EMC & Thermal Management
Cost-effective, compact, light EMI shielding for PCBs
As the most affordable way of resolving, mitigating and protecting against EMI issues, PCB level shielding clearly provides the greatest potential. However, the pitfalls of conventional technologies such as stamped metal cans have led the industry to ponder improved means of delivering board level shielding (BLS). As a consequence, the latest generation of low cost conductive coating solutions not only offer OEMs fully shielded components, but save on both premium board space and weight. Rachid Ait El Cadi, applications/process engineer, Parker Hannifin, Chomerics Division Europe, finds out more
E
lectronic product designers increasingly find themselves tasked with creating lighter, smaller formats. The knock-on effect of such a strategy is that signal integrity, durability and long- term reliability become even more challenged. With this in mind, in order to guarantee the superior performance that consumers demand, EMI and RF shielding require serious consideration. It is well documented that the
performance of a PCB and its components can be interrupted or limited by electromagnetic interference (EMI) or radio frequency interference (RFI). Only through adequate shielding of vulnerable parts can these adverse effects be avoided.
Conventional approaches Conventional shielding devices, like so- called stamped metal ‘cans’, are typically manufactured from materials that include
tin or zinc-plated steel, stainless steel, tin- plated aluminum, brass, copper-beryllium, nickel-silver or other copper alloys. However, tin-plated cold rolled steels are recognised as the most frequent selection for BLS thanks chiefly to their mechanical formability, environmental compliance and relative low cost.
Although metal cans deliver basic RF/EMI shielding, their inherent design introduces a number of limitations. For example, many are heavy and require a significant quantity of space on the PCB. Furthermore, as metal cans are usually soldered directly to the board, they can also complicate system design, place extra stress on parts and introduce unwanted cost for removal and reattachment whenever rework is required. Sure enough, there exist several advances on the basic metal can design, such as the use of surface-mounted clips that can be removed and reinstalled to offer access for rework and testing. Alternatively, a single-cavity fence with a removable cover can be used where greater access is needed. Two-piece solutions such as this can shield a single circuit area, with the fence soldered directly to the PCB, and a cover that can be lifted and reinstalled as required.
Other options
The next level up is to consider multi- cavity shields, which can lower the total cost and footprint of the metalwork by in effect ‘unifying’ several single cavity shields. Here, multi-cavity fence sets (having a proportionate number of cavities) can be built from scores of discreet components.
Chomerics CHO-SHIELD 571 and 604 provide an EMI shielding conductive coating for semiconductor packages
32 June 2016 Components in Electronics
Moving on, thermoforming options are available to support almost any design where more lightweight BLS solutions are required. Thermoforming technology leverages an innovative attachment mechanism with solder spheres to deliver an individual ‘snap’ action. However, this technology is patented, which could prove problematic in the event of obsolescence. Arguably a more radical process solution to metal stamping and thermoforming is photoetching, which allows the generation of complex shapes and features that cannot be replicated by alternative techniques without the use of costly tooling. BLS measuring from 0.18 to 0.5mm thick can be etched from materials that include brass, nickel-silver, copper or cold rolled steel. Tin plate is the standard finish, although so-called ‘tin whiskers’ can be a concern as these necessitate further surface treatments.
Conductive coatings Although there clearly exists a wide array of solutions, the onset of new BLS materials that form conductive and highly advanced coatings represents the future of shielding technology. Indeed, such coatings can be sprayed directly on to a PCB to build the necessary Faraday cage for EMI shielding. Applied in the correct way, conductive coatings can save valuable PCB space and reduce the overall cost of board level EMI shielding by replacing stamped metal cans. Two principal coating variants are available. The first, silver-filled epoxy conductive coatings, are based on a specially formulated polymer system with a coefficient of thermal expansion that can match most epoxy moulding compounds (semiconductor packaging materials). Offering superior adhesion and environmental stability, these two- part, silver-filled epoxy coatings are able to resist wave solder heat measuring more than 262°C. What’s more, the coatings contain anti- settling additives and are environmentally stable. Application can be performed using a standard air gun, spray gun/pressure vessel or robotic spray system to suit volume requirements, while pot life is around 12 hours.
The second type of conductive coating
is silver-filled polyurethane, which also offers impressive environmental stability, but even greater adhesion. These coatings are single-component and deliver long operating life at room temperature, and cure only when heated beyond 122°C.
Both of these silver-filled conductive coatings have been developed for use in high volume, accurate (no masking) spray applications in semiconductor production environments. Beyond facilitating compact board design, they provide first class EMI shielding performance (80dB+) at less than 25 µm thickness and application.
Conclusion
When it comes to board level shields, it’s no longer all about metal stamped cans. Instead, the latest BLS conductive coating materials provide the answer for a huge variety of production, performance and rework needs. Only by selecting the optimum EMI shielding technology at the design stage will signal performance be maximised. As a result, the first port of call for shielding solutions should always be a professional BLS specialist.
www.chomerics.com www.cieonline.co.uk
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