For decades, the power of computers has grown rapidly as designers have managed to place more and smaller transistors onto a silicon chip, doubling the number every two years, leading the way to increasingly powerful and inexpensive personal computers, laptops and smartphones. As the number of transistors has increased, more power is required to run economically, and subsequently the service temperature requirements of microwave absorbers used to mitigate unwanted energy has increased. Additionally, if the absorber is to be placed on the circuit board, the material needs to be able to withstand the solder reflow process.
Microwave absorbers based on several thermoset polymers can be used, however when the need for high volume manufacturing exists, this class of materials can become costly because of the associated machining or die cutting processes. Thermoplastic absorbers based on high temperature polymers offer a solution to both of these needs. Certain polymers are capable of withstanding 200° C long term in addition to the short duration, higher temperature solder reflow conditions. Thermoplastic based absorbers can also be injection molded rapidly in large quantities, making the process economical.
This article reviews one thermoplastic polymer in particular, polyphenylene sulfide, that when combined with a soft magnetic filler, carbonyl iron powder, provides excellent microwave absorptive properties along with high temperature stability.
Background
In 1979, as digital system interference in communication equipment was increasing, the U.S. Federal Communications Commission (FCC) required that the electromagnetic emissions of all digital devices be below certain limits in order to reduce the number of instances of EMI and “electronic pollution.” Other countries also imposed similar restrictions. Many manufacturers already had internal limitations established to minimize interference; however, this regulation resulted in increased interest and the development of many varied solutions to overcome EMI.
Designing compliant and effective devices has become an increasingly difficult task as electronics have evolved into smaller, multi-functional packages. Increasing clock speeds and subsequent higher frequencies have transitioned electromagnetic control microwave absorbers from shielding components because of the associated higher emissions at shorter wavelengths. Emissions at high frequencies are beginning to approach the physical dimensions of many microwave cavities, which can lead to cavity resonance effects. Standing waves exist inside the cavity if the largest cavity dimension is greater than ½ wavelength, making the enclosure act as a resonator, affecting circuit performance. If a noise source has a frequency that corresponds to a resonant point, a large field can be generated due to the multiplication or amplification effect if there is not a high rate of energy loss relative to stored energy.
To address cavity resonance, microwave absorbers can be inserted onto a wall or roof of the enclosure with a pressure sensitive adhesive (PSA) to absorb standing waves, thereby keeping the electronics performing optimally. Absorbers can also be placed at the source, i.e., directly on the radiating element, in order to eliminate coupling of the electronic field with the chassis, so currents will not flow into the chassis and set up circulating currents within it. For moderate to high power chips radiating unwanted energy, the need for absorbers that can withstand high temperatures is becoming necessary.
An electronic load is a device that simulates loading on an electronic circuit. It can be any electronic device connected to a voltage source such as a radio, antenna, computer, a resistance, etc. When discussing electromagnetic control, a load is a passive device which will reduce or change the unwanted microwave voltage, power, current or phase in a microwave circuit. The load will act as a power drain, or as a microwave absorber for unwanted electromagnetic energy, but can also act as a wave tuning component because of its intrinsic magnetic and dielectric properties.
Terminations are especially fabricated for use at microwave frequencies. Molded resistive wedges are commonly employed and consist of a dissipative material dispersed in a dielectric medium. In moderate to high power waveguide terminations, a wedge of lossy dielectric absorbing materials is used, shown in Figure 1. The length-to-base width taper of approximately 10:1 ensures very low return losses, mandatory in military radar systems and waveguides used in low signal to noise conditions. When high power is involved, resistance to high temperatures is important as the load can become extremely hot.
Microwave absorbers can come in a variety of forms, from rigid to flexible to foam. They can be made from virtually any polymer and contain a variety of fillers from magnetic or dielectric to control the electromagnetic performance. Magnetic fillers are most commonly used in enclosed electronic devices since the magnetic portion of the wave dominates in the near field. Carbonyl iron powder is commonly used when the frequencies of concern are anywhere between 1 and 40 GHz. Below this frequency, specialty alloys are used, and above this frequency dielectrics are typically employed.
Carbonyl iron powder is a highly pure iron prepared by thermal decomposition of highly purified iron pentacarbonyl. In the process, spherical particles form on a nucleus, thereby developing a shell structure. The particles give outstanding magnetization behavior for electronic applications and are frequently used as inductor core material in power supply converters. Typically, one desires the composite to be highly filled with carbonyl iron in order to attain good absorption or attenuation, although lesser loaded composites will exhibit resonances at higher frequencies, which can sometimes be advantageous.
