What’s the difference between Adhesiveless and Adhesive Polyimide (PI)?

Today I’d like to share what’s the difference between Adhesive PI and Adhesiveless PI.

Item 1: Adhesiveless PI stack up

Adhesiveless PI stack up
Adhesiveless PI stack up

As you can see in above stack up, there is no adhesive connect the Copper layer and Polyimide.

There are four advantages of adhesiveless PI:

  1. Thinner Thickness

As you know, the thickness of adhesive is around 12.5um, 15um, 20um. If you have strict requirement for the whole board thickness, it will be a good choice to use adhesiveless PI.

2.Better Bending

Adhesiveless substrate is thinner than adhesive substrate, so it’s bending is better.

3.  Heat resistance

Due to the adhesive have poor performance in heat resistance, so without the adhesive, the adhesiveless base material have better heat resistance.

Under the same temperature, such as 200℃the tearing strength of adhesiveless PI change a little. But the tearing strength of adhesive PI material rapidly decrease.

4.  Dimensional stability

The dimension of adhesiveless board change very small when the temperature increase. Even under the temperature of 300℃, it’s dimensional changing rate is no more than 0.1%.

Good dimensional stability will be great help for fine wiring process.

5.  Chemical Resistance

The adhesiveless substrate have good performance of chemical Resistance, it’s tearing strength have no obvious change with time increased.

The adhesive base material has poor chemical resistance, so it’s tear strength decreases greatly with the increase of time.

The only disadvantage for adhesiveless PI is it’s a little expensive than adhesive PI.

Item 2: Adhesive PI stack up

Adhesive PI stack up
Adhesive PI stack up

As you can see, the thickness of  2 layer adhesiveless FPC is 0.19+/-0.03mm.

The thickness of  2 layer adhesive FPC is 0.23+/-0.03mm.

If you need thicker thickness, the adhesive PI might can meet your requirement, the cost will be much lower than increase copper thickness or PI thickness.

There are two mainly advantages for adhesive PI:

  1. Thicker thickness.
  2. Cheaper price.

Anyway, if your project need thinner thickness or smaller line width and space (0.05mm), adhesiveless PI will be the best choice. Welcome to contact us at sales@bestfpc.com if you need to know more details.

What is the Dual access flexible circuit?

Have you ever been heard about the dual access flexible circuit? Why it named dual access flexible circuit? After you read my e-mail, you will find the answer.

Dual access flexible circuit board is similar to double side FPC, but it has different stack up to double side FPC. Why it named dual access FPC? Dual access can access to both side by copper, so it named the dual access flexible circuit. Dual access flex PCB only have 1 layer copper, and coverlayer on both side. They can connect both side by layout through coverlayer opening, and do not need to drill the holes like 2 layers flex PCB to connect both side.

Dual access flexible circuit samples picture:

Dual access flexible circuit sample
Dual access flexible circuit sample
Dual access flexible circuit sample
Dual access flexible circuit sample

Working principle: The coverlayer play the role as an insulator, the copper is a very good kind of conductor. One side is anode, and the other side is cathode. If they the current flow, they form a closed loop circuit.

Stack up of dual access flex circuit:

Stack up of dual access flex circuit
Stack up of dual access flex circuit

Dual access circuit advantages:

  1. It can be connected to both side through copper, and do not need to make a through hole. If it is 2 layers FPC. You have to make a through hole to connect both side.

2. The appearance will be more beautiful than 2 layers flex PCB. Because we can conduct by trace, it will be more beautiful than through holes to conduct.

Dual access circuit disadvantages:

  1. Complex production process: The production process is similar to 2 layers flex circuit, but it will be more difficult and complicate compared with 2 layers flex PCB. It is easy to scrap. And many factories can not make dual access circuit board due to production technique.
  2. Easy to wrinkle. Because the dual access is very thin. Only one copper and two coverlayers, it will be easy to tear up. When it comes to laminate process, is also very easy to laminate wrinkles and blister.

