Drill dirt and etch back to Rigid printed circuit board

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Drill dirt and etch back to Rigid printed circuit board

1, Overview


Drilling and denting is just an important step after CNC Drilling of PCBs, electroless copper plating or direct copper plating. To achieve a reliable electrical interconnection with flex-printed circuit boards, it is necessary to combine Just – flexible printed circuit board of its special material composition, for its main material polyimide and acrylic alkaline nature of the impatience, the choice of appropriate decontamination and etching technology. Just – flexible printed circuit board to drill dirt and etch back and break wet technology and dry technology are two, the following two technologies to discuss with you colleagues.


2, just – flexible printed circuit board wet to drill dirt and etching technology


Rigid-Flex PCB technology Wet-to-Drill and Recess technology consists of three steps:

L bulky (also known as swelling treatment). The use of alcohol ethers bulking syrup to soften the hole wall substrate, destroy the polymer structure, thus increasing the surface area can be oxidized, so that the oxidation is easy to carry out, the general use of butyl carbitol wall base material swelling.

L oxidation. The purpose is to clean the wall and adjust the hole wall charge, at present, the domestic tradition in three ways.


  1. Concentrated sulfuric acid method: Since concentrated sulfuric acid has strong oxidation and water absorption, most of the resin can be carbonized and dissolved in water to form alkyl sulfonated and removed, the reaction is as follows: CmH2nOn + H2SO4 – mC + nH2O In addition to the hole wall resin, the effect of drilling and concentrated sulfuric acid concentration, processing time and temperature of the solution. For the concentration of concentrated sulfuric acid except for drilling pollution shall not be less than 86%, 20-40 seconds at room temperature, if you want to etch back, you should appropriately increase the solution temperature and extend the processing time. Concentrated sulfuric acid acts only on the resin, the glass fiber is invalid, the use of concentrated sulfuric acid concave hole wall, the hole wall will have glass fiber head protruding, need fluoride (such as ammonium bifluoride or hydrofluoric acid) treatment. Fluoride treatment of outstanding glass fiber head, the process conditions should also be controlled to prevent wicking due to over-corrosion of glass fibers, the general process is as follows:

H2SO4: 10%

NH4HF2: 5-10g / l

Temperature: 30 ℃ Time: 3-5 minutes

According to this method, the rigid-flex PCB after drilling is drilled and etched, and then the hole is metallized. Through the metallographic analysis, it is found that the inner layer of the drilling mud does not completely disappear at all, which leads to the copper layer and the hole wall Adhesion is low, so in the metallographic analysis of thermal stress experiments (288 ℃, 10 ± 1 seconds), the hole wall copper layer off and lead to the inner circuit.


Moreover, ammonium bifluoride or hydrofluoric acid is poisonous and wastewater treatment is difficult. More importantly, polyimide is inert in concentrated sulfuric acid. Therefore, this method is not suitable for desmear and etchback of rigid-flex printed circuit boards.


Ⅱ. Chromic acid method: Since chromic acid has a strong oxidizing ability and strong etching ability, it can make the long-chain of macromolecule material in the pore wall to be disconnected, oxidize and sulfonize and produce more relatives on the surface Water-based groups, such as carbonyl (-C = O), hydroxyl (-OH), sulfonic acid group (-SO3H), etc., to improve its hydrophilicity, adjust hole wall charge, Erosion purposes. General process recipe is as follows:

Chromic anhydride CrO3: 400 g / l

H2SO4 sulfuric acid: 350 g / l

Temperature: 50-60 ℃ Time: 10-15min


According to this method, the rigid-flex PCB after drilling is drilled and concave etched, and then the holes are metallized. Metallization holes are subjected to metallographic analysis and thermal stress test, and the results fully meet the requirements of GJB962A-32 . The company is located in:


Therefore, the chromic acid method is also suitable for Rigid-Flex PCB de-agglomeration and etchback. For small businesses, this method is indeed very suitable, easy to operate, and more importantly, the cost, but the only method Unfortunately there is a toxic material chromic anhydride.


Ⅲ. Alkaline potassium permanganate method: At present, many PCB manufacturers due to the lack of professional technology, still follow the rigid multilayer printed circuit board to drill dirt and etch back technology – alkaline potassium permanganate technology to deal with just – Flexible printed circuit board, by this method to remove the resin after drilling the same time, the surface of the resin can be etched to make the surface uneven small bumps in order to improve the hole wall coating and the substrate binding force in high temperature and high alkali environment, The use of potassium permanganate oxidation to remove swell resin swell, the system for general rigid multilayer board is very effective, but for rigid-flex printed circuit board does not meet, because rigid-flex printed circuit board body insulation Polyimide material is not alkaline, alkaline solution to be swell even at least partially dissolved, not to mention the high temperature and high alkali environment. If this method is used, the reliability of equipment using the rigid-flex PCB will be greatly reduced even if the flex-printed circuit board was not scrapped at that time.

l and. After the oxidation of the substrate must be cleaned to prevent contamination of the activation process after the procedure, for which must go through the reduction process, according to the different ways of oxidation and use different reduction solution.

3, just – flexible printed circuit board to dry and etch dry etching technology


At present, the popular dry method at home and abroad is plasma to drill dirt and etch back technology. Plasma is used in the production of Rigid-Printed Printed Circuit Boards, primarily for detonating the wall of the hole and modifying the surface of the hole wall. The reaction can be seen in the highly activated plasma and the porous wall of the polymer material and the glass fiber gas, solid phase chemical reactions, the resulting gas products and some non-reactive particles are pumped by the vacuum pump process is a Dynamic balance of chemical reaction process.According to just — flexible printed circuit board polymer materials used are usually selected N2, O2, CF4 gas as the original gas, which played a clean vacuum N2 and preheating role.

O2 + CF4 mixed gas plasma chemical reaction formula is:


O2 + CF4 O + OF + CO + COF + F + e- + ….



Due to the acceleration of the electric field so that it becomes a highly active particle collides with O and F particles to generate highly active oxygen radicals and fluorine radicals and the like, the reaction with the polymer material is as follows:

[C, H, O, N] + [O + OF + CF3 + CO + F + …] CO2 + HF + H2O + NO2 +

The reaction of the plasma with the glass fiber is:

SiO2 + [O + OF + CF3 + CO + F + …] SiF4 + CO2 + CaL


So far, to achieve a just — scratch printed circuit board plasma processing.


It is noteworthy that the atomic state of O and C-H and C = C carbonylation reaction to increase the polar bond on the polymer bond, so that the hydrophilic surface of the polymer material was modified



O2 + CF4 Plasma-treated rigid-flex PCBs, which are then treated with O2 plasma, not only improve wall wetting (hydrophilicity) but also remove the reaction. After the end of the sediment and reaction incomplete product. Using plasma technology to decontaminate and etch the rigid-flex PCB, and after direct plating, metallographic analysis of the metallization and thermal stress experiments, the results in full compliance with GJB962A-32 standards.


4 Conclusion

In summary, whether dry or wet, if the main body of the system for the characteristics of the material, choose the right method, you can achieve rigid – Flex interconnect mother board to drill dirt and concave etching purposes.

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