How many types of stiffener for flex pcb?

Have you ever been faced don’t how to choose a suitable stiffener for your flex circuit board issue? Do you know t how many types of stiffener will be used for flex pcb? What is the reason why we need to add the stiffener on the flex PCB? Today let us study flex circuit board different types of stiffener together.

The types of FPC stiffener

In generally, there are only 3 types of stiffener we usually used. Polyimide(PI), FR4, and stainless.

1.Polymide PI stiffener:The tolerance can be controlled +/-0.03mm, high precision and have high temperature resistance(130~280 Centigrade)

2.Stainless Stiffener: Need to do assembly by hand and the process is more complicate, the cost will be a little higher.

3.FR4 stiffener: If the thickness is below 0.1mm, the tolerance can be controlled +/-0.05mm. If the thickness is more than 1.0mm, the tolerance is +/-0.1mm

Stiffener thickness:

1.PI stiffener thickness: 0.075mm, 0.1mm, 0.125mm, 0.15mm ,0.175mm, 0.2mm , 0.225mm, 0.25mm.

2.FR4 stiffener thickness: 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm~1.6mm

3.Stainless stiffener thickness: 0.1mm, 0.2mm

The disadvantages and advantages of FPC stiffener


Stiffener type advantages disadvantages
PI Small tolerance Not very hard
FR4 More thicker thinkness choice Bigger tolerance
Stainless Hard and stable Assembled by hand and hard to rework

You got it? If you have any comments, please kindly feel free to let me know. Please contact Best Technology at sales@bestfpc.com.

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What is the FPC tooling?

Recently, many customers asking me what is the flex PCB tooling included? And why we need to opening a tooling? How many types of tooling for flexible circuit? How to choose the suitable tooling for flex pcb?

Today, let’s learn about the Flex PCB tooling together. The FPC tooling is always used for cutting the FPC outline and coverlay. So it has included the FPC outline and coverlay tooling.

For samples production, normally we will open a tooling to save the cost and time for our customers, because the tooling will be scraped if you update the new version. So we will use the laser machine to cut outline and coverlay. But the productivity of laser cutting is longer than a tooling cutting the outline, that’s why we need to opening a tooling for volume production.

In generally, there are only four types of tooling in our production process.

1.Normal tooling, it is the most common used for most the flex pcb to cutting the coverlay and outline. The FPC appearance tolerance is 0.1mm, and the gold finger size tolerance range can be controlled at 0.15mm.

  1. Mediumn silking tooling: The precision is high, and the lifespan is better. The punching times is around 200,000 times. The punching effect appearance is good and smooth. The tolerance is 0.07mm. But the cost is high, and the tooling making time is longer than normal tooling
  2. Precision tooling: The surface is very smooth, it is usually used for tolerance very strict flex pcb. The gold finger tolerance can be controlled +/-0.05mm. But the cost will be also much higher than normal tooling.

4.Knife tooling: It is usually used for cutting the simple flex pcb, more than 500 mm length flex circuit board and adhesive. The tolerance is around +/-0.2mm.

Do you still have any questions about the flexible circuit board tooling? If yes, please feel free to contact Best Technology at sales@bestfpc.com. Looking forward to receiving your feedback.

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What is the difference between assembling flex and rigid boards?

Assemble components on a flexible printed circuit board (FPC) requires that when the smart wearable industry becomes more and more popular, due to the assembly space, the surface mount of SMD on FPC has become one of the development trends of SMT technology.
But flex PCB is more difficult to assemble than rigid boards, because it is not as sturdy to assemble. Today let’s learn something about the difference between assembling flex and rigid boards.
1.Soldering process
Like the rigid PCB process, the solder paste is covered on the flexible board and the rigid-flex board through the operation of the stencil and solder paste printer. But the surface of FPC is not flat, so we need to be fixed with some fixtures or stiffeners. Normally we will stick stiffener for component area of flex PCB, you can refer to below sample picture. Stiffeners contains PI stiffener, FR4 stiffener and Sheet steel stiffener.

