Advanced Materials

For PCBs and IC Substrates

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Beyond FR4 PCB Materials

In the past, many PCBs did not rely on specialty materials except for thermal and mechanical reasons. The electrical reasons for using advanced materials later became obvious in devices relying on fast digital signals for communication, such as telecom and networking equipment. Today, many products rely on high-speed digital interfaces, RF and wireless signals, and low-level analog signals all in the same system. What is the glue that makes it all possible? It’s the materials that make up PCB laminates and PCB stackups.

Almost rigid PCB materials are built from two common components: a resin mixed with a curing agent, and a woven glass fiber reinforcement material that ensures rigidity. This forms the most common class of materials known as FR4, which are the standard class of PCB materials used in most PCBs. Advanced PCBs and IC substrates require the more advanced materials we present in this guide.

Advanced FR4 Materials

Standard FR4 materials have typical Dk values from 4.2 to 4.8, and loss tangent (Df) value of 0.02. Advanced FR4 materials push Dk and Df values lower, with many advanced FR4 laminates having Dk < 4 and Df < 0.01. Some common materials include Isola Getek, Megtron 5 and above, and ITEQ IT-8350A. Some of the important characteristics of advanced FR4 materials include:

  • Dk values available from 3 to 4
  • Df values available below 0.01
  • Tight glass weave styles (e.g., 2116) or spread glass
  • Variety of epoxy/curing agent mixes for high reliability
  • High glass transition temperature (Tg) options available

Advanced PCB materials

More advanced systems that are fabricated with higher layer counts may require these advanced FR4 laminates to reduce conductor losses and keep the device fabrication process in the subtractive regime. There are also signal integrity benefits to lower Dk materials, primarily being related to reduced capacitive crosstalk and extension of channel bandwidths, both of which are required when digital signals require faster edge rates. Both factors enable the use of faster digital interfaces without requiring more expensive fabrication processes.

Polytetrafluoroethylene (PTFE) PCB Materials

PTFE PCB materials are used primarily in RF applications, where extremely low dielectric loss (low Df) is required for a device to function. Examples include wireless systems, specialized sensors, and radar PCBs, all of which may also have a digital section that provides fast data transfer.

These materials are designed to provide very low dielectric loss with a very broad range of Dk values available. Some products, such as Rogers 3000 series, are available in Dk ranging from 3 to 10. The ability to select a high Dk value with low loss allows the size of certain circuits or resonant structures to be tuned based on the wavelength of signals in an RF circuit, but without incurring additional loss. These materials are also available in unreinforced versions (without woven glass) in order to provide a skew-free solution, and they can be used in hybrid configurations where the PTFE laminate is used with standard or advanced FR4 materials.

Advanced PCB materials

One should note that commercially-supplied PTFE-based PCB laminates are only available in certain thicknesses, so not all products can be used in high-density systems or in high layer count PCB stackups. When high layer counts are needed, specialty HDI PCB materials are available that can be built up to high layer counts with advanced via transition options.

Materials for HDI PCBs and Substrates

Materials for high density interconnect (HDI) PCBs and IC substrates are different from standard and advanced FR4 materials in that they are compatible with high-density drilling and stackup fabrication processes. Specifically, these materials are laser-drillable for forming microvias, and they can be put through sequential lamination buildup processing to form a high layer count stackup. For IC substrate fabrication, materials should also be compatible with additive copper deposition and processing. Example material types include:

  • Conventional prepreg with square/spread glass
  • Low warpage BT-epoxy substrate
  • Multifunctional epoxy with non-woven aramid reinforcement
  • Polyimide with non-woven aramid reinforcement
  • Ajinomoto Build-up Film (ABF)
  • Uncured B-stage resin and copper foil (RCC laminates)

Advanced PCB materials

Blind and buried via types as commonly found in PCBs are not limited to the HDI PCB materials listed above. The materials above are intended for fine-line subtractive fabrication and laser drilling of blind/buried/skip microvias. However, standard materials could still be mechanically drilled if the drill size and via aspect ratio allow. For example, NWES has manufactured hybrid PTFE-FR4 stackups with mechanically drilled blind vias. Some input from the PCB manufacturer will be needed to ensure the fabricated structures in standard PCBs and HDI PCBs will be reliable.

Copper Foil and Glass Weave

Glass-reinforced composites used in PCB fabrication use a rigid base material coated with a copper foil, which is then etched away to form conductive circuits. In more advanced high-density PCBs and substrates, the copper may be deposited by an additive process, which is demanded in ultra-HDI and IC substrate fabrication. The copper foil type and glass weave style are both very important for ensuring signal integrity in advanced PCBs, and they are both an important part of the PCB materials selection process.

