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| EMI Shielding: Thermoplastic Compounds vs. Coatings – What is Better? I Findoutaboutplastics.com |
The topic of electromagnetic interference (EMI) gains more and more importance in the electrification of cars. In this blog post, we discuss the EMI shielding using either modified thermoplastic compounds and/or standard thermoplastics with coating.
Fresh-up on the basics of EMI
The radio frequency range of the electromagnetic spectrum (ranging from 10 KHz to 77 GHz) is mainly responsible for electromagnetic interference. As a consequence, device failures can happen by such interference. Looking at electric cars, the overall number of sensitive electronic components increases and with this the need for electromagnetic “noise” protection increases too.
When electromagnetic waves hit a material, three different reactions of the waves may happen [1]:
1) Absorption (energy transformation as the electromagnetic waves pass through the materials)
2) Reflection (the electromagnetic waves are reflected by the materials)
3) Transmission (the electromagnetic waves pass through the material without any energy transformation)
In general, the higher the power of a system, the higher EMI becomes (right thumb rule of current). Also, with higher power, the temperature in the enclosed housing will increase.
What are the main sources of electro-magnetic interference?
There are three different categories of EMI sources:
1. Inverters (low frequency),
2. DC Motors (low frequency),
3. In-car radio frequency (RF) as well as out-of car RF (high frequency).
Which parts need to be protected (EMI receptors)?
There are several parts of an EV which need protection: battery housings, housings for automotive electronics, infotainment enclosures, ADAS housings, and LIDAR housings.
Polymer Material requirements in EVs:
New applications in Electric Vehicles lead to an increase of electrical properties such as Comparative Tracking Index (CTI), flame retardant levels, and chemical properties. Those new requirements need to be considered in the
material selection of the before shown applications.
The ideal polymer compound fulfills:
- UL 94 V0 (0.8 -1.6 mm)
- Chemical resistance toward acids and oils
- EMI shielding (> 52 dB over different frequencies - 150 kHz -100 MHz)
- High CTI (> 600 V)
- Thermal conductivity (> 1 W/mK)
- Thermal shock resistance
- Overmoulding capability
- No electrical corrosion on overmoulded parts
- Lead-free reflow soldering (connectors)
- Cost competitive
Testing EMI shielding and automotive standards:
ASTM D4935 is used to estimate the shielding effectiveness (SE) of different materials. It uses a plane- and far-field electromagnetic wave.
Furthermore, in automotive the so-called CISPR 25 was introduced in 2016 by OEMs. In this standard, different shielding levels are defined, such as CISPR 25 A2 (70-40 dB) and CISPR 25 A1 (80 dB). In automotive, shielding targets are between 40 dB and 70 dB.
Shielding compounds vs. coating of plastics
Plastics are natural insulators and do not reflect, nor absorb EMI. Modification of the compound needs to be done using different fillers such as stainless steel fibers or nickel-coated carbon fibers. As a rule of thumb, one can state that by adding 20% carbon fibers into a compound, the same EMI shielding effect as with 10% metal fibers is achieved. Another way to achieve EMI shielding is over metal based coatings.
In a recent study conducted by RTP company [2], cost per piece of a plastic part (housing with 51 mm x 51 mm x 13 mm; wall thickness: 3.0 mm) with minimum 40 dB shielding effectiveness was shown.
The detail requirements and results of the study:
-Conductive coating methods: painting, metallic painting, and vacuum metallizing
-Compounds: ABS with 10% stainless steel fiber; ABS with 15% nickel-coated carbon fiber
-Part volumes: 10,000 and 100,000 pieces
-Coating costs include: apply coating; tooling/fixing/masking costs; scrape rate
-min. EMI SE: 40 dB
The cost per part for low and high volume parts can be seen in the chart below.
Conclusions:
Conductive coatings can be an option for higher volume parts. However, EMI shielding compounds show less costs for low and high volume parts. As we pointed out, compound requirements are changing. As soon as UL94 VO level is needed, additional flame retardants need to be added or the used resin needs to be changed to an intrinsic V0 grade such as
PPS. In such a case, costs for EMI compounds will increase and conductive coatings will be more competitive. There are also new players for conductive coating systems such as the Seal and Shield technology (Freudenberg Sealing) [3] or the Chrom-VI free process called Slotosit offered by company Schlötter [4].
Another innovative way of shielding is by using carbon fiber based thermoplastic composite tapes. A 0° / 90° carbon fiber PPS UD Tape allows a uniform EMI shielding while fulfilling thermal rating and mechanical stabilities.
Thank you for reading and you interest!
Best regards and #findoutaboutplastics
Herwig Juster
More on electrification:
High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have
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Literature:
[1] https://incompliancemag.com/article/the-basic-principles-of-shielding/
[2] https://www.rtpcompany.com/products/emi-shielding/
[3] https://www.fst.com/corporate/newsroom/press-releases/2019/freudenberg-emshielding-press/
[4] https://schloetter.de/slotosit/
