RFI shielding options
It is always better to design a PCB or equipment to include a shield, and not to try and retrospectively fit a shield to the PCB or equipment. Designing in a shield and including it on the bill of materials prior to EMC testing can give you a significant cost saving. Fitting a shield following initial EMC testing usually means a PCB re-design and a struggle to find adequate PCB real estate. In these cases the shield itself is often forced to take an odd shape which generally implies a more expensive component or press tooling.
levels of shielding
There are several different levels of shielding:
- Individual component
- Groups of component
- Sub-assembly
- Assembly
- Complete equipment
Where individual components or areas of circuitry are susceptible to radiated RF interference or are the main cause of radiated interference, a local PCB mounted enclosure is suitable and most cost-effective, both from a component and PCB assembly point-of-view.
Where the interference is likely to occur between two or more areas of circuitry on the same PCB or within the same equipment, then the use of several shields or one multi-cavity shield is generally recommended.
Examples of this include:
- Several different oscillators or digital clocks on the same PCB as sensitive receivers or measuring circuitry.
- Where the designer is trying to increase interchannel isolation between multi-channel devices operating at the same or similar frequencies
methods of shielding
The most common form of shielding is a metal enclosure, soldered to an RF ground plane which generally forms the sixth side of the enclosure. These can be individual or multi-cavity enclosures designed to suit the application and assembly method most suited to the product being designed.
Another method is to enclose the total PCB or in the case of equipment such as mobile phone handsets, the complete unit. This can be achieved using a total enclosure fabricated from metal or a die cast boxes, an EMC sealed cabinet or by plating or metalising the plastic enclosure the equipment is housed in.
If the equipment requires ventilation or has a large display area then mesh or honey comb fabrications are used. For the shielding of cables between equipments, the use of braiding within the cables and selected bonding of the braiding to the equipment is generally used although in larger systems, cable trunking may be used.
In domestic or office applications the used of plated plastic or metalised plastic enclosures are being used, where the level of protection is adequate for the intended use of the equipment and where a plastic case is part of the normal equipment being supplied.
shielding design
RFI enclosure design should always be considered alongside all the other design issues such as operating environment, overall volume of product to be manufactured, assembly method, test and inspection methodology, PCB and equipment layout, rework or adjustment requirements.
All too often the RFI enclosures, like power supplies, are left until the end of the design process which generally makes them more difficult to accommodate. As a result, they can then often interfere mechanically with other areas of the design.
Choosing the format
When choosing the format of enclosure to be used the following issues should be considered.
our standard cans
If a designer is unsure of the format of enclosure to use or has a series of different sizes of enclosure on the same PCB or equipment, then we offer the single development sheet, at a fixed price including tooling, where the engineer can mix and match different types of enclosure and different sizes on the one sheet, providing the metal type and thickness is the same for each of the enclosures or part of the enclosures.
Where a designer is sure of the need for an enclosure but does not want the expense of a bespoke one, Tecan offers a comprehensive range of standard enclosures, including the following:
four-sided enclosure with spring finger lid (SFL)
This enclosure consists of a four-sided fence, with an array of pins on the PCB edge, used to solder the fence to the PCB either by hand soldering, wave soldering or pin-in-hole reflow soldering.
A spring finger lid is generally associated with this enclosure. This is the best form of removable lid providing the fence height is sufficient to accommodate the fingers. Tecan has two standards of spring finger:
- Standard height: 3.7mm
- Low profile: 2.2mm
We also have an internal spring finger for situations where there is insufficient clearance on the outside of the fence to allow for external fingers. External and internal fingers can be mixed providing they are the same format on opposing faces.
four-sided surface mount enclosure with SFL
This enclosure is the same as the normal four-sided enclosure but without the fixing pins and is usually seam-soldered to a continuous track on the PCB. When considering the fence, height allowance must be made for the solder fillet at the base of the fence.
Another alternative to the continuous seam-soldered fence is to castellate the PCB board edge. This reduces the amount of solder used to attach the fence to the PCB as well as giving clearance for tracks to cross the fence boundary without specific track clearance mouse hole or multi layer PCBs.
Where the fences are required to be machine placed onto the PCB then a pick and place target may need to be provided, this would probably be combined with a punch and press format fence.
four-sided can with plain folded lid
The plain folded lid is generally less costly to produce, especially at the development stage. The only drawback to this design is that there is no guarantee of a connection between the lid and fence other than where the lidhold-down tags are placed. This may have an effect on the EMC performance of the enclosure. The lid hold down tags can either be the fold down type or the twist type, both options are suitable for up to five lid removals and replacements.
pip-lock lid and fence
An option for very low profile fence and lid sets is the pip-lock lid, where pips in the side walls of the lid lock into slots in the fence side walls. These can be applied to wall heights down to 1.5mm.
Consideration should be made to whether or not the lid and fence are placed as a set as a set using the pick and place facilities on surface mount assembly lines, the lid only needing to be removed lid should rework to the components inside the enclosure be required. This will need an array of apertures in the lid to allow the reflow heat into the enclosure to solder the electronic components inside.
five-sided enclosure delivered flat for self-forming
The most cost-effective enclosure for development and low volume production is the bend-line-formed five-sided enclosure delivered flat and tagged in sheets for the user to self-form as they fit them to the PCB.
The five-sided option is worth considering where the enclosure can be placed after testing, or if the PCB production yield is high. This can be supplied with soldering pins, spot welded corners or butted corners, as well as reflow heat holes.
why use chemical etching for RFI shields?
Chemical etching from Tecan allows for precise and intricate designs to be achieved with high accuracy. It ensures that shields are consistent in depth and uniformity, which is crucial for their effectiveness in blocking electromagnetic interference. Performance is further enhanced as chemical etching results in burr-free edges – essential to maintaining signal integrity and meeting stringent requirements for electromagnetic compatibility.
Additionally, chemical etching is a cost-effective manufacturing process, especially for producing RFI shields in large quantities. The ability to create complex patterns and shapes with minimal tooling costs makes it a favorable choice for many industries.
- faster production cycles
- minimal tooling costs
- design flexibility
- inclusion of complex patterns and shapes (e.g. features like access holes, ventilation, fold lines, logos, and part numbers) at no additional cost
- short timescales, even at high volume
- cost-effective