Copper foils are laminated onto PCB laminate layers and are subsequently etched to form traces. The copper starts with some level of roughness, which will creates greater skin effect losses than one would expect in smooth copper. In advanced PCBs, rolled annealed or reverse treated low-profile foils are demanded in order to reduce insertion loss when low-Df laminates are used.

Advanced PCB materials

Image source: Oak-mitsui, 80 First St, Hoosick Falls, NY, 12090. Link

To help minimize signal losses at high frequencies and bring conductor losses on par with dielectric losses in low-Df materials, smoother copper is demanded. In particular, smoother copper microstrips are preferred as minimal roughening will be required during fabrication of the surface layers. Platings can also impact losses, and active research into low-loss plating options with high reliability is ongoing.

Glass Weave in Advanced PCB Materials

The glass weave in PCB substrate materials is fabricated with a set of standard styles which are designated by a number. PCB laminates may incorporate multiple layers of glass weaves to build up the dielectric to the required thickness. The impact of the glass weave is primarily found in RF signal integrity and signal integrity of differential pairs in digital systems. In the latter case, differential pairs can experience excessive skew between each polarity portion of the signal, leading to excessive mode conversion and thus bit error rate increases.

Advanced PCB materials used in digital systems prefer to use spread glass, which collapses during pressing in order to eliminate glass voids in the PCB laminate material. RF PCBs operating at dozens of GHz can benefit from unreinforced PTFE materials mentioned above, although these create fabrication challenges due to layer-to-layer misregistration.

Advanced PCB materials

Additional materials used in PCB fabrication and assembly include the surface plating materials, conformal coating for PCB assemblies, embedded capacitance materials for power integrity, and underfill for BGAs and other leadless components. Experienced PCB designers and your layout services firm know how to balance the reliability, cost, SI/PI/EMI aspects of material selection, as well as where to procure circuit board materials for your next build.

Ajinomoto Buildup Film (ABF)

IC package substrates are built on top of Ajinomoto buildup film (ABF), a thin-film material that plays the same role as a buildup layer in HDI PCBs. This advanced material provides a platform for either subtractive or additive circuit fabrication and formation of microvias on vertical interconnects. ABF layers are laminated onto a rigid core layer, such as an organic FR4 core, BT epoxy, or even stiffened polyimide. ABF carries many of the advantages of advanced engineered PCB materials, such as:

  • Thin film thickness that enables high layer count
  • Low dielectric constant (Dk as low as 3.1)
  • Low loss tangent (Df as low as 0.014)
  • High Tg value (~170 °C)
  • Compatible with smooth laminated copper foils
  • Low water absorption (<0.6 wt% at 100 °C for 1h exposure)

Advanced PCB materials

Designers building high density PCBs can also use ABF as a buildup layer in sequential lamination, although this is less common than laser-drillable prepregs. Whether used as a buildup film in IC substrates or PCBs, the design process is very similar and the circuit/via formation capability is only limited by the drilling precision, imaging precision, and/or deposition precision in the fabrication process. ABF has become the dominant material used in IC substrates, but alternatives are being developed by innovative companies and may soon be commercialized.

Our Material Selection Process

We help you balance costs, capabilities, availability, and design for manufacturing during the PCB material selection process.
  • Contact us for a consultation and quote.

  • We determine primary design constraints for your advanced design, including signal integrity requirements and routing needs to inform material selection.

  • Contact with a fabrication house begins and a stackup is designed during this phase. The proposed stackup is qualified with fabrication and materials availability is verified.

  • Additional elements such as transmission lines, high-speed/high-frequency via transitions, and RF printed circuits are designed during the front-end engineering phase.

  • We perform a final DFM/DFA check and mechanical backcheck to ensure form factor needs are met and the manufactured design will be reliable before creating a PCB manufacturing files package.

Why Work With NWES?

  • Broad expertise - We're a digitally-driven remote-first organization with diverse talent and experience. We know your technology because we've used it and built it.

  • Manufacturing partners - We work with local and overseas CMs and EMS providers that are ISO-9001, AS9100, ISO-13485, IPC-A-610, NADCAP, and/or ITAR/JCP certified. We help you find the best option to produce prototypes and scale to volume production.

  • Supply chain management - We take a proactive approach on projects to ensure your design can be produced at the required volume. We'll manage procurement from major distributors or brokers throughout your project.

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