PCB Design for Manufacturing with Form/Met
Introduction
Form/Met® EMI shielding products enable OEMs to achieve electromagnetic compliance for their electronic designs while assuring economical volume manufacturing. OEMs are encouraged to understand the benefits of deploying the Form/Met® technology during initial conceptual development. Proper PCB design, as delineated below, provides electromagnetic shield designers and their clients with the assurance that prototype designs can pass EMC tests quickly with no need for board rework or less effective and more costly "fixes."
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Board Shield
A few simple rules applied during the early stages of shielding design and prior to initial board layout assures the easy use of Form/Met®.
Ground traces - Components and circuits known or suspected of creating potential compliance problems should be isolated from adjacent features on the board with the use of guard traces. Ground traces should be 1 mm in width or greater when dispensed adhesives are used for attachment (2 mm for pressure sensitive adhesives). If size constraints dictate smaller width traces, consult WaveZero for solutions involving sub-millimeter guard trace widths.
Component placement - Form/Met® EMI shields are often formed with a very small draft angle to facilitate removal from the thermoforming mold. To avoid interference of the electromagnetic shield sidewall with a component, the component should be placed a minimum of H/4 mm away from the closest edge of the trace where H is the height of the component. Consult WaveZero when tighter component placement is required.
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Enclosure Shield
Form/Met® enclosure shields surround the printed circuit board and fit within the inside surface of the plastic housing.
Housing dimensions - The spacing between the edge of the printed circuit board and the inner surface of the housing (X) should allow for the thickness of the Form/Met® shield. For a nominal EMI/RFI shield thickness of T, the spacing should be a bit larger to allow for manufacturing tolerances and to facilitate assembly. In a typical situation, where T is 0.010 inches (standard Form/Met® film thickness), and the designer believes that 0.003 inches is the preferred assembly tolerance, then X would be 0.013 inches.
"Z" stack tolerances - When designers develop electronic products with constraints on the overall product thickness, it may be possible to use designed-in features in the plastic housing and/or the Form/Met® shield to hold the Form/Met® EMI shield solidly against the ground traces of the PCB. This application eliminates adhesives and mechanical forms of attachment. Consult WaveZero for implementing this design attribute.
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Shield Assembly Processes
WaveZero delivers complete EMI shields to the OEM's manufacturer. If the manufacturer is not already a certified Form/Met® assembler, the manufacturer can be rapidly trained and certified. Virtually all manufacturers possess the necessary equipment for the assembly of Form/Met® EMI/RFI shields.
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