Solder mask Selection

The solder mask process is qnite complex. It reUes upon all PCB fabrication processes that preceded it and plays a key role in all processes after it has been applied, through assembly and the hfe of the PCB. As a result, it is recommended that the specifying authority choose companies that can provide technical expertise and troubleshooting assistance. Companies that offer service and experienced engineering personnel in the PCB fabricators are becoming scarcer, making the suppher s abUity to provide service a key issue in solder mask selection. 

Manufacture of Printed Wiring Boards. Printed wiring boards, or printed circuit boards, are usually thin flat panels than contain one or multiple layers of thin copper patterns that interconnect the various electronic components (e.g. integrated circuit chips, connectors, resistors) that are attached to the boards. These panels are present in almost every consumer electronic product and automobile sold today. The various photopolymer products used to manufacture the printed wiring boards include film resists, electroless plating resists (23), liquid resists, electrodeposited resists (24), solder masks (25), laser exposed photoresists (26), flexible photoimageable permanent coatings (27) and polyimide interlayer insulator films (28). Another new use of photopolymer chemistry is the selective formation of conductive patterns in polymers (29). 

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. 

If done as part of the electronic design automation (EDA) process using appropriate electronic computer-aided design (ECAD) software, the software wiU automatically assign copper directions to each component footprint, as weU as appropriate coordinates and dimensions. These may need adjustment based on considerations related to wave soldering, test points, RE, power and EMI/RFI issues, and board production limitations. Allowing the software to select 5-mil traces when the board production facility to be used can only reliably do 8-mil traces would be inappropriate. Likewise, the solder mask patterns must be governed by the production capabilities. 

Once the acceptability of the solder masks is defined, the PCB manufacturer selects the acceptable masks based on either preferred process (due to volume or cost) or the design characteristics interaction with the solder mask. These design characteristics include the following ... 

Fill Materials. Since the fill material is an additional fabrication material that becomes a part of the design construction, procurement documentation is required to specify a fill material type and thereby implement the via fill process. The selection and documentation of the fill material require the same consideration as the base laminate preference. This is especially critical when targeting a lead-free-compatible process. Currently, an industry-based material specification for via fill material does not exist.Therefore, specific fill-material brands may be named on the drawing, or some other form of user/supplier agreement must be established.The fabricator has preferences for the type of material used for via fill. Just as suppliers often have preferences for a specific solder mask brand, they also often prefer to use of a specific via fill material around which they have developed their principal processes. Supplier preferences can be driven by specific via fill material characteristics, such as accessibility, equipment compatibility, process supportability, plateability, and/or shelf/pot life. This may complicate source selection, or it might influence the use of a dedicated service center for the hole-fill process. The fabricator may not always know the reliability of its preferred material for a given via structure or end-use environment. 

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. 

Early solder masks could not be photoimaged— that is, placed in its exact location with a photographic process. They could not be hardened selectively in one area and then be removed from other areas. These masks had to be applied in the final pattern desired, typically through a patterned screen. The sUk screening process (the screens are actually nylon) is limited in that it can neither resolve the fine features nor meet the registration requirements and tolerances needed for today s PCB designs. The use of these materials is limited to lower technology products. 

Mixed Metals. Two good examples of the use of mixed metals are for panels that have been selectively solder-stripped and for panels that have had fingers or contacts plated with gold before the solder mask process. The surface preparation process must be selected based upon the most delicate of the metals on the panel. 

Material Selection Considerations—Solder Mask versus Specialty Inks... 

Solder Mask. The three major types of solder mask— liquid screen-printed, dry film, and liquid photoimageable (LPI)—come with different benefits and concerns from a reliability perspective. The solder mask material should be selected for its compatibility with the heat and solvent characteristics of the assembly process, its capability to provide good conformity over surface features on the PCB, and its ability to tent vias if required. Since many of these characteristics are product-specific, only a few general guidelines can be provided here. Where tenting of vias is required to keep solder, moisture, or flux from wicking up... 

PCB design and fabrication materials and methods used in circuit-board construction should be examined first. These consist of three critical elements pad size, pad plating (finish), and solder mask. Identifying these parameters dictates the materials and equipment selected to complete the process. 

The pallet, selective soldering pallet, or shield is a masking device meant to shield certain secondary (bottomside) components or features of the PWB during the wave soldering process. On its topside are a recessed nest for the PWB and retention features to hold the PWB within the nest. There are cutouts that expose those areas to be soldered to the flux and... 

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