How to choose the FPC stiffener?

 Flexible PCB -Flexible Printed Circuits are made of thin dielectric substrate & high ductility metal foil, flexible pcb can be bent or folded and used in static and dynamic applications. 

Do you know why the fpc need to add the stiffener?

Flexible-only PCBs can withstand fewer components than rigid-flex. The rigid part of rigid-flex can also be as complex as a traditional rigid Circuit Board. Stiffeners are occasionally laminated the same time as the coverlay.

When we need to add the Stiffeners ?

Components are near or in an active, or dynamic, flex zone
Component size and weight will import stress onto the flex
A large number of SMT pads make planarity critical
Connectors that require repeated insertions or assemblies require a stiffener, or something to help alleviate the stress of pads.

When we don’t suggest to to add the Stiffeners ?

 Small “static” components do not exert a large amount of strain on the flexible of the PCB.There are no components in the flex region.
 

How to choose the FPC stiffener?
 As we all know, the Stiffeners are typically made of FR4, PI(Kapton), stainless and alumimum stiffener.Then how we we too choose the right stiffener?

PI is often used when the end of the flex must be thickened and inserted into a ZIF connector. However, the use of Kapton(PI) requires a separate lamination cycle. For golden area, we will be need to add the PI stiffener to meet the total thickness 0.2mm, 0.3mm or 0.5mm.   

PI stiffener

FR4 stiffener can be done within the same lamination cycle. FR4 stiffener usually added for the USB connector area.

FR4 stiffener1

Stainless stiffener is not easy to transformation. And it has the good hardness. And it can play a conductive and inductive role.

Stainless stiffener

Aluminum stiffener has the best heat disspation effect, because it usually need to openning a tooling to cut the alumunum stiffener outline, then the cost is the highest.

Alumininum stiffener

Are 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 difference between FPC coverlay and EMI shield?

Covering is the film that covers and presses on the surface of the flexible circuit, usually polyimide resin, used for solder mask, etc., and has little effect on signal shielding. The Electro-Magnetic Interference (EMI) shield is generally attached to the coverlay, and the price of the EMI shield is much more expensive than the coverlay. What is the difference between FPC coverlay and EMI shield?

FPC coverlay, known as CVL in the industry, it has the same main function as the solder mask of PCB: 1) Protect the copper foil from being exposed to the air and avoid oxidation of the copper foil; 2) Cover for the areas which it do not need to do surface treatment. 3) The solder resist function in the SMT. The usual colors are yellow, black and white in Best Tech.

Coverlay
FPC with yellow coverlay

The EMI shield uses mixed composite metal as the shielding material and has excellent OSP performance and corrosion solvent resistance. It is superior to similar products in the market in shielding and winding characteristics. The EMI shield is suitable for flexible printed circuits. It has high shielding, bending and heat resistance. The usual color is black in Best Technology.

EMI material

After pressing the cpverlay, the FPC will press a layer of EMI shield as a conductive layer to shield the external electromagnetic interference. EMI shield is mainly used in 3C products such as notebook computers, GPS, ADSL and mobile phones, because they all produce noise due to high-frequency electromagnetic interference, which affects the quality of communication.

FPC with EMI shield
Rigid-flex with EMI shield

If you want to know more design guild about the EMI shield, please feel free to contact us at sales@bestfpc.com

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Why the FPC can not use HASL surface treatment?

The hot air solder leveling is a technology developed for the coating of lead-tin on rigid printed circuit boards. Due to the simplicity of this technology, it is also applied to flexible printed board FPC. But we usually don’t suggest to use HASL and suggest to use ENIG.

In modern market, there are no FPC manufacturer will be use HASL surface to instead of ENIG. Although ENIG will be expensive than HASL. Attached the ENIG FPC and HASL FR4 PCB pictures for your reference, please kindly check it.

