Solder masks

PVF resins have also been used in a variety of other appHcations, including conductive films (152), electrophotographic binders (153), as a component for inks (154), and in membranes (155,156), photoimaging (157), solder masks (158), and reprographic toners (159). 

Union Carbide Corp. Parylene conformal coatings, photoresists, developers, etchants, solder masks, potting compounds... 

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. 

Printed circuit board component such as laminates, conformal coatings, solder masks, masking tapes, component attachment adhesives, vibration dampers and photoresists. 

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). 

Luo S, Wong CP. Surface property of passivation and solder mask for flip chip packaging. Proceedings of the IEEE Electronic Components and Technology Conference 2001. 

UV Solder Masks as Insulators for Printed Circuit Boards... 

The effect of temperature and humidity on the insulation resistance of photopolymerized and thermally cured solder masks was determined. Dry film and screen ink solder masks were applied to a test pattern. The insulation resistance of the coated test pattern decreased with exposure to elevated temperature and humidity. Elevated temperature alone, however, had only a small negative affect on the insulation resistance. 

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. 

The ability of a solder mask to protect conductors from physical and chemical deterioration and to insulate adjacent circuitry is a major consideration of PCB manufacturers. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has defined the requirements for the qualification and performance of solder masks in the Standard Specification IPC-SM-840 (A). The specification defines classes (1, 2, and 3) to reflect progressive increases in sophistication, functional performance, and testing methods. These classes try to provide PCB manufacturers with assurances of reliability. For example, Class 1 requirements provide the reliability needed by commercial boards used in radios, televisions, and small appliances. Class 2 is for computers and Class 3 for military and life-dependent products. 

Ultraviolet light (UV) curable screen ink solder masks and photopolymerizable dry film solder masks have been available commercially since 1973 (5). These materials offer the PCB manufacturer many processing and production advantages relative to the conventional solvent-evaporative thermally cured solder masks. These products based on a new emerging technology have helped... 

Many studies have been concerned with electromigration on PCB s (8-11). Metallic (dendritic) growth of metal from one conductor to another can cause microamp current leakage or electrical shorts. Since high insulation resistance lowers the rate of electromigration, the insulation resistance of a solder mask and the factors which influence it are of great interest. This study was performed to isolate the affects of temperature and humidity on the insulation resistance of photopolymerizable solder masks relative to conventional solder masks. 

The typical UV curable solder mask formulation consists of the following. 

FOX UV Solder Masks Printed Circuit Board Insulators... 

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. 

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. 

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. 

SM-6 Dry film solder mask, all purpose aqueous developable. 

Generally, it is believed that many plastics classed as insulators become partial conductors when "hot and wet" (12). The UV and thermally cured solder masks in Table III show a dramatic loss of insulation resistance after seven days at 85°C and 95% relative humidity (RH). All the test boards recovered most of their initial insulation resistance after 24 hours at ambient conditions. Table IV shows that an additional exposure to 85°C at ambient humidity produces an increase in insulation resistance. 

Table V Illustrates the effect of cure and soldering on solder masks tested according to Class 2 conditions, 50°C, and 95% RH. Subjecting the test boards to hot solder (510°F) increases the insulation resistance.

The loss of insulation resistance of a solder mask determined at elevated temperature and humidity is dependent mostly on the moisture content of the air. Elevated temperature alone does not cause a dramatic reduction of insulation resistance. The soldering process increases the insulation resistance of the solder masks examined. It is postulated that hot solder seals the surface of the solder mask, rendering a better moisture barrier. This study suggests higher insulation resistance should be obtained from solder masks with higher hydrophobic properties. 

IPC Standard Specification IPC-SM-840, "Qualification and Performance of Permanent Polymer Coating (Solder Mask) for Printed Boards", Evanston, Illinois, 1977. 

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