With the technological revolution that integrated circuits represent, the design of printed circuit boards with noise reduction capabilities, capacitance (impedance) and proper signal conditioning that could comply with electromagnetic radiation rules became a must.
In such sense, as components also became smaller and the need for more adaptability of a board is smaller spaces, single or double-layered designs were not capable of providing expected effectiveness, giving birth to multilayer design, which considers boards with three or more layers, including, for example, 6, 10, 16 or even 20-layer PCB. In this article, we will explore and dig more on this last option and review some of its characteristics and applications.
Table of Contents
What is 20-layer PCB stackup
As described above, a 20-layer PCB will generally contain 22 copper cores (2 copper foils and 20 copper inner cores) between 18 and 35 µm thick. The copper mid layers must be stacked and prepared for noise and parasite signal reduction with the appropriate use of FR4 core layers and enough pre-preg cores to curate the overall stack. Typical pre-preg substrates are used and depending on the PCB type and profile (high current, High-TG, ENIG, rigid or flex…) special core substrates may also be applied (with some thickness variation) to prepare a harsh environment 20 layer PCB.
What are the applications of 20-layer PCB
The applications of 20-layer PCBs can go from the consumer market on high end white goods that require good EMC compliance and noise reduction capabilities, such as in motor controlled applications (drying machines, washing machines) to applications on computers (multilayered complex motherboards) and graphic cards.
There is also high demand on the industrial sector on boards that require good feedback loop processing. You may find 20-layer PCBs on industrial motor controllers for robots, cars and even extruder controllers.
Also, critical application equipment such as medical and military markets are well covered by 20-layer PCBs, especially on measurement equipment and navigation systems with complex signal integration requirements (such as AI and military automation).
How thick is 20-layer PCB
The answer to this question is very related to multilayer PCB stack up, in this case 20-layer PCB stack up, which will be described later.
This is much more of a match question that relates a summation of the copper, FR4 and pre-preg layers included into the main stack up. A 20-layer PCB consists of twenty layers of copper in between layers plus two copper foils on the top and bottom layers, which may have thicknesses between 18 and 35 µm. With around nine to ten pre-peg cores useful for PCB curation and FR4 cores for dielectric as well, a conventional 20-layer PCB (3 oz copper) may have thicknesses between 4.32 and 5.28 mm.
What are fabrication processes of 20-Layer PCB
Every multilayer design will undergo similar processes a typical PCB, but in this case the high number of layers require to deal with proper lamination and stacking with the appropriate insulation.
In the case of 20-layer PCBs, compliance of the design is assured before printing the PCB layers. Once the inner layers are printed, etching will be the process utilized to remove unwanted copper from the layers.
After visual inspection, the lamination process takes places, and consists of pressing the 20-layer PCB until one integrated board is finalized. Then, drilling (machining) and plating with electrolysis occur before some additional etching and visual inspection take place.
Finally, the desired surface finish is applied (HASL, ENIG…) and final inspection is performed before the PCB undergoes some assembly process or packaging.
What are the standards of 20-layer PCB quality
The purpose of a multilayer PCB is to reduce space while conforming to high speed and signal integrity rules. This is no exception for a 20-layer PCB.
In this case, a good 20-layer PCB must be able to provide the desired signals without signal propagation or delay issues. Also, it should be rigid enough as per its design condition and resistant to the temperatures and environments it was designed for.
Finally, it should contain the proper stack up for a well dielectric/signal matching as per the previously defined stack up considerations.
What are design specifications of the 20 layer PCB
Well, in this case, the most obvious is that of having twenty layers. The proper signal simulation and integrity checks have been done (at least for ten Gigahertz).
Via support is also a must (both blind and buried) and will be depending on the impedance matching considerations. As general rules, via hole size must be of 0.2 mm and trace width and separation must be no smaller than 0.1mm.
Finally, the mechanical holes should also comply with the separation rules and the component placement should comply to the IPC standard for effective results.
Is the 20-layer PCB cost effective
High-level boards take into account multilayer design above ten layers; such is the case for 20-layer PCBs. The costs both in design and manufacturing are superior due to the end application of the devices they will be used for but also the amount of expertise through design technicians and high technology involved in their fabrication.
Hence, a proper balance that meets the market demands and 20-layer PCB fabrication must be totally paired in order to save the non-returning expenses and the post-production costs of a board of such kind.
Conclusion
Versatile high-level multilayered boards will become a higher trend as technology and computational performance demands for greater number of signals and less propagation issues throughout them on reduced spaces with thermal resistance. 20-layer PCBs represent the use of edge technology to prepare highly complex boards that comply to noise and electromagnetic radiation rules. Hopefully this article can provide an insight on their fabrication and applications and design considerations.