The polymers employed in the composites are for the most part the binder that holds the filler together and are therefore selected based on their flexibility, thermal stability, compressibility, machinability, etc. The polymers often used are commonly thermosets such as epoxies or silicones. These materials are liquid in the uncured state and can therefore accommodate reasonably high filler loadings, thereby providing the necessary electromagnetic control, although large volume manufacturing of parts with specific shapes can become costly.
For large volume applications, thermoplastic polymers offer a more cost effective alternative because the secondary injection molding process is rapid and the cost per part decreases as the volume increases. Once the initial outlay of the tool cost is realized, the piece price can be minimal. Various thermoplastic based absorbers are on the market filling this niche, including polypropylene and thermoplastic elastomers (TPE) for lower thermal requirements. However as previously mentioned, the need for higher thermally rated materials is on the rise because of the close proximity of components and higher power requirements.
Several high temperature thermoplastic polymers for microwave absorbers have been investigated to fill this need, specifically polyphenylene sulfide (PPS) and liquid crystal polymer (LCP).
The Experiment
PPS is a semi-crystalline polymer offering excellent high temperature resistance, chemical resistance, flowability and dimensional stability. PPS has a repeating molecular structure, as shown in Figure 2. It has a high melt flow index thereby having a very low melt viscosity, allowing for high filler loadings. It is very brittle, but this becomes minimized when filled. The polymer is also inherently flame retardant making it ideally suited for electrical applications.
LCP is also a semi-crystalline polymer, having long, rod-like molecules that are ordered in the melt phase, unlike other polymers whose chains become entangled in the molten state. A representative LCP structure is shown in Figure 3. The polymer has a very high heat deflection temperature, near 300° C, high melt flow, combined with high mechanical strength and dimensional stability. Like PPS, LCP is inherently flame retardant and promoted as being able to withstand solder reflow conditions.
In order to compare the two thermoplastic absorbers, it was necessary to make a number of sample batches of composite material with a compounding process and then produce test pieces with injection molding. The compounding work was done on a Micro-18 Leistritz co-rotating twin screw extruder and the injection molding was completed on a 40-ton injection molding machine. Specialized tools were used to create small samples (1.0 mm x 22.9 mm x 2.5 mm) for electro-magnetic testing and traditional tensile bar tests pieces for measuring mechanical properties.
A total of 23 individual batches were compounded, injection molded, and tested. For PPS, this consisted of five batches with a volume loading of 48.5 percent carbonyl iron powder, five batches with a volume loading of 50.3 percent and one batch each with a volume loading of 10, 20, 30, and 40 percent volume loading.
Nine batches were prepared using LCP as the binder, with five batches having volume loading of 48.5 percent carbonyl iron powder and one batch each with a volume loading at 20, 30, 40, and 50.3 percent volume loading. The small sample pieces were tested in the x-band frequency range (8 – 12 GHz) using a waveguide test fixture and a network analyzer, shown in Figure 4. The sample holder, which can be seen in testing position in Figure 4, is shown on its own in Figure 5. The density of each small sample piece was measured using a pycnometer and the volume loading of carbonyl iron for each sample piece was calculated using the known densities of the components.
Using magnitude and phase of both transmission and reflection data, the real and imaginary components of the relative permeability (u*) and permittivity (e*) were calculated. Using these parameters the attenuation at 10 GHz for each sample was then calculated. Attenuation is a measure of how much the energy of a wave propagating through the material is reduced and is expressed as a rate per unit distance, usually expressed in dB/in or dB/cm. Attenuation in dB/cm is given in Equation 1.
The Results
The loading levels at 48.5 percent and 50.3 percent were prepared because these levels of CIP are known to provide excellent attenuation in other composites. The lower loadings were prepared in order to establish a relationship with attenuation. The natural log of attenuation forms a linear relationship with the volume loading of a two component composite, therefore using the experimentally determined volume loadings and the calculated attenuation values for every sample piece, this relationship was determined for both the PPS and LCP composites, shown in Graphs 1 and 2.
Although it had been assumed that the carbonyl iron powder content was overwhelmingly the driving factor in attenuation, it can be seen that the thermoplastic binder played a significant role in attenuation from the difference in linear relationships shown above. Another way to examine this difference is a side by side comparison of attenuation for the LCP and PPS samples with the same volume loading. This can be clearly seen for samples where the measured density was less than 4 percent off from the theoretical value. The LCP samples had an average attenuation of 47.7 dB/cm while the PPS samples had an average attenuation of 60.4 dB/cm.
