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Metal Stiffener Rigid-Flex PCB

  • metal-stiffener-rigid-flex-pcb

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OVERVIEW

Item Rigid-Flex PCB
Max Layer 36L
Inner Layer Min Trace/Space 3/3mil
Out Layer Min Trace/Space 3.5/4mil
Inner Layer Max Copper 6oz
Out Layer Max Copper 3oz
Min Mechanical Drilling 0.15mm
Min Laser Drilling 0.1mm
Aspect Ratio(Mechanical Drilling) 12:1
Aspect Ratio(Laser Drilling) 1:1
Press Fit Hole Ttolerance ±0.05mm
PTH Tolerance ±0.075mm
NPTH Tolerance ±0.15mm
Countersink Tolerance ±0.15mm
Board Thickness 0.4-3mm
Board Thickness Tolerance(<1.0mm) ±0.1mm
Board Thickness Tolerance(≥1.0mm) ±10%
Impedance Tolerance Single-Ended:±5Ω(≤50Ω),±10%(>50Ω)
Differential:±5Ω(≤50Ω),±10%(>50Ω)
Min Board Size 10*10mm
Max Board Size 22.5*30inch
Contour Tolerance ±0.1mm
Min BGA 7mil
Min SMT 7*10mil
Surface Treatment ENIG,Gold Finger,Immersion Silver,Immersion Tin,HASL(LF),OSP,ENEPIG,Flash Gold;Hard gold plating
Solder Mask Green,Black,Blue,Red,Matt Green
Min Solder Mask Clearance 1.5mil
Min Solder Mask Dam 3mil
Legend White,Black,Red,Yellow
Min Legend Width/Height 4/23mil
Strain Fillet Width 1.5±0.5mm
Bow & Twist 0.05%
Table of Contents
Primary Item (H2)

While stiffeners are not an integral part of the rigid-flex PCB design, their use becomes vital in some cases where rigidity or strength enhancement is needed. These are selected on the basis of the application of the rigid-flex PCB. 

Rigid-flex PCB, as we know, is an amalgamation of two types of circuits - rigid and flexible. While the former makes the circuit strong, the latter is responsible for adding flexibility to the PCB board. But, as opined earlier, some applications may demand more sturdiness and stability, and these can be furnished with the help of an additional material - a stiffener. 

So, what is a stiffener? To help you have a better understanding, we have a comprehensive article on PCB stiffeners, their types, advantages, and much more. We will also shed light on how they help the said PCB perform better, with a special focus on metal stiffeners.  

What are PCB stiffeners?

These are materials or metals that help enhance the rigidity as well as strength of a part of the PCB. Once stiffener is applied, connecting to the other rigid parts becomes easier. This prevents bending and damage to the board. 

Remember, the normal thickness of a PCB stiffener is 0.002″ to 0.059″.

Types of stiffeners

Stiffeners in a PCB are of various types, including:

  • Polyimide
  • FR-4
  • Stainless steel and 
  • Aluminum 

Of all the above, the last two are metals and are used in rigid-flex PCB if the application requires so. Now, let's study these two one by one. But before that, it is important to know the reasons why stiffeners are used in PCBs.

Benefits of Stiffeners 

When the PCB components are positioned in the flexible zone, they exert pressure on the flexible PCB and, therefore, require a rigid surface. Stiffeners provide them with this rigidity so that components can be placed easily and comfortably.  

While traditional PCBs had restrictions due to shape and design, modern-day PCBs exhibit flexibility and can be made to fit into smaller spaces. Stiffeners help them achieve these benefits without losing power. This is how stiffeners help.

  • Support those parts of the board that contain through-hole components.
  • Add support to the connectors and the PCB components.
  • Make sure that the flexible parts remain within the rigid areas as needed.
  • Help in making it easier to handle thin and delicate circuit boards.
  • Maintain the thickness of the flexible printed board circuit.
  • Fulfill all the specifications of the Zero Insertion Force (ZIF) connector.
  • Maintain certain areas of the flexible board flat and firmness.
  • Help in making the bend radius of the circuit bigger where the rigid and flexible parts meet. This prevents the flexible part from getting strained when the circuit is bent.
  • Help arrange and preserve arrays by extending into the array.