ENIG
HASL

The hot air leveling is to immerse the board directly in the molten lead-slot, and the excess solder is blown off by hot air. This condition is very demanding for the flexible printed board FPC. If the flexible printed board FPC cannot be immersed in the solder without any measures, the flexible printed board FPC must be clamped to the middle of the screen made of titanium steel. Then, it is immersed in the molten solder. Of course, the surface of the flexible printed board FPC is also cleaned and coated with flux.

Due to the harsh hot air leveling process conditions, it is easy for the solder to be drilled from the end of the cover layer to the underside of the cover layer, especially when the bond strength between the cover layer and the copper foil surface is low, and this phenomenon is more likely to occur frequently. Since the polyimide film is easy to absorb moisture, when the hot air leveling process is used, the moisture absorbing moisture will cause foaming or even peeling of the cover layer due to rapid heat evaporation, so it is necessary to dry and prevent moisture before the FPC hot air leveling management.

As we all know, the Gold is a better finish for the corrosive environment if it is clean going into the field. HASL will typically use a very corrosive flux to prepare the surface and if this is a lead free HASL or leaded it does not matter, both are susceptible to corrosion.

These must be very clean going into the fielded environment or just the residue from the HASL process can set up corrosion cells. The plating process for the gold finish is not as difficult to clean and the residues are typically very low even with RO water rinsing.

If you would like to learn more details or you have any different opinions, please feel free to contact us and let us know at sales@bestfpc.com

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What is the differences between FFC and FPC?

Have you ever been heard about the FFC? Do you would like to learn about the FFC usually will be used in which kind of field? What are the differences between FPC and FPC?

FFC means flat flexible cable, it consists of multiple flat conductors insulated with a flexible plastic film that is laid out with conductors that are straight. It’s shape fixed as a rectangle. The pitches are available such as 0.5mm, 1.0mm and 1.25mm. Pitch means the distance between the center of conductor to the center of the conductor next to it.

ffc

What kind of FFCs usually used for PCB? It’s typically have a stiffener attached to the opposite side of the contact portion side to be inserted into a connector. They are simple one-to one connect jumpers. The FFCs are widely used in LCDS, mother boards, touch screens, cameras and other various devices.

FPC means flexible printed circuit board. FPC consist of copper, coverlay and PI. Can be customize as different patterns and multilayers flex pcb.

fpc

The mainly differences between the FFC and FPC?

1. Base material different: FFC base material is PET material, the range of use temperature from 85 to 165 °F. FPC base material is PI material. Attached the difference between PI and PET difference blog for your reference, hope you will be interested in . https://www.bestfpc.com/blog/2020/09/26/what-are-the-differences-between-the-poyester-pet-and-polymidepi-material%EF%BC%9F/

2. FFC width depends on the connector need to assembled pitch. FPC can be customized by all kinds of shapes. That is why the FFC you have seen is the rectangle shape, and there are all kinds of outline FPC.  

3. FFC is can be designed as 1 layer, but FPC can be designed as 1~10 layers.

4, FFC surface color usually is white, FPC is yellow, black or white.

If you have any query about FFC? Please feel free to contact Best Technology at sales@bestfpc.com

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Did you heard of etch 6L FPC to be 2L FPC?

We received a special project, one of our customer need 2L FPC, and want the thickness between two copper layers to be 0.15mm. Because this customer is specialized in RF product, he need so thick dielectric(0.15mm) between two copper layers.

What does it means?

  1. Firstly, for adhesive material, it means “adhesive + Polyimide + adhesive” should meet 0.15mm. If Adhesive is 25um, two adhesive layer totally 50um, the minimum Polyimide thickness will be 4mil.
  2. Secondly, for 4mil Polyimide, it usually refers to adhesive-less material. So if deduct 50um adhesive, it require 6mil adhesiveless material. 6mil adhesiveless PI is very hard to purchase, need to customize, have MOQ limit, and lead time will be very long.

Is there any solution to meet customer’s need?

After discussed with our engineers and manufacture department, we put forward one suggestion. Draw an stack up with 6L FPC, it’s made up of three double-sided FPC. Etch out one copper layers in TOP and Bottom double-sided FPC. And for the middle double-sided FPC, we’ll etch out two copper layers, only leave Polyimide and adhesive. 

