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.

Imaging Process for Flexible Circuits

One of the first steps in manufacturing a flexible circuit is to build a circuitry pattern. In order to define the circuit pattern correctly, one needs to understand the basic process for building a circuit. Processes tend to fall in one of the following two categories:

1. Subtractive

The substrate begins with copper bonded to the dielectric, the unnecessary metal is removed; the remaining metal defines the circuit traces. The basic process steps for a subtractive process are:

Create resist pattern

Etch away exposed metal

Remove resist

Continue to process

Many possible methods are adopted for the creating the resist pattern, the following are two of the more common ones.

I. Screen Print

Screen print resist pattern

Cure resist

Etch

Remove resist

II. Photo Image

Apply resist coating

Photo expose image onto resist

Develop away unexposed resist

Etch

Remove resist

Screen printing stands for a tool made by forming a negative of the desired pattern on a woven screen mesh. The resist material is pushed through the open mesh and forms the circuit pattern on the substrate. The initial state of the resist material is normally a paste or semi-liquid. After the material is applied to the substrate, the resist needs to be cured. The cured resist protects the covered copper areas during the etching process.

In regards to photo imaging, a photo tool is created that is the negative of the desired pattern. The photo tool is placed over the resist coated substrate and flooded with a UV light. The photo tool allows the light to cure the resist where the pattern is to be created and blocks the light everywhere else. The UV light causes a chemical reaction in the resist and transforms it to have the chemical and physical properties to withstand the etching process. The developing solution strips away all the “un-cured resist” exposing the bare copper to be etched away.

2.Additive

The substrate begins with just the bare dielectric (possibly with a seed coating), the metal is added to the surface to directly form the circuit pattern. The basic process steps are:

Apply conductive material

Cure conductive material

Continue to process

The additive material is usually a paste and is often applied by screen printing. The applied paste is then cured to reach its optimal properties. Materials that have outstanding conductive properties tend to require very high temperatures to cure. The dielectric substrate for flexible materials is a limiting element for the temperature level and tends to restrict the types of additive materials that are used to create a circuit pattern. Generally speaking, the electrical, physical and chemical properties of this type of circuit are significantly lower than the properties achieved via the subtractive process.

The reason for the problem of development is not clean and has Residual glue in FPC?

How to deal with these situation?

  1.  How to deal with these situation?There have glue mark and greasy dirt on the boards in prepared processing stage, it need to clean the plate thoroughly.
  2. The temperature is too high and time is too long in the stage of Pre-heating, so it need to control the temperature of the pre-drier, adjust it on normal temperature and strengthen the control of time.
  3. The exposure energy is too high on the stage of exposure, or the shading rate is poor of the base boards.
  4. The developing liquid’s concentration and the temperature are too low, so it need to check the concentration and adjust the temperature to normal timing.

FPC Typical Production Process

FPC Typical Production Process

Cutting -> NC Drilling -> Black Holes/Cu Plating -> D/F Lamination -> LDI Exposure ->Develop/Etch/Stripping -> Automatic Optical -> CVL Lay UP -> CVL Lamination -> Hole Punching -> Surface Treatment -> Screen Printing -> Open/Short Testing -> Adhere Stiffener -> Outline Punching -> Final Inspection

FPC Typical production process flow chat

FPC Typical production process flow chat

 Meanwhile we also can visit our website for single side Flexible PCB,dual access Flexible PCB,double side Flexible PCB or flex rigid PCB.

Differences between Roll Anneal Copper Foil and Electrolysis Deposition Copper Foil

Differences between Roll Anneal Copper Foil and Electrolysis Deposition Copper Foil

Roll Anneal Copper Foil (RA copper):

Formation: Roll copper block many times, and then do the annealing treatment with high temperature. The shape of its crystal is lamellar structure.
Advantage: Soft, smooth surface, suitable for flexible PCB and small lines.
Shortage: high cost

Electrolysis Deposition Copper Foil (ED copper):

Formation: Melt copper into dilute sulfuric acid, it becomes bluestone solution.
Through a high electric field, copper attach on the metal drum. With the rolling of metal drum, thin copper foil finally formed.

Advantage: Low cost
Shortage: Unsuitable for small lines; bad bending resistance.
Here is the cross section observation of RA Copper and ED Copper:

roll anneal copper foil and electrolysis deposition copper foil