Guide to PCB Thickness
When manufacturing PCBs, Rush PCB Inc. considers PCB thickness as a very important factor, as it affects the power handling capacity of the board. As PCB design software does not provide any direct control over the printed board thickness, customers must instruct us about their requirement of circuit board thickness.
Thicker copper on the circuit board increases its price, but there are times when a thick copper is necessary. The increase in cost is not only due to the cost of raw materials, it is also due to more time and effort necessary in processing thicker copper.
Constituents of PCB Thickness
To understand PCB thickness, it is necessary to know what forms the thickness of the board, and its constituents. The PCB thickness specified by the customer is the thickness of the board after fabrication is completed. Nominal thickness of copper clad laminate is:
For glass fiber boards, the thickness is made up of the individual thicknesses of the glass fabric, epoxy, and the copper foil. For multi-layered boards, the thicknesses of the constituents of individual layers make up the total PCB thickness. Of these, the copper foil thickness is very important.
PCB Copper Thickness
PCB manufacturers specify copper thickness in ounces. The copper thickness varies depending on the type of board they are making:
- External copper thickness for Regular boards — 1 oz. / 1.37 mil / 35 µm
- External copper thickness for Switching Power Supply Boards — 2 oz. / 2.74 mils / 70 µm
- External copper thickness for high-current power boards — 3 oz. / 4.1 mils / 105 µm
- Internal copper thickness for multi-layered boards — 18 µm or 8 µm
In a printed circuit board, a major portion uses the 1 oz. copper thickness, of course, depending on the purpose of the board and the strength of the signal voltage and current. For the parts that handle excessive current, the manufacturer may use 2 oz. or 3 oz. copper thickness. To improve the yield, PCB manufacturers use the same thickness of copper foil on both sides of the inner layers.
Printed circuit boards with ultra-high-density patterns may use 12 µm copper thickness to make thin boards.
Rush PCB Inc. recommends using as much space as possible between copper features. This is because for the manufacturer, it is easier to etch traces and spaces of equal width than it is to etch thinner spaces between features.
The general rule is to distribute the copper as evenly as possible throughout the circuit board—not only with regard to the thickness of copper on each layer, but also its distribution across the layers, as far as possible.
As etching and plating are organic processes, the manufacturer has no precise control over where the process is removing or adding the copper. Although a mask protects the intended image from the etchant, the chemical dissolves the copper at different rates depending on the position of the feature on the board, placement of the board within the tank, and the density of the copper features on the board.
As the manufacturer agitates and circulates the chemical solution during etching and plating to minimize inconsistencies, PCB boards with drastically different copper densities can be problematic. Therefore, designers must distribute the copper across the board rather than create large open spaces with isolated copper features.
PCB Core Thickness
The core of the PCB, also known as the substrate or dielectric, is an insulating material separating two conductor or copper foil layers. Therefore, the core is a copper-clad material with a constant thickness that the manufacturers use during the fabrication of a printed circuit board.
For two-layer printed circuit boards, the central substrate will have two copper layers usually known as top and bottom layers.
Substrates are usually made in standard thicknesses of 1.6 mm, for 2-, 4-, and 6-layer boards. For higher number of layers in a board, the manufacturer may reduce the thickness.
Characteristic impedance or mechanical reasons may call for a thicker substrate of 3.2 mm, or a thinner substrate of 0.2 mm from the manufacturers.
Multi-layered printed circuit boards may have two or three pre-preg layers. These are glass fiber fabrics filled with resin. Manufacturers place them between two cores and press them together with the cores.
While cores retain their thickness even after pressing, pre-pregs change their thickness. Manufacturers can therefore, use pre-pregs to change the thickness of the PCB. The PCB thickness with pre-pregs depends on its processes, and on the ratio of copper and non-copper areas on the layer.
Thickness of Internal Layers of PCB
Apart from the sequencing of the layers, manufacturers can customize the stack-up by defining the internal PCB thickness. This is the vertical distance between the layers consisting of the copper tracks. However, the manufacturer cannot decide the internal thickness at random.
The PCB manufacturer arrives at the internal thicknesses because of the specific construction sequences of the circuit board. That means, to achieve a certain thickness in a printed circuit board, the manufacturer must build it with a certain structure.
Standards for PCB Thickness
There are various standards for printed circuit boards, including IPC/JPCA, JIS, JPCA, UL, and many more. Depending on the materials the manufacturer is using and the standard to which the PCB confirms, its thickness may vary.
Apart from the thickness of the substrate, the thickness of the copper foil, thickness of the plating, thickness of pre-preg, thickness of surface finish, thickness of solder mask, also contribute. Moreover, it depends on the material the manufacturer is using for the circuit board.
As PCB design software have no in-built provision to define or control PCB thickness, Rush PCB Inc. recommends explaining the required PCB thickness to the manufacturer. Therefore, if the customer wants a control on the copper thickness on the PCB board, they can write the requirements to let the manufacturer use a copper clad with the proper copper weight.