Learn the parameter value of the solder mask dielectric constant.
Find out if it’s affecting circuit performance.
Explore ways to reduce the impact of dielectric loss.
The idea of putting on a face mask used to be appalling. I couldn’t bear the idea of having a slimy piece of mask covering my face. However, as age took its toll on my aging skin, I was convinced that I ought to give it a shot.
To my surprise, using face masks does produce improvement after all. While you won’t be treating a PCB with face masks, it has another type of mask put on, i.e. the solder mask. Often taken for granted, it will be great to know if the solder mask dielectric constant is affecting your circuit’s performance in any manner.
What is Solder Mask Dielectric Constant
Have you ever wondered why most PCBs are green in color? That’s because a layer of liquid photoimageable polymer and solvent was applied to it. This layer is also known as the solder mask for PCB designers. Solder masks provide the generic green color for a PCB, although you could opt for different colors these days.
The fundamental role of the solder mask is to protect copper traces from getting corroded. It also helps to prevent excessive solder from accidentally shorting traces together. At a glance, solder masks seem to be a brilliant invention and a must-have for PCB manufacturing.
However, many of us designers overlooked a single property of solder masks, which may return to haunt us in some scenarios. The solder mask dielectric constant. You may have been familiar with the term dielectric constant. It’s a property that indicates the ability of a material to store charges between two points of charged materials.
Dielectric constant is often mentioned in capacitors, as it determines the amount of charge a capacitor could store. The same parameter is also applicable to solder masks that are often spread across the entire PCB. Often, the value is around 3.3 to 3.8, when rated at 1 GHz.
How Solder Mask Dielectric Constant May Affect Circuit Performance?
Solder mask dielectric constant will not be a concern if you’re designing low-frequency PCBs. You may also be spared the issues resulting from solder mask dielectric constant if you’re working on a high speed, high-frequency design. If you’re placing the high-speed circuitry in the middle layers, there won’t be any issues.
However, the combination of solder masks and high-frequency RF traces on the outer layer often results in a dielectric loss. This is also most likely to cause an increase in the insertion loss of the circuit, particularly for microstrips or grounded coplanar waveguide structures. Naturally, the performance of the RF circuit will be significantly affected.
Solder mask dielectric constant introduces dielectric loss to high-speed RF traces.
Mitigating Dielectric Loss At High Frequency
By now, it’s obvious that solder masks and high-frequency traces aren’t the best companions. If you’re having RF traces on the outer layers, you’ll want to avoid placing solder masks on it. Without the introduction of solder mask, and it’s lossy property, you’ll have better performance in terms of transmission.
Avoid solder masks in the same area of RF traces.
Alternatively, and if it’s unavoidable, you can have patches of solder masks at areas where SMT components are populated. These tiny areas of solder masks tend to have little or no impact on the performance of high-frequency circuits. This is due to the fact that the wavelength of these patches of solder masks is less than 1/10 of those of the transmitted frequency.
With that said, you don’t want to overdo with patching solder masks in large amounts. In numbers, they will still degrade the performance of the RF signals from the microstrips. You can’t take chances when using solder masks on a high-frequency PCB. Adequate modeling and simulation are needed to get a better picture of how the dielectric constant is affecting the circuit.
A PCB design software with a high-speed analysis tool is needed to ensure RF transmission is not affected by the solder mask. You should take advantage of the Cadence SI tool to perform analysis on high-speed circuits and work with the layout advantages that Allegro PCB Designer offers you.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.