一、The problem of line deformation caused by poor etching of multilayer circuit board lines
When etching the outer layer of the multilayer circuit board, if the copper foil ridge line penetrates into the board resin quite deeply, residual copper may remain in the dense circuit area after etching. These phenomena may not be easy to detect after etching, but in the chemical After the nickel immersion gold process, it may be found that there are deformed lines or metal areas on the edges of the lines or pads. This problem is sometimes considered to be a problem of residue or poor washing, but it is actually a problem of circuit etching or improper selection of copper skin.
二、Poor tin stripping may cause exposed copper on the gold surface
After etching, it is necessary to pay attention to whether there is still light gray intermetallic metal that has not been stripped. If it is really not removed, brushing, pickling, and micro-etching may not be able to completely remove it, which will inhibit the start of the nickel immersion gold reaction. If the reaction cannot be started at all, copper leakage on the gold-plated surface may occur.
三、The problem of residual copper on the wall of the copper-free through hole
The current method of copper-free through-holes is mainly to etch and remove the copper after plating, or use the cover hole process to prevent the holes from being plated with tin, and then etch to remove the copper. However, the etchant cannot remove the special metal, so nickel and gold will still be adsorbed on the hole wall during the process. This is an immediate problem for products that do not require metal in the walls of these holes.
At present, some multi-layer circuit board manufacturers have launched the so-called chemical copper process without the trouble of electroless nickel immersion gold. Copper-free via fabrication troubles. However, such an approach will have potential crises of insufficient chemical copper activity and hole breakage, and the operating range of chemical copper will be narrowed. Some manufacturers also adopt the method of removing special metal, adding a chemical solution for removing special metal after the tin stripping tank, but this method must increase the setting of the chemical solution tank in the current process, and the operating cost will also increase.
At the same time, most special metal removal systems have the risk of corroding copper, and some so-called special medicines have patent and cost problems. Another method is to passivate the special metal layer in the hole with a mercaptan solution before stripping the tin, so that the subsequent nickel immersion gold process cannot be used. However, if the mercaptan treatment is not rinsed cleanly, the residue will be brought into the tin stripping tank and the copper surface will be stained with sulfide. The sulfur on the copper surface is fatal to the reaction of nickel, so it is very difficult to prevent the problem of copper exposure. As a result, the exact solution for copper-free vias is still under development.
