User:Realhon5/sandbox

'''

PCB Inner Layer Imaging Process
Disclaimer: This content is intended only for internal use at Continental Automotive and shall not be distributed outside of Continental Automotive.

The multi-layer PCB is manufactured from inner to outer. After board cutting process, the bare board will be sent to inner layer imaging process.

Imaging technology introduction
By removing the unwanted copper of the bare board, the PCB circuit pattern is created. This is called the subtractive method. It is widely used in the PCB industry for the imaging.

In the inner layer process, the etching resist material will be coated evenly on the copper clad laminate surface. The resist material is often photosensitive which can be applied into pattern transfer with photo tooling and UV exposure machine. After etching the unwanted copper, the inner layer is finished.

Typical process flow
After board cutting, the bare board will be sent to the inner layer imaging process:

(Incoming)à Pretreatment à Dry film laminationà Artwork checkà UV Exposureà First Article checkà DESà (next process)

12314124124124

Equipment and tooling

 * Clean room

In order to fulfill the material control requirement and to reduce the defect rate, a class 10,000 clean room should be set up. Typical temperature control should be 20+/-2C and the humidity should be 50+/-10%. Positive air pressure is a must for the clean room to the external.


 * Plotter

A laser plotter is applied for transferring the inner pattern on to silver halide film. It is connected to the CAM data station. And it may not be setup in the inner layer clean room due to the higher temperature and humidity requirement of the silver halide film and medicine.


 * Pretreatment machine

Either mechanical treatment or chemical treatment machine will be applied to clean the bare board surface and to gain a rougher surface for the dry film lamination.


 * Cut-sheet laminator

An automatic machine can laminate the dry film and cut the film into right size for the panel.


 * UV exposure machine

This machine provides the UV energy for the dry film for reaction. Collimated exposure machine is applied for the fine line product. The scatter light system needs strict process control to ensure the quality.


 * DES machine

This machine is widely used for combining the Developing-Etching-Stripping process together to reduce the handover problem.


 * Artwork tooling

For the inner layer imaging, the book printing method is applied. 2 pieces of silver halide film (e.g. Layer 2 and Layer 3) are combined to be a book for production. Pressing offset must be calculated and added into the film at the sample stage. The offset happens during the pressing stage due to the heat treatment, and the inner pattern has to be adjusted to fulfill the final pattern size requirement.

Pretreatment
The inner layer manufacturing process starts from the pretreatment process. The purpose of this process including 2 parts:

To remove the grease, salt and oxidation on the copper surface;

To reshape the copper surface and get large surface area to enhance the adhesion to the dry film.

The bare board must go to pretreatment before dry film lamination. Any dust or residue on the copper surface will lead to open/short problem after etching.

There are generally 3 types of pretreatment:

Mechanical: A brush only cleaning system. It is easy to use but need frequent maintenance for the brush. It is not suitable for the fine line the thin products due to its surface is not quite even.

Chemical: A chemical treatment system. The copper surface could be evenly treated and is good for producing fine line products. Its limitation is not quite adhesive to the dry film and the surface is quite easy to get oxidation. And its cost is high not only for the chemicals but also the waste treatment.

Pumice: A pumice spraying treatment system. The copper surface could be evenly treated and it is good for producing the fine line products. The adhesion capability of the treated surface is good and not easy to get oxidation. Its limitation is residue problem. High water pressure treatment should be applied to reduce the residue.

General control items:

Visual inspection: To check the copper surface by visual inspection after pretreatment;

Water break test: After pretreatment, to dip a sample board into the water pool and pull it out. There is a water mask on the copper surface. This test is to count the water mask breaking time to test the cleanliness of the copper surface. Over 18 seconds is a must for a pumice treatment surface.;

SEM (Scanning Electronic Microscope): This is one method to check the roughness of the copper surface;

Drying test: To press a sample board onto a white paper to test if the drying system could fully dry the board.


 * ==== Dry film lamination ====

After pretreatment, the board is clean and the further processes are in the clean room. An automatic cut sheet laminator could be connected directly with the pretreatment line. The purpose for this process is to press the dry film evenly on to the bare board with high pressure hot rolling. The dry film is water soluble, UV reacted and etched resist material.

There are a few steps in using the automatic cut sheet laminator:

Alignment: The board will be aligned to the central;

Lamination: The dry film will be rolled on to the bare board.

Cutting: The edge of the board is auto detected and the dry film is cut according to the length of the PCB.

Cooling: The board is cooled down by a cooling section or just be hold in the production line. Cooling before exposure is a must for the heat is affecting the adhesion of the dry film.

