Why Multilayer PCBs Always Have Even Number of Layers

Article By : Suresh Patel, Mer-Mar Electronics

The multilayer PCB solves many performance issues due to the added number of layers in the stack-up.

The multilayer printed circuit board (PCB) solves many performance issues due to the added number of layers in the stack-up. It is not just the extra space but issues due to signal integrity, thermal management, and EMI shielding are optimized by using multilayer PCBs. Also, the fabrication of a multilayer PCB becomes more advantageous with distributed circuitry and copper balance in different layers of the stack-up.

Multilayer PCB stack-up can enhance the energy distribution in the circuit board, support high-speed design, and reduce the EMI issues. It also offers other advantages like reduced noise and improved electromagnetic compatibility. These advantages have enabled the usage of Multilayer PCBs in advanced applications. With the latest process techniques, it is possible to fabricate a PCB with a very high layer count of around 100 and above. But the most commonly used multilayer PCBs are of 8, 10, or 12 layers.

Factors to be considered while designing a Multilayer PCB stack-up:

  • The end application and its complexity influence the board density.
  • Maximum operating frequency of the circuits involved.
  • Expected turnaround time.
  • Estimated manufacturing cost and available budget.

The above aspects drive the number of layers required and are always an even number. It is not that the odd number of layers cannot be used. One layer more will not affect the production cost instead provides a lot more advantages. So, a design requiring 3 layers can be better fabricated using 4 layers. We shall discuss why multilayer PCBs have an even number of layers and the problem associated with an odd number of layers in PCB stack-up.

In the multilayer PCB fabrication process, several layers are pressed together for long hours at an extremely high temperature under pressure. If it’s an odd layered PCB, due to the standard symmetrical requirement, one of the copper layers is etched away. This may result in board warping. Also, the odd layered PCB may create uneven weight during the copper plating stage and result in irregular plating issues. This non-standard, odd-layered stack requires an extra core process for layer bonding and adds to the manufacturing cost. The odd layer issues discussed above in different stages of fabrication can degrade the PCB performance. So, it’s always recommended to use an even number of layers in the PCB stack-up.

Advantages of even number of layers in Multilayer PCBs:

  • Reduced Manufacturing cost: An odd layer PCB may save the cost of material for one layer but the processing cost increases significantly compared to an even layered PCB. The core structure requirement for odd layered PCB increases the production cost significantly. An even layered PCB can save these costs and reduce the overall manufacturing cost.
  • Improved Performance: The extra processing of outer layers due to odd numbers can scrape the surface and result in etching errors while even layered PCB can avoid such defects and improve efficiency.
  • Even layers maintain the stack-up symmetry and are considered as the best design practice to avoid mechanical issues like bending or warping.
  • Multilayer PCBs with even stack-up provide mechanical stability which can further speed up the automated assembly process like the pick and place method.
  • Copper balance is easily achieved in even layered PCBs. This is one of the recommended techniques in high-speed design to control signal integrity issues.

The above list of benefits indicates why the multilayer PCBs always have an even number of layers in their design. In case of high dielectric space requirements or certain signal routing strategies, an odd layered stack-up is essential based on the end application. In such situations, a set of guidelines are recommended to optimize PCB production.

Best practices to follow while designing an odd-numbered Multilayer PCB:

  • If the stack-up has odd signal layers and even power planes then splitting the layer into two can enhance the quality and reduce the turnaround time.
  • To improve the stack-up balance, insert a blank layer in the centre of the stack-up. This improves the performance and is suitable in mixed circuit design.
  • If there are odd power planes and even signal layers, then add a GND plane in the centre of stack-up. This provides immunity against EMI issues.

In general, Multilayer PCBs have several advantages like small size, lightweight, and reliability. Though certain specific applications use odd layered PCBs, the most widely accepted stack-up design in Multilayer PCBs is one with an even number of layers.

Many electrical performance issues can be solved using a multilayer PCB. Adding multiple GND planes in the stack-up can avoid damages due to EMI issues. A well-designed power distribution method includes complete GND planes and metal power planes instead of routing them as traces. A multilayer stack-up can help in efficient heat dissipation also.

Multilayer circuits with an even-numbered stack-up have benefits during the PCB fabrication. The additional circuit area can assist in balancing the copper weight of each layer with its mirror layer. This symmetrical balance increases the board consistency which can avoid the “potato chipping” effect of PCB bending during manufacturing. Multilayer PCBs with an even number of layers are easy to manufacture with better yield. The performance quality improves with an even layered PCB design and hence it is an extensively used stack-up scheme in the Multilayer circuit boards.

The complexity involved in Multilayer PCB design makes it costly and demands an experienced designer and manufacturer to build the product. Also, the turnaround time is longer for Multilayer PCB production. Yet, the demand for these PCBs is growing immensely in the electronics industry. Multilayer PCBs are used in consumer electronics, automotive, medical devices, aerospace, etc where product performance is the key factor. Hence, even layered PCBs are the best choice in these quality-driven applications.

An experienced contract manufacturer will be qualified in handling the intricacies of a multilayer PCB. Using the right material and following the quality process can assure a better yield in PCB production. It is always recommended to interact with your CM from the initial stages of manufacturing. This will allow you to understand their expertise and limitations while building high-density multilayer PCBs.

 

About the Author

Suresh Patel has worked as a Sales Engineer and other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business. You can connect with him on Twitter.

 

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