  1. SMT component placement

Under the current trend of miniaturization of SMT components, small components will cause some problems during the reflow soldering process. If the flexible circuit is small, extension and wrinkles will not be a significant problem, resulting in smaller SMT frame or extra mark points. If you do not want to stick stiffener on the bottom side of components, maybe you need flexibility after assembly. So SMT fixture would be a good choice, you can refer to below pictures for fixtures(fixture vs fixture with flex PCB).

  1. Reflow soldering process

Before reflow soldering, the flexible circuit must be dried. This is an important difference between the flexible circuit and the rigid PCB component placement process. In addition to the dimensional instability of flexible materials, they are also relatively hygroscopic. They absorb water like a sponge. Once the flexible circuit board has absorbed moisture, it has to stop reflow soldering. Rigid PCB also has the same problem, but it has a higher tolerance. The flexible circuit needs to be preheated and baked at ~225° to 250°F (like below pictures). This preheating and baking must be completed quickly within 1 hour. If it is not baked in time, it needs to be stored in a dry or nitrogen storage room.

If you would like to know details about the assembly of flex PCB, please feel free to contact Best Technology at sales@bestfpc.com

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What is the difference between solder mask and coverlay?

Have you ever been have trouble in selecting the solder mask and coverlay when you are design a flex pcb? Solder mask and coverlay has the same basic function of insulating, do you know the difference between solder mask and coverlay? Is it possible to make the solder mask and coverlay overlap together for insulating? Do you think the flexible solder mask is the same as rigid printed circuit solder mask?

1)Design: Actually, coverlay is widely used for flexible printed circuit board. Because of most of flex pcb will need to add the PI/FR4 stiffener and EMI shield. But if there are BGA pads and dense solder mask opening area, limits the minimum size of openings and the minimum web of material then can be retained between adjacent openings. I would like to share a coverlay and solder mask overlap together FPCB with you. The coverlay is applied first and the solder mask second with a slight overlap onto the coverlay to prevent any gaps in the combined layer. This kind of design have much higher requirements for our production to ensure your functional and reliable design. Below picture is a coverlay and solder mask overlay together FPCB, hope you will be interested in.

If the coverlay opening is square, the minimum size for coverlay is 0.6mm, and space is 0.25mm. Otherwise you need to print solder mask. If the coverlay opening is round, the mimimum size for coverlay is 0.25mm, and space is 0.15mm. Otherwise you need to print solder mask. Because too small coverlay webs will be easily damaged or broken during manufacturing process, and the limited of adhesive may not allowed for a good lamination.  Coverlay openings may also exhibit a small amount of adhesive squeeze out into the openings. Then the resulting design rules for coverlay are significantly different than those for solder mask. Attached the coverlay square and round opening picture for your reference, please kindly check it.

2)Thickness: Below is the pure green solder mask flexi pcb picture, it do not need to add the stiffener for PCBA (SMT assembly), and there are no BGA area, why it choose to use the green solder mask instead of coverlay? The normal coverlay thickness is 27.5~50um. And the solder mask thickness is 8~12um. Due to the customers need the ultra thin flex pcb. Then we suggest the customer to use the solder mask instead of the coverlay.

3)Color: The most popular coverlay color is yellow, we usually prepared the yellow, white, black coverlay in stock. Solder mask is available in a wider range of colors: green, black, white, yellow, etc. Please kindly note that the flex pcb solder mask is quite different to rigid pcb. The flex pcb solder mask cost is much higher than FR4 PCB.Because the flexible circuit board solder mask the withstand bending and high temperature is better than Rigid FR4 PCB. But the coverlay and solder mask for flex price is the same for us.

If you have any comments, warmly welcome you to contact us. Once we received your news, we will reply to you immediately. Looking forward to receiving your inquiry.

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What are the differences between the Poyester (PET) and Polymide(PI) material?

The flex pcb raw material and stack up will decide the flex circuit quality. According to the market demand, there are three types of base raw material used for Flexible circuit board. Poyester(PET), adhesive Polymide (PI) and adhesiveless Polymide (PI) . Do you know the difference between the PET and PI material? If not, let us learn about it together now.

