Picking A PCB For High Reliability
High reliability is a goal and desire for all designers and end-users of high-frequency printed-circuit boards (PCBs). Since all of the components mounted on the PCB depend on it, it is expected to deliver dependable and consistent performance over time. But depending on the operating conditions, it can sometimes be difficult to achieve. A high-temperature environment, or conditions of high humidity, can challenge the stability and reliability of the best-engineered PCB materials. Of course, some PCB materials hold up better than others under challenging conditions and some deliver outstanding reliability for a wide range of circuits. It can be helpful to know what to look for in a PCB material when high reliability is critical, and what types of operating conditions can put the reliability of a PCB material to the test. When considering different PCB material choices, it can be useful to know which material parameters and characteristics can provide insight into the expected reliability. In an attempt to help, the next two Blogs will explore PCB material reliability: this blog, Part 1, will review some of the general obstacles for a PCB material to achieve good long-term reliability while the next blog, Part 2, will take a close look at how the characteristics of one particular PCB material add up to good long-term reliability.
Nothing lasts forever, not even a PCB. But a well-engineered PCB material can provide reliable and consistent performance for a long time and across a wide breadth of operating conditions. For many applications, such as for circuits and systems in medical, military, and space applications, PCB material reliability is critical. Anything that can cause a change in the PCB material’s performance or behavior, such as a change in dielectric constant, can be considered a threat or challenge to the long-term reliability of the PCB material.
Elevated temperatures, and the high power levels that can produce them in a high-frequency circuit, may pose the greatest challenges to the long-term reliability of any PCB material. Any PCB laminate consists of different materials with different properties, including dielectric materials and conductive metals, such as copper. These materials expand and contract at different rates with temperature, respectively, resulting in thermally induced stresses on junctions between different materials.
Thermal expansion occurs at different rates through the different dimensions of a PCB material, described by a parameter known as coefficient of thermal expansion (CTE). It is not unusual for a PCB material to have different CTE values through its x and y directions—the width and length—compared to its z dimension or thickness. To improve reliability, PCB materials are often formulated for a dielectric with CTE in the x and y directions matched to that of copper (about 17 ppm/°C), so that the conductive metal and dielectric materials expand and contract similarly with temperature, avoiding stress from thermal junctions.
But this may not be the same in the z direction, where plated through holes (PTHs) are drilled and metalized through the thickness of the material to form signal and ground interconnections as well as to connect different layers of multilayer circuits. The metal/dielectric junction for these structures must also endure thermal stresses, but may not enjoy the same matched CTE values as in the other two PCB dimensions. High reliability depends on the durability of these different metal/dielectric junctions over time and temperature. For that reason, a PCB material engineered for reliability usually features a CTE in the z direction for minimal dimensional changes in PTHs with time and temperature.
Design engineers typically sort through potential PCB material choices by their performance levels, such as the dielectric constant or loss at a target frequency. But when picking a PCB material for high reliability, it is necessary to compare another set of parameters. In its simplest terms, a PCB material built for good long-term reliability should exhibit stable mechanical and electrical characteristics with temperature. A PCB material’s parameters with close ties to reliability include CTE, dissipation factor (Df), and glass transition temperature (Tg).
The Df, which is the loss attributed to the dielectric material, can impact reliability at high operating temperatures and power levels, since high loss will contribute to high material temperatures and added thermal stress. The glass transition temperature, Tg, is something of a warning point for reliability, since it represents the temperature at which dramatic changes can occur in a PCB material’s CTE behavior. Above this temperature, a material can become mechanical and electrically unstable. Although it may be necessary to exceed the temperature for short-term processing steps, it is a temperature that should not be exceeded for any length of time to ensure good long-term reliability.
Another concern for the long-term reliability of some PCB materials is the effects of oxidation, which can cause a small increase in the dielectric constant of a PCB material over time, especially for thermoset PCB materials. The dielectric material covered by copper is protected against oxidation, but the uncovered dielectric material is subject to oxidation and “darkening” in color over time, especially at elevated temperatures. Some thermoplastic PCB materials, such as polytetrafluoroethylene (PTFE), are relatively immune to oxidation, but can be impacted by CTE and other reliability concerns. Oxidation, which is accelerated at higher temperatures, can also cause a small increase in dissipation factor and the loss of transmission lines.
Oxidation can also cause an increase in the loss of transmission lines, although this change is relatively small over time and at elevated temperatures. It has also caused small increases in dielectric constant over time and temperature of a few percent or less, which can be a concern for some circuits where dielectric constant is critical, such as resonators and filters. Of course, oxidation will not occur where oxygen is not present, such as in space in satellite-communications (satcom) applications. When oxidation is a concern, a circuit can be stored in an oxygen-free environment, such as under nitrogen or in a vacuum.
Numerous standards laboratories, including Underwriters’ Laboratory (UL), have studied PCB reliability. UL’s long-term thermal rating for PCB materials is known as the relative thermal index (RTI). It refers to the maximum temperature that a material can handle indefinitely, without compromising performance or critical material properties. It is backed by another UL rating, the maximum operating temperature (MOT), which refers to a particular PCB construction. In that particular PCB configuration, it is the highest temperature that the circuit can withstand without enduring changes in performance or material properties. The MOT can never exceed the RTI for a given material.
Design engineers must review the requirements of an application when sorting through their PCB material choices. Some applications may not have the environmental/performance demands of others and not require the same PCB materials to ensure high reliability. Because PCB material is a complex issue, the next Blog, Part 2 on PCB reliability, will explore how these general guidelines for reliability in Part 1 can be met by a specific PCB material, Rogers RO4835™ laminate. This laminate was formulated specifically to improve the long-term aging characteristics of the company’s popular RO4350B™ laminate.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.