Rigid-Flex circuit board Decontamination and Concave erosion technology

- Feb 11, 2020-

Decontamination and concave erosion is an important process before electrolesscopper plating or direct copper plating after the NC hole of the Rigid-Flex PCB. In order to realize reliable electrical interconnection of PCB, it is necessary to combine the special material composition of PCB with Polyimide for its main material amines and acrylics are resistant to strong alkaline properties, select suitable decontamination and concave erosion technology. There are two methods for decontamination: wet and dry .


The technology of wet decontamination and erosion of newly printed circuit board consists of the following three steps


1.Expansion (also known as swelling treatment). The base material of pore wall was softened by alcohol-ether bentonite to destroy the polymer structure and then to increase the oxidizable surface product, so that its oxidation is easy to carry out, generally using butyl alcoholization to make the pore wall substrate swelling.

 

2.Oxidation. The purpose is to clean the hole wall and adjust the charge of the hole wall. At present, there are three traditional ways in China.


1)Concentrated sulfuric acid method: because of its strong oxidation and water absorption, it can carbonize most resins and form alkyl sulfonates dissolved in water The reaction formula is as follows: the effect of CmH2nOn+H2S04=mC+nH2O on drilling of pore wall resin and concentration of concentrated sulfuric acid, treatment time and solution Temperature. The concentration of concentrated sulfuric acid used for decontamination shall not be less than 86%,20-40 seconds at room temperature, and the temperature and elongation of the solution shall be appropriately increased if cavitation is to occur Long processing time. The concentrated sulfuric acid only acts on the resin and does not work on the glass fiber. After the hole wall is eroded by the concentrated sulfuric acid, the glass fiber head will protrude from the hole wall It needs to be treated with fluoride (e.g. ammonium fluoride or hydrogasic acid). Fluoride treatment of prominent fiberglass head, should also control the process conditions to prevent Glass fiber corrosion caused by core absorption, the general process is as follows: H2S04(10%)+NH4HF2(5-10G/L)  Temperature:30C Time:3-5 minutes. 


According to this method, ammonium fluoride or hydrofluoric acid are highly toxic, and waste water treatment is difficult. More important is polyimide in concentrated sulfuric acid, so this method is not suitable for decontamination and concave corrosion of rigid-flex printed circuit board


2) Chromic acid method: because the chromic acid has the strong oxidation property, its erosion ability is strong, therefore it can make the pore wall polymer material long chain disconnect, and produces the oxygen .To form more hydrophilic groups on the surface to improve its hydrophilicity, adjust the pore wall charge, and achieve the purpose of removing pore wall drilling and erosion. General process formulas are as follows:CrO3:400g/L;H2504:350g/1 Temperature:50-60℃ Time:10-15 minutes.In accordance with this method, the perforated rigid-friction printed circuit board is de-duged and corroded, then the hole is metallized, and the metallographic analysis and thermal stress experiments are carried out on the metallized hole. Therefore, the ming acid method is also suitable for the decontamination and reetching of rigid-scratch printed circuit boards. For small enterprises, the method is very suitable, simple and easy to operate, and more importantly, the cost, but the only regret of the method is the presence of the toxic substance chromic anhydride.

 

3) Alkaline potassium permanganate method: at present, many PCB manufacturers still follow the rigid multilayer printed circuit board decontamination and concave corrosion technology-alkaline potassium permanganate technology to deal with rigid-flex printing circuit board because of the lack of professional technology, by this method, the resin surface can be etched to produce small uneven pits on the surface, so as to improve the bonding force between the pore wall coating and substrate. If this method is adopted, even if the newly printed circuit board was not scrapped, the reliability of the equipment using the newly printed circuit board will be greatly reduced.


3. Neutralization. The base material after oxidation treatment must be cleaned to prevent contamination of the activated solution of the following process. For this purpose, it must go through the neutralization and reduction process and select different neutralization and reduction solutions according to the different oxidation methods.


At present, the popular dry process at home and abroad is plasma decontamination and reetching technology. Plasmas are used in the production of rigid-friction printed circuit boards, mainly for the hole wall decontamination and modification of the hole wall surface. The reaction can be seen to be highly activated by the gas product produced by the gas-solid chemical reaction of the plasma with the pore wall polymer material and glass fiber, and the process of partially unreacted particles being removed by the vacuum pump is a dynamic chemical reaction equilibrium process. According to the polymer materials used in the rigidly flexed PCB, N2, O2, CF4 gas are usually selected as the primary gas, in which N2 plays the role of cleaning and preheating.

 

At this point, the plasma processing of the rigid printed circuit board is realized It is worth noting that the carbonylation of O and H and C in the atomic state increases the polar group on the polymer bond and improves the hydrophilicity on the surface of the polymer material.


The rigidly flexed PCB treated with O2CF4 plasma and then treated with O2 plasma cannot only improve the wettability (hydrophilicity) of the pore wall, but also remove the reaction. After the end of the fluid and reaction of incomplete midway product. The metallographic analysis and thermal stress experiments of the metallized holes were carried out after the rigid-printed circuit board was treated with the plasma technology of drilling and cavity erosion, and the results were quite consistent.

 

To sum up, whether it is dry or wet, if the characteristics of the main materials of the system, choose the appropriate method, can reach the grid rigid Purpose of decontamination and concave etching of interconnect motherboard.