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.

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

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.

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

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.

What do you know about the connection ways of PCB panel?

As we all known, some customer need to do following assembly in their side when received the bare boards. In order to facilitate customers for subsequent assembly, we’ll ship out boards in panels. So, what’s the difference between FPC and PCB paneling connection mode?

  1. For FPC , we often use connecting tabs to connect the boards. 
FPC connecting tabs
  • For Rigid PCB, we often use V-cut and connecting tabs(with stamp holes) to connect the boards.
PCB V-cut
  • For Rigid-flex PCB, we’ll use connecting tabs(without stamp holes) in flex part, and use connecting tabs(with stamp holes) in Rigid part.
connecting tabs(with and without stamp holes)

Take the Rigid-flex PCB for example, if the boards are easy to fallen from panels when do assemble, how to improve the design?

There are two direction we can go, one solution is add more connecting tabs in paneling, another solution is broaden the width of connecting tabs.

Hope our suggestion are helpful for you. If any further question want to discuss with us, pls feel free to contact us at sales@bestfpc.com.

Why the FPC or rigid flex add the EMI shield?

Do you know what is the EMI shield? Do you know what is the EMI shield used for  and what kind of products need to add the EMI shield? Today let’s know why the FPC or rigid flex add the EMI shield together.

What’s EMI?

When your flexible PCB or rigid-flex PCB application requires limits in electromagnetic and /or electrostatic interference, EMI shielding should be applied.

Electromagnetic interference (EMI) also called radio-frequency interference (RFI), when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade and effect the performance of the circuit or even do not get work. EMI is caused by electrical pulsing.

What’s is our EMI shield?

With EMI shielding film design, the Flexible PCB has best flexibility as well as the thinnest overall thickness. Compared to silver ink printing and copper layer design, the flex circuit cost will be lower, and the shielding capability will be in a higher degree. No need extra shielding material, like metal cover. EMI shielding film such as Tatsuta can replace the copper layer design, then to save cost without compromising on the flexibility of the board.

EMI consists of an additional layer of selectively specialized laminate on the surface of the coverlays. It has 3-layer construction consisting of an electrically conductive adhesive, a reinforced layer and an insulation layer. The insulation layer is black in color as well as friction resistant. The EMI shielding film is laminated under heat and pressure. The conductive adhesive flows into the coverlay opening which then adhere and electrically connects to the ground. Attached the EMI shield stack up for your reference, hope it will be helpful to you.

EMI shield

The normally used EMI shield is TSS200 from Toyochem. The thickness is 200um. It has the excellent flexibility, electrical conductive reliability, chemical resistance, non-Halogene and UL type. If you would like to learn more details, please contact with me.(sales@bestfpc.com), we will be very glad to share the TS 200 EMI shield specification with you. Attached some of our FPC and Rigid flex with EMI shield for your reference, hope you will be interested in.

Rigid-flex pcb with EMI shield
FPC with EMI shield

EMI Shielding application filed

Our EMI shield required customers are mainly specialize in wireless communication, 5G phone, radar, drone, camera and new energy automobile. EMI shield is becoming more and more popular in the designs of flexible printed circuit and rigid-flex printed circuit board. Many flexible PCB or Rigid-flex circuits are sensitive to either absorbing or emitting EMI. If EMI shield is out of control, it can negatively impact the performance of the design and or effect the circuit function.

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.

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.

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