Factors Affecting the FPC Connector

FPC connector is used to the connect LCD (Liquid Crystal Display) to drive circuit. It is mainly applied to various digital communication devices, portable electronics, cartronics and so on, which has become an indispensable component in digital domain. However, it is undeniable that the performance of FPC connector is often affected by various factors. And the main points are as follows.

FPC with Connectors

Quantity of Solder When Welding

In the welding of FPC connectors, it is necessary to pay attention not to adding too much solder in reflow soldering when welding the connector on the FPC, otherwise, the connector terminal cannot be welded stably and insufficient soldering phenomenon may also occur.

Number of Usage Times

When the FPC connector has been used for a long time, its lifetime will absolutely be shortened due to the wear of the needle core. So, do not insert the FPC connector when it is not needed, which helps to extend the service life of the connector.

To sum up, there are two major points that will affect the FPC connector, including the quantity of solder when welding and the number of usages times, which are necessary to be paid attention to in order to have a better performance of the FPC connector.

And Best Technology is experienced in the electronics industry so that if you have any questions about Flexible Printed Circuit, you are welcome to contact us.

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What is the difference between solid copper and grid copper?

There are two different design of copper trace, namely solid copper and grid copper. Do you know what is the difference between solid copper and grid copper?

Solid copper has the dual functions of increasing current and shielding, but if solid copper is used for wave soldering, the board may lift up and even blisters. Low-frequency circuits have circuits with large currents, such as commonly used copper foil. Below is a sample picture of 2 layers FPC, the top layer is copper trace, the bottom layer is a solid copper/copper foil.

The grid copper is mainly used for shielding, and the effect of increasing the current is reduced. From the perspective of heat dissipation, the grid is good (it reduces the heating surface of the copper) and plays a certain role in electromagnetic shielding. But we need to pay attention to the spacing of the grid: if it is too small, this may cause inaccurate alignment of the top and bottom copper layers especially for multilayer board; If it’s too big, there’s no shielding. Below is a sample picture of 4 layers FPC, Layer one, layer two and layer four are grid copper layer.

Different copper trace designs also have an effect on impedance. For the same projects and materials, if the reference layer is solid copper and grid copper, the impedance they reach may differ by 20-30 ohms. It also has a certain relationship with the trace width and space of the grid copper layer. In general, the greater the impedance requirement, the grid copper should be selected, and the solid copper should be selected when the impedance is small.

If you have any question about solid copper or grid copper when you design, please feel free to contact us at sales@bestfpc.com

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What should we pay attention to FPC with dome switch design?

Applications for flex circuits boards incorporating dome switches are extremely common now. The combination allows for reduced space requirements and design flexibility in many handheld devices. When comes to a FPC design, or some extremely complex rigid-flex circuit board designs. Any additional connectors or cabling between a rigid PCB and the dome switch area is eliminated thus saving space/weight and additional assembly costs. Then more and more applications require a dome switch mounted to a flexible circuit board.

Flex circuit board with metal domes

Gerber Layout Requirements

Creating the Gerber data set for a flex PCB applications is straightforward with only a couple of additional considerations. The footprint for the dome switch is defined by the supplier and will vary depending upon the specific switch selected.

A key element is to define the coverlay opening so that the entire footprint is exposed rather than having independent openings for the center and surrounding contacts. This prevents the taller height of the adjacent coverlay from interfering with the operation of the switch ensuring a reliable actuation. Also, having two separate openings is not manufacturable when using a polyimide coverlay or soldermask.

Top side coverlay openning

A second element is including the vent hole. This prevents air entrapment which will negatively impact the switch operation/feel and potentially cause the overlay layers to separate from the flex circuit. It is preferred to have the vent hole as a plated hole in a 2-layer flex design. For a 1-layer design, it will be non-plated.

How the flex circuit board will be mounted in the assembly and what the sealing “IP” rating requirements are, to prevent the ingress of moisture, dirt and etc. This will define the amount of spacing required from the edge of the switch footprint to the outline of the flex circuit. 3mm spacing is a typical minimum spacing that will allow the dome switch overlay to seal to the flex circuit and may accommodate and additional sealing methods used in the final enclosure. More space may be required depending upon the application.

