Advanced Materials

For PCBs and IC Substrates

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.

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.

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.

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.

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.

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.

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.