The advantages and disadvantages of Polyester(PET) and Polymide(PI).

Attached is a form for your reference, hope it will be helpful you. And you can choose the right raw material according the below form.

 Polyester(PET)Adhesive
Polymide
Adhesiveless
 Polymide
Flexibility (2mm radius)BadGoodBest
Tear strength800g500g500g
Strip strength in the air1050N/M1750N/M1225N/M
Eatching>=20%BESTBadGood
Working temperature85~16585~165105~200
ChipBadGoodBest

The advantages of PET material: Production process is less, price is cheaper, lead time is shorter.

The disadvantages of PET material: Polyester material can not pass through soldering flow. Then it can not be working in high temperature environment.

The advantages of polymide (PI): It’s easy to bend. Can work in high temperature.

The disadvantages of Polymide (PI): The price of raw material is much higher than Polymide.

Below picture is the single sided FFC, ENIG 3u’’, 10pin, 0.5mm pitch, and it is made of Poyester(PET) material

BFS17386-B

Below picture is a 1layer flex pcb, 4 mil PET material, print silver ink, carbon finishing mask, insulation( protective coating), carbon resistance less than 500 ohm.

BFS18205

Below picture is 1L FPC, it is made of adhesive polymide raw material. ENIG 1u’’, 1oz Cu, 1mil PI, 1mil coveraly, and the total thickness is 0.13mm+/-0.03mm.

BFS20032

It is a single sided FPC, ENIG 1u’’, 1/2ozCu, 1mil adhesiveless PI, 1/2mil coverlay.

BFS20209

If you have any questions or would like to learn more, warmly welcome you to contact us at sales@bestfpc.com

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What’s the difference between rigid-flex PCB and flex PCB with FR4 stiffener?

In the production of FPC, normally we need to add different stiffener to meet the total thickness or hardness, such as PI stiffener, FR4 stiffener and stainless steel stiffener. But the FR4 stiffener after laminated is very similar to the rigid-flex PCB, especially there are also have drilling holes in FR4 stiffener.

Do you know how to identify FR4 stiffener and rigid-flex PCB? Please refer to the below picture. On flex PCB, you can see there are many PTH holes, maybe these boards are need to insert connectors and connect it to other products. So they need the hardness in this area.

From the left sample, we added the FR4 materials on bottom side, there are also have vias on stiffener, but no copper in vias, so this is FR4 stiffener. It can only increase the hardness of flex. From the right sample, the green part are still FR4 materials, but we plated copper for all vias, so it’s a rigid-flex PCB.

FR4 stiffener VS Rigid-flex PCB

Rigid-flex board has many production processes, high production difficulty, low yield rate, so its price is relatively expensive and the production cycle is relatively long than flex PCB. If the FPC with FR4 stiffener can meet your requirements, then it would be a better choice for you.

If you have any other question about stiffener or rigid-flex PCB, please feel free to contact us at sales@bestfpc.com

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Flex Circuit Specific Terms and Definitions

Recently, I was having a few discussions with someone very new to flex design. I was trying to be helpful and going through several things that are specific to flex and rigid flex design which should be considered as you start your first design. So, this blog is written for those of you who are new to flex and rigid flex, and may be unfamiliar with some of the terms that get volleyed around in conversation.

Flexible Circuit(flex pcb, pcb flex, flexible pcb, flex circuits):  

A pattern of conductive traces bonded on a flexible substrate. There are several different substrates available, the most common being polyimide. Different than rigid materials, these laminates will have rolled annealed (RA) copper for improved flexibility.

flex pcb
flex pcb

Rigid-Flex PCB(rigid flexible pcb):  

This is a hybrid construction, using flex materials in areas that need to bend or flex and rigid materials in areas with dense component areas, surface mount components on both sides of the PCB, and applications with higher layer count, dense routing areas.  Most common rigid materials can be incorporated into a rigid flex construction.

rigid flex pcb
rigid flex pcb

Flex Tails:  

Typically refers to the areas of flex extended out past the rigid portions of the rigid flex. This may be one flex region, or several bands of flex areas that extend in various directions. Rigid flex is often used to solve packaging issues and connect on multiple planes. Flex tails enable this.

