Designing a PCB for modern wireless devices means you have to deal with alot of EMI. Board level shields are the most effective way to stop cross-talk at the source. If you dont plan for your PCB shield early, you might face expensive redesigns when you hit the compliance lab.
Every year, electronics get smaller and faster. This creates a messy environment on your PCB where one component affects another one through radiated noise. While using an emi gasket on the outer enclosure is great for passing overall emissions, a board level shield (BLS) is what actually keeps your internal signals clean. If you have a sensitive GPS module next to a noisy processor, the shield is the only thing that keeps them from fighting with eachother.
One-piece shields are cheaper but you cant see the parts inside after soldering. Two-piece shields have a frame and a removable lid. This is alot better for debugging and repair because you can just pop the lid off if you need to measure something on the board during your testing phase.
You need to define the X, Y, and Z height spefically. Alot of times, engineers forget to check the component height inside the shield. Its also important to think about the keep-out area around the shield frame so the soldering machine has enough room to work without hitting other parts on the board.
Surface mount is the most common now. You can use standard tape and reel for high speed assembly. Some older designs still use through-hole pins if the environment has alot of vibration. Shield clips are another cool option where you just snap the shield onto the board after its already been through the oven.
One of the biggest issues with PCB shields is co-planarity. If the bottom of the shield is not perfectly flat, some parts wont touch the solder paste and you get a gap. This gap doesn't just look bad, it also lets RF energy leak out like a tiny antenna. Standard spec is usually 0.1mm flatness. If you are ordering custom shields, you should ask the factory how they measure this everytime because its the number one reason for assembly line failures.
Another thing to think about is "venting". If you have a solid metal box over a chip that gets hot, the heat has nowhere to go. You can add small holes (perforations) to the lid to let air move, but dont make them too big or the EMI will leak out. There is a balance between thermal performance and shielding effectiveness that every engineer has to find for their spefic project. Alot of mobile devices use very thin nickel silver for this reason because it solders well and is very strong even when its thin.
Through-hole shields are great for rugged designs but they take up space on both sides of the board. SMT shields are much more popular because you can put components on the back of the board directly under the shield area. Most high-volume manufacturing uses SMT shields with pick-and-place nozzles. If your shield is too big, the nozzle might not be able to lift it, so you have to design a flat spot on the top of the lid for the vacuum cup to grab. If you forget this, the factory might have to place them by hand which costs alot of money.
Some designs use "Shield Clips". These are small metal clips that you solder onto the board, and then the shield lid just snaps into them. This is amazing for repair because you dont have to desolder anything to get to the components. However, clips don't provide a 100% continuous ground seal like a full frame does. If your frequency is very high, the small gaps between the clips might allow enough noise through to fail your CISPR tests. Its always a trade off between serviceability and performance.
Before you finalize your PCB shield, check these 3 things:
Don't let EMI ruin your PCB design. Our team at 3G Shielding has helped thousands of engineers protect their sensitive electronics with high-quality board level shields. Whether you need a standard size or a full custom solution, we are here to help.
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