OSP, an abbreviation for Organic Solderability Preservative, it is a method for Printed circuit board Plating. It uses a water based organic compound that selectively bonds to copper and provides an organometallic layer that protects the Copper for Soldering.
What's the difference between "μm" and "oz" for copper weight?
35μm=1oz, 70μm=2oz, and so on
What's the difference between "mm" and "inch" for length?
1mmm=0.039inch, 2mmm=0.078inch, and so on