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[Industry] Technology 丨 rigid-flex printed circuit board to drill and etch technology

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点击次数:240 更新时间:2019年07月01日19:29:12 打印此页 关闭

1 Overview


Drilling and etching is an important process before electroless copper plating or direct electroplating of copper after rigid-flex printed circuit board numerical control drilling. In order to achieve reliable electrical interconnection of rigid-flex printed circuit boards, it is necessary to combine The rigid-flex printed circuit board has a special material composition, and the suitable material for drilling and etching is selected for the characteristics of the main material polyimide and acrylic acid which are not resistant to strong alkali. The rigid-flex printed circuit board is divided into two types: wet technology and dry technology. The following two technologies are discussed together with the peers.

2 rigid-flex printed circuit board wet method for drilling and etching

The rigid-flex printed circuit board wet-to-drill and etchback technology consists of the following three steps:



l bulky (also called swelling treatment). The alcohol wall-based bulking agent softens the pore wall substrate, destroys the polymer structure, and further increases the surface area that can be oxidized, so that the oxidation is easy to proceed. Generally, butyl carbitol is used to swell the pore wall substrate.



l Oxidation. The purpose is to clean the wall of the hole and adjust the wall charge of the hole. At present, the domestic traditional three ways.




I. Concentrated sulfuric acid method: Due to the strong oxidizing and water absorption of concentrated sulfuric acid, most of the resin can be carbonized and formed into water-soluble alkyl sulfonate, and the reaction formula is as follows: CmH2nOn+H2SO4--mC+nH2O The effect of the resin contamination of the hole wall is related to the concentration of concentrated sulfuric acid, the treatment time and the temperature of the solution. The concentration of concentrated sulfuric acid used for drilling is not less than 86%, 20-40 seconds at room temperature. If it is to be etched, the solution temperature should be increased and the treatment time should be extended. Concentrated sulfuric acid only acts on the resin and is ineffective for the glass fiber. After the concentrated sulfuric acid is used to etch the pore wall, the glass fiber head protrudes from the pore wall and 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 should also be controlled to prevent wicking due to over-corrosion of the glass fiber. The general process is as follows:


H2SO4: 10%


NH4HF2: 5-10g/l


Temperature: 30 ° C Time: 3-5 minutes

According to this method, the rigid-flex printed circuit board after punching is drilled and etched, and then the hole is metallized. Through metallographic analysis, it is found that the inner layer is not completely penetrated, resulting in the copper layer and the hole wall. The adhesion is low. For this reason, in the metallographic analysis of the thermal stress test (288 ° C, 10 ± 1 sec), the copper layer of the hole wall falls off and the inner layer is broken.


Moreover, ammonium hydrogen fluoride or hydrofluoric acid is highly toxic, and wastewater treatment is difficult. More importantly, the polyimide is inert in concentrated sulfuric acid, so this method is not suitable for the drilling and etchback of the rigid-flex printed circuit board.


II. Chromic acid method: Because chromic acid has strong oxidizing property and strong etching ability, it can break the long chain of high molecular material in the pore wall, and oxidize and sulfonate, and generate more pro on the surface. A water-based group, such as a carbonyl group (-C=O), a hydroxyl group (-OH), a sulfonic acid group (-SO3H), etc., thereby improving the hydrophilicity, adjusting the pore wall charge, and achieving the removal of the pore wall and the concave and concave The purpose of the eclipse. The general process recipe is as follows:


Chromic anhydride CrO3 : 400 g/l


Sulfuric acid H2SO4: 350 g/l


Temperature: 50-60 ° C Time: 10-15 min

According to this method, the rigid-flex printed circuit board after punching is drilled and etched, then the hole is metallized, and the metallized hole is subjected to metallographic analysis and thermal stress test. The result is in full compliance with GJB962A-32 standard. .


l Neutral. The oxidized substrate must be cleaned to prevent contamination of the activation solution in the subsequent process. For this purpose, a neutralization reduction process must be performed, and different neutralization reduction solutions are selected depending on the oxidation mode.

3

Dry-flex printed circuit board dry method for drilling and etching

At present, the popular dry methods at home and abroad are plasma de-soiling and etch-etching techniques. Plasma is used in the production of rigid-flex printed circuit boards, mainly to the wall of the hole and to modify the surface of the hole wall. The reaction can be seen as a gas-solid phase chemical reaction between the highly activated plasma and the pore wall polymer material and the glass fiber, and the generated gas product and a part of the unreacted particles are pumped away by the vacuum pump, which is a process. Dynamic chemical reaction equilibrium process. According to the polymer material used in the rigid-flexible printed circuit board, N2, O2, and CF4 gases are usually used as the original gas. Among them, N2 plays the role of cleaning vacuum and preheating.

[C、H、O、N]+[O+OF+CF3+CO+F+…] 

 CO2+HF+H2O+NO2+……


 

The reaction between the plasma and the glass fiber is:

 SiO2+[O+OF+CF3+CO+F+…] SiF4+CO2+CaL


So far, the plasma processing of the rigid-flexible printed circuit board has been realized.




It is noteworthy that the atomic state of O undergoes a carbonylation reaction with C-H and C=C to increase the polar group on the polymer bond, thereby improving the hydrophilicity of the surface of the polymer material.




The O2+CF4 plasma-treated rigid-flex printed circuit board, which is treated with O2 plasma, can not only improve the wettability (hydrophilicity) of the pore wall, but also remove the reaction. The sediment and the intermediate product of the incomplete reaction after the end. After the rigid-flex printed circuit board was processed by plasma technology and subjected to direct electroplating, the metallized holes were subjected to metallographic analysis and thermal stress tests, and the results were in full compliance with the GJB962A-32 standard.



4

In summary, whether it is dry or wet, if the appropriate method is selected for the characteristics of the main material of the system, the purpose of drilling and concave etching of the rigid-flex interconnecting mother board can be achieved.


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