IPC has no specific definition for a Fine Line in printed circuit boards, but people in PCB industry generally define fine lines as boards with 2 through 5 mil line and spacing. When your board’s minimum line and spacing is 3/3mil, then this is a fine line board.
High density interconnect (HDI) PCB technology, including fine lines and spaces, is a key enabling technology for the next generation of portable electronic devices. This technology offers many advantages over conventional technologies, including circuit board size reduction, increased routing, and lower fabrication costs.
For HDI boards containing fine lines and spacing less than 4mil, laser direct imaging (LDI) is the best option. However, cupric chloride etching is a much more controllable process and with very little effort and the most basic equipment, line, and space sizes down to 2 mil can easily be achieved using cupric chloride.
With the need for fine lines and spaces comes the need for ultra-precise registration in the drilling process. With laser drilling and the automatic registration of the drilling pattern to optical targets (drilled or etched) is this drilling technology’s main advantage. With the individual registration and the adaptation of the drilling pattern to the dimension of each circuit board, pad sizes could be reduced dramatically. Microvia target lands have a diameter between 300 μm and 250 μm, and they will continue to drop.
Without investing in the proper technologies, it is difficult for a PCB manufacturer to produce fine line and space printed circuit boards down to 2 mils.