By doing this, it finally will only leave two copper layers, and the thickness between two copper layers is 0.15mm. (As below stack up show)

To be honest, when customer put forward to his design requirement, I was amazed, 0.15mm material is so hard to purchase. But our engineers and manufacture department find a good solution for our customer. Our customer are satisfied with our work.

If you have similar need, pls feel free to contact us. We can help you make it come ture.

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How much do you know about FPC surface finish?

How many types of FPC surface treatment have you ever seen? FPC & Rigid flex is unlike Rigid FR4 PCB, there are less types of surface treatment in flexible PCB manufacturing, and most of flex pcb defult ENIG 1u’’ for flex pcb and rigid flex pcb. Please refer to the following FPC surface finishing:

  1. Electroless nickel immersion gold (ENIG or ENi/IAu), also known as immersion gold (Au), chemical Ni/Au or soft gold, is a metal plating process used in the manufacture of flexible printed circuit boards (FPCs), to avoid oxidation and improve the solderability of copper contacts and plated through-holes. It consists of an electroless nickel plating, covered with a thin layer of gold, which protects the nickel from oxidation. The gold is typically applied by quick immersion in a solution containing gold salts.

In generally, we will default as ENIG 1u’’ for customers if there are no any special requirements from our customer. There are also some special requirements from our customer, such the whole FPC surface area need to ENIG, it is very expensive. And there are also some customers may need hard gold plating (30~50u’’)for gold finger area.  

Hard-gold-plating

The advantages of Immersion gold( ENIG) will have very smooth surface, long storage time, easy for PCB soldering; suitable for fine pitch components and thin PCB board. It is more suitable for FPC just because of the thinner thickness.

2.Organic solderable protective layer (OSP) This process refers to cover the exposed bare PCB copper surface with a specific organic material.

OSP

Advantages: Can ensure a very flat surface, in line with environmental requirements. It’s cheaper compared with other surface finish.

Disadvantages: Once removed, the bare copper is exposed and subject to oxidation. Need to use conventional wave soldering and selective wave soldering process PCBA, does not allow the use of OSP surface treatment process. And it has a very limited shelf life.

2. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) , ENEPIG is an acronym for Electroless Nickel Electroless Palladium Immersion Gold. This type of finish offers abundant benefits that make it suitable for a variety of applications.When we consider the final finish performance in a variety of different assembly methods, it can be seen that ENEPIG is suitable for a wide range of assembly requirements. The only one disadvantage of ENIPIG is the price is the most expensive.

ENEPIG

In conlusion, we sumarized the OSP and ENIG and ENEPIG some characteristics as a form as below, please kindly check it. Sincerely hope it will be helpful for you. If you have any quetions or would like to learn more details, warmly welcome you to contact us at sales@bestfpc.com

suface-finish-form

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Do you know why the golden finger need to add the PI stiffener?

Stiffeners are an important element in most flex designs and as such need to be engineered into the design and fully documented in the data set to ensure the form fit and function of the finished flex circuit parts. Some designs may have complex stiffener requirements that may impact the manufacturability of the flex circuits and may create added complexity in the component assembly process. For these designs we recommend that our customers consult with an engineering team to ensure the flex circuit is manufacturable and will meet your requirements.

Several days before, I received an rigid flex pcb inquiry form our customer, required to add a FR4 stiffener for golden finger area to meet the total thickness 0.3mm. Attached the picture as below, please kindly check it.

Rigid flex pcb add stiffener for golden finger area Gerber file picture

Do you know it is not available to add a FR4 stiffener for golden finger area? Because of  the FR4 stiffener is too hard, it will be bad for golder finger connector to a ZIP( Zero Inserion Force). Because the FR4 stiffener tolerance is bigger than PI stiffener, if the stiffener thickness it a bit of big or small. It will be very easy to lead to the golden finger is not contact well and maybe because of it’s too loose to fall out. Or it is not able to insert to the connector due to it is too thick.

