As complexity and density increase, the long-term reliability of RF/microwave circuit components becomes more difficult to characterize. Printed circuit boards (PCBs) contain many active and passive components whose performance changes over time and over operating ambient temperature. In addition, the substrate materials of the PCB, such as the dielectric, copper foil conductors, solder mask, and final plating, will also change over time, and the operating environment will have an impact on this time. At higher frequencies, changes in electrical performance, such as power and efficiency losses, may occur over time. This effect may occur in both short-term and long-term effects. Long-term changes in circuit materials and PCB performance are caused by thermal effects, such as operating in high temperature environments.

Short-term exposure to high temperatures, such as those experienced during reflow soldering during PCB assembly, generally does not affect the electrical performance of circuit materials or PCBs. However, electrical performance can be affected when the temperature exceeds the relative thermal index (RTI) of the circuit material or the maximum operating temperature (MOT) of the PCB. Temperatures above the decomposition temperature (Td) of the circuit material, even for just a few minutes, can cause changes in electrical performance. RTI is a temperature-based parameter for circuit materials that indicates the maximum temperature that a circuit material can withstand without degradation of one or more of its key characteristics. MOT is a circuit-level parameter certified by Underwriters Laboratories (UL) that applies to the entire PCB, including dielectric and conductor layers. The two parameters are similar in that both indicate maximum temperatures, but RTI refers to the maximum temperature of the circuit material, such as the laminate itself, while MOT applies to the maximum operating temperature of the completed PCB and the circuit material after it has been processed into a PCB. The MOT of a circuit will not exceed the RTI of its substrate, as UL will not issue an MOT for a circuit that exceeds the RTI of its material.

High frequency PCB laminate materials are made of dielectric materials and copper foil as conductors, based on thermoplastic or thermosetting materials. Thermoplastic materials are usually soft or flexible, while thermosetting materials are harder and more rigid. Thermoplastic materials can be heated to melting or reflow temperature, but thermosetting materials cannot be heated to reflow temperature. When the temperature is high enough, thermosetting materials will decompose.

Thermoplastic materials used for RF/microwave/ millimeter wave PCBs are usually based on polytetrafluoroethylene (PTFE). Although other materials can also be used alone or in combination with PTFE as high-frequency circuit substrates, many RF/microwave/millimeter wave PCBs use PTFE in some form. Thermosetting materials used for RF/microwave/millimeter wave PCBs are generally based on hydrocarbon resins or polyphenylene ether (PPE or PPO) polymer resins with good dimensional stability and cost advantages.

PTFE-based thermoplastic circuit materials are highly regarded for their stability and small change in electrical properties over long-term use and high temperature environments. In contrast, circuits made of hydrocarbon or PPE-based thermoset circuit materials will change their electrical properties over time and temperature, and the magnitude of this change will depend on the specific circuit material composition.

For circuit materials that are nearly pure PTFE, such as Rogers’ RT/duroid® 5880 laminate, the electrical properties are very stable over long-term use and at elevated temperatures (above room temperature or 25°C). For materials that combine PTFE with other materials to adjust the dielectric constant (Dk) or provide the performance required for a certain circuit (such as mmWave frequencies), the performance can vary over time and temperature due to the differences in the other materials. For example, Rogers’ RO3003™ material is a circuit material based on a PTFE resin system with ceramic fillers and other additives for automotive radar and mmWave frequency band circuit applications. As shown in Figure 1, it exhibits different aging characteristics than materials based on nearly pure PTFE.

As shown in Figure 1, both materials show little change in heat aging properties: less than 1% change in Dk or relative dielectric constant (εr). Both materials initially show a decrease in Dk associated with drying out the materials at 150°C. Although both materials are low hygroscopic, at a microscopic level they carry some moisture prior to testing. When moisture is removed from the materials at elevated temperatures, the Dk decreases. The RO3003 laminate has a more complex PTFE formulation than the RT/duroid 5880 material and responds differently to elevated temperatures and drying. However, the less than 1% change in Dk for both materials is considered very robust for long-term aging at 150°C.

Thermosets will experience a greater change in Dk under long-term high temperature exposure than thermoplastics. However, the amount of Dk change is closely related to the thermoset formulation, and the reasons for Dk changes in thermosets are very different from those for thermoplastics.

