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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The different design way of pluggable connector on FPC

Flexible printed circuit board (FPC) is widely used in smart phones and LCD TVS. With the extensive use of electronic products in modern society, the demand for FPC circuit board increases greatly. More and more FPCS require connectors to be assembled at both ends, easy to connect with other devices or PCB board, and FPC can be bent in the middle, reducing the space required for assembly, see below FPC samples with different connectors.

However, FPC assembly materials are not as easy as PCB, because FPC assembly requires stiffeners or fixtures on the bottom of component areas. Especially for assembling the pluggable connectors, it needs strong support and good connectivity. Do you know the different designs of the connector area and what are the differences between them?

The first and most common design, we add a FR4 stiffener on bottom side, drill holes on FR4 stiffener in the corresponding positions of the holes, noted that there is no copper for vias of FR4 stiffener, the function of FR4 stiffener is to support the component on top side, it often used for plug-in connectors.

The second design is to make the stiffener as a single-sided FR4 PCB, there is one layer of copper, and copper plated for vias. Then laminated FPC and stiffener together, and filled with solder paste in vias when assembling connectors, the connector’s pin and stiffener can also conduct except support function. However, the FR4 stiffener and FPC copper traces are not conductive.

The third design is to make it as a rigid-flex PCB, 2 layers FPC with 1 layer FR4 PCB. Although there are only vias on FR4 PCB, but the PCB through hole can be conductive to the FPC copper traces. This design has good stability and conductivity, but it is the most expensive, the cost is 3 times higher than the first design, and 2 times higher than the second design.

Do you have any FPC project need to do assembly together? Best Technology can provide FPC, FR4 PCB, Rigid-flex PCB and SMT service, 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 ‚ÄúIP‚ÄĚ 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 Р1.5 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‚Äôs navigational signals. This is the reason why EMI/EMC study and analysis is important. Does your product‚Äôs 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¬į 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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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‚Äôt suggest to to add the Stiffeners ?

¬†Small ‚Äústatic‚ÄĚ 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‚Äôs 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 ¬įF. 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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