Flex Circuit Board Materials

The flex circuit board materials required for a specific design are usually defined by design elements other than that required to meet the dome switch specifications. In most designs, a flex tail is incorporated which then extends out to then into a connector on the control PCB. The bend requirements of this tail area, if excessively tight or complicated, we suggest to use of thinner than standard flex materials to reliably meet the minimum bend radius. There are wide variety of materials available. If the tail is to plug into a ZIF connector then an added polyimide stiffener, in the ZIF contact finger area, is required to meet the connector specifications. If the connector is an SMT or PTH male/female header type, then an FR4 stiffener will be needed to support the connector.

Stiffener & PSA Requirements

Many flex circuits with dome switches require additional stiffeners or PSAs, pressure-sensitive adhesives. An additional stiffener may be necessary to provide support for the force applied when actuating a dome switch if the enclosure is not configured to do so. These stiffeners would reside behind the switch area and be thick enough, 1mm – 1.5 mm, so as not to allow any deflection.

The ressure-sensitive adhesive resides behind the switch area and can be used in conjunction with a stiffener if a specific thickness is required to fit the enclosure. The most common PSAs used are 3M 467 and 3M 478 for flex circuits that do not have any component assembly and 3M 9077 for designs that require SMT assembly. Other adhesives are also available for unique requirements.

Flex circuit board with stiffeners.

Flex circuit board with PSA.

 

The FPC with metal dome design process is not overly complex but does have several elements that need to be evaluated and incorporated to ensure a successful design. Best Technology can support a dome switch project by providing complete or partial design services as part of our user interface product line. Please feel free to contact us at sales@bestfpc.com if you have any questions or require design support in developing a dome-switch-based flex circuit.

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What you need to know about PTH and NPTH holes?

Plating copper through-holes also named vias is a requirement for double-sided and multilayer circuits. Becuase one copper layer to another copper layer stack up as below, the midlayers has a Polymide(PI) regardless of adhesive or adhesvieless stack up.

2 layers adhesiveless stack up

Copper is a conductor, Polymide(PI) is an insulator. Then we need to drill a PTH hole to make the copper layer to copper layer connect together. Below picture is a multilayers flex pcb with PTH holes.

4 layers FPC with PTH holes

NPTH (Non Plating Through Hole) refers to a hole without copper in the borehole wall. It is generally used as the positioning hole and screw hole of PCB. The hole diameter is usually larger than PTH. The easiest way to distinguish between PTH (Plating Through Hole) and NPTH (Non Plating Through Hole) in PCB is to see if there are any traces of plating on the borehole wall in the PCB. Please kindly check below picture.

Holes

The advantages of hole plating

The point of plated through holes is so you can use both sides of your printed circuit board and connect to other layers of the board. The plating on the through holes is copper, a conductor, so it allows electrical conductivity to travel through the board.

Non-plated through holes do not have conductivity, so if you use them, you can only have useful copper tracks on one side of the board. You cannot connect to the other side or other boards because there is no way for electricity to travel through. You can use non-plated through holes either to affix a PCB to its operational location or to mount components, but not to connect to other boards or the other side of the board.

The risks of hole plating

All products that contain printed circuit boards are subject to the thermal cycling effect. When we power them up, they heat up until powered down, which is when they cool. As the product is heating up, so is the printed circuit board inside it. Over time, with the board continually heating up and cooling down, the copper of the plated through hole can become fatigued and crack.

The thicker the copper plating the through hole, the longer it can go through this thermal cycling without cracking. Since this cracking will ultimately lead to failure, the life of the printed circuit board in the product is linked to the thickness of the copper plating of the through hole.

Plated through holes for IPC class 3 different types:

IPC Class 1: The least thick and the shortest-lasting, usually reserved for consumer electronics that are likely to become obsolete in a couple of years.

IPC Class 2: Longer-lasting, continuous-use holes for products like computers or copy machines that will be in frequent operation for five years or more.

IPC Class 3: The thickest and longest-lasting of plated through holes, for products that are expected to last ten years or more.

