Recent years have been witnessing improvement in electronic information technology and increasingly higher demands from people on their applications of electronic products. Complex structure and multiple functions of electronic products lead printed circuit boards (PCBs) to move towards new directions. On one hand, increasingly more integrated components in electronic products and miniaturization of overall size make circuit boards be of high density, high accuracy and high integration, which arousing a transition from simply multilayer PCBs to HDI PCBs.
On the other hand, electronic products call for high-speed, low loss, low delay and high-fidelity (HiFi) signal transmissions and have to be compatible with high-consumption environment for high powers, requiring PCBs to be involved with a high-frequency and high-dissipation thermal design and high-quality signals transmission.
To cater to the demands and market trend, technologies concerning high frequency, high thermal dissipation and high density interconnect designs have been receiving the most attention in modern PCB industry and will be a leading development trend in the future.
In this article, we take an 18-layer PCB (a multilayer circuit board containing participation of numerous advanced designs including multiple back drilling, high aspect ratio resin-plugged via and high-thermal dissipation via) with high-frequency material as a study example, to detail technologies applied in the process of this circuit board fabrication, such as resin-plugged via, stacking up, high-density drilling and back drilling.
High-Frequency and High-Speed Multilayer PCB Structure Features and Materials
In order to be fully aware of structure features of high-frequency and high-speed multiplayer PCBs, the following table and figure can be referred to.
High frequency, signals with a frequency band to be at least 300MHz (equivalent to wavelength no more than 1m), can be further classified into medium frequency (MF) and very high frequency (VHF) according to different wavelengths. Electromagnetic waves whose wavelength is at least 1GHz are usually referred to microwaves. As PCB’s majority functions are determined by substrate material, substrate material featured by high frequency will be applied in high-speed and high-frequency PCBs. High-frequency substrate material has to meet the following demands:
a. Dielectric constant (Dk) should be small and stable (generally speaking, the smaller, the better). Based on principle which signal transmission rate is inversely proportional to square root of material dielectric constant, high dielectric constant tends to cause delaying signal transmission.
b. When it comes to signal transmission quality, dielectric loss (Df) should be small as well. The smaller Dk is, the smaller signal loss will be.
c. Copper foil should have a surface with low roughness to avoid impedance control mismatching and signal loss caused by skin effect.
d. Substrate material for high-frequency and high-speed PCBs should feature low hydroscopicity. Dielectric constant of water is 70 and this number will be increased when substrate absorbs more moisture. Thus, modification of impedance control will lead to badly-performed signal transmission.
e. Copper foil should conform to relatively high peel strength that mustn’t be compromised just due to low surface roughness.
f. Substrate material should also perform excellently in terms of size stability, heat resistance, chemical resistance, impact strength and manufacturability.
In conclusion, much care has to be taken with above aspects and items considered when deciding suitable substrate material for high-frequency and high-speed multilayer PCBs. For example, the following table details comparison between substrate materials provided by different distributors.