Weiterentwicklungen in der Technologie



	Technology Transitions and the Comparative Properties of PCB Materials for Commercial Microwave and High Speed Digital Applications Microwave printed circuit boards have traditionally been limited to microstrip and stripline designs. The traditional dielectric substrates used were GP, GR (random fiber reinforced), and GY, GX, and GT (woven reinforced). All of these materials a based on polytetrafluoroethylene (PTFE or Teflon®) resin chemestry. These substrates were engineered to provide the optimum electrical desires, primaly in the form of very low dissipation factors for good loss performance. Regrettably, along with the good loss performance, came poor mechanical and thermal stability due to the very high concentration of PTFE resin. The woven structure of the GY and GX materials offered some mechanical advantages over the GP and GR systems, but usage in complex designs (particularly multilayer designs) was limited by poor mechanical and thermal stability. Complex, high density, multi-layer digital circuit boards have been manufactured for years with technology transitions toward smaller plated- through-holes, thinner dielectrics, finer lines/ spaces, and higher layer counts. Digital substrates have kept pace with these developments by transitioning to higher temperature resins to provide better plated-through-holes reliability, and more consistent thickness and dielectric constant control for controlled impedance. However, the digital substrates do not exhibit the loss performance required to function at microwave frequencies. There has been a significant development in PTFE-based material technologies to meet the mechanical and thermal stability requirements for complex multi-layer and double-sided circuit boards. These materials provide a "bridge" for the microwave and digital worlds. High reliability plated-through- holes are now possible. By adjusting the composite construction of PTFE resin and reinforcement concentrations, the new materials exhibit consistent dimensional stability, rigidy, and low thermal expansion rates. The excellent loss performance and consistent dielectric properties are maintained. These properties allow the new materials to be used in both high-speed digital and microwave applications. The electrical properties are superior to any digital substrate, and depending on the reinforcement technique being used, the cost of new materials is much closer to the digital substrates than traditional PTFE substrates. Taconic has engineered two PTFE-based substrates with enhanced technical properties. TLE-95 substrates are maufactured in thicknesses from .004" (0.1mm) with a dielectric constant of 2.95 +/- 0.05. TLE-95 substrates are intended for complex multi-layer microwave and high-speed digital circuit boards. TLC substrates are specifically designed to meet the low cost objectives for newly emerging commercial rf/microwave applications, TLC substrates are manufactured in thickness .0145" (0.37 mm) with Er=2.70, and .020" (0.50 mm) with Er=3.0 and .031" (0.80 mm) and thicker with Er=3.30 and 3.20. Both materials exhibit excellent mechanical and thermal stability and cost less than traditional PTFE substrates. A comparison chart and graphs for materials including microwave and digital substrate properties are available by contacting Taconic´s Customer Service Department. The data is taken from suppliers` literature. The list is not inclusive of all available materials. The comparisons are made of similar materials that are readily available and meant for similar applications. Properties of Materials When choosing a material, a designer must consider the importance of dielectric constant, loss tangent (dissipation factor), thickness, tolerances, and consistency, thermal and mechanical stability, ease of circuit fabrication, and cost. The scope of this paper is to compare general material properties and their impact on circuit designs. The category of "Digital Materials" is included since there are some rf and microwave designs attempted with these materials in a frequently futile effort to reduce or control cost. Electrical Properties Er-Tolerances The dielectric constant (Er) is a primary component in determining width and length of lines on a printed circuit board for a given frequency. One of the most important considerations for a designer is the consistency and tolerances of the Er. Less control of the er will result in inconsistencies in circuit performance. Traditional microwave materials and most of the new "bridge" materials have tight er tolerances of +/- 0.02 to 0.05. The digital materials have tolerances of +/- 0.15 to 0.30 and are not usefull for most microwave designs. Temperature Effects The er is influenced by temperature change. For microwave designers, the characteristic room temperature PTFE change in er is well known. The significance of the change is design and frequency dependent. The high resin content GY and GP/GR materials exhibit an absolute er change of .045 (2.5%) over the extreme temperature range of -60 to 200 °C. The "bridge" materials with lower or "dilluted" PTFE resin systems yield considerably less change in er over temperature. Rogers 6002 exhibits less than .5% change. The TLE-95 and TLC substrates exhibit less than .75% change. When considering the effect of traditional PTFE substrates` absolute change of .45 in Er on microwave designs, it is easy to understand why the digital substrates with Er variables of +/- 0.2 over temperature and lot to lot variations of similar magnitude are less than useful for rf/microwave designs. Another important consideration is the effect of temperature on line length. In controlled impedance and microwave applications, changes in line length result in phase changes. It is appropriate to highlight this point along with the effects on Er as choosing the material with the lowest Er change over temperature may not perform as well as material with a lower coefficient of thermal expansion in the X-Y plane. For example, a recent study of a design with long line length has shown TLX material to have less phase change over temperature than 6002. Loss Performance The dissipation factor (Df or loss tangent) of a given substrate is perhaps the most critical variable to a microwave designer. It is important to note the frequency at which a material is tested since the Df increases with frequency. The standard test methods use 1 MHz and 10 GHz frequencies. New cellular communication systems are being upgraded from 450 MHz to 900 Mhz and 1.8 GHz, and new wireless and PCS/PCN applications are developing at these frequencies and above. The use of low-cost FR-4, BT/epoxy and cyanate ester materials in base station power amplifiers has proven deficient as their poor loss performance renders them nearly useless at these frequencies. Additionally, their poor control of er and thickness cause reliability problems. TLE-95, and particularly TLC substrates, deliver