Technically Appropriate Material Choices Are Key to Success

Estimated reading time: 5 minutes

Materials are no longer a passive part of the design; they play an active role in the manufacturability, reliability, and speed of a PCB. Editor Nolan Johnson and Mike Creeden, founder of San Diego PCB Design, discuss several key characteristics that designers should consider in their material selection process.

Nolan Johnson: Mike, could you introduce yourself and tell us about what you’re working on?

Mike Creeden: I’m the founder of San Diego PCB Design, and I had the opportunity to sell the company to Milwaukee Electronics/Screaming Circuits, so we’ve joined the Milwaukee Electronics family of companies. I also serve as an EPTAC instructor for the IPC CID and CID+ programs as an MIT (master instructor).

Johnson: When in the design cycle should PCB development people consider material selection?

Creeden: That’s a great question. It should be within the first days of starting your project because that’s the best time to establish that things will be designed “correct by construction.” I do not want to design a product and then consult my fabricator in the last days of development. If the material is not in stock, we may be delaying the procurement of what is probably a late design, anyway. You’d want to make sure that it’s in stock, and you’d also want the coaching from your fabricator and supply chain to make sure that you’re making correct selections. When you do this late in the process, selections may happen quickly, and there may not be enough time to ensure that it’s an appropriate material selection. There’s always a material technology decision, and it should be made early in the design process. The earlier, the better because this allows time to change it if that is required.

Johnson: You’re making the point that material is becoming increasingly important. With that in mind, and especially for anybody who’s newer to this, why is material important?

Creeden: If you’ve ever seen lightning strike in the air, you’re probably seeing it from the cloud to the ground. Or if you’ve ever seen static electricity, when the lights are low, you can see the spark fly. That’s a good visualization to understand every time you’re routing a trace. Historically, circuitry traces were DC in nature, and their environment didn’t matter as much. Now, you are managing an electromagnetic field. The field is capacitive, and that’s best (high capacitive) when a trace is close to its return path. It’s also magnetic, which is inductive.

That is how a signal propagates (low inductance) down the line. You’re also managing an EM energy field; you’re not just connecting two points with a trace. The energy field is not in the trace. Rather, the trace and it’s return path—typically a GND plane—serve as reference points; thus, the energy exists in the dielectric material between them. Therefore, the material with all of its parameters are an integral part of the performance of the circuit.

The material’s electrical properties are measured by the dielectric constant (Dk) denoted as ϵr. This measures how well energy will permeate through the material at different frequencies. Also, they are measured by the dissipation factor (Df) also known as loss tangent. This measures how much energy can be dissipated or lost into the material. The energy field travels within the dielectric material. The material can resist the flow of energy, and each material has a known measured rate. As a reference, air has a dielectric constant of ϵr = 1.

Your average FR-4s have a dielectric constant of approximately ϵr = 4. Faster circuitry requires less resistance and less energy loss, so you see high-speed material go down to the range of ϵr = 3.

With circuitry achieving increasingly faster speeds, most people equate speed to the circuit’s frequency, but it must be understood that the burst of energy delivered in every pulse as measured in the rise time (Tr) as related to voltage/frequency. That is when the signal transitions from zero to its voltage and that burst of energy defines its field. To manage that, you must understand that the material selection is an integral part of the function of the circuitry. These are all factors that engineers and designers must take into consideration from day one.

Material plays an important factor, but the designer must practice good design skills to ensure that signal integrity issues are not created by violating a signal’s return path, impedance matching, or crosstalk to another signal. Another electronic consideration is the consistency of the weave pattern because of the Dk difference between weave and resin, so as a solution, they have what’s known as spread weave. They spread the weave out to get a consistent Dk, which is essential for the performance of differential pairs of traces.

Mechanical and physical properties affect the structural integrity and manufacturing process. With the glass transition (Tg), the material’s resin will transition from a hard to a rubbery state as a factor of temperature. That’s imperative when you’re considering high layer count boards. The dielectric material typically is comprised of a glass weave, which expands in the X-Y axis, and a resin that expands in the Z axis. The Z-axis expansion is measured as the coefficient of thermal expansion (CTE), which threatens the structural integrity of the via plating, and the vias are the most vulnerable entity on a PCB. The other physical property to be considered is the thermal decomposition temperature (Td) when you start doing HDI boards where you have multiple lamination (thermal) cycles to accomplish the construction. The material can breakdown due to the excessive amount of heat from thermal excursions during all phases of manufacturing, test, and environmental stresses.

Do your research and make sure you understand, based on your application, whether the material’s physical and mechanical properties suit your requirement and the manufacturing process. Fabrication is where the material will probably see its first and worst thermal excursions. The amount of weave plays a part in the drilling in some of the HDI laser vias too.

To read the full article, which appeared in the June 2019 issue of PCB007 Magazine, click here.