The natural reaction of thermoset circuit materials at elevated temperatures is oxidation. Oxidation is slow at room temperature but is accelerated at elevated temperatures. Oxidation of thermoset substrates is limited to the depth of the oxide’s penetration into the material, and the reaction at the surface of the material changes as more oxide builds up on the surface until the oxidation process stops. For thermoset materials, the speed of the oxidation process and how deep the oxide penetrates the material will depend on the material’s formulation. For example, there are many types of antioxidants (AOs) that can be included in a formulation to slow the oxidation process, and some are more effective than others depending on the material formulation.

Figure 2 compares two thermoset hydrocarbon laminates, one with poor oxidation resistance and one with the most resistant AO content to minimize oxidation effects and provide robust long-term aging performance. The benefit of adding AO can be seen in the stability of Dk over time. The data in Figure 2 is the change in Dk over time measured by the X-band fixed stripline resonator test method under high temperature conditions. The test materials were fully exposed to the environment, and the data shown is extrapolated to a longer time using the Arrhenius equation. This accelerated aging method allows long-term thermal aging effects to be estimated without the need for long-term testing. The test data compares a hydrocarbon laminate to a material with long-term stable aging resistance, Rogers RO4835™ circuit material. RO4835 material inherits the characteristics of RO4350B™ laminate and has exactly the same electrical performance. As a reference for the time scale in Figure 2, 1.0E+05 hours is equal to 11.4 years.

Figure 1: Long-term aging of RT/duroid 5880 laminate and RO3003 laminate dielectrics

Figure 2: Comparison of long-term aging of thermoset hydrocarbon circuit materials at 25°C

The Dk test of this aging test is based on the X-band fixed stripline resonator test method defined in IPC-TM-650 2.5.5.5c to evaluate the Dk change of the material. The test frequency is 10GHz, and the copper foil of the tested sample needs to be completely etched away, leaving only the dielectric material. When the circuit material is aged in the form of a circuit, the aging effect is different because the copper layer can protect the dielectric material and prevent oxidation. The amount of protection depends on the different structures shown in Figure 3.

Figure 3: A simplified side view showing different RF structures and how oxidation (yellow) infiltrates the thermoset dielectric material.

Figure 4: Bare dielectric (fully etched) compared to a 50 Ω microstrip circuit on a 20-mil RO4350B laminate (Std) and a 20-mil RO4350B LoPro laminate (LoPro).

The depiction in Figure 3 shows approximately how oxidation forms on dielectric materials. An oxide layer forms on the bare substrate surface, and some of the oxide layer even reaches below the copper foil conductor. In other words, most of the oxidation is on the surface of the material, and as the oxidation gradient accumulates, a small amount of oxidation will enter below the surface, but this oxidation below the surface will gradually decrease.

Circuit material parameters Dk and dissipation factor (Df) will increase in the presence of oxidation. Several factors will determine the extent to which oxidation affects the RF/microwave/mm-wave electrical performance of the circuit material. Thinner dielectric circuits will be more affected because oxidation will account for a larger proportion of the overall dielectric material.

Oxidation affects different structures of RF/microwave/millimeter-wave circuits differently, depending on the electromagnetic (EM) fields in the circuit. For example, for microstrip, most of the EM fields are located between the bottom of the signal conductor and the top of the ground plane, and there are strong fringing fields on the left and right of the signal conductor, and the signal will change due to sufficient oxidation. Although microstrip may not be significantly affected by oxidation, the effects of oxidation can be detected at high millimeter-wave frequencies. Thicker microstrip circuits have less of this effect than thinner microstrip circuits.

It is evident from the location of the signal conductors that stripline structures are generally not affected by oxidation (Figure 3). However, for microstrip line edge coupling circuits, the coupling field is located on the surface of the circuit substrate and approximately below it and intersects with the oxide layer. Oxidation will reduce RF/microwave/millimeter wave performance. The effects of oxidation may also be significant in grounded coplanar waveguide (GCPW) circuits on thin substrates when operating at millimeter-wave frequencies .