Banding:  

Often used to accommodate a shorter flexible area if there is no room for a service loop. Instead of using the full width of the flex region between areas for each inner layer, the area can be divided into smaller bands of equal width for each inner layer, eliminating buckling and stress in that area.                                                                   

Pouch:  

This is a protective barrier material often used in rigid-flex fabrication. Often, this is a coverlay material used to protect exposed flexible materials during processing and is removed from the flexible portion of the board before shipment.

Coverlay:  

A layer of insulating material applied to the flexible circuit to insulate the conductor pattern. Coverlay is typically a layer of polyimide with acrylic adhesive. Film based coverlay is much more flexible than cover-coat materials and highly recommended for dynamically flexing applications or flex that will have a tight bend radius. It is important to be sure to spec enough adhesive to fully encapsulate the copper conductors.

Bend Radius:  

This is the ratio of the bending radius measured to the inside of the bend to the overall thickness in that area.  Typically, recommendations for non-dynamically flexing designs is 10:1 for single and double-sided construction, and 20:1 for multilayer construction. These can be exceeded but should be evaluated carefully. Dynamically flexing applications should be discussed with your fabricator for a recommended stack up.

I always wrote a blog about how to calculate the bending radius of fpc.

Button Plate:  

Fabrication process to selectively electroplate copper to vias and onto the pads capturing the vias. The remaining copper traces do not have electrodeposited copper, increasing the flexibility of the circuit.

I-Beam Effect:  

Stacking conductors on adjacent layers directly on top of one another, increasing the stiffness of the circuit in the bend or fold areas. Staggered conductors are recommended if possible, to retain the maximum flexibility of the circuit.

I hope this helps explain some of the common terminology with flex and rigid-flex materials and design. Please reach out to me with any questions for further information!

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We will be exhibiting at electronica Munich 2020

Best Technology, a China- based assembly manufacturer will be exhibiting at electronica Munich 2020 in Bavaria, Germany at booth No. 623(Hall B1) from November 10, 2020 to November 13, 2020(Western Time). Actually, we have participated in this exhibition for continuous 2 years. Here are some pictures for your reference.

pcb exhibition
pcb exhibition
pcb exhibition
pcb exhibition
pcb exhibition
pcb exhibition
pcb exhibition
pcb exhibition

At our exhibition booth, attendees will be able to learn more about our company for PCB manufacturing and PCB assembly technology.

For interested professionals, Best Technology had on hand experts to offer detailed insights and experience on PCB assembly, covering PCB prototype and low/high-volume production. For more information on PCB assembly and fabrication, welcome to visit our booth.

You will see 10mm to 1,500 mm flexible circuit board, from 2 layer rigid-flex circuit to 50 layers, 1/2 OZ copper to 3 OZ, 0.15mm extra thin FR4 PCB to 30 OZ heavy copper, turn-key service from components sourcing,board fabrication(FPC, PCB, MCPCB, Ceramic PCB) and final assembly with programming and testing and metal dome that is used in conjunction with a printed circuit board, flex circuit, or membrane, become normally-open tactile switches. we have four types of metal domes: four legs with size from 5mm-20mm; triangle with size 4mm-12mm; round dome with size 4-16mm, oblong dome with size 3.7mm-58.1mm, and we also make dome with adhesive, named dome array.

You are warmly welcome to see how we can manage the components wastage and efficiency to help save your time, money and energy.

About electronica Munich:

Electronica is the international trade show for electronic components, systems and applications and shows the full range in all its diversity in width and depth as the world’s leading trade fair. It reflects the high degree of innovation throughout the industry. Exhibitors and users in the areas of systems, applications and technologies of electronics can receive the information of the latest innovations and developments in the fields of system peripherals, power engineering to printed circuit boards or EMS.

