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How to choose the copper weight of a PCB

Printed circuit boards are composed by substrate and layers of copper foil. When the PCB is single layer, the copper foil will be only placed in the top or the bottom of the PCB, and when the board has two layers, the PCB will have copper foils in both top and bottom sides.
Typically, the thickness of the copper layers is specified in copper weight with oz/ft2, but commonly is named only as oz. But also, sometimes, it is specified in length, in µm, following the relationship that 1 oz/ft2 is equal to 17.5 µm. 


Different thickness of the PCB copper layers

Typical values of the copper layers thickness are 0.5 oz, 1 oz and 1.5 oz. This is the thickness of the base copper before any kind of metallization. When via holes are placed in the PCB, in the metallization process, the thickness of the copper layer typically increases to the double of its initial value. For this reason, the typical thickness of the finished copper layers are 1 oz, 1.5 oz and 3 oz. This increment always happens in the outer layer copper thickness. The outer layer is the top or the bottom when dealing with single and two layers boards, but it is also the external layer when dealing with multilayer printed circuit boards.

When a multilayer board is manufactured, the internal copper layers are called inner copper layers and, in this case, the thickness is not increased because the metallization only affects to the external layers. This is completely true when you work with through via holes, in which first, the multilayer is created, and the via holes and metallization is performed over the final stack up. If blind or buried via holes are required this will change. The boards that will have this kind of via holes have to be drilled and metallized before to assembly the final stack up and, in this case, the inner layer copper thickness can have the same value as the outer layer copper thickness. 

It is very important to have in mind all these aspects to know what will be the final thickness of your PCB.


Applications of the different copper layer thicknesses

Typical applications use the lower copper thickness for different reasons. Most important is due to the different chemical processes when removing copper to define the circuit in the substrate and, also during the metallization, it is easy to obtain better traces when the copper thickness is lower. Due to these processes it can be seen that the width of the trace can be slightly different between the bottom and the top of the trace, wider in the bottom. This effect is increased with the thickness of the copper. For this reason, for RF of digital applications in which the width of the traces is very important for a good performance, 0.5 oz as base copper thickness is selected.

Thicker copper thicknesses are selected for applications in which the voltages, currents or RF power are higher. High copper thickness helps to deal with the high temperatures that can occur due high current or power levels. For this reason, 1 oz or 1.5 oz base copper thickness is used for high power amplifiers, batteries, power supplies, in which many amperes are flowing through the traces and many heat has to be dissipated. In case a thin thickness is selected the trace can burn. New demanding advances in term of power dissipation are using thicker copper layers of up to 20 oz to improve performance.

For some RF applications, the selection of the copper thickness can help to improve the performance, for example in filter parameters, when working with coupled lines, a thicker copper layer can improve the results with respect the thinner one.

And that is all. In this post we explain you how to order your PCBs.

 2018-11-13 11:11
 Help    Engineering   
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