When complex circuit designs and multiple connections are required, it is not enough with single or two-layer boards and more complex PCBs are required combining several two-layer boards, called cores, to finally build a multilayer PCB. In order to create the final stack up of the PCB, not only typical boards with substrate and copper foils are used, also prepreg laminates are used. The prepreg is a layer of uncured substrate, for example FR4, without copper, that is used to glue together two different printed circuit boards. In the multilayer you can have only a top layer in which you will place components and the bottom only as a ground layer, or you can used both, top and bottom, to place components.
Multilayer circuits are composed by many layers. In pcbGO we typically offer up to 6 layers in our online form, but if you require more layers, please contact us by e-mail. Each two layers is a core with substrate and two copper foils in the top and the bottom. In order to build the stack up one or more prepreg layers are placed between each PCB for two main reasons, the first one is to glue together each different PCB and the second one to isolate the boards and avoid short circuits between them.
The multilayer can be composed by cores with different substrate height and copper thicknesses. Also, the prepreg laminates can have different thicknesses. Typically, the outer cores height is selected to have the optimum parameters for your design, for example in RF, the height that allows you to have an optimum 50 Ohm lines width to correctly place the components. On the other hand, the core of inner layers, that are used to distribute DC and/or RF signal from the top to the bottom of the multilayer is typically selected to have low thickness, so the selected lines for the distribution are thinner, to comply with the corresponding optimum impedance, and more space can be used for the distribution.
Typically, the copper thickness of the inner layers of the multilayer is selected to be higher than the outer layers to give a higher robustness to the multilayer and because typically, these inner layers are not metallized.
In pcbGO, we offer 0.5, 1, 1.5 and 2 oz of copper thickness as copper base for the outer layers that finally is converted to 1, 2, 3 and 4 oz after the metallization process. For the inner layers you can select between 1 and 1.5 oz copper thickness. If you require higher thickness, please contact us by e-mail.
Regarding the thickness of the multilayer, you can select a standard thickness from 0.6 to 2.4 mm, with a tolerance of 10% due to the different process steps required to construct both, the individual PCBs and the final multilayer.
The multilayer PCB fabrication will be performed depending on your requirements. If your PCB only have through via holes, each individual PCB is manufactured without the via hole process. The multilayer is built by placing together all the boards and applying heat and pressure. Finally, over all the stack up the via holes are created and the metallization process to create the electrical connection between the top, bottom and inner copper layers is performed. This is the reason that, for the outer layers, the top and bottom will have different copper thickness than the inner ones.
If blind or buried vias holes are required, your multilayer stack up the process will be more complicated since each board will have to be drilled and metallized separately and if through holes are also required a new drilling and metallization process will be required. All these processes increase notably the price of this kind of multilayer boards.
If you liked this post, you can help us enormously by sharing it on Facebook, Twitter or Google+. Even you can subscribe to our newsletter!