Solder mask Application

Colorants. Pigments or dyes are used to ease the inspection of the PCB after solder mask application. The colorant content is limited by competition with the photoinitiators for the UV light. Too high a level of colorant can result in a loss of cure speed. 

If the hole is simply coated with solder mask and the solder mask is not developed from the hole, the air trapped in these small holes may expand and create a bubble or blister, or may erupt, exposing the copper in the hole to potentially corrosive chemistries. These holes should be developed cleanly so that they receive the final finish, should be protected before solder mask application with an inert final finish or be completely plugged. 

TABLE 33.3 Key Variables for Curtain Coating Solder Mask Application and the Effect If the Process is Operated at the Extremes of That Variable... 

Debubbling time With some solder mask applications, it is important to have some time (usually 5-10 min. maximum) for the ceating to debubble before the tack drying occurs. This allows time for the bubbles to break and for the coating to heal itself. 

PVF resins have also been used in a variety of other applications, includ ing conductive films, electrophotographic binders, as a component for inks and in membranes, pholoimaging, solder masks, and reprographic loners. 

Solder masks (solder resists) are protective polymer coatings used on printed circuit boards (PCB s) to prevent solder bridging between conductors during soldering (1-3). The selective application of the solder mask limits the amount of conductor area in contact with the molten solder, minimizing solder usage and slowing the rate of conductor metal contamination in the solder pot. 

Functional Fillers. These relatively inert materials are dispersed in the formulation to develop the desired rheology and body needed for the application of the solder mask. 

The test boards were inspected visually, cleaned, and baked dry before application of solder mask. The various solder masks were applied and cured according to methods specified by the solder mask supplier. The solder masks tested are described in Table II. A typical insulation resistance determination is performed by placing five test boards in the test fixture and following the measurement procedure. The test fixture is sealed in the humidity chamber and placed in the oven. The specified conditions (Table I) are controlled by the oven temperature. 

High-temperature resists have been formulated from a group of polyesters based on styrylpyridine, which are thermally stable up to 450°C. Such resists have found applications as fine-line solder masks or when the polymer is intended as a permanent eomponent of the final deviee, he it as insulator, interlayer dielectric, or a-particle harrier. ... 

Adhesion to metal is sometimes difficult to achieve. However, due to the development of adhesion promoters industrial applications on metal exist. Electronics applications are important photoresists (both wet and dry film), solder masks, potting compounds, and conformal coatings are products based on UV-curable materials. 

Fine patterns of polymeric material are widely used in the electronics industry. They are required in the manufacturing of integrated circuits, printed circuit boards and solder masks for the latter. However, photopolymerization finds only limited application in this area. Photocrosslinking and photochemical enhancement of the rate of dissolution are primarily used. Larger structures can also be made by screen printing. In this technique, the photopolymerizable material is deposited pattern-wise and then irradiated in an overall exposure. 

Materials Type class and grade of materials, including color if applicable. Plating and coating material(s), type, thickness, and tolerances. Solder mask and marking inks type, minimum thickness, and permanency. 

Hole Size. Holes in boards that have been coated with LPI solder mask will have some solder mask material in the holes after application. Typically this material must be removed in the development process. There are very significant differences in how easily different solder mask materials develop from holes. There is also a difference in the amount of solder mask material that is in the holes between appUcation methods, with spray and curtain coating leaving less material than screen printing. 

Many localities have strict air emissions regulations that limit the materials and the amount that may be exhausted to the atmosphere. Individual processes or pieces of equipment may require a permit to exhaust any volatile materials. Hazardous air pollutants (HAPs) and volatile organic components (VOCs) are of particular concern. Limits on these parameters can limit mask selection and application processes. For example, curtain coating and spray mask application are not viable in some areas due to the higher percentage of solvent needed for proper application. Screening ntihzes solder mask at a much higher percent solids (less dilution), so air emissions may be only half that of other methods. 

Military Applications. The military specification that applies to solder mask is MIL-P-55110. Since the mid-1990s, the revisions of this document refer to the IPC-SM-840 specification for solder mask requirements. 

Other Customer Industry Application Specifications (AABUS). Often PCB customers have special requirements for their solder mask. These can be a modification of the IPC criteria or some additional requirement not included in the SM-840 document. There is a term used to describe these requirements as agreed between user and supplier (AABUS). The key word here is agreed, since both parties must be completely aware of the requirement and what it takes to achieve the required result. 

Flip-chip assembly, with its finer pitch, has placed further restrictions on the maximum mask thickness. These thickness restrictions effectively eliminated dry film solder mask in most apphcations and relegated it to a specialty product for special applications. 

Choose solder mask that is compatible with the existing or proposed application method. Not all solder masks can be screen printed, sprayed, and curtain-coated. Consult with the solder mask supplier for the appropriate solder mask for your application method. 

The choice of the final surface finish for the copper has an effect upon how the solder mask performs. And, with the elimination of lead in solder, the compatibility of new final finishes needs to be confirmed before their implementation. Most of these finishes are applied after the solder mask so the process for mask application can have a significant impact upon final quality of the PCB ... 

Liquid Masks. Solder masks applied in liquid form can be either photoimage-able or not. Most solder mask in use today is photoimageable, meaning that after application it will be patterned in a photolithographic process. A non-imageable product must be applied only to those areas to be coated, and areas where metal or base material must be exposed cannot be coated. 

Application—Liquid Photoimageable (LPI) Solder Mask. LPI solder masks are applied typically by screen printing, spray, or curtain coating. Although each method has pros and cons, all are used successfully in the industry. 

Dry-Film Solder Mask (DFSM) Application. The process for DFSM differs from LPI solder mask only in the application step. Surface preparation, exposure development, and cure are the same as for LPI solder masks. 

Roll Lamination. Lamination of DFSM with a standard hot roll laminator is unacceptable for all except the least demanding applications. It is impossible to laminate a flat film of solder mask to PCB topography without trapping air along circuits. There are vacuum hot roll laminators that will work satisfactorily, but these are not widely available. 

UV and Thermally Curable Solder Masks. Because these products are not photoimageable, they must be applied with patterned screen printing in the exact pattern that is needed on the finished PCB. After their application, they are cured by the appropriate cure recipe as dictated by the supplier. There is no opportunity to develop off solder mask from areas where it is unwanted, so registration of the screen to the panel is very important. 

In SMOBC processing, the metal-plated resist is removed to present a flat, clean copper surface for solder mask definition. Tin/lead alloys can be stripped in oxidizing fluoride solutions such as fluoboric acid and hydrogen peroxide or ammonium bifluoride with hydrogen peroxide or nitric acid. (Caution machine construction must be made compatible with fluorides by elimination of titanium and glass components.) Commercial formulations are available to be used inline after the etch machine rinses. Accumulations of spent solution or filtered lead-fluoride deposits must be treated as hazardous waste and have been accepted by solution vendors for treatment and disposal costs. Modern applications usually use lead-free tin plating resists, which can be fluoride containing as previously discussed, or compounds of ferric chloride... 

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