Key Elements of an Ideal PWB Material from Rush PCB Inc.
Printed Wiring Materials (PWBs) from Rush PCB Inc. are available in huge diversity and wide range of applications. This has made the concept of any ideal laminate very fuzzy, to say the least. For instance, a material suitable for a high temperature application may be impractical for high-speed digital design such as in a microwave. Therefore, although it is possible to list potentially important properties, each design team will need to establish a prioritization among them since no one material will present the optimal value for all properties that might be considered to be important. Essentially, Rush PCB Inc. presents some common ingredients or properties of PWB materials listed below, from which designers can make their choices during the design phase.
Tg—Glass Transition Temperature
Polyimides with Tg of 250°C or above are suitable for the highest temperature systems. Designers consider Tg as a rough indicator for total Z-axis expansion and hence, a proxy for reliability indication for plated through holes. For applications where either in-use temperature or process temperatures (or both) are less demanding, manufacturers offer a wide variety of epoxy systems with Tg in the range of 170°C.
Tg offers a good frame of reference for polyimide and traditional epoxy materials. However, it is not reliable for characterizing the reliability of non-traditional resin systems and highly filled systems used in high frequency and low loss applications where the PWB is a composite of various components.
Td—Thermal Decomposition Temperature
Depending on the chemical composition of PWB materials, this property may vary greatly form the mid 300°C range for several epoxy systems to over 400°C for some polyimides. When a PWB material reaches its Td temperature, it begins to degrade thermally. In general, data sheets list Td temperature as one at which the material loses 5% of the original weight due to decomposition.
However, the onset temperature at which significant weight loss of PWB material begins to occur, presents a better indicator of performance. The reason being by the time the material has lost 5% of its weight from decomposition, it might be totally unsuitable for any application.
Loss Tangent and Dielectric Constant
A signal passing along a transmission line on a dielectric material loses its power. Loss tangent is a measure of how much power the signal has lost. Dielectric constant is a measure of the speed of an electric signal as it travels in a dielectric material, relative to the speed of light in vacuum—the dielectric constant of space (vacuum) being defined as 1.00. Therefore, a higher dielectric constant for the PWB material implies a slower propagation speed through it.
Etching causes shrinking in all laminate materials to some degree, while consistency of both process and product depends on the consistency in registration. A suitable PWB material would be one that shrinks minimally when etched, has consistent and reproducible shrinkage that allows predictable factors for artwork compensation. Ideally, the PWB material should not require any artwork compensation, and should always register properly, without requiring any compensation while drilling. The IPC test of dimensional stability is at best a measure of the actual registration of a specific board design, and registration continues to be a major fabrication concern for HDI and high layer count designs.
CTE—Coefficient of Thermal Expansion
Designers must match the expansion requirements of PWB materials to the expansion requirements of devices to be mounted on the surface, claddings, and the thermal planes buried in the interior. For instance, a CTE of 6 ppm/°C is ideal for leadless ceramic chip carrier attachments, and laminates such as Arlon’s 45NK woven and reinforced with Kevlar with low resin contents are suitable. Rush PCB Inc. tend to use other material also, such as nonwoven aramid reinforcements, copper-invar-copper distributed constraining planes, and quartz reinforcements for achieving values as low as 9-11 ppm/°C. This represents a substantial improvement over conventional polyimide or epoxy laminates and such PWB materials have proven consistent and acceptable in a variety of SMT designs.
With increasing density of components on a board, required for meeting the demands of improved functionality, as against a steady decrease of the overall surface area of PWBs, the watt-density of power the PWB generates also increases. As critical devices could fail at rates doubling for every 10°C increase of temperature, this pushes designers to use PWB materials with high thermal conductivity to remove heat directly from devices placed on the surface of the board.
However, designers must do this without allowing the board to suffer in terms of dielectric and electrical properties. While traditional polyimide or epoxy systems have thermal conductivity values between 0.25 and 0.3 W/m-K, Rush PCB Inc. targets thermal conductivity figures in the 1.0 to 3.0 W/m-K range, to achieve significant reduction in the board surface temperatures, especially near active devices.
Expansion in the Z-Direction
For the ideal PWB material, expansion in the Z-direction must match that of copper within the PTH to avoid damaging the plating inside the holes during thermal excursions in processes such as solder reflow. Typically, standard materials exhibit CTEs of 50-60 ppm/°C, when the operating temperature is below Tg, while this increases to roughly four times higher when the temperature crosses Tg. PWB materials with high Tg value, such as Polyimides, show a lower overall Z-direction expansion than typical epoxy systems do.
Compatibility with Lead-Free Processes
PWB materials compatible with lead-free processes need to withstand higher soldering and reflow temperatures associated with lead-free solder systems—typically 30 to 50°C higher than traditional lead-tin systems. Manufacturers usually characterize PWB material suitable for lead-free systems in terms of Tg>155°C, Td>330°C (for 5% decomposition), and overall CTE<3.5%.
This classification includes materials such as Polyimides as being lead-free compliant, while Rush PCB Inc. is using newer generations of epoxy systems to meet additional requirements. Lead-free systems are inherently more complex, as several current laminate materials can survive its applications, but some devices mounted on the boards may not even have been tested at the highest lead-free temperatures. Therefore, the trend is to use materials that have a margin of safety in the range of higher temperatures as seen during lead-free soldering.
Also Read; PCB Testing: Why is it Important?
Green Prepreg and Laminate
Green PWB materials are typically compliant to UL-94 V0 flammability ratings specifically achieved without the use of brominated flame-retardants. Although most manufacturers use the brominated bisphenol-A for FR-4 systems, there is a movement towards non-brominated systems wherever possible, as these are more environmentally friendly.
Meeting Environmental Regulations
Several countries are now complying with regulations such as those modeled on RoHS and WEEE from the European Union, or are in the process of developing their own. The cost of compliance to multiple independent and different regulations becomes a significant part of the cost of a finished PWB.
Ideally, manufacturers should be able to process the laminate and prepreg in simple ways using regular methods of photo-imaging, etching, processing through wet chemicals, and traditional techniques of lamination. High process yields and design flexibility demands the ideal material be used in multiple designs with high probability of success. Although most manufacturers still define normal processing as being suitable for the conventional FR-4 material, Rush PCB Inc. uses advanced materials in high performance PWBs that require more than the normal ideal processing.