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Coplanarity is defined as a measure of how close a given surface is to a perfect 2D plane and is often referred to as flatness. This is an important factor when designing board level shielding and determining the solder thickness required to ensure the performance of the EMI/RF shield. If not properly soldered, there could be gaps that could lead to EMI/RF leakage and weak solder joints.
Factors Affecting Coplanarity
Shape: The design of the shield will directly affect its coplanarity. Rectangular shields have four surfaces/ walls touching the circuit board, and the distance between all four surfaces and the circuit board is the coplanarity. However, sometimes a rectangular shape is unsuitable for the application, and a more complex perimeter is required. More complex shapes will increase the amount of walls required for the shield, which can increase the coplanarity.
Size: The size of the shield also affects its coplanarity. Larger shields will have a
larger coplanarity.
Manufacturing Method: The standard method for manufacturing a sheet metal board level shield is to cut the blank and form the walls one at a time using fixtured bases and blades on pneumatic presses. However, this means that the walls are formed independently from one another and can have slight variations, affecting how flat the shield can be to the circuit board. When designs that are more complex are formed this way, there are even more walls, and the stack up of tolerances for each increases the coplanarity more. A tooling investment that can form multiple walls at one time can improve the tolerance stack up depending on the design.
Handling and Packaging: Coplanarity can also be affected by how the parts are handled during shipping. Having the parts protected during shipping will help maintain the manufactured coplanarity. Trays and tape and reel are the most common forms of protection for board level shielding.
Recommendations for Achieving Proper Coplanarity
Soldering Fixtures or Weights: In many cases, the design of the shield is set, and the manufacturing method cannot improve the coplanarity. In these cases, we recommend using weights or soldering fixtures to retain the shield to the circuit board surface and achieve proper solder wicking.
Conclusion
In summary, coplanarity is a crucial factor to consider when designing and manufacturing board level shielding. By taking into account the various factors that can affect coplanarity and using the appropriate techniques and tools, you can achieve the desired level of flatness and ensure the performance of the EMI/RF shield.
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