Plastics: Shielding Solutions (Jan. 2008)
January 2, 2008
Fabricating electronic device housings out of plastic offers several advantages, including design flexibility and inherent coloring, but polymer has an Achilles heel when it comes to electromagnetic interference (EMI), which passes through even the hardest plastic like ghosts through walls. Unchecked interference can cause a range of problems, from mild annoyance in a consumer electronic device to a life-threatening malfunction in a sensitive piece of medical equipment.
The issue has become of greater importance to design engineers due to the growth of electronic devices in general, and of wireless devices in particular. And increasingly tighter European regulations targeting interference make it an issue that cannot be ignored. Those regulations make specific demands regarding both a product’s emissions of EMI and its immunity to EMI.
The only way to plug the EMI holes in plastic is with some type of conductive material, which can be either embedded inside the polymer material, or applied after molding to the plastic part. There are different approaches to both concepts, and design engineers need to understand all the tradeoffs involved with each before settling on the optimal solution for their product. The following issues need to be examined:
- The level of immunity defense needed.
- The level of needed emissions limitations.
- Cost targets for materials.
- Assembly/manufacturing costs and need for secondary operations.
- Effect on molding of the part and tooling life.
- Design flexibility for the part, particularly regarding color options.
- Effect on the recyclability of the part.
- Environmental considerations involved with any post-molding operations.
Essential to pondering the first two points is establishing what mandates must be met and how do they limit ones choices. EMI emissions and a product’s immunity against them are regulated in different ways in different places around the world. Fabrizio Montauti, vice president of engineering for WaveZero, a Sunnyvale, Calif., manufacturer of shielding technology that coats products with metallic films, says that in the U.S. the FCC regulates EMI emissions, but does not require products to be certified against their immunity to these radiated fields. This is not true however in Europe where the EU’s EMC directive has stringent rules against about EMI. In a nutshell, the directive states that the devices are “secure in, and cause no harm to, the environment into which the device will be placed,” says Larry Rupprecht, Manager of Conductive Materials Development, RTP Co., Winona, Minn.
Most regulations describe the maximum allowed level of emission within the frequency range from 30 MHz to 1 GHz, which is the range at which most electronic equipment operate. However, the frequency range used by electronics is expanding to higher frequency ranges such as 10 Ghz or even 40 Ghz, according to Eric Jiang, product manager of Faradex, SABIC Innovative Plastics, EMI Shielding Compounds, SABIC Innovative Plastics is formerly GE Plastics, with U.S. headquarters in Pittsfield, Mass.
Designing an electrically quiet circuit board is a good practice for reducing emissions, but by itself does nothing to shield the electronics to interference from other nearby or far-away systems. Unfortunately, one of the all-too common problems today is that engineers focus their attention on the control of emissions, while ignoring immunity, says Montauti. This is acceptable design practice when the product is limited to the American market, but it becomes a liability when the product is sold on the international market, he says.
While metal parts block EMI inherently, plastic parts must rely on the addition of some conductive material to stop EMI from coming in and going out. The three basic means of achieving this include:
Each of these various shielding methods have their pluses and minuses and tradeoffs may need to be made. What is optimal for one application may not be optimal for another, so designers need to understand how those pros and cons apply to their particular application.
“The best way to reduce the level of emission is to aim for an optimal design of electronic device, including the design of circuits, placement of components, and filters,” says Jiang. “When these methods are not adequate to meet the requirements, an enclosure can be used such as a Faraday cage around the electronic device to shield against EMI.” A Faraday cage, which is also known as a Faraday shield, is a term for an enclosure formed by a metal or conducting material to block electrical fields from escaping or entering the barrier from either side. The traditional shield is a fabricated metal barrier, which is considered an effective way to achieve shielding. The downside can be added weight, cost and cycle time to the product.
Shielding by dispersing conductive fillers into compounds is an option offered by suppliers such as RTP and SABIC. According to Jiang, moldable plastics with shielding properties can improve cycle times, logistics and overall system cost can be improved when the enclosures are made from inherently shielded thermoplastics.
Available EMI shielding thermoplastics fillers include those based on the dispersion of stainless steel fiber, carbon fiber, graphite, nickel-coated carbon fiber, and silver-based additives and other fillers in a thermoplastic. It is the inherent conductivity, volume percentage and dispersion of the shielding additive in the thermoplastic that affects the part's volume conductivity. In inherently shielding thermoplastics the volume conductivity determines the overall shielding effectiveness. Typically, with this technology shielding values ranging from 40 dB to 60 dB can be obtained under far field conditions, which is sufficient for most consumer applications, says Jiang.
Carbon filler is often used, especially when color is not important, because of cost. Carbon filler will color most plastic black or grey. While carbon-based polymer additives are cheaper than other fillers, and do provide increased shielding effectiveness, Rupprecht says that carbon cannot compete with the metal and metalized additives. The metal fibers, he says, reflect and shield wave energy better than carbon.
Metal fibers also work better than powders and particulates, he says. A metal coating provides a metal skin as a shield for reflecting and absorbing. A metal-additive filled plastic makes use of an internal matrix of overlapping shields to accomplish the reflecting and absorbing actions. This internal matrix is more effectively formed with high aspect-ratio fibers as compared to metal powders or particulates. The lower loading of high aspect-ratio fibers provides an economic advantage versus higher loadings of low aspect-ration powders and particulates, says Rupprecht.
An issue when adding additives to plastics is that some of the material’s properties can be altered. Generally speaking, impact performance will typically get reduced when fillers are added. Carbon filled thermoplastics have higher stiffness, higher HDT and smaller mold shrinkage. Stainless-steel-fiber filled thermoplastics have similar mold shrinkage, stiffness, HDT as unfilled resin, says Jiang.
