Printed Circuit Design & Fab - July 2008 - (Page 37)

EnvironMEnt dling, dispensing, curing, waste handling, testing, scrap rates and warranty/ reliability/ durability in the final end use. In FiGurE 2 the market share for the different types of coating materials is shown. Because of their low purchase price, acrylics are the largest volume coating used, but a low initial cost does not tell the whole story. A solvent-based coating for instance, may offer a very attractive price per kilogram, but may require multiple application passes to obtain the same cured coating thickness – and therefore level of protection – as a solventless material. Additionally, there are costs associated with shipping, handling, storage, and the use of flammable and often toxic solvents. Moreover these same solvents transform into vapors during curing, and the vapors can often present additional issues to deal with. Also, most solvents are considered Volatile Organic Compounds (VOCs) and, in addition to having toxicological properties, they are recognized as contributors to ozone depletion and/or global warming. Finally, there may be considerable costs associated with the waste disposal of uncured (and sometimes cured) material. Likewise, materials that require a curing oven not only add capital costs when setting up a production line, but also incur additional energy costs and likely add work for the HVAC of the manufacturing area. Ovens also add cycle time and work-in-process (WIP) costs that must be added to the total cost-of-ownership equation. The example in FiGurE 3 compares the total cost of two materials. Coating material 'A' had a lower price per kilogram, but ended up with higher processing costs and more rework and scrap costs. When the total cost to coat a board was calculated, the two materials had almost identical cost. Other factors can include the cost of quality problems, measured by first-time reject and rework rates. On the other hand, improved profit margin in the form of superior reliability and durability for the end product can be achieved and enhanced by making the correct conformal coating selection. Besides cost of purchase, performance criteria certainly must be met. Even here, just meeting product specification minimum requirements may not be an adequate criterion for choosing one coating versus another. For instance, a minimum coating-thickness requirement must be met to ensure adequate protection. However, nearly all coating application methods will produce varying thicknesses on a populated board. Vertical surfaces, wires, sharp edges and solder joints often will allow a coating to slump and sag, leaving far less coating and protection in those areas. Boards may require differing levels of protection, depending on the environment in which they will be used. While board assemblers may try different coatings in pursuing a more perfect match of performance and value for a given application, there are additional costs when multiple coatings are used on a production floor. In some cases, using one product that meets the most demanding requirements for all coating lines may actually incur lower total costs when viewed through the larger picture of an entire plant’s operation. Besides cost and performance, an increasing concern for lower environmental impact is affecting board assemblers’ decision making when evaluating conformal coating choices. Solvents. Most production lines capture only a fraction of the solvent vapors given off by many coatings,and this applies particularly to acrylic coatings. These solvents are typically vented to the atmosphere where they are usually counted as greenhouse gases. Additionally, most solvents have noxious odors that are quite objectionable to workers – and to neighbors as well. In addition to solvents within the coating formulation, some products, such as parylene, requires an adhesion promotion step that can involve large quantities of solvent that are released into the air and must be either captured or scrubbed before it is vented into the atmosphere. Solventless coatings create far fewer volatiles or vapors, and therefore may contribute far less to environmental problems such as global warming. Parylene, urethanes and silicones are commonly available in solventless versions. In some cases, silicones are available in solvents that are not considered VOCs or greenhouse gases. Toxicity. While nearly all materials can be considered as having at least some toxicological effects in certain concentrations or exposures, some coatings are recognized as needing greater care when handling, applying, curing and in the disposing of waste materials. Some acrylics and many urethanes can present significant challenges. Parylene and silicone have some of the least issues in this regard. ! "#$%&''()*!%$'+'!()%,-.&/! 0,12213,&!'$,4&)+! 51).,()*6!%-#()*6!1).! 5171#.$-'!81'+&! #&%$4`!,$''!1).! #&*-,1+($)!%$2:,(1)%&;!! FiGurE 2. Conformal coating market, material type utilization. (Courtesy Prismark Partners llC) JULY 2008 FiGurE 3. The total cost of ownership may be similar between coatings with considerable differences in purchase price. printEd circuit dESign & fAB 37

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