Page 46


www.us- tech.com


December, 2016


Using Conformal Coating to Protect Sensitive Electronics


By Manuel Schwarzenbolz, Product Manager, Rehm Thermal Systems


that today’s electronics function properly in a vari- ety of harsh conditions. Despite the unrelenting miniaturization of components, a suitable coating application method is required to continue keeping PCBs safe from moisture, dust, vibration, and other ambient influences. During the development of a PCB layout. A frequently overlooked factor is that application-specific system limits also apply to the coating process. If this is not taken into considera- tion early on in the development process, selective coating can become a significant challenge. As a result, complex PCBs may have to be


T


masked through a time-consuming manual process or complete PCBs may need to be sent out to an external coating supplier in spite of an existing, automated in-house coating system. Rehm Thermal Systems designed its Protecto coating system in cooperation with KC-Produkte GmbH to provide conformal coating without additional out- sourcing expense. The highly-flexible customer- specific layout and its various application methods allow it to be used with many different types of PCBs. The company also offers individualized sys- tems with up to four different application methods for specific requirements.


Targeted Prep and Planning Before the protective coating is actually


applied, numerous factors must taken into account. The conditions that the boards will be subjected to should be evaluated, including ambi-


o give electronic devices the longest possible operational lifespan, conformal coating was developed to protect components, and ensures


the entire board. Depending on the application method, the lay-


out should include a minimum clearance between the areas to be coated and the areas to be left uncoated, because inadvertent application of the coating within the specified exclusion area might occur. Due to the capillary effect, even an extremely


ducibility and the curtain nozzle can coat large surface areas quickly and with little spatter. The jetting valve is highly versatile. In combi-


nation with the patented StreamCoat® nozzle, it offers the user a very broad range of applications. It’s also the only extremely narrow nozzle offered by Rehm. With an outside diameter of just 2.5 mm (0.1 in.), the nozzle is capable of advancing deep into highly-populated areas with tall components and tight spaces.


Types of Application Diaphragm valves belong to a generic product


Rehm’s StreamCoat nozzle reaching between tall components.


small contact surface between the protective coating and a plug housing, for example, is enough to result in complete wetting of the periphery and the pins, which results in functional failure.


Choosing Materials Factors that determine the choice of coating


material include viscosity, environmental consider- ations such as evaporating VOC regulations, and the availability of the right dispensing equipment. Drying ovens that use IR radiators, convection or UV radiation restrict the selection of possible pro- tective coatings because of the drying process itself. Above all, however, the application methods of the coating system dictate which materials are best. Rehm designed its Protecto system to be


PCB with applied protective coating.


ent temperature, temperature fluctuation, UV radiation, atmospheric humidity, dust, and vibra- tion. Among other things, which areas of the PCB it will be absolutely necessary to cover with the protective coating need to be specified, as well as which areas must not be coated under any circum- stances. Plugs, terminals, test contacts and ground pads may not be coated as a rule, because wetting them with the coating may lead to malfunction of


adapted as flexibly as possible in various manufac- turing environments. The basic model with one applicator is an entry-level system to get started with conformal coating. However, with up to four different applicators equipped simultaneously, the system can be used to coat several PCBs at the same time or to use up to four separate materials without requiring any changeover time. A diverse range of applicators and nozzles for


the coating process is available. As with the choice of coating system, the process and materials dic- tate the types of applicators. Diaphragm valves are well-suited for more aggressive media. Needle valves are rugged and have minimal dead space. Atomizer valves are preferred for coating large surfaces. Piston valves are especially useful for highly-viscous dam materials. Eccentric worm gear valves ensure high levels of volumetric repro-


Customized coating system with room for four applicators.


thickness of the dam, can be varied easily. Typical applications for diaphragm valves are in the low- to medium-viscosity range. Rehm offers a special material needle for these types of valves, which


Continued on page 52


group including time/pressure-controlled valves. Where the coating of electronic PCBs is concerned, diaphragm applicators are used mainly to apply various dam materials and to encapsulate small components. During dam material application, the distance from the dispensing needle to the surface of the component must be selected correctly. If the distance is too small, the nozzle might sink into the freshly-applied material causing it to become smeared when the nozzle is removed. An exces- sively large distance is also unfavorable because it can result in uncontrolled application of the dam material. In particular where travel around a curved contour is involved, the results may deviate significantly from the planned coating application. Also, the speed of the axis system must be


matched to material pressure and quantity. If the axis system moves too fast, material flow is inter- rupted at the applicator and the dam will have gaps. Typical process speeds are within a range of 10-100 mm/s (0.4-3.9 in./s). By varying material pressure and selecting the right material needle, the quantity of applied material, and thus the


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84