Page 64 Continued from page 62
the EMI shield coating, typical device laser marks could still be read.
Tilted Spray Considerations Initial attempts with spray coating of EMI
shielding materials also focused on vertical spray-
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erage has slowed the acceptance of spray-coated EMI shielding materials. In order to improve the sidewall coverage
with spray-coated materials, Nordson ASYMTEK began testing tilted spray coating. By tilting the spray-coat applicator, sprayed particles are focused more directly toward these vertical sur- faces to increase the chance of adhesion to the sidewall of the target component. In the company’s exploratory test-
Table 1: Test results with dummy coupon 2 (0.2 in. [5 mm] pitch).
ing of the fluid. With a vertical spray, the fluid is atomized at the spray nozzle and applied to the target component in a conical pattern, with the bulk of the particles and adhesive traveling in a vertical direction toward the target. As such, top surface coverage is very good and
can be created with a uniform thickness using con- trols in the spray coating equipment. However, this vertical dispensing method limits application of the shielding to the sidewalls of the components, thus creating an incomplete Faraday cage and leaving the components susceptible to EMI. In past testing with multiple fluids from various sup- pliers, such as Tatsuta, Parker Chomerics and Clariant, vertical spray coating of the fluids would often lead to a sub-50 percent ratio between top layer thickness and sidewall thickness of the spray-coated fluid. Limited sidewall thickness cov-
ing, the spray valve was tilted to a 30° angle from the vertical position. The design of the tilting system enabled the applicator to tilt in either the x or y axes to allow for spray cov- erage along all four sides of a component device. How - ever, even with a tilted spray, neighboring devices created trenches and ob - struc tions preventing uni- form coverage of the side- walls of a device. As a result, the company ex - plored various pitches be - tween devices to determine the minimum distance re -
quired between neighboring devices to achieve target sidewall coverage.
Test Setup In the initial test setup, Nordson
ASYMTEK chose two types of dummy test coupons, both 0.4 x 0.4 in. (10 x 10 mm) with 0.05 in. (1.25 mm) thickness to confirm that sprayed fluid adhered consistently to both types of common component surfaces. “Dummy coupon 2” was an FR-4-type material. “Dummy coupon 3” was a coupon with sample molding com- pound exterior. In this setup, devices were mount- ed with a set 0.2 in. (5 mm) spacing between them. In the secondary test setup, the company
wanted to determine the minimum pitch between devices to achieve coverage improvement. In this test setup, dummy coupon 3 devices were mounted
Table 2: Test results with dummy coupon 3 (0.2 in. [5 mm] pitch).
ing thickness was approximately 13 µm. Sidewall thicknesses varied from 7 to 9 µm depending on the line speed. The ratios of sidewall-to-top-layer thicknesses ranged from 53 to 71 percent. For both components, higher line speeds resulted in better coating thickness ratios and thinner top layers. Figure 3 shows example images from cross-
sectioned samples for both dummy coupons. It can be seen that coverage across sidewalls and the top surface is complete. It is important to note from these images that the sprayed shield material also
Continued on page 68
October, 2017 Improving Sidewall Coverage with Tilt Spray Coating
at 0.02, 0.04, 0.08, and 0.12 in. (0.5, 1.0, 2.0, and 3.0 mm) pitches. In each test, multiple line speeds were tested
to adjust the resulting coating thickness for both the top and sidewall. The more time that a target surface was exposed to the spray from the spray applicator, the more fluid and particles adhered to the target surface. Higher line speeds generally resulted in thinner coatings, while slower line speeds resulted in thicker coatings. The results from the 0.2 in. (5 mm) pitch test
for both dummy coupons 2 and 3 resulted in simi- lar thickness results between the component types. Depending on the line speed, top layer coat-
See at productronica, Hall A4 Booth 311
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