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QUESTION: What are the new low cost material choices for wireless applications with the performance of PTFE and fabrication ease of FR-4? ANSWER: Materials with various dielectric constants and dissipation factors are being used in many commercial wireless applications. Materials with high dielectric constants tend to be best suited for lower frequency, where minimizing circuit size may be desired. As frequencies increase, selection of lower dielectric constant substrates is preferred since circuit losses are lower for these materials. Table 1 outlines various wireless applications and the frequencies at which they operate. It also provides an indication of the typical maximum frequency of operation for RF/microwave circuit board laminates based on their dielectric constant.
Table 1 Polytetraflouroethylene (PTFE)-based substrates have satisfied the electrical needs of these applications but at a fabrication cost premium due to the need for special surface processing associated with plated through-hole manufacturing. This processing is usually accomplished using highly reactive sodium naphthalene etchants which are costly and also restrict the number of printed circuit board fabricators willing to process PTFE based-laminates. Epoxy/glass laminates, like FR4, are also being used in some applications in order to keep fabrication costs at a minimum. The low cost of the material itself, 3 to 4 times lower than PTFE/woven glass based laminates, and the easy availability of highly automated circuit board shops make this laminate a candidate when initiating designs for wireless commercial applications. FR4 may be used when frequencies are low enough, typically below 1 GHz, and the performance of the circuit is not too critical. Due to the high tolerance of dielectric constant and dissipation factor for these laminates, these applications would require tuning and compensation to take into account the above factors. Rogers RO4000® series High Frequency Circuit Materials establish a new paradigm for high performance laminates. RO4000 material was developed to combine the high frequency performance comparable to woven glass PTFE substrates with the ease of fabrication associated with epoxy/glass. RO4000 series laminates are not based on PTFE but are a woven glass-reinforced hydrocarbon and ceramic thermoset material with a high glass transition temperature (Tg > 280°C). Unlike PTFE-based microwave material, no special through-hole treatments are required. RO4000 circuit processing costs are comparable to epoxy/glass laminates. There are currently two versions RO4003™ and RO4350™. The flame retardance of RO4350 has been enhanced through additional processing. Electrical and mechanical properties of RO4000, commercial PTFE/woven glass (TFG) and epoxy/glass materials are given in Table 2.
The RO4000 series offers the designer an excellent combination of electrical properties. IPC-TM-650 Method 2.5.5.5. was utilized to obtain an accurate picture of the performance of the material. This method utilizes a two-wavelength-long, 10 GHz resonator etched onto one side of an 0.008” thick piece of dielectric. The material under test is clamped on either side of the resonator at 500 psi. The resonator is loosely coupled so as to measure material and not circuit Q, figure 1. Resonance exists at four nodes which occur approximately at 2.5, 5, 7.5 and 10 GHz. These frequencies cover the majority of modern wireless applications.
Figure 1: Stripline Resonator The frequency of each resonant node is measured using a scalar network analyzer. Dielectric constant is derived using the standard equation for wavelength, adjusted for fringing effects. The Q of the material is calculated by dividing the resonant frequency by the 3db bandwidth and correcting for copper losses. The IPC test methods manual provides specifics on conducting the test. The dielectric constant change versus frequency can be determined by comparing the measurements at the four nodes and is essentially zero, figure 2.
Figure 2: Dk Change vs. Frequency Thermal coefficient of dielectric constant for both RO4003C and RO4350B exceeds the stability of PTFE/woven glass laminates, providing the needed temperature performance for critical components such as filters and oscillators. This material attribute was determined using the IPC test fixture in a thermostatically controlled oven, figure 3. For the temperature range of -25°C to 150°C, the change is closer to 1.5% of the nominal value at 25°C. The RO4000 family is better suited for temperature-sensitive applications, like VCO’s, than PTFE/woven glass materials.
Figure 3: Dk vs. Temperature Insertion loss was determined by measuring the electrical loss of two different lengths of 50 Ohm line. The insertion loss of the shorter line was subtracted from the long line’s in order to give the insertion loss of the differential length. This loss was then divided by the difference in length to obtain the insertion loss per inch. This was done for both the RO4000 materials as well as the Epoxy/glass. The insertion loss of RO4000 is clearly superior to epoxy/glass substrates, figure 4. This low loss performance extends the useful range of these materials well above 20 GHz.
The success of a modern wireless high volume application hinges on the repeatability of the components used to affect the design. Key properties such as dielectric constant and thickness must be statistically controlled in an effort to reduce tuning and test time. The tighter the control on key laminate properties the larger the process window afforded the designer. Selection of materials from suppliers that use statistical process control on key laminate properties also allows the designer to better model and predict the performance of the design. This reduces the prototype phase and shortens the time needed to bring product to market. Figure 5 illustrates the statistical process controls on RO4350 series laminates by presenting a histogram on dielectric constant (3.48±.05).
Circuit Board Fabrication The mechanical properties of the RO4000 series laminates facilitate a wide variety of construction and assembly practices. The combination of a thermoset resin system and a woven glass reinforcement provides for a rigid laminate with low X and Y coefficients of expansion. The rigidity of RO4000 provides a firm platform ideal for pick-and-place or other forms of high volume manufacturing processes. Applications requiring high interconnect densities usually resort to multilayer printed circuit boards. These PCBs are often made entirely of the high performance material to ensure mechanical compatibility. The X and Y expansion characteristics of RO4000 provides a suitable physical match to epoxy/glass laminates. Hybrid constructions or combinations of high performance laminates and epoxy/glass substrates have become an increasingly utilized alternative to lower overall costs. The DC, control and digital signal paths are designed onto the lower cost epoxy/glass and the RF or microwave signals are carried on the RO4000 layer. The surface of RO4000 does not require special surface preparation to promote wetability for plating solutions. Standard FR4 prepreg can be used to bond RO4000 to epoxy/glass. A hybrid MLB can be manufactured without the need for exotic processes such as sodium or plasma etching or the need for high temperature bonding films as would be the case if using PTFE/woven glass materials, figure 6. The result is a high performance, low loss material which can be fabricated using standard epoxy/glass processes offered at competitive prices.
Figure 6: Typical RF/Digital Multilayer Construction The selection of an appropriate bonding film must be given careful consideration. The electrical loss budget on certain applications may be limited causing one to erroneously conclude that a low loss bond film is essential. Low loss films similar to FEP are more expensive and require costly surface preparation which would diminish RO4000’s fabrication cost advantage. The thin cross section of the FR4 bonding layer degrades the loss performance of typical constructions by a surprisingly small amount as illustrated in figure 7. Selection of a bonding film should be examined by each application.
Figure 7: Insertion Loss of RO4003 50 Ohm stripline Conclusion RO4000 laminates deliver the performance of PTFE but fabricate like FR4. Designers currently using woven PTFE can lower their overall cost by eliminating the need for exotic surface processing. FR4 consumers struggling with the impact of dielectric constant and thickness variations can step up to high performance laminate without also impacting their circuit fabrication costs. Competing in the Wireless market presents the designer with many challenges. RO4000 will help to confront those challenges.
RO4000, RO4003, RO4350 are licensed trademarks of Rogers Corporation. |
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