2 Layer Flex PCB Stackup

The relevance of the printed circuit board (PCB) continues to skyrocket. The advancement in technology makes it possible to do a lot with PCBs.

One area is the PCB stack-up the PCB, and the stack-up comes in numerous layers. However, we will discuss the two-layer flex PCB Stack-up in this piece.

Several questions surround this topic, and you will get the answers you need in this article. So let’s dig right into it.

What Does A 2 Layer Flex PCB Stack-up Mean?

It’s best to understand what a flex PCB means. A flex PCB is a type of circuit board that comprises flexible electronic materials.

So, a 2 layer flex PCB stack-up is an arrangement or pile of flexible circuitry. That is, there are 2 layers of the flex PCB.

2 lay Flex PCB Stack-up

Why Choose Polyimide Over FR-4 For 2 Layer Flex PCB Stack-up?

The FR-4 and Polyimide are crucial materials in PCBs. And it is true that FR-4 is less expensive compared to Polyimide.

But why do people still choose Polyimide over the FR-4?

Polyimide is more suitable for flexible circuits due to it’s unique qualities. Let’s go through a brief comparison of both materials;

• Flexibility: Flexibility is the fundamental, and Polyimide posses greater flexibility than the FR-4
• Durability: Although the FR-4 is reasonably strong, yet it has its limitation as it faces physical stresses and heat. However, Polyimide performs better in these circumstances.
• Heat Resistance: FR-4 isn’t the best choice for situations of high heat dissipation. In constrast, Polyimide has great resistance to heat as it accommodates a wide range of temperatures.
• Polyimide makes it easier for future replacement of components due to its flexibility.

What Are The Specific Elements Of A 2 Layer Flex PCB Stack-up?

Many things come together to make the 2 layer flex PCB stack-up an element of great value across several industries.

These will include design, the thickness of the materials, etc. However, some specific elements are crucial to the make-up of the 2 layer flex PCB stack-up.

Let’s have a view;

• The Coverlays
• The No flow prepreg
• The Flex cores

What Are The Materials For A 2 Layer Flex PCB Stack-up?

The following materials make up the 2 layer flex PCB stack-up.

Stiffeners

The stiffener helps in providing mechanical support to the board. In addition, it makes some parts of the board stable, and as a result, soldering circuit components becomes easier.

The stiffeners suitable for the 2 layer flex PCB stack-up include:

• Polyimide
• Stainless
• FR4
• Aluminum etc

Flex Cores

The flex core is another essential material for the 2 layer flex PCB stack-up. The flex cores may be adhesive or adhesiveless.

In terms of thickness, we have copper weights. In the manufacturing process, one can use the Electrodeposited Copper foil(ED copper) and Rolled Annealed Copper foil (RA).

The electrodeposited copper has a better conductivity compared to the rolled annealed copper. On the other hand, the rolled annealed copper has better extensivity.

So your choice will depend on what you want for the flex PCB.

Soldermask

We can also call the solder mask a coverlay. It is a layer of polymer that protects the PCB from contaminants, dust, etc.

It also shields the copper from oxidation and shorts.

The adhesive types for the solder mask include acrylic, epoxy, etc.

Let’s see some other materials;

• Electromagnetic Interference protective films
• Pressure Sensitive Adhesive
• Epoxy strain reliefs

What Are The Differences Between A Flex PCB And A Rigid PCB In Terms Of A 2 Layer Flex PCB Stack-up?

Of course, the flex PCB and the rigid PCB are not the same. We will highlight their differences using the following headings.

Flex PCB

Rigid PCB

Flexibility

Just as their mane implies, the flex PCB is flexible. You can bend the flex PCB to reasonable lengths. As a result, they can fit into numerous designs and applications.

On the other hand, the rigid PCB is not flexible. As a result, they attract a few limitations to specific designs and systems.

Cost

Although the flex PCB is versatile, they accrue so much cost. Little wonder the rigid PCB is more popular due to its low cost.

Other differences include;

• The flex PCB feature in designs that require few spaces compared to the rigid PCB, which features in applications with larger spaces

What Are The Benefits Of A 2 Layer Flex PCB Stack-up?

On a general scale, the PCB stack-up provides immense benefits. However, the 2 layer flex PCB takes these advantages to a whole new level.