If you have any questions or would like to learn more details, please feel free to contact us.

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!

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

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 Âœ 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 ÎŒm=.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.

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.  

FPC Flexibility Influencing Factors

During PCB design, FPC flexibility plays an important part and FPC flexibility is influenced by the followings:

(1)From the perspective of FPC material itself, FPC flexibility is influenced by the followings:

First, molecular structure and direction of copper foil(i.e. copper foil types);

Folding strength of RA copper is obviously superior to ED copper.

Second, thickness of copper foil;

In terms of the same type, the thinner the copper foil, the better the folding strength.

Third, types of adhesive used by substrate;

flex pcb
flex pcb

As a general rule, the flexibility of epoxy glue is better than that of acrylic adhesive. So, if high flexible material is required, epoxy glue is recommended. Moreover, the adhesive with high tensile modules can improve flexibility.

Fourthly, the thickness of adhesive;

The thinner the adhesive, the softer the material. Thin adhesive can improve FPC flexibility.

Fifth, insulation substrate.

flex pcb
flex pcb

The thinner the insulation substrate(PI), the softer the material. Thin insulation substrate can improve FPC flexibility. And FPC flexibility will get better if PI with low tensile modules is used.

In conclusion, for material, type and thickness are the most important 2 factors influencing FPC flexibility.

(2)From the perspective of FPC technology, FPC flexibility is influenced by the followings

First, symmetry of FPC combination;

After substrate is pasted with coverlay, good symmetry of material on both sides of copper foil can improve flexibility. This is because they bear the same stress when bending. PI thickness on both sides of PCB tends to be the same, and thickness of adhesive on both sides of PCB tends to be the same.

Second, control of lamination technology.

During coverlay lamination, the adhesive is required to be completely filled into the middle of the trace, and there can be no delamination. If there is delamination, it is equivalent to bare copper bending, which will reduce the number of bending.

If you have any queries, comments or suggestions on FPC flexibility, feel free to contact us, and we have 24-hour online customer services.

How to calculate bending radius of fpc?

 The most frequently asked question we received regarding fpc is “how much can I bend a fpc?” So, we would like to share how to calculate bending radius of fpc with you today.

fpc
fpc

When fpc is bending, the stress borne by both sides of center line is different. Pressure is inside of bending face, while tension is outside. The stress has something to do with thickness and bending radius of fpc. Excessive stress will lead to delamination of fpc, fracture of copper foil and so on.Therefore, the laminated structure of fpc should be arranged reasonably in the design, so that the lamination at both ends of the center line of the curved surface should be symmetrical as far as possible. At the same time, the minimum bending radius of fpc should be calculated according to different applications.

Situation: the minimum bending radius for single-sided fpc:

 Bending Radius for Single-sided fpc
Bending Radius for Single-sided fpc

  The calculation methodR=c/2[100-Eb/Eb]-D
WhereinR=Minimum Bending Radius(unit:”m); c=Copper Thickness(unit: ”m); D=Thickness of Coverlay(unit:”m); EB=Copper Elongation Desired(Measured as a Percentage)
Copper elongation desired also differs among various types of copper.

A.The maximum copper elongation desired for RA Copper ≀16

B.The maximum copper elongation desired for ED Copper ≀11

Moreover, value for copper elongation desired for the same material will be different in different applications. For one-time bending, the limit of the critical state of fracture is used (for RA Copper, the value is 16%). For bending installation design, use the minimum deformation value specified by IPC-MF-150 (10% for RA Copper).For dynamic flexible applications, copper elongation desired is 0.3%. For magnetic head applications, copper elongation desired is 0.1%. By setting the copper elongation desired, the minimum bending radius of fpc can be calculated.

