Many systems are currently using very high data rates in the 1-10 Gb/s range, and planning for systems with even higher data rates is on-going. These very high data rate signals are usually differential, but any small amount of imbalance will create common mode noise, potentially requiring additional shielding at GHz frequencies. The very high frequencies require traditional metal shield air vent areas to have very small holes, inhibiting air flow. Honey comb filters can be used, but the added cost is often a factor that makes their use undesirable.
Increasing the metal thickness of the air vent area can increase the amount of shielding, allowing larger holes (more air flow) without adding significant cost to the product.
Thick Air Vents
Figure 1 shows the general geometry of holes used for this study. The array of holes was in a 1 mm stamped metal sheet as the typical air vent. The number of holes was varied to maintain a close similar percent open area for the air flow. Naturally the air flow is greater when the holes are larger. The metal between the holes was also 1 mm wide. Hole sizes of 4×4 mm, 5×5 mm, 7×7 mm, and 9×9 mm were included in this study.
Figure 2 through 5 shows the results for different thickness of the metal for the various hole sizes. As expected, the smaller hole sizes provided more shielding than the larger hole sizes. Also, increasing the thickness of the metal provided significant improvement in the shielding performance.
Figure 6 shows a summary of the thickness of the metal vs. shielding performance at 1 GHz. For example, if 40 dB of shielding is desired at 1 GHz, but a hole size of 4×4 mm is too small for the required air flow, then the hole size can be increased to 7×7 mm if the metal thickness is increased from 1 mm to 4 mm.
Separate Metal Panels
Since the air vent holes are usually stamped out during manufacturing, thicker metal may not be able to be stamped as easily. If multiple 1 mm stamped metal air vents are stacked, the thicker metal can be obtained. Since it is likely that some of the stacked metal panels may not make a good electrical contact, a small space was inserted between stacked panels. To observe the results, Figure 7 shows the case for 5×5 mm holes and two 1 mm thick panels now with space between them, or between 1-5 mm of space. The additional spacing actually increases the amount of shielding observed. Small spacing has no impact on the air flow and helps the shielding.
Summary
High frequency shielding usually requires extremely small holes in the air vent area restricting air flow. Creating thicker panels allow the hole size to be increased without the normal reduction in shielding performance.
Dr. Bruce Archambeault is an IEEE Fellow, an Adjunct Professor at Missouri University of Science & Technology as well as a IBM Distingushed Engineer Emeritus. He teaches short courses in EMI/EMC design and is the author of the book “PCB Design for Real-World EMI Control.”