Tensile bar samples from both polymers at the 48.5 percent volume loading were used to determine average expected tensile as well as to help determine thermal stability. Samples underwent thermal aging at set temperatures for 500 and 1000 hours and then measured for tensile strength. By exposing test pieces to a series of elevated temperatures, the relationship between the rate of degradation and temperature can be determined. This test procedure is outlined in International Standard (ISO) 11346.
Different samples also underwent a slightly exaggerated solder reflow process simulation; spending five minutes in a furnace at 280° C, slightly over a typical maximum temperature of 260° C. For each condition, five samples were taken and the results averaged, shown in Tables 1 and 2.
There is no recorded result for the LCP samples which underwent 1000 hours at 230° C because all samples were too weak to withstand the pressure applied to test pieces in the grips of the tensile testing machine.
Physical changes to the thermally aged samples were also noted. All thermally aged samples, both PPS and LCP, underwent a color change becoming darker, shown in Figure 6. The PPS samples developed a brittle outer layer from oxidation. Some sections of this outer layer would flake off during tensile testing, but in general this layer remained intact on samples. This layer could in part explain the increase in tensile strength values measured for most of the PPS samples. The LCP samples which experienced higher temperatures (230°C and 280°C), developed small bubbles on the surface.
unaged PPS, aged PPS, unaged LCP, aged LCP)
Conclusion
It is known that polymers filled with carbonyl iron will exhibit inferior thermal resistance compared to the virgin polymer because of the oxidation of the carbonyl iron. Polyphenylene sulfide (PPS) is a high temperature polymer, although actually rated below LCP. However, when filled with carbonyl iron, it was shown to demonstrate excellent long term thermal stability as well as surviving solder reflow conditions because of its inherent chemical and thermal stability. Liquid crystal polymer (LCP), is one of the highest temperature polymers, capable of withstanding temperatures up to 230° C long term. However, when filled with carbonyl iron to function as a microwave absorber, it was unable to withstand continuous exposure at 200° C. This composite also was unable to withstand solder reflow conditions, yet unfilled material performs extremely well.
PPS is a polymer made up of alternating sulfur atoms and phenylene rings in a para substitution pattern, as shown in Figure 1. These highly stable bonds give the polymer stability toward thermal degradation and chemical reactivity. Also because of its molecular structure, PPS tends to char during combustion, making it flame retardant. Loading the PPS polymer with carbonyl iron powder allows it to function as an effective microwave absorber, essentially comparable to other carbonyl iron-filled materials with the advantage that it offers excellent short term and long term thermal stability. It has been demonstrated that these composites can also withstand solder reflow conditions as well as being resistant to long term exposure to 200° C. This composite can therefore realize applications as an absorber at the board level. Being thermoplastic, this material can be easily injection molded into complex parts in large production quantities.
PPS absorbers provide a variety of outstanding properties such as:
- High service temperature of 200° C
- Withstand solder reflow conditions
- UL94 V-0 flame rated
- High iron loadings translating to excellent RF properties
- Complex 3D shape designs
- High modulus and creep resistance
- Chemical resistant
These high performing absorbers based on PPS see applications not only as waveguide terminators in the form of a wedge, but can be molded into a variety of shapes for applications such as caps and covers, RF filters and cavity resonance absorbers, shown in Figure 7.
References
- Paul, Clayton R. 2006. Electromagnetic Compatibility. Hoboken: Wiley.
- Gruenwald, G. 1993. Plastics: How Structure Determined Properties. Munich: Hanser.
- Dixon, P. “Theory and Application of RF / Microwave Absorbers.” Emerson & Cuming Microwave Products Tech Notes. 2012.
- Shahrooz, S. and Ramahi, O. 2004. “Electromagnetic Interference (EMI) Reduction From Printed Circuit Boards (PCB) Using Electromagnetic Bandgap Structures.” IEEE Transactions on Electromagnetic Compatibility, Vol. 46, November 2004.
Robert Boutier joined Laird in 2008 as a product development manager where he led the development of thin film microwave absorbers. His current focus includes thermoplastic absorbers and hybrid materials for control of both electromagnetic energy and heat. He can be reached at robert.boutier@lairdtech.com.
Andrew Labak has been employed by Laird as a research engineer since 2011. His research is focused on processing and molding of thermoplastic materials for control of EMI and 3D printing novel electronic structures with advanced materials. Andrew can be reached at andrew.labak@lairdtech.com
