A Printed Circuit Board is a board with copper foil(s) on it. This is generally called a Copper-clad Circuit Board. There are different materials used for the Board itself, like Bakelite, Glass fiber (eg. FR4), Phenolic, etc. These boards come in various thickness (eg. 0.8mm, 1.0mm, 1.2mm, 1.4mm, 1.6mm, etc.), with 1.6mm being the most common and standard configuration used.
There are boards that only have Copper foil on one-side and there are boards that come with Copper foil on both sides of them. These are termed as Single-layer and Double-Layer respectively. For more than 2 layers, multiple boards can be combined or so to say, “stuck” together with insulation materials (eg. Prepreg) to yield what’s called a Multi-layer Circuit Board.
These boards, are mostly of Bakelite or Phenolic material, having copper foil on only one side of the board. Bakelite boards are rigid, while Phenolic varieties have flexible boards as well. In Single-layer boards, the customer’s design is etched on only 1 layer that has the copper foil, and the other side of the board is used for component mounting. Today, the general complexity of circuits dictate use of more number of Layers and so there is a limited or very particular market for Single-Layer boards.
These boards, are mostly of Fiberglass (FR4) or Phenolic material, having copper foil on both sides of the board. Fiberglass boards are mostly Rigid, while Phenolic boards have flexibility. The customer’s pattern is etched on both sides of the board to make the tracks / wires on the respective copper foils, and any connection that needs to be made from one layer to the other, is done by Plated Through Hole (PTH) Vias or Pads. PTH Vias / Pads are basically holes drilled through the top to the bottom of the board, and the foils on both end are connected by a conductive plating in the barrel of the hole. Because the Double-sided Board give twice the area for the same board size, and because the tracks can be interleaved (connected with PTH vias), it is a better alternative to Single-sided boards AND more suitable for relatively complex boards.
In order to increase the area for wiring, multiple Single or Double-Layer boards are stacked up together. These Single or Double-layer boards are stacked up with insulating layers between each board and they are stuck together firmly. The total number of layers on a board tells us how many tracking layers there are including the two outermost layers, and this is usually an even number (eg. 2, 4, 6, 8). Theoretically, the number of layers could go upto 100, however such combinations are not usually made. Most HDI motherboards are usually 6 to 8 layer boards. Although it is difficult to see how many layers there are in a PCB with naked eye, it can be seen through the aide of a microscope OR a Layer numbering scheme if adopted on the PCB as annotation.
When a Multi-layer board is sent to us for fabrication, the usual data for this is sent as a spreadsheet or table with the Gerber files. This usually looks something like this:
Most Circuit design and Layout CAD software have a Layer Stack Manager tool or option where hey can select and specify the individual Layers, their sequence and their size / thickness. As an example, Altium Designer has the following Layer Stack Manager where the layers and their details can be specified.
Similar to the Layer Stacking details, there are specifications of Via hole details as well. As mentioned in the “PCB Drilling” document, there are either Full Through Vias, Blind Vias OR Burried Vias. The following table shows these:
Via-8 is a Full Through Via, while all the other Vias are actually Blind vias. There is no Burried Via shown in this table. So this table is also necessary and whichever CAD design software you are using, it should be able to generate a Via detail file, usually this is covered in NC Drill.
The NC Drill file also tells us which holes are supposed to be PTH and which are Non PTH. So for example the following image shows where and why the NPTH holes could be used – to assist in PCB Fixation OR for a particular component fixation.
Common issues and questions with Multi-Layer board designs
- Sometimes the CAD software will not make Copper pads in middle layers (or even sometimes outer layers) for a Via. In such a case we have to put some copper area there for making a through plating of the via.
- Do not leave open-ended traces on your PCB design unless they are intentional antennas. If they are open ended inputs, they would be floating and result in spurious noise pickup and excess power dissipation in your digital circuitry. If they are left open-ended near to another copper feature, they could result in a short-circuit in the etch process.
- Sometimes, we get designs where the inner layers have sparse tracking. This will generate high quantity rejects during lamination. So please try to minimize open areas on your designs, specially ones where all the design is on any one side of the board while other sides remain empty.
- If you require a certain thickness for inner copper layers, for example you require 35um copper thickness, we suggest to use 1oz Base copper and finished thickness controlled according to IPC Class 2.
- Please use a Copper Pull-back from Board edges and all Router mill and V-Cut feature lines, otherwise the Copper will be exposed and cut through by the milling or V-cut machine. We generally use a minimum 8mil shave off of copper if your designs do not have a safe pull-back.
- We use a different software, basically a CAM software as compared to your design software which is a CAD software, for processing Gerber files. Therefore, we will always make a CAM import of your gerbers, and send to you for verification, along with any questions and highlights that might need your confirmation before order processing.