Everything is changing in high-performance substrates, from materials technology to engineering priorities.
Materials science is a quiet contributor to high-performance electronics, playing a huge yet mostly unseen role, enabling everything from smartphones and automotive systems to 5G radio access, communication infrastructures and high-performance computing systems. That conventional-looking circuit board, which the user may never see or think about, is anything but ordinary underneath. While ICs sit serenely on the surface, the processes going on inside couldn’t happen without the engineering that created the substrate beneath. Demand for improvement is ever-present, and many avenues may be explored in search of a solution.
At microwave and RF frequencies, managing signal energy is a critical imperative that directs substrate-engineering choices. Signal propagation repeatedly realigns the dielectric’s molecular dipoles, converting a proportion of the energy into heat that dissipates into the ambient environment. The effect is small, and general-purpose resins that tend to have a large dipole moment cause minimal losses at low frequencies. At GHz frequencies, however, the losses are significant and must be addressed.
Materials science has produced more suitable resins, engineered for low polarization to preserve energy within the signal. Ultimately, PTFE – which has effectively zero dipole moment – has the lowest loss of the materials currently available. Moreover, introducing extra materials to the resin, such as ceramic fillers, gives extra control over properties like thermal performance. Care is needed to ensure particles are of consistent size and evenly distributed throughout the material, however.
As the industry develops extremely low-loss resins, signal integrity degradation due to the glass-fiber reinforcement within the substrate becomes more significant and demands attention. In particular, the feature sizes of high-speed circuits have become comparable to the dimensions of the glass weave in the X-Y plane. Changing the conventional weave to a symmetrical square pattern gives a valuable improvement in signal integrity. An alternative is to remove the glass content altogether, which is driving the development of advanced film materials that can deliver even better high-frequency performance.
Successfully reducing substrate-related losses then shifts attention to the copper traces and – at high signal frequencies – the skin effect that concentrates signal propagation into a small area at the perimeter. Effectively shrinking the conductor’s cross-sectional area increases the impedance and, therefore, also signals energy loss. At frequencies above 1GHz, the effective skin depth becomes less than 2.1µm. Adding to the challenge here, the typical surface roughness of the copper traces is by tradition intentionally up to about 6µm to assist adhesion to the prepregs. Because the skin depth is less than the roughness, high-frequency signals experience scattering as well as high impedance.
To deal with this, the barrier layer – applied to protect and preserve the copper traces in the factory and the field – is the next aspect of the stackup to undergo scrutiny. Dealing with signal issues calls for a smoother, flatter profile than can be achieved through conventional tinning applied using processes like hot-air surface-leveling (HASL) or roller tinning. Gold plating can provide excellent smoothness and ensure solderability, but is expensive due to gold’s relative scarcity. Hybrid electroless nickel/immersion gold (ENIG) introduces a nickel barrier about 4-7µm thick, which is needed to prevent the gold from diffusing into the copper, and permits a thinner gold layer. The immersion-gold process results in a deposit thickness of about 0.05-0.2µm, lowering the cost premium. An additional electroless palladium layer may also be deposited (ENEPIG), which permits the gold layer to be thinner still at about 0.03-0.05µm. The palladium brings further advantages, such as increased solder joint strength and preventing the black-pad effect that can result from nickel’s phosphorous content.
At microwave frequencies, however, nickel’s magnetic properties become problematic and can interfere with circuit function. We could consider thicker gold plating, albeit at a price premium. Nanotechnology may be the answer, as research has shown some success depositing a nano-organic layer to prevent diffusion. The gold plating can also be just a few nanometers thick, deposited using a cyanide-free process.
In the search for the performance improvements needed to build multi-GHz systems, advanced materials science is transforming the board’s composition. We will likely need to rely even more heavily on this know-how in the future as we seek solutions that fulfill sustainability goals in addition to satisfying the established design triangle of solvability, manufacturability and performance. Sustainability is the new fourth dimension, squaring that triangle and demanding consideration for the long-term environmental impact of everything we produce. The question for engineers is no longer simply whether we can make it, but whether we should make it, and how long it can last without harming the future. Investors and innovation funds demand strong answers.
We will find them by improving the constituent materials and production processes, and through proper consideration for end-of-life, including disposal and recyclability, to avoid simply adding to the pile of waste. In the quest to overcome the technical challenges, it’s increasingly important to steer away from rare materials or materials that are difficult to recycle. Finding elegant solutions that meet requirements, including preserving our well-being and avoiding negative impacts from production, use and disposal, holds the key to successful innovation in this field.
As we address these issues, we can anticipate future generations of products made possible with advanced PCBs enabled by cutting-edge materials science. Superficially standard, but very far from mundane in every way.
Alun Morgan is technology ambassador at Ventec International Group (venteclaminates.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column runs monthly.