Media Characteristics Decryption

In the material aging process and test conducted, the X-band fixed stripline resonator test method is used to determine the characteristics of the dielectric material. Since the dielectric material is completely exposed to the environment during aging, all surfaces of the dielectric material are oxidized, as shown in Figure 3. That is, when the material is placed in the fixed stripline resonator to form a stripline resonance, the eight surfaces of the dielectric will be oxidized. Depending on the test material, the amount of material that needs to be placed in the fixture is different and the oxidation is also different. However, compared with the test in the form of a circuit, the oxidation of the material in this measurement method is much greater.

To protect against the effects of oxidation, RO4835 circuit laminates are formulated with an optimal combination of AO additives. In addition, RO4835 circuit laminates use the same formulation as traditional RO4350B circuit materials, which have been a reliable circuit material substrate for high-power RF/microwave circuits. Due to the optimal combination of AO additives, RO4835 laminates take 10 times longer than RO4350B laminates to reach the same oxidation level.

In fact, RO4350B laminates show good long-term aging performance in high-power applications, except for oxidation differences. Generally, long-term aging issues with RO4350B laminates are related to circuits with coupling characteristics, such as directional couplers. To better understand the role of circuit characteristics in the long-term aging performance of circuit materials, materials with circuit characteristics are compared to the same dielectric material processed in the same way but without copper (the copper is completely corroded away and there is no circuit characteristics).

This comparison used a variety of circuit structures using Rogers’ RO4350B laminates and RO4350B LoPro® laminates, including 50 Ω microstrip circuits, microstrip edge-coupled bandpass filters, and microstrip step-impedance low-pass filters. RO4350B LoPro laminates are identical to the company’s RO4350B laminates, but with smoother, low-profile copper foil to reduce insertion loss at RF/microwave/mmWave frequencies.

Figure 4 compares several long-term aging effects for two circuit materials. The sample with the largest change in Dk over time is the one with the copper completely etched away (marked as fully etched), i.e., the bare dielectric material. This bare material, with no circuitry on it, was characterized in an X-band fixed stripline resonator fixture at +50°C. The circuit tested with 50 Ω microstrip has much less variation in Dk over temperature.

Samples evaluated for long term aging using transmission line circuits at elevated temperatures were found to take much longer (10-100 times) to reach the same level of oxidation as samples where the copper was completely etched away. The amount of oxidation and Dk change for samples at various temperatures is also shown in Figure 4. If the test circuits were fabricated on the same material but with different thicknesses, the oxidation effects could also be different.

Studying aging effects with low-pass step impedance filters is similar to that of transmission line circuit samples. Microstrip edge-coupled bandpass filter samples are more affected by oxidation. It still takes 3-5 times longer to reach the same oxidation level as the completely etched copper (bare dielectric) samples. Tightly coupled circuits are more affected by oxidation than loosely coupled circuits. In addition, the oxidation time span varies with the test temperature, with the largest difference in circuit material Dk due to oxidation at the highest temperature.

Solder mask ink, parylene coating and moisture-proof insulating glue are used in circuits to reduce the effects of thermosetting oxidation. Solder mask inks can significantly reduce the effects of aging, but often also reduce RF performance. When applied at thicknesses of 25 μm or above, Parylene coatings also help to minimize the oxidative effects of long-term aging. HumiSeal also helps minimize the effects of oxidation, and while there are many types to choose from, some are more effective at reducing the effects of aging.

in conclusion

Circuit materials are exposed to high temperatures for short periods of time during processing and for longer periods of time during use. The effects of long-term high temperatures are often seen as oxidation accumulation on PCB thermoset dielectric materials, which can cause a shift in the Dk of the circuit material. Determining how sensitive a circuit material is to the effects of long-term aging requires precise and careful testing, as different measurement methods and techniques can yield very different measurement results when characterizing high-frequency circuit materials. Selecting the right test method, whether it is material-based or circuit-based testing, provides reliable data when modeling Dk and circuits.

TMM® 3-D Compression Molded Microwave Materials

TMM® Thermosetting Microwave Material is a patented product of Rogers, a ceramic and thermosetting polymer composite material designed for high-frequency applications. The material is available in PCB laminate form or 3D form by compression molding, making it an ideal material for innovative design applications. Its key features are extremely low TCDk, a dielectric constant that can be controlled between 3 and 12, low CTE, high chemical resistance, and the ability to be molded into a variety of shapes.