Metallic Stiffeners for Rigid-Flex PCB

Stainless Steel Stiffener

Made up of iron (Fe) and carbon (C), stainless steel alloy is a popular form used in the production of stiffeners. It has 10.5% chromium and nickel, with 0.2 to 2.11% of carbon. It is also known as rustless steel or corrosion-resistant steel. 

In some rigid circuits, the designers are required to ascertain the protection of the PCB from physical damage. This is where the stainless steel stiffener comes into play. 

The unique design is brought to life by sintering technology that uses stainless steel powder fused to form a billet. The process does not end here, as more steel can be added to increase its strength. However, these are mostly used in applications that require only surface mounting and no drilling.

To make the connection strong, silicone serves the adhesive's role and is applied in military electronics, ruggedized systems, and medical devices. What makes stainless steel stiffener perfect for these industries is its ability to handle diverse environmental challenges, including extremely high temperatures, vibration, and rough handling. 

Furthermore, stainless steel stiffeners are extremely light in weight and last really long. These can be coated on the surface or during the assembly process. However, these come at a high price and, unless needed, should be avoided. 

Stainless steel stiffener in a Flex PCB

The thin sheets of stainless steel can bend, and this makes them the perfect candidate for use as a stiffener in a flex circuit PCB. Just cut it using shears and nibblers or a laser to create the desired shape.

In a flex PCB, when a designer wants to achieve rigidity but has limited space, stainless steel is the right option. Both polyimide and FR4 fail to provide the as these are limited due to the stiffener thickness. Stainless steel, on the other hand, is just perfect as it is thin and provides the required thickness. 

Advantages of stainless steel stiffeners

  • It is extremely non-susceptible to damage that may occur due to high moisture. 
  • Good conductor of heat allows heat dissipation from PCB components
  • Stainless steel can take upon a lot of heat compared to standard steel, both high as well as low
  • Allows customization as it can be formed into different shapes and sizes
  • It can be made thinner for space-constraint applications 

Disadvantages of stainless steel stiffener

  • Although thinner than other stiffeners, it still has more weight
  • Attaching steel to the board is not easy and requires special adhesives
  • High plasticity makes machining steel really tough 
  • It is more expensive than most stiffeners as the raw material comes at a high price. 

Aluminum

Another metal stiffener for rigid-flex PCB is aluminum. It is made up of bauxite, appears silvery white, and is soft. It is considered better than glass or epoxy stiffeners due to its heat dissipation and malleability. So, for applications that generate a lot of heat, aluminum stiffeners are just perfect. These not only handle heat well but can also turn into a heat sink. 

Using these will give you peace of mind, knowing that the components and connectors are safe as the heat gets evenly spread out. This is also expensive in comparison to FR4 or Polyimide material. 

Advantages of Aluminum:

  • It is really good because it makes thin materials much stronger.
  • The metal is cheaper than using polyimide films.
  • Aluminum doesn't weigh much, and it is lightweight.
  • You can shape aluminum however you want because it is easy to work with.
  • It is great at getting rid of heat because it is good at conducting heat.

Disadvantages of Aluminum:

  • Sometimes, aluminum can change shape when the temperature fluctuates, which might cause problems.
  • Putting parts of aluminum together can be a little complicated and may need some specialized treatment.
  • Aluminum can also start to rust if it is not protected well
  • Stay alert when handling flexible aluminum because it can bounce back if it is hot.

These are used in military electronics, microwave ovens, and cellular phones. These can be shaped according to the shape and size of the PCB.

