Assembly Planbook - April 2008 - (Page 54) Dispensing various chemistries. Test and compare a sample of each coating on your application before making a final selection. This up-front work will ensure that you protect your device with the best coating available. held aerosol can. For larger jobs, a spray gun can be used inside an enclosed booth. Both methods are hard to control and are prone to “shadowing”—the formation of voids under components. Overspray is also an issue. The health hazards of Application Methods manual spraying are obviA brush is the oldest and ous, and manual masking simplest method of applyand demasking is required. ing conformal coatings. The As a result, spraying is coating is applied to the brush most often done with semiand then painted onto the automatic or fully automatic assembly. Like all manual equipment. Selective sprayprocesses, results are opering with automatic equipator-dependent. Depending ment allows assemblers to on the coating, the brush apply coatings with great method can present a hazprecision and edge definiard to the operator. Manual tion. Most selective coating masking and demasking is Most conformal coatings are sprayed onto electronic assemblies using systems provide closedrequired to protect areas that semiautomatic or fully automatic equipment. loop control over the mateshould not be coated. rial, the needle aperture, and Dip coating can be done manually Conformal coatings can also be the movement of the spray head. Such or automatically. With this method, the sprayed onto assemblies. Like dip coat- equipment doesn’t completely elimiassembly is submerged in a vat of coat- ing, this can be done manually or auto- nate the need for masking, but it does ing material. When no bubbles come to matically. For small jobs, the coating greatly reduce it. Because the matethe surface, the assembly is removed. can be dispensed from a small, hand- rial is dispensed more precisely in an This sounds simple, but in enclosed space, the process truth, the speed of immerpresents almost no hazard sion, the dwell time, and to the operator. the speed and angle of Some machines are removal from the vat must equipped with more than be precisely controlled to one dispensing head to obtain an even, quality apply more than one matecoating. rial or coating pattern at Like brush coating, manthe same time. An atomually dipping assemblies izing spray head can prois operator-dependent and vide general coverage, presents potential health while a needle dispenser hazards. As a result, the procan inject the coating into cess is usually done autodeep recesses and around matically. With a machine, tall components. Each head the operator can set speeds, can move simultaneously angles and dwell times. in all three axes, with anguHowever, assemblies lar deflection of up to 30 still must be loaded and degrees to reach all surunloaded manually, and faces on an assembly. special tooling is needed Automatic coating to hold each assembly. equipment can be linked And, whether the assemto a shop floor network for blies are dipped by hand Vacuum deposition of Parylene is a unique process. The devices to be coated off-line programming and are placed in one chamber, and a powdered raw material, known as a dimer, is or with a machine, manual placed in a second chamber. The dimer is heated, causing it to sublimate into statistical process control. masking and demasking is a monomeric vapor. This vapor is transferred into the other chamber, where it With an integrated a bar required. code reader, the coating condenses onto the parts, forming a thin film. 54 ASSEMBLY / April 2008 www.assemblymag.com http://www.assemblymag.com
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