Dynamic flexibility: In these applications, copper functions through deformation. For example, the phosphorus and copper metal dome in the IC card seat, i.e. the part where the IC card contacts with the chip after being inserted, and the metal dome is constantly deformed in the process of inserting. This kind of application is flexible and dynamic.
If you have any queries or comments on bending radius of fpc, contact us anytime.

Do You Know the Standard Flex Stackup for 1 Layer, 2 Layers, or Multilayers?

Standard flex stack up for 1 layer flex pcb:

1 layer flex pcb stackup
1 layer flex pcb stackup

This is the simplest flex pcb. Generally, base material and adhesive and copper foil are used as raw materials, or protective film and adhesive are used. Firstly, copper foil needs to be etched to get traces required. Protective film needs to be drilled to expose pad. Both are combined by using rolling process after cleaning, then the exposed pad should be electroplated with gold or tin for protection. In this way, the flex PCB is finished and can be punched into small flex pcb.

Standard flex stack up for 2 layers flex pcb:

2 layers flex pcb stackup
2 layers flex pcb stackup

When the traces of flex pcb are too complicated, the 1 layer flex pcb can not meet the needs of wiring, or needs copper foil for grounded shield, 2 layers flex pcb or multilayers flex pcb can be used.

Standard flex stack up for multilayers flex pcb:

The via holes are added to make multilayers flex pcb different from 1 layer flex pcb, so that copper foil in each layer can be connected. Generally, for base materials and adhesive and copper foil, the first processing technique is to make via holes. The base materials and copper foil should be drilled firstly and then electroplated with copper of certain thickness after cleaning. Thus the via holes are finished. The subsequent manufacturing techniques of multilayers flex pcb are almost the same with 1 layer flex pcb.

For example, here is the standard flex stack up for 4-layer flex pcb

4 layers flex pcb stackup
4 layers flex pcb stackup

There may be some differences among the stackup for 1 layer flex pcb, 2 layers flex pcb and multilayer flex pcb, but similarities also exist among many manufacturing techniques of flex pcb. Different techniques are just added in some basic places, so as to be applied in different fields.

If you have any queries or comments about flex pcb, welcome to contact us anytime.

How to Improve Reliability & Flexibility in an 8 Layer Stackup Flex PCB?

When designing a flex PCB stackup, you may face some restrictions in enhancing the reliability of the board. Today, we will discuss the ways to improve the reliability of a board by increasing the flexibility and improving design considerations of a multilayer PCB stackup. These steps can be implemented for PCB stackup designs as seen in 8 layer flex PCBs.

flexible circuits

flexible circuits

Improve Flexibility by Reducing Overall Thickness

All or some of the below mentioned steps can be executed to lower the overall thickness of the PCB. A thinner PCB stackup will allow it to be more flexible.

1.Reduce the base copper weight.

2.Reduce the adhesive thickness.

3.Reduce the dielectric material thickness.

4.Using adhesiveless base materials can reduce the thickness of the substrate by 1-2 mils, which is almost 25-50Όm.

5.Make use of selective plating to avoid copper plating the conductors.

Improve Reliability by Reinforcing the PCB

Here, the boards, components, and the layout should be designed to deliver a robust structure that can withstand flexing.

1.The weight on both sides of the PCB should be equal. This includes the weight of the conductors and the material.

2.The conductors on different layers of a PCB should not be placed on top of each other. Instead, they should be placed in a staggered pattern.

3.Conductor thickness should always be constant in and near the bend areas.

4.There should be no plated-through holes in the bend areas.

5.There should be no discontinuities near the bend.

6.Traditional copper can be proved to reduce the flexibility of a PCB. Opt for screened-on coatings like silver epoxy.

7.For slits in the circuit, reliefs or tear stops need to be built into the PCB.

flexible pcb

flexible pcb

Tight tolerances will have to be put in place to ensure that your stacked up PCB can provide flexibility as well as reliability. Several of the above-mentioned steps can easily be incorporated in the designing process itself. Your PCB manufacturer should be able to help you with the most appropriate improvements for your application.