Technically Appropriate Material Choices are Key to Design Success

Estimated reading time: 11 minutes

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.

Johnson: Are there other specifics in materials that designers should be considering?

Creeden: Absolutely. Another major consideration nowadays is thermal properties. There are many different market spaces where the end user will place their circuit board in a high-temperature environment. For thermal concerns, we’re seeing a lot of people using different polyimide materials, such as automotive or military under the hood where the temperatures are extremely high.

We’ve talked about the dielectric material, but the other major part of circuit material is the copper. Copper has been our conductive metal of choice going way back. The metal comes in two forms. There’s an electrodeposited type of copper both in the holes and on the surface, but there’s also a base copper, which is either rolled on to prepreg or clad right onto the core laminate itself. Copper is used because it is highly conductive with low resistivity and will transmit voltage/current. It’s affordable, available, and relatively easy to manufacture. The base copper comes in different thicknesses. When you feel it, you’re amazed at how easily you can distinguish the weight by holding a couple of sheets of different weight. And copper has an ability for thermal dissipation, which is helpful. If you put a power and ground plane layer next to each other (close so that they couple well) and they’re both thick layers of copper, you can typically carry more current, so that’s another property it would have.

But when the copper is put onto the prepreg, it needs to bond. When it’s laminated, the resin needs to adhere to the metal. Usually, the metal has a smooth side and a rough side, and the rough side gives you that adhesion required to bond to the material, which will hold it down. And that’s measured as a peel strength. This could be realized if you need to do solder rework, especially if the pads/lands are small, they’ll have the potential to peel right off the board, which would destroy it. So, they have this rough side to hold it down.

If you look at it in a cross-section under a microscope, you’ll see how rough it is, which is problematic from a high-speed perspective. And when I explained how the field goes between the bottom side of a trace and the ground plane underneath it, most of the energy is on the bottom side of the trace, which is the rough side. Therefore, the topology of that rough copper is not good for what’s known as the skin effect where the electrons are bumping on the surface of the copper facing the ground plane. As a solution, the industry has produced very low-profile copper, but it’s threatened by that adhesion and peel strength challenge. I’m seeing engineers consider putting the ground plane opposite the smooth side of the copper, trying to guide the wave towards the smooth side. Not all circuits can do that, but that is one way to get around it if you’re stuck with the rough side of the copper. Again, getting low-profile copper with good adhesion is the advancement that we need with materials today.

In addition, the devices and circuitry are getting smaller and smaller. The standard pin pitch of BGAs are 1.0 mm, but BGA pin pitch keeps getting smaller (0.8, 0.65, 0.5, 0.4, 0.35, 0.25, and smaller). Consequently, high-definition and rectangular, square-edge traces are important. The traditional fabrication process is subtractive where you etch in, resulting in an interconnect with a trapezoidal geometry. However, it is difficult and inaccurate to create a microtrace that is less than roughly 88 µm (0.0035) with a trapezoidal geometry. The definition becomes imperative and should have well-defined, squared edges. This is accomplished using some additive or semi-additive methods. Some of the dry-film photoresist materials are used for that, and we’re going to see that become more and more prevalent as people must use microtraces and microfeatures. Ormet’s sintering paste is one of the best innovations for any-layer HDI vias along with innovations with DuPont’s conductive inks, which will help define the printed electronics needs that are now happening.

Johnson: That’s very comprehensive, Mike. Is there anything else you’d like to mention?

Creeden: There’s always something else to consider. The inquisitive mind of a design engineer is always trying to look and consider everything, and that’s where diligence pays off. It’s a relentless profession where the pursuit of excellence is what makes us move forward. And ensuring your materials are appropriate for your circuit is no longer a thing of the future. It’s our present challenge, and we should make it our success story.

Johnson: Thank you, Mike.

Creeden: Thank you, Nolan. I appreciate this opportunity to serve.

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