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