Let’s see some of the mind-blowing benefits of the 2 layer flex PCB stack-up

Flexibility

Of course, it gets its name “flex” because it is flexible. The 2 layer flex PCB stack-up is flexible. You can bend, fold and manipulate the circuit to fit several applications.

The 2 layer flex PCB stack-up gives the manufacturer the ability to fit the circuit into any device. This contrasts with situations where the manufacturers have to build the device around the PCB.

Smaller Weights

The 2 layer flex PCB stack-up is lightweight due to its components. As a result, its usage promotes the existence of lighter electronic products.

Manufacturers and even consumers all prefer electronic devices of smaller weights, and the 2 layer flex PCB helps this course.

High Resistance

One major challenge with circuits is their reaction to environmental factors. However, the 2 layer flex PCB has a high resistance to environmental factors such as radiations, heat, and chemicals.

Little wonder it has a broad application in external devices like automobiles.

Greater Connectivity

Of course, the quality of connection of the circuit’s components plays a significant role in the efficiency of electronic devices.

The 2 layer flex PCB provides good connectivity of components irrespective of length and size.

Durability

The flexibility, ability to absorb shock, and resistance to environmental influences gives the 2 layers Flex PCB good durability.

What Are The Features Of A 2 Layer Flex PCB Stack up?

The 2 layer flex PCB is an outstanding PCB type that flourishes in extreme temperature and limited design spaces.

From the background, it comprises of polyimide base and two conductive layers. However, let’s take a deeper dive to see some other features.

• It has connections established by plated through holes
• It has stiffeners, pins, and connectors which acts as means of support
• The covering of vias is possible using exposed pads without covers

What Are The Applications Of A 2 Layer Flex PCB Stack-up?

The unique qualities of the 2 layer flex PCB stack-up make it a popular demand across the world of electronics.

As a result, its application is quite numerous, and it’s time we take a look at a good number of them.

Radio Frequency Identification (RFID)Systems

The RFID is a system of identification that uses radio waves. The 2 layer flex forms the circuity of this system. With the RFID tracking patients and livestock become easy.

Solar Cells

Solar as a source of power continues to accrue attention and relevance across the globe. Several solar cells have the 2 layer flex PCB stack-up as their circuitry.

Other applications include;

• Usage in robotics and several artificial intelligence systems
• Application in making circuits for digital cameras
• It has great application in the automobile industry
• Good usage in the design of novel forms
• Mobile devices
• Production of flexible sensors
• IC packaging
• Health devices such as hearing aids, exercising devices
• Aero space application involving power and ground plane
• The dense surface mount assembly
• Production of shielding or protective devices etc

What Are The Guidelines For Preparing A 2 Layer Flex PCB Stack-up Board Outline?

We often give attention to the prowess of the 2 layer flex PCB stack-up; however, its efficiency depends on several factors.

And one factor is the board outline. An accurate board outline sets the tone for producing an efficient 2 layer PCB stack-up.

Hence, the following are guidelines for best results. First, however, we will lay out the guidelines in two sections: board outline and sample outline.

Board Outline

This is the first section of the whole mix. First, it’s best to present the picture you have in mind on paper. Then, it involves cutting a piece of paper into the shape of the board design.

For best results, you should represent rigid areas and stiffeners with cardboards. So when you have all these in place, follow the steps below.

• Start your layout, but do not route
• Draw the board outline
• Identify and draw the areas of different thicknesses
• Carry out a preliminary component placement
• You must check if the components will need reinforcements from the component placement, i.e., stiffeners.
• Draw out the rigid areas and stiffeners

Sample Outline

After the board outline, the next step is the sample outline. It involves the appearance of the sample, which should be simple as possible.

• In a bid to minimize material wasting, use the folding technique in designing the outline
• It’s best to increase the tear resistance. You can do this by adding copper to the corners of the outline
• Now it’s time to consider the radius of the outline’s inner angle. However, for best results, the greater radius will increase tear-resistance and reliability
• In the case where the circuit requires dynamic bending, a radius greater than 1mm is best.
• A hole of diameter 1.5mm and above should be in place at the ends of the slitting and slotting

Note: If there will be surface mounting(SMT) of the panel, it’s best to design fiducial marks as it will eradicate misalignment.

What Are The Guidelines To The Thickness Of A 2 Layer Flex PCB Stack-up?

