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July 2014 • SMT Magazine 43 forgiving nature [17] . Acrylics dry rapidly, reach- ing optimum physical properties in minutes, are fungus resistant and provide long pot life. Additionally, acrylics give off little or no heat during cure eliminating potential damage to heat-sensitive components. They do not shrink during cure and have good humidity resistance and exhibit low glass transition temperatures. Polyurethane coatings are available as either single or two-component formulations [14] . Both formulations provide excellent humidity resis- tance and far greater chemical resistance than acrylic coatings. Single component polyure- thanes, while easy to apply, enjoy long pot life but sometimes require very lengthy cure cycles to achieve full or optimum cure. Two compo- nent formulations can reach optimum cure properties in as little as one to three hours with the assistance of heat. However, when compared to single component formulations, two-compo- nent formulas can have a relatively short pot life sometimes making them difficult to work with. Since polyurethanes are polymerized and cross- linked in place, they have excellent resistance to chemicals, moisture and solvents. They are available in tough, abrasion-resistant varieties and also in low modulus varieties for extreme temperature ranges. Polyurethanes have good adhesion to most materials and provide for a robust coating process. The material is difficult to remove following cure except by thermal or mechanical means. The parylene coating is chemically inert and moisture resistant [14] . Very thin, uniform layers can be applied to the surface with no pinholes or voids. Parylene coating has a high dielectric strength. Due to the nature of the deposition process used to apply the coating, there are no volatiles generated. Parlylene coatings are ex- tremely low weight and yet have the highest modulus of the three coatings being examined. The coating process must be performed in batch mode, using specialized coating equipment. Re- work is difficult, and a microabrasion process is usually required to remove the coating. The spray process used at the company for application of the acrylic and polyurethane coatings is automated with a rotating spray head. The motion of the head is designed to cover a given width from all angles. Masking is required to keep coating out of areas that should not be coated. Parylene is applied at room temperature with deposition equipment that controls the coating rate and ultimate thickness. Polymer deposition takes place at the molecular level in three stages. The raw material dimer is vapor- ized under vacuum and heated to a dimeric gas. The gas is then pyrolized to cleave the dimer to its monomeric form. In the room temperature deposition chamber, the monomer gas deposits as a transparent polymer film. In addition to tin whisker growth on com- ponent leads and coatings, the risk of tin whis- ker growth on braiding and wires is also of con- cern. Hillman et al. indicated a low risk for tin coated copper wire, braid and cable following exposure of samples to 85°C and 85% relative humidity [18] . Scope and objective This study was designed to examine the ef- fects of tin whisker growth on the three coat- ings, applied to test coupons at varying thick- nesses. In addition, the tin whisker growth on braiding, stranded wire and solid single strand wire with pure tin coating was also monitored to determine the risk of use in high-reliability products. Examination of Conformal Coating as a Mitigating Material for Tin Whisker Growth Test coupons consisting of two types of base material (Copper C110 alloy and Alloy 42) were electrodeposited with a layer of "bright tin" plating. Copper C110 and Alloy 42 are com- mon base metals utilized for component leads. After plating and prior to conformal coating a quantity of the plated coupons were scratched to simulate those found during handling and shipping conditions, and another quantity of plated coupons were bent (without scratches) to induce tensile and compressive stresses on the plating. All of the test coupons were then conformal coated on approximately half of the surface with the other half remaining un- coated. The coupons were masked, coated, and then demasked to ensure the coating thickness was uniform and there was no thinning at the TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues fEATURE