Printed Circuit Boards (PCBs) in the past were simple, providing the basic functions through leaded components and copper traces connecting them. Today, PCBs are far more complex structures offering a variety of types including odd shapes and options of flexibility. However, the most common form of PCBs the electronic industry uses today are multi-layered.
Single and two layered PCBs are still in use, but the electronics industry limits their use to simple electronic circuits with only a few components. With the growing functionality and complexity, along with the shrinking dimensions of electronic devices, the PCB designer is left with no other options than to use miniature surface mountable components and multilayered PCBs.
The electronic industry terms a PCB with more than three layers of conductive copper foil as multilayered. In simpler terms, consider multi-layered PCBs as containing several layers of single-sided circuit boards, with the manufacturer laminating and gluing them together with layers of heatprotective insulation (prepreg) between them around a double layer board. It is common to have two layers on the top and bottom sides of the PCB for electrical connections to the rest of the device.
Fig 1: Cross-Section of Multi-Layered PCB.
Within the multi-layered PCB, vias make the electrical connections between the different layers. Vias can be of different types—plated through holes, blind, and buried. This method of making multilayered PCBs leads to the generation of highly sophisticated and complex PCBs of varying sizes.
Fig 2: Interconnections Within Multi-Layered Board.
Evolving changes in the electronic industry leading to progressive sophistication over time created the requirement of PCBs with increasing complexity. However certain design constraints were necessary to limit problems such as stray capacitance, noise, and crosstalk. Such design constraints often made it impossible to obtain a satisfactory level of performance from either a single or even a double-layered PCB. This led to the evolution of multi-layered PCBs.
The popularity of multi-layered PCBs is increasing in the electronic industry, as they pack the power of double-layered boards within a format a fraction of their size. The total number of layers typically varies from four to twelve and more, often coming in even numbers, as odd numbers of layers often leads to issues like warping. While being more expensive and labor-intensive to produce, the electronics industry is using increasing amounts of multi-layered boards as they are extremely beneficial compared to the simpler single- and double-layered varieties.
Benefits of Multi-layered PCBs
- Smaller Size (Compared to Single- and double-layered PCBs)
- Lower Weight
- Better Quality
- Improved Durability
- Enhanced Flexibility
- Better Performance
- Controlled Impedance
- EMI Shielding
- Reducing Wiring Harnesses
Smaller Size: The most prominent benefit of multi-layered PCBs is their ability to drastically reduce the board size compared to single- and double-layered PCBs. As they are made of many layers, multi-layered PCBs are inherently smaller than other PCBs while still retaining similar and most often, exceeding the functionality of other PCBs.
Lower Weight: As their size is smaller, the weight too is lower for multi-layered PCBs. Moreover, multi-layered PCBs do not need the multitude of connectors necessary when the design uses singleand double-layered boards.
Better Quality: A designer has to put in a good amount of planning and work in creating a successful multi-layered PCB. Similarly, the manufacturer also needs to apply carefully controlled processes for fabricating the board. The net effect of these considerations is that multi-layered PCBs tend to be of a better quality and more reliable when compared to their simpler variants.
Improved Durability: By the nature of their construction, multi-layered PCBs tend to be more durable. As manufacturers fabricate them using heat and pressure to bind the different layers together, multilayered PCBs are able to withstand their own weight reliably. Each multi-layered PCB has several layers of insulation between the copper layers, and prepreg bonding agents and protective material bind all of them, which increases the overall durability.
Enhanced Flexibility: The designer has the enhanced flexibility of increasing or decreasing the number of layers—within the design cost constraints—while keeping the overall size the same. This is a highly desirable feature considering the complex nature of design necessary for modern devices.
Better Performance: With highly dense assemblies, multi-layered PCBs incorporate multiple layers within a single structure. As the different layers are close together, signals have to travel smaller distances and hence encounter lower resistance. This allows the electronics to achieve higher speeds and greater capacity despite the smaller overall size.
Controlled Impedance: As the distances between layers are small, it is easy for designers and manufacturers to control the impedance of specific traces by enclosing them within adjacent layers of copper. This is a very important feature of multi-layered boards in applications involving highspeed and high-frequency signals.
EMI Shielding: According to many international standards, modern electronic equipment must be EMI compatible, meaning they should neither disturb nor be disturbed by other equipment operating nearby. By arranging the placement of shielding layers appropriately within a multilayered PCB while designing it, a designer can effectively control the EMI compatibility through shielding.
Reducing Wiring Harnesses: Although it is possible to design with single- and/or double-layered boards, most often the design requires multiple such boards interconnected with wiring harnesses. Using one multi-layered board instead not only simplifies the design, but also reduces the number of wiring harnesses necessary, leading to a drastic reduction in the possibilities of failure.
Disadvantages of Multi-Layered PCBs
Although multi-layered PCBs have several benefits making them suitable to a wide variety of advanced applications, their drawbacks can make these PCBs unsuitable for a number of applications, especially those involving low cost and complexity. Some of these disadvantages are:
- More Expensive
- Limited Availability
- Higher Designing Skills
- Complicated Production Methods
- Higher Fabricating Times
More Expensive: In comparison to circuit boards of comparable complexity and functionality made with single- or double-layers, multi-layered boards are more expensive. With some designs it might be possible for the designer to set off the increase in cost against the reduced size, as the multi-layered board is smaller.
Limited Availability: The expensive machinery necessary to produce multi-layered boards is one of the biggest issues with manufacturers. However, the high demand for multi-layered PCBs is prompting most manufacturers towards fabricating these boards with cheaper machines. This is a major reason OEMs requiring multi-layered PCBs must be careful while selecting a capable contract manufacturer.
Higher Designing Skills: Multi-layered PCBs require careful planning and higher skills during design. These are necessary as multi-layered boards require extensive interconnections between layers, but at the same time the designer must also mitigate impedance and cross-talk issues. For a perfectly functioning board, the designer must overcome all design issues in it.
Complicated Production Methods: Compared to fabricating single- or double-layered boards, fabricating multi-layered boards involves more complicated production methods. There are more chances of making errors while following these processes, and manufacturers must install statistical process control methods to overcome them.
Higher Fabricating Times: Complicated production methods lead to higher fabricating times. Therefore, lead times for multi-layered PCBs are almost always higher than those for fabricating single- and/or double-layered PCBs. OEMs need to factor this increased lead-time when calculating the time-to-market for their products.
Applications of Multi-Layered PCBs
All complex electronic equipment use multi-layered PCBs. Applications range from computer motherboards to smartphones to wearables, primarily for their attributes of space-saving and providing higher functionality. Apart from computers, other fields making use of multi-layered PCBs include:
- Data storage
- File servers
- Cell phone transmission
- Signal transmission
- GPS technology
- Cell phone repeaters
- Satellite systems
- Industrial controls
- Test equipment
- Hand held devices
- Heart monitors
- X-ray equipment
- Atomic accelerators
- Cat scan technology
- Fiber optic receptors
- Fire alarm systems
- Space probe equipment
- Nuclear detection systems
- Weather analysis
- Battery chargers
- Automotive electronic systems, and many more.