Patterning processes of PWBs

For patterning (or imaging) with negative mask, the dry film photoresist method is the most popular in the PWB industry. Alternatively, the less expensive but lower resolution screen printing imaging technology can be used. Screen printing is the main imaging process used for solder resists, as well.

The advantages of dry film photoresist are as follows:

- fine lines definitions;

- plated circuitry has straight sidewalls;

- excellent conformance to artwork dimensions;

- fast set up and turnaround time on small jobs;

- hole tenting provides a photoresist seal over a hole, so it eliminates the need of hole plugging.

The following disadvantages should be mentioned:

- dry film photoresist is expensive to buy;

- large capital investment is required to purchase processing equipment;

- dry film leaves a monomolecular layer of adhesion promoter on the copper surface from which it has been developed, and this layer must be chemically etched off. For this purpose ammonium per­ sulfate, sulfuric acid, hydrogen peroxide, sodium persulfate can be used.

The key steps of dry film photoresist processing: lamination, exposure, developing.

Before resist lamination, panels should be baked shortly. Moisture would prevent good bonding and contribute to resist lifting. Aside from mois­ture removal, there is another reason for baking: photoresist bonds best to a warm copper surface. The heat may be supplied by a hot shoe lami­nator or by a hot roller laminator. Panels to be laminated must be baked (at 80-100 degrees centigrade) and laminated while they are hot. The laminating pressure and conveyor speed are also important factors.

 

First step of negative mask preparation: dry-film photoresist lamination

 

 

 

After laminating the panel is exposed using the phototool or artwork for masking. Important elements of a good exposure are the cleanliness and the correct develop­ing. It is necessary to control humidity and temperature according to the film manufacturer’s limit. The phototool (or photomask) must be fitted correctly to the panel, with its good contact to the dry film coated panel during vacuum pump down and exposure. The emulsion of the phototool must be placed directly against the photoresist covered sheet for maximum resolution and accuracy.

Exposure is a fairly important factor, because almost every aspect of dry film imaging has an effect on exposure. Step wedges are useful to determine correct exposure.

Photoresist is a combination of monomers, free radical photo initi­ators, plasticizers, dyes, adhesion promoters and a methacrylic binder which holds it all together. When this resist film is exposed to UV radiation the photo initiators generate free radicals. A free radical chain reaction (polymerization) is set off. What had formerly been a soft, gelatinous film, easily washed away by the developing solvent, has now become a hard, chemically resistant surface, suitable as plating or etching resist.

 

Second step: photoresist exposure using phototools (photomasks)

 

 

 

By the developing process, the unexposed resist is washed away in conveyorized or non-conveyorized developer. Nowadays, fully aqueous developing photoresists are in use to meet the environment protection requirements.

 

Third step: photoresist development

 

 

 

Pattern plating is followed by removing the mask and etching of the copper. During this process the tin or tin-lead coating will protect the PWB circuitry against the etchant.

 

Negative mask removal: photoresist stripping after pattern plating

 

 

Screen printing is also a very important imaging process used in PWB manufacturing. Opposite of dry film, which is capital intensive, screen printing is labor intensive. Manual screen printing needs better skilled operators, than automatic screen printing.

Screened-on resist generally has better adhesion and better toler­ance for unfriendly environments, than does photoresist. Screened-on resist generally is easier to strip, strips cleaner and faster than does photoresist.

 

Principle of screen printing

 

 

 

Screen printing is an old process, however it has taken giant technological steps forward. Nowadays stainless steel or polyester screens, UV exposure, rubber or plastic squeegee are used. There are different types of stencil medias: indirect, direct or direct/indirect. Direct/indirect system's advantages over indirect system are the longev­ity of the stencil and its ability to hold sharp lines. The main advantage of direct system is that the stencil thickness can be controlled as required.

Lot of manufacturers have automatic screen printers. Advantages to using this type of equipment are:

- the pressure used during the printing is uniform and consistent;

- the squeegee angles with respect to the surface of the panel is consistent;

- the throughput is higher.

In the PWB industry plating resists, solder resists and legend resists are applied by screen printing.