rigid-flex board.


Bottom Line

PCB stiffeners enable the flex circuit to function more effectively and go through the assembly process flawlessly. There is a wide range of possible combinations with various types of PCB stiffeners depending on the end design. If you are not certain how PCB stiffeners will work with your flex printed circuit board design, make sure to quote PCBONLINE. Our specialist will go into detail about the process and discuss using PCB stiffeners effectively. Contact PCBONLINE via email info@pcbonline.com now to learn more.

Here is a question, and you can kindly tell me in the comments.

My Question:
As a PCB manufacturer, we follow your thickness requirements to make the FPC stiffener. Usually PI is of the thickness same as the FPC board, stainless steel a little thicker than the FPC, and the FR4 much thicker, but on the design side, how do you decide its thickness?

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