The concept of thickness is crucial for flex PCBs because too much thickness will disturb their flexibility.

The bend radius the circuit requires goes a long way in determining the thickness of the flex PCB. But if the circuit requires greater thickness, make use of stiffeners.

But the question is, how do you know the thickness your circuit requires? Check out the following pointers that answer your question.

• Dielectric thickness
• Design of materials
• Adhesive thickness
• Base Copper weight
• Copper layer count
• Material thickness

Bonus:

For high-quality results, you can check out the following hints.

• Whenever you use a 1/2oz copper, it should go along with a 1/2mil coverlay. This will result in an adhesive of 15μm between the Polyimide and copper. Then, high bonding strength.
• When using 1oz copper, follow through with 1mil coverlay.
• Make do of the 1/2oz copper to avoid short circuits.
• For high-temperature resistant flex PCBs, the FR-4, PI, and stainless steel stiffeners will do.
• Use PET stiffener in the case of low-temperature resistance

What Is The Vias Design For A 2 Layer Flex PCB Stack-up?

Vias are small drilled holes cutting through two or more adjacent layers. They serve as an electrical connection between layers of copper in the PCB.

When it comes to the use of vias in flexible PCBs, one has to be careful. This is because Vias in flex are highly susceptible to peeling.

 Vias Design for a 2 Layer Flex PCB Stack up

Now, let’s look at the Vias design for a 2 layer flex PCB stack-up.

• The annular rings of the Vias should be reasonably large
• The vias design should have a teardrop
• Ensure to add anchors or tabs to the vias as it helps prevent peeling
• Do not place vias on flexed areas
• It’s quite risky to place vias at the edge of a reinforcement. This will cause cracking.

What Is The Role Of Polyimide In A 2 Layer Flex PCB Stack-up?

Polyamide is another name for Polyimide. They are polymers whose chemical structure comprises imide monomers. Polyimide can be natural and synthetic.

The demand for polyamide in manufacturing a 2 layer flex PCB is high. Of course, this will be due to the vital role it plays as a component of the flex PCB.

Therefore the role of Polyimide includes the following;

Flexibility

Polyimides are flexible materials; that is why they feature in the production of flex PCBs to promoter flexibility.

High Tensile Strength

Polyimides have no issue with warping. They have top tensile strength, which is a critical factor in the durability of the 2 layer flex PCB stack-up.

High Resistance to Chemicals

One key advantage of the 2 layer flex PCB stack-up is that it is highly resistant to environmental factors. However, the presence of polyimides makes it all possible.

They are highly resistant to chemicals and other environmental factors.

Thermal Stability

Temperature is one major challenge for electronic components. However, polyimides show excellent stability within a broad temperature range.

As a result, they make the 2 layer flex PCB stack-u thermally stable. Since they are thermally stable, there is no fear of thermal damage.

High Resistance

Polyimides have high resistance to physical stress; as a result, they boost durability. In addition, polyimides ensure the PCBs do not change their shape over time.

What Are The Things To Consider Before Designing A 2 Layer Flex PCB Stack-up?

For best practice in terms of stacking up your flex PCB, there are a few “must-know.”These things boost the building an efficient stack-up.

Let’s have a look at what they are;

The Bendability of the Flex PCB

Of course, this is one of the primary aims of working with a flex PCB in the first place. Two questions are crucial at this stage:

To what extent will the PCB flex? And what is the amount of times the PCB will be flexing?

Also, these concepts go a long way to establishing the stack-up boards as dynamic or static.

In the case of a static board, you should expect the PCB to flex less than a hundred times in its life cycle. On the other hand, flexing will occur countless times in a dynamic board.

In summary, identify the bending radius as early as possible.

Materials

What materials make up the flex PCB that you want to stack. In this concept, the things you should consider includes;

• Flex cores
• Are your flex materials adhesive-based or adhesive less?
• Polyimides, copper, etc.

Flex Trace Routing

This involves the circuitry layout. Here the bending radius comes to play.  Ensure you keep traces perpendicular to the entire bend.

Placement of Flex Layers

The best move is to place flex layers at the center of the stack-up. The big question is, why?

• Boosts impedance in the flexed areas
• Makes manufacturing simple
• It wards off the outer-layer plating exposure.

Other things to consider include;

Holes and Pads: You should anchor every hole in the board. And ensure the pad sizes are reasonably large.