Normally, we usually to use make it as 0.2mm,0.3mm,0.5mm(FPC+PI stiffener) for golden finger area. Attached one of our FPC +PI stiffener for golden finger picture for your reference, please kindly check it. Are you have similar design? It is 2layer red oil flex pcb. 1/2oz copper, 1mil PI, red solder mask instead of coverlay, board thickness 0.14mm, ENIG1u’’, minimum copper plated 25um, white silk screen, 3M467 tape on bottom side. Add the PI stiffener on the golden finger area to meet 0.3mm total thickness.

FPC+PI stiffener for golden finger area to meet 0.3mm total thickness

By the way, are you have any trouble in sourcing the connector for match the FPC golder finger? If yes, pleas also feel free to contact us. We can according to your gerber file to advise the right connector part number for you. If you would like to learn more details, please free to contact us by e-mail sales@bestfpc.com.

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What we do to meet your impedance control?

Have you designed the flex pcb or rigid flex required the impedance control? Recently, there are more and more customers are required the impedance controlled circuits throughout the industry in a wide range of applications. And also need a minimum bend radius. It gives a big change to improve our production capability.

Attached a picture for 7 layers HDI rigid flex pcb and impedance control required picture for your reference, please kindly have a look at it.

Rigid flex with impedance control

Line widths and spacing, and the copper thickness of the flex layers interact to achieve the impedance values required. Normally, we will adjust the trace spacing and width to meet our customers impedance control requirements. Thinner copper allows for a thinner line width/spacing and a thinner flex core thickness. This results in a thinner flex area, which will have the highest degree of flexibility and the tightest min. bend capability. The minimum trace width and spacing we can make is 2/2 mil for flex pcb. If the customers are required 2/2mil trace width and spacing, we will use 1/3 oz Cu. The following are typical line width and spacing for the more common impedance values when using 1/2 OZ copper:

50 Ohm Single Ended: 0.004” line.

90 Ohm Differential Pair: 0.004” line/0.0055” spacing.

100 Ohm Differential Pair: 0.004” line/0.006” spacing.

120 Ohm Differential Pair: 0.0037” line/0.0075” spacing.

If 1 OZ copper is required, usually due to a higher current carrying requirement on non-impedance lines, the above line widths are not valid as a 0.004” line width is below the manufactured limit of 1 OZ copper. This requires the line widths/spacing as well as the flex core thickness to increase, which negatively impacts flexibility.

Polymide flex materials are very well suited for impedance-controlled designs. The material is homogenous, has a low DK (3.2-3.4), is very uniform, and has tightly controlled thickness. And we usually use the polymide base material and coverlay to start production. There are many factors will affect the impedance control.

If you would like to learn more details,please feel free contact us at sales@bestfpc.com. We have impedance control tester in our factory. And we can calculate the impedance control details for you for free.

Impedance control tester

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How to avoid fake exposed copper for Flex-Rigid PCB?

Did you meet the situation? When your Flex-Rigid PCB product produced, but PCB vias has fake exposed copper phenomenon, which might caused worse problem. (Such as below picture show)

Detailed picture for fake exposed copper

Today we’ll share some info with you, wish it can be helpful to know more about it.

Why fake exposed copper happens? There are two main reasons:

  1. Reason one: Just use screen to plug via holes in the solder mask process(have not used aluminum pluged), which caused the solder mask poor in via holes and the hole edge have fake exposed copper.

2) Reason two: When the boards printed not so well on first time, need to be washed and perform a second printing. If the solder mask in holes didn’t clear up, it will cause the solder mask cannot plug through the holes, plug holes not full, then fake exposed copper occurs.

fake exposed copper

How to improve this situation?

1) Suggestion one: Plug vias with resin, and then do the solder mask. It can avoid fake exposed copper.

2) Suggestion two: Can considering change the stack up, such as change 4 layer flex-rigid PCB(1L PCB + 2L FPC + 1L PCB), replace the TOP/Bottom PCB layer with FPC. And use coverlay, the coverlay can cover the vias, which can help solve this problem.

If you have any further question, pls feel free to contact us.

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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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