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5.3 Copper Thickness as Ounces of Copper
For example, the options for www.PCBway.com are shown in Figure 5.1. The price for 5 boards, 100 mm x 100 mm with the default values and 6 mil line and space, is $5. If the 5-mil line and space is select, the price increases to $40.
Figure 5.1 Example of the features PCBway is capable of fabricating at no extra charge.
Unless you have a strong compelling reason otherwise, do not use a line width or space narrower than 6 mils. If you do, your board may cost more, and you may not be getting any additional value for the higher price.
Watch this video and I will walk you through evaluating the fabrication capabilities of a few fab shops.
5.3 Copper Thickness as Ounces of Copper
The line width of copper is measured in mils or mm. The minimum line width commonly used is 6 mils or about 0.15 mm. The thickness of copper can also be measured in units of mils or mm. However, it is more commonly measured in units of ounces or oz.
In the early days of printed circuit board fabrication, the plating thickness of copper on a board was measured by weighing the board.
The weight of plated copper per square foot of board area on the board is related to the copper thickness based on the density of copper:
Weight Weight density A t or t
density A
= =
The units are a little unusual. The weight will be in oz. The area, A, will be in ft2 and we want the thickness in microns. This means the units for density should be in units of oz/ft2-u. We can do the units conversion for density very easily in steps:
3 2 2 2 4 2
density 8.9gm / cm= 0.035 oz / gm 30.5 cm / ft 10 cm / u− =0.029oz / ft −u This makes the thickness for 1 oz copper to be,
2 2
Weight 1oz
t 34 u 1.3 mils
density A 0.029 oz / ft u 1ft
= = = =
−
This thickness will vary depending on the density of copper and is often reported as between 1.2 mils to 1.4 mils in thickness.
If it is important to know the precise thickness of the copper, the traces should be cross sectioned.
For example, a plating weight of 1 oz of copper per square foot of board area corresponds to about 34 um or approximately 1.3 mils thick plating. This means ½ oz copper has a thickness of 17 um or about 0.7 mils.
A standard thickness for copper on outer layers is 1 oz. In some processes, all signal traces on outer layers are plated up an
additional 1 mil when the through-hole vias are plated. This means the signal traces are actually 2.3 mils thick.
In other processes, the board surface is coated with a plating resist, except where the through-hole vias are. The via barrels are the only features plated with extra copper, leaving the traces as the original foil thickness of 1 oz.
Inner layers in a four or more layer board use copper foil typically at either ½ oz or 1 oz thick. The inner layers are not plated up when through-hole vias are exclusively used for the board.
5.3 Copper Thickness as Ounces of Copper 125
Selecting a thicker copper for an inner or outer layer will generally cost more. For example, with PCBway, moving from 1 oz copper to 2 oz copper on an outer layer, the price of 5 boards increases from $5 to $49. Unless you have a strong compelling reason otherwise, stick with the default values of 1 oz copper.
Generally, there are three reasons thicker copper might be needed in a design.
Obviously, thicker copper will have lower electrical resistance than thinner copper. Those applications where very low resistance is needed, generally either high current or when very low DC voltage drops are required, might benefit from thicker copper. However, a similar benefit might be gained by just using a wider trace at no added cost.
In addition to lower electrical resistance, the other electrical benefit with thicker copper is higher current carrying capability before a significant temperature rise. Current loads to a specific device of more than 40 A DC current might benefit from copper foil thicker than 1 oz copper. Again, the same benefit might be possible with a wider trace with no added cost.
In many applications, the copper planes in a board provide not just electrical conductivity but also thermal conductivity. The planes, and even wider traces, can be used as heat spreader elements.
When you have a high-power consumption device, like a transistor or MOSFET operating in the linear regime, dissipating 1 watt or more, it might be useful to engineer a wide surface trace under the component or a solid plane under the component that is well thermally coupled to the component to suck the heat away and spread it to a larger area from which normal air convection can remove the heat from your board. Thicker copper might help remove more of the heat in these applications.
Whenever making the decision to use thicker copper compared with the standard 1 oz copper, it is important to compare the value returned at the higher cost. If there is no added value with the more expensive feature, why would you want to pay more and get nothing in return? This is a waste of money.