General control items:

Holding time: The holding time from dry film lamination to the exposure is normally within 24 hours (or other requirement for different dry film). There is dry film residue problem after long time holding.

Rolling pressure: Too high pressure destroys the dry film. And there is adhesion problem if the pressure is too low.

Exit temperature: The exit temperature just after lamination is the quality criteria of the dry film lamination. Different dry film has a different exit temperature requirement.

Visual inspection: Foreign material on or under the dry film will be checked.

Artwork preparation

Before going to the UV exposure, the artwork tooling is prepared.

The film with circuit pattern is often called artwork. The inner layer film is plotted by the plotter which is connected to the CAM station. It is the original silver halide film. The artwork must be checked before exposure for mass production to prevent the batch open/short problem. The artworks are often checked by visual. General data such as adjusted length of the board, light density, defects of film dents, scratches, pin holes, color variation are filtered in this process.

Recent development for the artwork checking is to use the film checking machine. And it is a similar machine to the AOI to reduce the manual mistakes.

UV exposure

The continued process of dry film lamination is UV exposure process. The inner layer image is transferred from the artwork to the dry film with the UV energy.

Reactions:

The monomer of the dry film is a liner type molecule. Free radical will be produced after UV exposure and trigger the cross-linking reaction. With under UV light (wave length from 310~430nm), it will transfer the energy to the Monomer (free radical).

Molecule structure will be changed from liner type to cross 3D type. And finally the dry film is hardened and resists etching.

Book printing method:

2 inner artwork films are paired and turned into a printing book. The board is inserted into the book for exposure. And that’s the common way for the inner layer mass production.

General control items:

UV energy: Different dry film requests different UV energy. And it is commonly checked by a Stouffer 21-step film.

Uniformity: The exposure uniformity of the machine is checked by the UV energy meter. Over 80% is required for the machines.

Exposure time: By UV exposure, the artwork life is reducing and light density is becoming lower. The book printing time for the silver halide film is controlled according to the film requirement.

Holding time: The time from exposure to develop is controlled to prevent the dry film residue problem.

First Article check

Before going directly into a mass production for etching, first article control is a common method for inner layer process.

2 kinds of first article control:

Exposure first article: After exposure, the board is sent for developing but will not go for further etching. Unwanted dry film is removed and the circuit pattern is remained on the board for protection. The problems can be corrected in this stage for the dry film could be washed away.

Etching first article: After the whole process including the etching, the unwanted copper is removed and the pattern is created. The problems could not be repaired and has to scrap the boards for most cases.

Recent development of checking the article is by using AOI machine to replace the manual visual inspection.

DES

It is the acronym for the Developing –Etching-Stripping process:

Developing: The un-exposure part of the dry film will be washed away by sodium carbonate solution. And the exposure part of the dry film is the designed circuit part and will be kept.

Etching: After the unwanted dry film is removed, the copper surface is exposed. Going through the etching process, the unwanted copper will be removed by copper chloride and hydrochloric acid solution. The pattern will be remained because the harden dry film still remain on the board and protect the copper.

Stripping: The harden dry film could only be removed in this stage by sodium hydroxide solution.

General control items:

Developing point: It is the quality item to test the developing effect of the developing machine. By checking the 100% dry film developing of the board, the developing effect is fully assured if the point is at 50~70% range of the tank.

PH: >10 is required for the developing. Lower value may cause dry film residue problem.

Nozzles check: Nozzles must not be blocked and it’s very critical to the etching uniformity.

Special process

A degrease process may be added before the pretreatment process if there is a baking process after board cutting. Its purpose is to remove the base material residue on the copper surface due to the baking. This kind of residues left on the board before dry film lamination and may cause the short problem after etching.

Out put

The inner layer core is created.

Typical defects

There are 2 typical defects happen in this process are open and short:

Open

name="OLE_LINK2" name="OLE_LINK1"

Dry film adhesion is not good;

Dry film wrinkle and bubble;

Foreign material or dry film residue;

Insufficient exposure energy;

Over developing;

Over etching;

Handling scratch.

Short

The root cause may be related to the below items:

Insufficient light density of artwork;

Scratch on the artwork;

Under developing;

Dry film residue

See also

Further comments or links

Note

Footnotes

Reference

IPC-T-50H

External Links

http://en.wikipedia.org/wiki/Printed_circuit_board

http://www.pcb007.com/

Disclaimer: This content is intended only for internal use at Continental Automotive and shall not be distributed outside of Continental Automotive.

'''