IPC Class I and II plated through holes require an average thickness of 20 microns, with spots no thinner than 18 microns, while IPC Class III holes require an average of 25 microns, with spots no thinner than 20 microns.

If you have any questions about PTH or NPTH holes, please contact us at sales@bestfpc.com

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What should we pay attention to EMI shield design?

Electromagnetic interference (EMI) is associated with every electronic device we use nowadays. If you turn on your radio set and TV simultaneously, you will experience the noisy disturbance from TV interfering with the radio signal and vice-versa. We can also experience this when we board a plane and are asked to switch off the electronic devices by the crew. This is to avoid interference of mobile and electronic device signals with the plane’s navigational signals. This is the reason why EMI/EMC study and analysis is important. Does your product’s radiation disturb other devices present nearby?  

EMI Shielding Design Challenges

As we all know, the flex circuit EMI shielding added will create multiple design challenges that require careful review to ensure a successful part number. All EMI shielding will increase both the total flex circuit board thickness and cost. The thickness increased is most often the critical issue. The normal EMI shield thickness is 22um, but we also has 10um thickness EMI shield. It can easily lead to the bending effect get worse. This creates a reliability/mechanical breakage concern. The added cost is also should concern. The

Shielding is often combined with other electrical requirements; the most common is controlled impedance. This further increases the flex thickness and compounds the challenge of meeting both the electrical and mechanical design requirements.

The flexible circuit industry has multiple solutions that can be applied, which will eliminate both the absorption and or radiation of interference noise.

What should we pay attention to EMI shield design to avoid the interfering?

  1. Keep your signals separate. Keep high speed traces ( clock signals) separate from low speed signals, and analog signals separate from digital signals.
  2. Keep return paths short.
  3. Route differential traces as close as possible. This increases the coupling factor, bringing influenced noise into the common mode which is less problematic for a differential input stage.
  4. Use vias wisely. Vias are necessary because they let you take advantage of multiple layers in your boards when routing. Designers must be aware that they add their own inductance and capacitance effects to the mix, and reflections can occur from a change in characteristic impedance.
  5. Avoid using vias in differential traces. If you must, use an oval anti-pad shared by the two vias to reduce parasitic capacitance.
  6. Singled sided FPC EMI shield is not working, you need design it as a dual flex pcb at least or double sided or multilayers FPC. Below is a dual flex pcb with EMI shield stack up.
dual flex pcb with EMI shield stack up

7. Avoid sharp right-angle bends. Capacitance increases in the 45° corner region changing the characteristic impedance and leading to reflections. This can be mitigated by rounding right angles.

8. EMI minimum solder mask opening should be more than 0.8mm, and the solder mask area need to far away from the trace more than 0.2mm. Below design solder mask opening is less than 0.8mm, it will be not able to arrange production.

EMI open solder mask design issue

If you have any EMI shield design questions, warmly welcome you to contact us. Once we received your news, we will reply to you immediately..

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Which kind of FPC can be replaced by FFC?

FFC (flexible flat cables) are a type of ribbon cable, so named due to their wide flat structure. They are usually a straight connector, without any additional components. The lines are straight and the spacing is fixed.

Standard FFC BFS17386-B

FPC is made of polyimide material and etched copper conductors. FPC can accept more severe tolerance than FFC and is also available in 0.3mm thick versions. There are many different kinds of FPC, including single layer, double-sided, multilayer flex circuits.

Standard Fpc BFD21076

The cost of FFC will be much lower than FFC, and the flexibility and connectivity are almost same. Normally the FFC are used in plug connectors, there are golden fingers on both side, golden finers can be in different directions; all traces are straight and the spacing is fixed, like below picture. If your FPC project is similar to this design, no component needed, then you can design it as a FFC.

FPC with EMI shield BFD17016

The black film on above FPC, it’s EMI shield, if you want FFC to have a shielding effect as well, we can add the aluminum foil on FFC, like below picture.

FFC with aluminum foil BFS20233

If you have similar design or want to know more details about flexible flat cables, please feel free to contact us 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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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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