Welcome to visit us at Booth 623 from November 10, 2020 to November 13, 2020(Western Time).

electronica 2020
electronica 2020

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Copper Thickness Requirements for Flex Circuits

If an end user will specify the copper thickness of a printed circuit, there must be many reasons. For example, current carrying capacity, but copper thickness also directly impacts thermal performance and impedance. All these are vital properties, which have a great influence on the functionality and reliability of a flexible circuit.

flexible circuits
flexible circuits

At the point, it is important to understand the functional needs driving a copper thickness requirement.

Some of the common functional requirements could be:

1.Minimum thickness in a connector area to assure robust contact.

2.Adequate current carrying capacity directly related to the cross sectional area of the trace.

3.Proper conductivity, a function of cross sectional area and metal type of the trace.

4.Proper impedance in high speed circuits driven by the cross sectional area of the copper, the surrounding dielectric constant, and distance from signal trace to ground plane.

5.Thermal properties directly related to metal type and trace profile.

Copper weight is used in the industry as a “thickness” measurement. Circuit manufacturers commonly purchase copper foil with descriptions of ½ ounce, 1 ounce, 2 ounce and so on. The number is the weight of copper in a square foot of foil. Also, +/- 10% is the industry accepted tolerance for copper foil thickness from the material supplier.

flexible pcb
flexible pcb

Drawing specifications will frequently define a flexible PCB copper thickness using weight. For example “circuit to be 1 ounce copper”. This can lead to some ambiguity, as copper plating on double sided circuits can easily add an ounce of copper to the surface of a trace. So by specifying thickness in this fashion, it is not clear if this is intended as a finished thickness or an original thickness. Additionally, controlled impedance designs work best when copper plating is restricted to the vias with no copper plated on the surface of the traces. This will minimize trace thickness variability and suggests a specific product category requiring a process known as “Pads Only Plating” or “Button Plating”. For controlled impedance designs, one of these terms should be called out in the drawing notes.

What affects final copper thickness is the variety of manufacturing processes that add or subtract copper thickness. Micro-etching is a common “cleaning” process used to prepare a surface for plating or coating. This process removes a small amount of copper. Likewise copper plating will add thickness. The circuit fabricator will directly measure added (or subtracted) thickness in mils (1 mil =.001”) or microns (25 μm=.001”).

The most accurate method for determining thickness is to do a micro section. This is a destructive test, so it is common to use coupons located in unused areas of the processing panel. These coupons are located and sized to be “representative” of the circuitry copper thickness. Copper thicknesses will vary slightly across a panel depending on current density from electroplating. Current density can be a function of the copper trace pattern so differences among various part numbers will occur. As a general rule, copper plating thickness will tend to be thinner on the outer edges of the panel and thicker toward the center.

In summary, when defining the specific copper thickness for an application it is highly recommended to start with a discussion of the myriad functional requirements. Also, the manufacturer can help recommend copper thicknesses and tolerances as well as the best methods for measurement.

Best Technology is the professional vendor of flex circuits, from 1 layer to 10 layers, 2 layers rigid-flex circuits to 16 layers rigid-flex circuits, and one-stop service including components purchasing, assembly, IC programming, testing. Choose us, you can always enjoy our best service at a good price.

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FPC Coverlay Opening FAQ

Since electronic products tend to be shorter, smaller, lighter and thinner, flex pcb is more and more popular with people and the demand is increasing.

FPC coverlay is a functional film normally used in PCB industry, and it is used to protect copper foil from oxidation, cover for surface treatment subsequently and play the role of solder mask in SMT process.

flex pcb
flex pcb

Speaking of fpc coverlay opening, that is, at bare copper foil point, open a hole in coverlay according to the shape of design solder pad, so that fpc traces or traces at golden finger or solder pad can be exposed, which contributes to subsequent surface treatment by connection and placement.

flex pcb
flex pcb

We often adopt 2 ways for fpc coverlay opening. One is to open a tool. This is the first choice, very suitable for production in large batch. The other is uv laser cutting, very suitable for prototyping and production in small batch.

These are what we have sorted out about fpc coverlay opening. Hope it is helpful to you. If you are still confused, please contact us, we have professional staff who can answer your inquires for 24 hours a whole day.  

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