high performance and high reliability at a cost that is often competetive with, or lower than the digital materials when total package cost is concidered. Moisture Absorption Moisture absorption will effect the circuit board´s processability in the fabrication process. The PTFE/woven glass substrates exhibit exceptionally low moisture absorption, and require little or no conditioning during fabrication to eliminate trapped process chemicals in drilled holes. This translates directly into improved reliability in plated through holes. Some of the digital substrates, and particularly the ceramic PTFE substrates, absorb substantial moisture during processing and require costly, time consuming bake cycles during fabrication to remove the moisture. Outdoor applications such as direct broadcast satellite televisions LNB´s, PCS/PCN antennas, automotive radars, and other benefit from PTFE/glass low moisture absorption. Their properties remain stable over changing moisture conditions, while the digital and ceramic filled PTFE substrates´ properties change with moisture absorption. Mechanical Properties Flexural Strengh Flexural Strengh is an indication of the stiffness or rigidity of a material. In general, circuit fabrication and component mounting assembly operations are easier with rigid materials. TLC and TLE-95 substrates exhibit 35 - 40 Kpsi flexural strengh, making them highly suitable for high volume production processes. Peel Strength Copper foil peel strentgh should typically be greater than 6 Ibs/in ( 1.0 N/mm) to maintain good integrity through fine line circuit processing and soldering operations. Higher peel strenghts are necessary if multiple soldering operations are to be performed. The PTFE/woven glass constructions, with pure PTFE resin at the surface, yield excellent peel strenghts. Ceramic loaded PTFE constructions have poor bonds. Thermal Properties X, Y, & Z CTE The coefficient of thermal expansion (CTE) is very important for plated- through-hole designs. Reliability of copper plating will depend on the board thickness, copper elongation, and the material Z-axis expansion. The environmental conditions for the board are also important. Considerations must be given to whether the boards will need to be reliable for soldering operations or continuous thermalcycling such as aircraft or automotive applications. There are complex mathematical models used to predict PTH reliability. A material expansion rate of 70 ppm/°C will yield excellent reliability for a board. 100" (2.54 mm) with hole diameters .030" (.76mm) , cycled over the temperature range -55 to 125 °C several hundred times. Polyimide, cyanate ester, 6002, TLC, and TLE-95 meet this creteria. For relative comparisons, a Z-axis expansion percentage can be used extrapolating along a soldering temperature of 260°C. As a general guidline, 3% expansion is considered reliable for most designs and environmental conditions. Traditional microwave materials are deficient and limited for use in plated-through-hole designs. The traditional digital materials, with the exception of FR-4, are very reliable. TLE-95 and TLC substrates have expansion rates of 2% at 260°C, making them extremely reliable for PTH designs. TLE-95 has been designed into multilayer applications for high speed digital work stations and high frequency chip test modules. TLC is also used in 4 and 6 layer designs. One recent application utilizes TLC for a pager base station. The design is a 4 layer board combining TLC and FR-4 to handle the microwave and digital circuitry while maintaining a low board cost. The .062" (1.6mm) boards were thermal cycled 350 times from -55 to 125°C and showed no degradation of the plated through hole or electrical integrity. An important material property that is often overlooked is the X - Y expansion rate. The development of surface mount devices and the increases of their size requires temperature stability in the X - Y plane in order to maintain solder joint reliability. The ideal X - Y coefficient of thermal expansion is 6-8 ppm/°C to match the ceramic devices. The lower the number is for materials, the more reliable the solder joint will be. General Processing Any new material introduced to the pcb market will need to be proven reliable through the pcb manufacturing processes. General acceptance of new materials, regardless of the attractive electrical, mechanical, or thermal properties, depends on the relative ease to process boards. PTFE based substrates have traditionally been niche market materials processed by pcb shops which specialize in producing these boards. The PTFE based materials have been used with proven processing techniques for over 30 years. In recent years, the development of the direct broadcast satellite (DBS) television market has brought high volume PTFE circuit boards into the mainstream circuit processing shops. This trend is continuing as more high volume commercial applications develop. The basic processes which need to be changed to manufacture PTFE, as compared with FR-4, are: drilling, deburr, through-hole pre-treatment, and routing. Drilling, deburr, and routing are easily adjusted to accommodate the characteristics of PTFE and utilize existing equipment. Treatment of the resin for the subsequent plating process requires special processing conditions. Either a sodium based chemical process or plasma can be used. Using sodium based chemestry had been a drawback for PTFE to gain acceptance with the volume circuit processors. Plasma processes can be used to treat all types of circuit materials including PTFE. The development of plasma processes for PTFE has been an important breakthrough leading toward greater acceptance. The technique for building multi-layer PTFE-based boards is somewhat different but is developing as the need becomes more common. Bonding films with different melt temperatures have been used for years to build stripline boards. The same techniques can be used to build multi-layers. The bonding films available have melt temperatures of 90, 121, 181, 260, and 305 °C. Fusion bonding requires laminating above PTFE melt temperatures of 345°C. Most circuit board fabricators are working within 181°C and 260°C films. Hybrid board constructions combine PTFE and FR-4 laminates using FR-4 prepeg as the bonding mechanism using standard FR-4 lamination cycles. Conclusion PTFE based materials such as TLE and TLC meet the new high-speed digital and microwave design requirements for electrical, mechanical, and thermal reliability. The cost of TLE and TLC materials are significantly lower than the traditional microwave materials and are competitive with the high-speed digital materials such as cyanate ester or BT. The well established techniques for processing PTFE/woven glass materials has led to immediate acceptance of TLE and TLC in the market. Since the TLE and TLC are more forgiving in processing, the cost to produce and assemble circuit boards should also come down.