Ground Planes: Employing a cross-hatch pattern for the ground planes will help curb the problem of reduced flexibility and overweight.

What Is The Cost Of A 2 Layer Flex PCB Stack-up?

The cost of a 2 layer PCB stack-up falls within the range of $0.50 -$25. However, there are a few cost drivers that we should consider.

The Board Materials

In the regular scope of PCBs, the FR-4 substrate will be a perfect fit. However, for a flex PCB, the FR-4 substrate takes second place since there is more demand for polyimides substrates.

The polyimide materials produce the flex substrate, which a more expensive than the regular FR-4.

Another point of discussion is the adhesive concept. Adhesive-based materials are less expensive compared to adhesive-less materials.

Layer Count

An increase in the number of layers increases the cost of the stack-up. This is because manufacturers employ more lamination processes, more materials, and a longer work time.

Market action predicts the increase of 35 to 40% in cost when moving from a single layer PCB stack-up to that of 2 layers.

Shape and Size of Board

The shape and size of a PCB are a function of its surface aea. And large surface areas accrue more cost.

Therefore stacking flex PCBs of large areas will cost more than usual.

Copper Thickness

An increase in layers will increase copper thickness. When manufacturers use thick copper layers, it will require prepregs for lamination processes.

Thick copper also increases paces between traces. Coupling all these factors together tends to push up the cost of the Flex PCB.

Trace Width and Spacing

The trace width acts as a propagation channel for signals. Therefore, designs that require tight traces will increase the cost of the PCB. This is because it makes it more difficult for the manufacturers to etch the traces.

Drill hole Sizes

Small drill holes are quite challenging for the engineers, hence increasing the cost.

Type of Stiffeners

Stiffeners are an essential part of the PCB, but the type of stiffener influences its cost. The FR-4 is the regular stiffener that features several flex PCBs.

But some designs require other types of stiffeners like stainless steel or aluminum. Both stainless steel and aluminum are more expensive than the FR-4.

What Are The Rules That Guides A 2 Layer Flex PCB Stack-up?

Stacking up your PCB is not a process with standard guidelines. Below are the rules that guide an effective 2 layer PCB stack-up.

• Integrate the mass and power planes together.
• Place signal layers close to each other.
• Route high-speed signals. This should be on the intermediate layers. This will place the ground plane as shields against radiation.
• Always place each signal layer adjacent to a plane
• If possible, make use of multiple ground planes. Because they will help reduce ground impedance.
• Ensure that the configurations are symmetrical
• Make the insulation between a signal layer and adjacent plane thinner. This will improve its performance against noise.
• You can prevent deformation by applying cross-section

How To Calculate The Bending Radius Of A 2 Layer Flex PCB Stack-up?

Fortunately, calculating the bending radius isn’t as difficult as it may look. And of course, it is a very vital aspect of this subject matter.

To calculate the bending radius of the 2 layers Flex PCB, you have to use the formula below

2 Layer = Flex Thickness x 12

How Does The Institute Of Printed Circuit (IPC) Affect The 2 Layer Flex PCB Stack-up?

The Institute of Printed Circuit is a body that connects players in the electronics industry. They provide guidelines concerning every form of PCBs and even give a quality testing standard.

We can not undermine the weight of testing the 2 layer flex PCB stack-up. It is a way to check the product’s quality. There are several guidelines and testing measures across the industry, but the IPC is a force to reckon.

Let’s see the different aspects of the IPC’s influence on the stack-up.

IPC-2223C

This provides guidelines on sectional design standards. It guides in the choice of adhesive materials, Flex vias, and plated through holes.

IPC-6013

This provides a whole lot of standards, which includes;

• Qualification specifications
• Performance specifications
• Test methods such as bend testing, thermal testing, and impedance testing
• Quality conformance test

IPC-FC-234

This document covers the usage of pressure-sensitive adhesives (PSA). The documents give a standard for everything concerning the adhesive, such as type, strengths,  limitations, etc.

IPC-2221

This covers component mounting and the interconnection of structures.

Testing the stack-up will be dependent on its application. However, for testing any form of PCB, document IP-FC-234 and IPC-2223 are the standards.

Giving good adherence to the guidelines in the documents will ensure a 2 layer flex PCB stack-up at top capacity.

For all your 2 layer flex PCB stack-ups, contact Venture Electronics now.