Metal and metal-coated additives typically provide less mechanical improvement than traditional glass fiber or carbon fiber reinforcement. However, most parts that require shielding do not typically require mechanical strength. If they do, then the plastic compound can be altered to improve strength. When strength is critical to the application, modification of the plastic compound will provide the needed performance, says Rupprecht.
The compounds can be “tuned” to different frequency ranges and also incorporate other properties such as fire retardance. For instance, RTP, recently introduced a family of thermoplastic compounds that were specifically created for use on electronic devices that use the Bluetooth wireless communications protocol. The compounds are tuned to allow transmission of Bluetooth frequencies from the device, while blocking other EMI emissions and providing immunity from EMI from other electronics.
SABIC Innovative Plastics’ also recently released a new compound to its LNP Faradex line. The company’s approach is to infuse stainless steel fibers into the plastic compounds so that the finished product features inherent EMI and radio frequency interference. The new line of compounds has resulted in equal shielding ability as past products, but with less of a stainless-steel fiber load, which the company says helps keep costs and part weight down. The proprietary filler technology also increases modulus to enable strong and durable thin-wall parts. The five new LNP Faradex compounds are alloys of polycarbonate and acrylnitrile butadiene styrene (PC/ABS).
Metals serve as a key ingredient in the processes offered by WaveZero, including PVD and electroplating. The California company offers functional thin films of metal coatings that shield EMI. The PVD coating process deposits a thin metallic film by the condensation of a vaporized form of the material onto various surfaces. This process permits the creation of conductive coatings, 0.5 microns to 6 microns thick, that do not chip, peel, or flake.
WaveZero also offers the Form/Met EMI suppression process, which Montauti says are the optimal choice for new product development and design teams. The shields are created by thermoforming and die cutting a lightweight plastic substrate that conforms to the interior shape of the housing. The substrate is then metallized, creating a form-fitting shield that is dropped into housing. It can also be shaped just to cover the circuit board. Designed as a separate component that fits in perfectly, the barrier eliminates the need for coating the part after molding and also eliminates the need to use conductive fillers in the polymer. The shield is easily removable for recycling of the device after disassembly at the end of its lifecycle. A plated part or a part filled with stainless steel fibers is not.
The diversity of these approaches requires design engineers to establish their priorities and understand the tradeoffs with each method, but they must also keep abreast of new techniques and materials that may change the calculation.
The range of shielding methods today provides a solution for nearly every application. And, tomorrow, materials and methods will have even more capabilities. Jiang says that future materials will likely include some with high microwave absorption function in a low frequency range. Another future innovation may include a long-conductive fiber reinforcement process that will give better EMI efficiency.
Rupprecht adds that, something that is impossible today becomes can become state-of-the-art tomorrow, and that new materials and processes will continue to evolve to meet challenges in cost, performance, and processing. “For example, commercial nano-materials for shielding are not yet available, but it seems that nanotechnology can improve on the aspect-ratio related issues of cost/content and performance.”
For more information, email:
RTP Co.: email@example.com
SABIC Innovative Plastics: firstname.lastname@example.org
Sidebar: Get the Lead OutEver since the European Union’s Restriction of Hazardous Substances Directive (RoHS) went into effect in 2006 -- with stricter guidelines approaching in 2010 - OEMs have been scrambling for ways to remove banned substances such as lead from their products.
Since 2001, Thogus Products, a custom injection molder in Avon Lake, Ohio, has been developing a process to replace lead with tungsten-filled polymers for shielding certain specialized applications. Lead, which can block electromagnetic energy waves, can also block radiation energy. That is why lead is so often used with X-ray and CT scan machines to protect the patient as well as the caregiver. Working with compounder PolyOne of Cleveland, and more recently GE Plastics (now SABIC), Thogus Products has integrated the tungsten material into several plastic materials including Nylons (6, 66, and 12) and certain TPEs and are working to incorporate the material into higher heat resins, says Matthew Hlavin, vice president of sales and marketing for Thogus.
Tungsten-filled polymer has the same specific gravity as lead, 11.0 specific gravity, and it shields radiation at the same attenuation, Hlavin says. Thogus has been able to hold tolerances of the injection moldable material to +/- 0.002 in., and has made wall sections of about 1/2-in. thick. The smallest part molded by Thogus has been 20 g and the largest was 26 lbs.
The ability to mold the component can reduce the number of overall parts needed, reduce secondary machining operations, and other costs related to making products from lead, according to Hlavin. This helps to offset the additional costs of tungsten. Typically, lead sells for under $1 per pound, whereas tungsten can cost between $30 and $80 per pound depending on purchase quantities, he says. Tungsten is also an abrasive material that will wear on tooling.
Hlavin says that because of the toxic nature of lead and the cost of recycling the non-RoHS compliant material out of discarded equipment, the next couple years will see growth in the use of the tungsten based material in finished products. The tungsten-filled polymers can often be directly substituted for lead in applications such as collimators, X-ray tube housings, and other medical-shielding applications. To date, the company has used the technology on more than 50 different applications.
One early application was the lining of a collimator for a CT scan machine where a tungsten-filled polymer part replaced an aluminum fabricated housing that was lead. Thogus worked with GE Plastics’ (now SABIC) LNP Thermocomp HSG (high specific gravity) high-density compounds on the collimator for GE Healthcare’s OEC 9800 X-ray machine. Collimators absorb stray radiation and limits X-ray exposure.
According to Clare Frissora, market director, Healthcare, GE Plastics, the transition from machined and stamped lead to injection-molded engineering resins enabled tighter tolerance specifications and greater part consistency, enhancing the performance and safety of the X-ray equipment. It also helped avoid secondary operations required with lead. Also, combining multiple components into one part helps reduce manufacturing time, system cost, and complexity, she says.
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