Other Metallic Stiffeners

Copper and brass are also used as metallic stiffeners. But there is a difference. Both these metals are used to make busbars. Busbar is a type of PCB stiffener and has a dielectric coating that helps in conducting power. It comes in different shapes, such as flat strips, rods, and bars. Note that these busbars conduct more electricity than a trace.

Copper, as a metal, is light in weight and is a good conductor of heat. To attach it to the flexible circuit, soldering is a preferred bonding method. No plating is required. Copper is used as a busbar because it can handle high temperatures really well besides conducting electricity.

Another metal that is used in making busbars is brass. It is an alloy of copper (Cu) and zinc (Zn). Brass combines with copper and aluminum (used often) to keep the metal less flexible as 

Is required by the application. 

These two are essential components of a busbar. However, one must not forget that while every busbar is considered a stiffener, not every stiffener is a busbar.

Prerequisites for Using Metal Stiffener Rigid-Flex PCB

The metal stiffener should fit the design requirements of the PCB design. To ensure this, the following points are extremely vital.

  • The chosen stiffener needs to be made of sheet metal materials and should not conduct electricity.
  • When you apply it on one side of the rigid PCB with the flex circuits, it should not bend the rigid PCB permanently.
  • It should be thinner than 0.5 mm but not thicker than 2 mm.
  • There should be no sharp edges.
  • It should not hamper the working of the rigid PCB when you put it on one side with the flex circuits.
  • The stiffness of the metal should range between 0.25 - 0.75 MPa. 
  • It should work well with the other components and layers used in rigid-flex circuits.

The surface of the stiffener should be smooth to avoid messing up the rigid-flex circuit board during manufacturing.

How to Integrate Metal Stiffener in a PCB Design

Positioning of the layer

These are placed on both sides of the circuit board. This helps in maintaining the layers of the PCB and avoids expansion and contraction due to high heat and humidity. 

Adhesion

Both stainless steel and aluminum require special adhesives. The two most common adhesives used include thermally bonded adhesives or PSA (Pressure-sensitive adhesive). Thermally bonded adhesives come as a double-sided tape or a paste. PSA is a self-stick adhesive that only requires pressure to form a bond and requires water or heat to activate it. 

You can also use silicone as an adhesive. It attaches well to the stainless steel. There are two ways to attach it to the board: manually by using double-sided tape or with heat and pressure that laminates it. 

Contouring of the stiffener

Usually, laser machines are used to cut the materials so that they can fit the shape of the circuit precisely. 

Mounting 

To place components, make holes, pads, and cutouts. These help connect the flexible board to the rigid board.

Flex-to-rigid transition

While adding stiffeners, make sure there is enough space around the edges of the stiffener. This will help the flexible circuit to bend and transition into the rigid part easily.

Automated machine producing green electronic parts in a factory setting

Tips to Add Metal Stiffeners to a Rigid-Flex PCB 

Below are some points to help you make the process easy and workable:

  • To ease the strain in flex PCBs, it is important to have a minimum of 0.762 mm of overlap between the stiffener and the exposed coverlay.
  • Using a thermal adhesive may cost more than using a pressure-sensitive adhesive. 
  • Apply the stiffener on the PCB side that has plated through-hole (PTH) components. Doing so enables you to insert the component leads through the holes and also reach the solder pads on the flex circuit. 

Common thicknesses of metal stiffeners used in rigid-flex PCB

Stainless steel: 0.15mm and 0.2mm

Aluminum: 0.2mm, 0.25mm, 0.3mm, 0.35mm, 0.4mm, 0.5mm, 1.5mm, 0.15mm and 0.45mm 

Parting thoughts

We believe you are now well-versed with the metal stiffeners used for rigid-flex PCBs. Both aluminum and stainless steel are alloys with excellent mechanical properties that make them suitable as stiffeners for the rigid-flex PCBs.

While their abilities can’t be denied, it is important to contact a trustworthy and reputable PCB manufacturer to ensure your final product is the best in the market. Make sure they have ample experience to help you achieve the desired result.

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