Although it may not feel like it, the PCB manufacturing industry is approaching its 100th anniversary – a remarkable milestone. It’s hard for me to believe, especially having spent 40 years in the business myself. The idea of reaching a century of innovation in this field once seemed impossible. It almost feels like we should begin the story with, “A long time ago, in the 1930s,” when PCBs first came to light during WWII. Yet here we are, on the brink of celebrating this incredible achievement.
In this blog, we will discuss the implementation of inner layers and specialty processing related to inner layers only. It wasn’t until the 1950s that the multi-layer printed circuit board was developed. With the addition of internal layers came additional processing as well as some trial and error. We will look at inner layer evolution, specialty processing related to inner layering, and what the difficulties are from a manufacturing perspective.
What Are Inner Layers?
Inner layers, often referred to simply as “inners,” are the copper-based conductive layers sandwiched between the outermost surfaces of a PCB. These layers form the heart of a multi-layer PCB and are integral to achieving complex circuitry in a compact design. Inner layers are used to route signals, provide grounding and power planes, and enable the integration of advanced features like controlled impedance and shielding.
Circuit board internal layers routing to a BGA.
Their introduction marked a turning point in PCB design, allowing engineers to overcome the limitations of single- and double-sided circuit boards. By embedding traces within the board, inner layers maximize the available surface area, enabling the placement of more components and the addition of intricate wiring without increasing the overall size of the PCB. This innovation paved the way for smaller, lighter, and more efficient electronic devices, which were critical for advancements in industries like aerospace, automotive, military, and consumer electronics.
PCB Internal Layer Processing
The inner layers were a unique addition to PCB product lines from the start. When technology began to expand it was evident that the land would quickly become an issue. Through hole technology became combined with inner layer copper plating in the holes to make the connections. This allowed grounding and power planes to be internal as well as additional traces to be buried inside the PCB.
While perfecting the craft of multi-layer products was a bit of a challenge its success made it possible for smaller in size PCBs to fit into products that were also smaller in size. True to the demands of consumers, military, automotive, and aerospace as well as smaller, lighter, and more portable, were the goals of mankind. The choice was clear, and something had to give. Either the size of the part had to be increased or implementing a process for more layers of circuitry was needed. We already had the 1- and 2-layer recipes, so adding more materials should be easy enough. Cores, prepreg, and lamination cycles grew from the concept.
What Is The Inner Layer Process?
The core material made up of copper and resin laminated together in different thicknesses is processed as 2 side copper layers. For example, a 4-layer circuit board has one inner layer core, layer 2, and layer 3. The core is prepped for smoothness and cleanliness, it then has a film applied to the surface using heated rollers.
Once the film is applied an image is then placed onto the film using UV lighting or laser-direct imaging. The image is then processed through a developing solution to remove the non-circuitry imaged area leaving behind only the circuitry or copper. The unwanted copper is then etched away leaving behind the pads, circuitry, and bare laminate, this core is completed and ready to go to lamination for layup.
In the 1960s, surface mount technology was introduced by IBM, first on the top or component side and then added to both sides. As technology requests pushed the capabilities of circuit boards, the land was still the problem, more wires, more components and no room for placement were a growing issue. Unless the land issue was solved, we would then have to increase the board's physical size or layer count. With the mounting technology came more possibilities. As technology grew SMT, the robotic age, automation, speed, and control of circuits were another consideration for everything to work as planned.
Impedance and Controls
Inner layers allowed technology to sore, along with SMT and components, the possibilities were endless. As equipment improved and ideas expanded manufacturing was pushed to spend, machinery, people, shifts, expansion, and factories were thriving and busy. The term impedance was coined as a thing in the 19th century, it wasn’t until the mid to late 80’s that it was actually gaining traction.
So, what is impedance? Simply put, it is how an electrical circuit resists the flow of electricity. We measure the impedance in Ohms, controlling the signals being sent as a measure for the devices to work properly. For example, an electric guitar played through an amplifier to a speaker, without the proper wiring the sound could be weak and muffled vs. crisp and vibrant. Controlled impedance was perfected in the mid to late 1980s and continued to expand in the 1990s. Now that part doesn’t seem that long ago now, does it?
An important part of impedance, signals and technology are the means of the travel through the circuitry and connections. In most cases, PCBs are connected through layers by plated through holes that pierce the board from top to bottom. Over the years as we began to require PCBs to do more, having land to add all the components applied was still an issue. Sure, you can have capacitors, resistors, QFPs, BGAs, etc., but all those need space and connections and the only way to do it is through traces and holes. What about these wires connecting from point to point? Well, we didn’t have enough land to do it all.
Buried and Blind Vias - Why Are They Used?
At the end of the 20th century, via holes took on a more important role. The via, insignificance of typically feeding power from top to bottom, some of the time used as a testing point at assembly, was about to change. Along with more ideas and manufacturing challenges, what if we could bury the via internally or partially way through the top to mid or bottom to mid layers?
This concept made it possible to add more to the circuit board and maintain a smaller physical size. The job of the via was about to change, still carrying the connectivity but changing the size, filling the holes and plating over the top of it would improve the land problem, allow more functionality, and push manufacturers to a new process. With the specialty processing and equipment, the via is virtually invisible from the outer layer. When the via is placed within the SMT device area and processed accordingly the assembled part can be placed onto the pads without soldering issues.
Today, via-in-pad processing is used more widely to solve constraint issues and the inner layer functionality, vital to the successes of nearly every electronic product we touch in our homes, businesses, and life.
Summary
As technological advancements continue in other ways to evolve, it is more likely that we will continue to push forward manufacturing challenges after the fact. If there is one thing I would express to all engineers during development projects, it would be to consult with your trusted PCB supplier. Ask for performance checks and balances to be provided, DFM analysis on data, and listen to suggestions made. At Epec our engineering staff is available for free DFM analysis, consultation, and manufacturing suggested stack-up review during the quote process, online and available 24/7.
Key Takeaways
- Inner Layers Revolutionized PCB Design: Inner layers in multi-layer PCBs enable advanced circuitry in compact designs, providing critical signal routing, grounding, and power planes for modern electronic devices.
- Inner Layer Processing Overview: Inner layers start as copper-clad cores that are processed using imaging, etching, and lamination techniques, forming the foundation of multi-layer PCBs.
- Impedance Control: Controlled impedance, essential for signal integrity, became a key feature in PCBs in the 1980s, enabling precise electrical performance for advanced applications.
- Buried and Blind Vias: The introduction of buried and blind vias allowed for increased circuit density while maintaining small PCB footprints, solving space and connectivity challenges in multi-layer designs.
- Impact of Via-in-Pad Processing: Via-in-pad processing improves functionality and assembly reliability by embedding vias directly under components, reducing constraints and optimizing land use.
- Technological Evolution in PCBs: Advances like surface mount technology (SMT), automation, and specialty via processing have continuously pushed the boundaries of multi-layer PCB manufacturing.
- Consultation is Key: Partnering with PCB suppliers early in the design process ensures manufacturability, cost efficiency, and optimal performance through services like DFM analysis and stack-up reviews.
- Epec’s Support: Epec offers free 24/7 engineering support, including DFM analysis and performance checks, to help engineers optimize their multi-layer PCB designs.
