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www.us-tech.com
October, 2017
EMI Shielding: Improving Sidewall Coverage with Tilt Spray Coating
By Mike Szuch, Akira Morita and Garret Wong, Nordson ASYMTEK; Mike Sakaguchi and Hiroaki Umeda, Tatsuta Electric Wire & Cable Company Limited
E
MI shielding has always been an important component of RF devices, but the demands of today’s Bluetooth, Wi-Fi, 3G, LTE, wearable,
and IoT devices have created a special set of hur- dles for the manufacturing engineer. These devices rely on different radio frequencies that can inter- fere with one another and cause unwanted electro- magnetic interference (EMI). In the past, “metal cans” or “lids” were used to shield entire groups of sensitive components from interference. This is no longer feasible, as manufacturers continue to design thinner, smaller devices that leave very lit- tle space to house electronic components. As a result, it has become increasingly important to develop reliable EMI shielding solutions at the individual component level through the applica- tion of thin conductive coatings.
Like sputtering, however, one of the signifi-
cant concerns with spray coating has been reduced coverage along the sidewalls of components when material is sprayed from a vertical position. New material formulations are overcoming prior mini- mum thickness limitations of spray-coated materi- als, allowing coating thicknesses below the 25 µm level that allow proper legibility of laser markings on coated components. Nordson ASYMTEK has partnered with a
number of fluid formulators to explore the use of a tilted spray applicator to improve directional spray accuracy and sidewall coverage. One such compa- ny is Tatsuta Electric Wire & Cable Company Limited. Nordson ASYMTEK studied the applica- tion of Tatsuta’s AE5000A-5 EMI shielding mate- rial from a tilted spray applicator. The company specifically analyzed the sidewall thickness relative to the top layer thickness of the sprayed material.
Spray-Coated EMI Shielding Spray-coating technology for thin lay-
Figure 1: sample test performance of Tatsuta AE5000A-1 fluid relative to a tape-adhesive EMI shield material, SF-PC5600.
Multiple technologies have been explored to
apply EMI shielding to individual components. The leading three technologies are sputtering, plating and spray coating. Currently, sputtering is the most popular method. However, it is also the most expensive method and has some limitations when coating the sidewalls of components. Reduced sidewall coverage not only affects the overall package shielding performance, but may also lead to issues with shield grounding and reli- ability.
Plating has also been viewed negatively due
to environmental considerations, capital expense and requirements for masking to prevent plating the wrong areas. In contrast, spray coating has been viewed as an attractive solution due to the relatively low capital equipment investment, high productivity (UPH), ability to accommodate multi- ple components and coating patterns, and the reduction of waste when applying expensive EMI shielding materials.
ers of material has been around for decades, primarily used for applying environmental encapsulation and protective coatings to PCBs. Application of this technology to EMI shield spray coating has been highly desir- able as the cost of coating equipment is typ- ically one-tenth the cost of equipment for alternate technologies, and can be just as productive as the more expensive options. The smaller size of the equipment
(compared with PVD sputtering or electro- plating equipment) and the ability to inte-
grate the equipment in-line with a curing oven for the sprayed material allows for significant automation. Early tests with spray-coated materi- als, however, revealed two notable problems: less than 50 percent sidewall thickness compared with top surface thickness and EMI shielding perform- ance requiring thicker coatings. Spray-coated shielding materials are made
up of many small particles and attach to the target surface with a bonding epoxy or adhesive. Spray- coated materials have larger grain boundaries and gaps between conductive particles, due to the adhesives used to bind the particles together and to the target component. Surface-level contacts between particles assume the conductivity of the coating, creating a Faraday cage. But, the physical contacts provide less con-
ductivity than covalent bonding, sputtering and plating. This, in turn, leads to degradation in elec- trical performance of the coating and generally requires a thicker coating than covalent bonded
materials in sputtering and plating processes, ensuring conduction and avoidance of “pinhole” failures. Pinholes are gaps between particles that can result in areas where EMI radiation can pene- trate the shield and reduce shield efficacy. New fluid formulations with different particle shapes and atomized spray-coating technology help to address the challenges of reducing coating thick- ness, while avoiding pinholes through consistent particle distribution.
New Material Formulations For recent tests, Nordson ASYMTEK used
Tatsuta AE5000A-5, which is revised from the company’s AE5000A-1 fluid. This fluid has been shown to provide good EMI shielding performance results at various thicknesses. Figure 1 shows sample test performance of the AE5000A-1 materi- al at 16 and 25 µm thicknesses relative to a tape- adhesive EMI shield material, SF-PC5600. Target performance for an EMI shielding
material is to achieve greater than 50 dB at fre- quencies of 1GHz and above for radiation. More recent testing of EMI shielding is examining per- formance at lower frequencies where inductive and
Figure 2: tilted spray coating applies fluid at an angle to the top and sidewalls of component surfaces.
conductive interference from neighboring devices on a PCB or flex circuit have a more pronounced effect on overall device performance. One additional consideration for targeting
thickness of the EMI shielding materials is the leg- ibility of any pre-existing laser marking on the tar- get device after coating is applied. Laser marking post-coating is not feasible, as the laser marking would create holes in the EMI shield. In earlier testing performed by Tatsuta, it was determined that at a maximum target thickness of 15 µm for Continued on page 64
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