Decontamination and pitting is an important process before chemical copper plating or direct copper plating after NC drilling of soft and hard PCB. In order to realize reliable electrical interconnection of rigid Flex PCB, it is necessary to select appropriate decontamination and pitting technology according to the special material composition of rigid Flex PCB and the characteristics of polyimide and acrylic acid, which are not strong alkaline resistant. There are two kinds of technology to drill and dent rigid flex printed circuit board: wet process technology and dry process technology. The following two technologies are discussed with colleagues.
The wet drilling and pitting technology of rigid flex printed circuit board consists of the following three steps:
1. Swelling (also called swelling treatment). The pore wall base material is softened by alcohol ether swelling agent, the polymer structure is destroyed, and the surface area that can be oxidized is increased, so that the oxidation can be easily carried out. Generally, butyl carbitol is used to make the pore wall base material swell.
2. Oxidation. The purpose is to clean the pore wall and adjust the charge of the pore wall.
(1) Concentrated sulfuric acid method: due to its strong oxidation and water absorption, concentrated sulfuric acid can carbonize most resins and form water-soluble alkylsulfonates for removal. The reaction formula is as follows: cmh2non + H2SO4 -- MC + nH2O is related to the concentration of concentrated sulfuric acid, treatment time and solution temperature. The concentration of concentrated sulfuric acid used for drilling waste removal shall not be less than 86%, 20-40 seconds at room temperature. If concave corrosion is required, the solution temperature and treatment time shall be appropriately increased. Concentrated sulfuric acid only works on the resin, but not on the glass fiber. When concentrated sulfuric acid is used to etch the hole wall, there will be a protruding glass fiber head on the hole wall, which needs to be treated with fluoride (such as ammonium hydrogen fluoride or hydrofluoric acid). When fluoride is used to treat the protruding glass fiber head, the process conditions shall also be controlled to prevent the core absorption caused by the over corrosion of glass fiber. The general process is as follows:H2SO4:10%
NH4HF2：5-10g/l
Temperature: 30 ℃ time: 3-5 minutes
According to this method, after drilling the rigid flex printed circuit board, the drilling dirt and pitting were removed, and then the hole was metallized. Through metallographic analysis, it was found that the drilling dirt in the inner layer was not completely removed, which resulted in low adhesion between the copper layer and the hole wall. Therefore, during the thermal stress test (288 ℃, 10 ± 1s) by metallographic analysis, the copper layer on the hole wall fell off, resulting in the inner layer breaking.
Moreover, ammonium hydrogen fluoride or hydrofluoric acid are very toxic, so it is very difficult to treat wastewater. More importantly, polyimide is inert in concentrated sulfuric acid, so this method is not suitable for drilling and pitting of rigid flex printed circuit board.
(2) Chromic acid method: due to its strong oxidizability and strong etching ability, chromic acid can break the long chain of polymer materials on the pore wall, and produce oxidation and sulfonation, and generate more hydrophilic groups on the surface, such as carbonyl (- C = O), hydroxyl (- OH), sulfonic (- SO3H), etc., so as to improve its hydrophilicity, adjust the charge on the pore wall, and achieve the removal of drilling contamination and Purpose of pitting. The general process formula is as follows:
Chromic anhydride CrO3: 400 g / L
Sulfuric acid H2SO4: 350 g / L
Temperature: 50-60 ℃ time: 10-15min
According to this method, the holes of the rigid flex printed circuit board after drilling are removed from the dirt and pitting, and then the holes are metallized. The metallographic analysis and thermal stress test of the metallized holes are carried out, and the results fully conform to the gjb962a-32 standard.
Therefore, chromic acid method is also suitable for the drilling and pitting of rigid flex printed circuit board. For small enterprises, this method is very suitable, simple and easy to operate, and more importantly, the cost, but the only regret of this method is the presence of toxic chromic anhydride.
