Solder mask Types

Information This drawing describes the unpopulated printed board and all features that become part of the board. It may contain specific design requirements such as material requirements, multilayer stack-up diagram, dielectric separation between layers, controlled impedance requirements, solder mask type, nomenclature color, location, size requirements for the fabricator s ID,... 

Screenable inks have a resin or polymer base and are of three types organic solvent soluble, aqueous alkah soluble, and permanent. Primarily because of pollution requirements and higher solvent costs, the aqueous types have come into greater use. The permanent types are used as solder masks or for marking the boards. Uv-curable inks are also in use. 

Uses Epoxy-a lic in photo resists, solder masks, conformal coatings, overprint varnishes, paper coalings in clear coatings-type applies. Features Enhanced impact resist., flexibility, and resist, to thermal shock Properties Gardner 7-8 color m.w. 1500 vise. 5800 cps (65 C) acid no. 2.4 tens. str. 2700 psi tens, elong. 38%... 

The emissivity of the object is a unitless number that describes how efficiently a surface radiates. It varies from 0, which is a surface with no radiation that is perfectly reflective, to 1, which is a perfectly radiative and absorptive black surface. The emissivity of the surface is a function of both the type of material that composes the surface as well as the roughness of the surface. Typical solder mask material has an emissivity ranging from 0.85 to 0.95. Typical exposed Cu trace has an emissivity ranging from 0.1 to 0.3 depending on the roughness and oxidation condition of the Cu. 

Type of finish and quality requirements (hot air solder leveling [HASL] versus organic sol-derability preservative [OSP], solder mask). 

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. 

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. 

Two basic types of spray equipment have been used for spray coating PCBs with solder mask, HVLP (high volume, low pressure), and electrostatic. HVLP spray systems are most common. Single- and double-sided spray units are available. 

High humidity is a significant cause of reliability problems because many corrosion mechanisms require water to operate. A humid environment is an excellent source of water, even when it is not condensing. Polymers commonly used in PCBs are hygroscopic that is, they absorb moisture readily from the environment. This phenomenon is reversible the moisture can be driven out of the PCB by baking it. The amount of moisture absorbed and the time to reach equilibrium with a humid environment depend on the laminate material, its thickness, the type of solder mask or other surface coating, and the conductor pattern. 

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... 

One of the major differences between flexible circuits and rigid circuit boards is coverlay. In addition to the solder mask used for assembly, coverlay is the mechanical protector for the fragile conductors on flexible circuits. Film-based coverlay and flexible solder mask have been standard materials for traditional flexible circuits. Several types of photoimageable coverlay materials have been developed to satisfy the fine-resolution requirements of FCDI flexible circuits ... 

Acrylic or Epoxy/Dry-Film Type. Acrylic or epoxy/dry-fibn types were the first materials developed to act as the flexible photoimageable coverlay for HDI flexible circuits. They have the same product concept as the photoimageable solder mask of rigid circuit boards. The same vacuum laminators and imaging equipment are available for these materials. 

Figure 5.1 Disassembled view of the spectroelectrochemical cell. (1) Tightening brass cap (threaded inside). (2) Brass ring required to tighten the cell. (3) Working electrode (brass rod with platinum soldered to the base). (4) Auxiliary electrode platinum wire with the tip made flush to the teflon base of the cell. (5) Pseudoreference electrode silver wire, also made flush to the teflon. (6,7) Luer-lock-type injection ports. (8) Cell body, top part aluminium, lower part teflon. (All three electrodes and both filling ports are press fitted into the cell body, so that they can be replaced if needed.) (9) Teflon spacer, determines the pathlength of the cell and masks the reference and counter electrodes from the incident beam. (10) Calcium fluoride window (Wilmad, standard 38.5 x 19.5 x 4mm). (11) Rubber gasket. (12) Hollow brass cell body with threaded inlet and outlet ports (Swagelock) for connection to circulating bath. (13) Two-mirror reflectance accessory (Thermo-SpectraTech FT-30). (14,15) Mirrors.

On the RTV-SM assembly, component location and position had a strong effect on time-to-first-failure for PLCC-84 and LCCC-44 devices. While there was a wide range of performance exhibited across the eight solder alloys, a similarly wide variation in performance was observed among the various component sites on the same board. Furthermore, a wide variation in performance was also observed at all locations across the three replicate boards used to test each solder. It was concluded that this position effect, coupled with general data variability, overshadowed the effect attributable to the solder alloy alone, and masked the ability to distinguish one solder alloy from any other. None of the Pb-free alloys exhibited catastrophic failure or distinguishably poorer performance than the other Pb-free alloys or the eutectic Sn-Pb control. Component type determined the failure time and mode. There were no clear differences between solder alloys or PWB surface finishes under the vibration conditions studied. 

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