Flex Circuit Shielding Design Options

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Figure 3: Silver paste screened over a coverlay showing the via stitches down to a ground line—another coverlay goes on top to protect the silver.

• EMI Thermoplastic Shielding film—latest technique. This technology is the newest and most flexible shielding technique. The ultra-thin, ultra-flexible materials were developed for the cell phone market. The manufacturing process has been dramatically improved, making it only slightly more expensive than the older technologies. North American flex shops who use this material are pushing their customers in this direction. It is the thinnest and most flexible. It is easy to apply and creates a 360 degree shield similar to a silver paste or stitched copper via.

Figure 4: Material stack of the thermoplastic shielding film.

Figure 5: Thermoplastic shielding film applied to circuits. Easy for your vendor to apply, but material is still expensive.

Shielding—Design for Manufacturing—a Review of what we learned

Most of the North American flex suppliers can provide all four solutions mentioned below, but they will steer you to the technique they are most comfortable with. The most important design decisions for shielding flex circuits are flexibility/bendability and cost.

Here are my impressions:

• Adding layers of copper to shield, while adding potentially excellent shielding characteristics, adds cost, weight and thickness, and can prove to be a disaster when trying to bend the circuit. This method requires via stitching to prevent “leakage.”
• Adding cross-hatch layers can provide excellent shielding characteristics again, but still adding layers, weight, and will affect flexibility. This also requires via stitching to prevent “leakage.”
• Conductive paste allows better flexibility and lighter weight, and better coverage of shielding. Requires via stitching.
• Thermoplastic shielding film is slick stuff. Easy to apply, thin, light, and bendable. It allows no side leakage. It was originally made for cellphones so the material has been proven, but is more expensive (by 25%) than shielding with normal/older techniques.

Mike Morando is VP of sales and marketing for PFC Flexible Circuits Limited.

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