In this tutorial we are going to learn about WRITE A NOTE ON BALANCED PCB ?
Designers can be tempted to design PCBs with an odd no. of layers. If the additional layer is not needed for routing, why use it? Won’t eliminating the extra layer minimize PCB thickness? If the PCB has one less layer, the cost should be lower…shouldn’t it? But generally , adding a layer can help reduce costs.
Cost Advantages of Even Layer Count Boards The cost of the raw materials is slightly less for a PCB with an odd nuo. of layers than for a PCB with an additional layer, due to the prepreg & foil for the additional layer. However, the processing costs can be significantly higher for an odd layer count board with foil on 1 side & core on the other. The inner layer processing costs are identical, but the foil/core construction dictates additional processing costs for the external layers.
The odd layer count PCB incurs additional cost for a non-standard layup process and additional pressing materials like those used for a core construction board. The mfg yields are also lower for boards with a
core instead of foil on the external layers. The external core requires additional processing prior to lamination, & is subject to increased risk of handling damage like scratches or voids etched into the copper.
Balancing Layer Counts to Avoid Warp
The best reason to avoid designing a board with core on one side & foil on the other is the potential warping of the PCB. Through multilayer lamination we can make PCB cool, it can warp to relieve the different lamination stresses on the foil side and the core side. Generally if board thickness increases then shoud be Boards constructed with core on one side & foil on the other side warp,. The key to eliminating warp is to create a balanced stackup. Although some warped boards can meet specifications, the cost of processing them increases because inefficient processes are used to continue mfg a warped board. Quality is compromised because these boards require special fixtures and processes for assembly, and component placement is less accurate.
Modifying Designs to Achieve Even Layer Counts
When a design appears to require an odd number of layers, several methods are available to balance the layup & avoid significant processing costs & warp problem. In order of preference, the following methods can yield more balanced boards:
1. Add an additional signal layer and use it. This method is generally applied to boards with an even number of planes & an odd number of signals. The additional layer does not add to the cost of the board, & it can improve delivery & quality.
2. Add an additional plane layer. This method is generally applied to boards with an odd number of planes and an even number of signals. An easy way to create a balanced layup without compromising the electrical requirements of the board is to duplicate a ground plane in the center of the stackup. Route the board with the odd number of layers, then duplicate the ground in the center of the board & relabel the remaining layers. This has the same electrical effect as using thicker copper for the ground plane.
3. Add a blank signal layer near the center of the stackup. This minimizes the imbalance & gains the quality and processing benefits of a foil construction board. Route the board with the odd number of layers, then create a blank layer with no circuitry as a signal layer & relabel the remaining layers. Adding a blank signal layer also works with microwave boards & other mixed dielectric boards where prepregs have different electrical properties than the core material.
Benefits of Balanced Designs: PCBs with an even number of layers minimize cost, warp, delivery, & quality issues. By using the options presented in this note, a board designed with an odd number of layers can be modified to balance the stackup & minimize the risks associated with an odd number of layers.