Usually in America copper weight is the unit of measure used to specify the thickness of copper on a PCB. It is the number of ounces on 1 square foot area of copper. This means that 1 oz of copper is approximately 35 um or 1.37 mils (.00137 inches).
One reason the thickness of copper is important is because it is one of the main factors of how much electrical current the trace can handle. There are many good online calculators to determine how many amps a trace can handle given the width and thickness.
Another reason is you can use the ground planes of a board as a heat sink so the thicker the copper - the more heat it can take away. If you are hand soldering components, large thick copper areas without adequate thermal reliefs can make soldering difficult. If the design has impedance requirements the copper thickness is one of the factors that influence impedance.
When manufacturing a circuit board the copper is etched away so the thicker the copper the more that must be etched. As seen in the picture below the etch process takes away the sides of the traces, pads, and planes with more taken away from the top than the bottom. To deal with this loss of copper the PCB manufacturer will increase the size of the features. The thicker the copper the more the copper features will be increased. This means that the thicker the copper the more spacing is required between copper features.
Given all of the above info what does that mean when laying out the board? If your design is carrying large amounts of current or needing thermal considerations that require more copper you must maximize the spacing between features. If the copper thickness is not the limiting factor then going with thinner copper will make the board easier to be made.
At Royal Circuits we have core material as thin as 1/2 oz and have built boards as thick as 10 oz copper. On the fab information you will want to specify minimum copper thickness.
These are the minimum spacing for different copper thickness:
- 1/2 oz .003 inches (3 mils)
- 1 oz .004 inches (4 mils)
- 2 oz .005 inches (5 mils)
More than 2 oz addressed on individual design basis. The larger air gap the better.