Introduction To Desmear and Pitting Technique Of Rigid-Flex PCB

Desmear and pitting is just after the Rigid-Flex printed circuit board NC drilling, chemical plating of copper or copper plating directly in front of an important working procedure, just flexible printed circuit board to achieve reliable electrical interconnection, just flexible printed circuit board must be combined with its special materials, polyimide materials for its main body and the characteristics of acrylic acid is not resistant to strong alkali, choose suitable to desmear and pitting. There are wet technique and dry technique for the drilling and fouling removal of rigid-flex printed circuit board. The following two kinds of techniques are discussed together with your colleagues.

The wet technique for rigid-flex printed circuit board desmear and pitting consists of the following three steps:

1. Puffing (also known as swelling treatment).

Alcohol ether leavening solution is used to soften the base material of pore wall, destroy the polymer structure, and then increase the surface area that can be oxidized, so that the oxidation can be carried out easily. Butyl carbidol is generally used to make the base material of pore wall swell.

2. Oxidation.

The purpose is to clean the hole wall and adjust the charge of the hole wall. Currently, there are three traditional methods used in China.

(1) Concentrated sulfuric acid method: Because concentrated sulfuric acid has strong oxidation and water absorption, it can carbonize most of the resin and form water-soluble alkyl sulfonates to remove, the reaction formula is as follows: CmH2nOn+H2SO4–mC+nH2O hole wall resin drilling fouling effect is related to the concentration of concentrated sulfuric acid, treatment time and solution temperature.

The concentration of concentrated sulfuric acid used to remove dirt should not be lower than 86%, 20-40 seconds at room temperature, if concave corrosion is needed, the solution temperature should be appropriately increased and the treatment time should be extended.

Concentrated sulfuric acid only plays a role in the resin, ineffective to the glass fiber, using concentrated sulfuric acid concave etching hole wall, hole wall will have glass fiber head protruding, need fluoride (such as ammonium hydrogen fluoride or hydrofluoric acid) treatment.

When using fluoride to treat protruding glass fiber heads, the process conditions should also be controlled to prevent core-suction due to excessive 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, to desmear and pitting, and then the holes were metallized. Through metallographic analysis, it was found that the drilling dirt in the inner layer was not completely removed, resulting in low adhesion between the copper layer and the hole wall. Therefore, during the thermal stress experiment of the metallographic analysis (288℃, 10±1”), the copper layer of the hole wall fell off, resulting in the inner layer breaking.  Moreover, ammonium hydrogen fluoride or hydrofluoric acid are extremely toxic, making wastewater treatment difficult. More importantly, polyimide is inert in concentrated sulfuric acid, so this method is not suitable for rigid – flexible printed circuit board to remove dirt and concave corrosion.

(2) the chromate method: because of chromic acid has strong oxidizing, its erosion ability is strong, so it can make the hole wall polymer material long chain disconnected, and oxidation and sulfurization, generated in the surface more hydrophilic groups, such as carbonyl (- C = O), hydroxyl (OH), sulfonic group (- SO3H), etc., so as to improve its hydrophilicity, adjust the hole wall charges, and to remove the purpose of the wall of hole drilling sewage and pitting.

General process formula is as follows:

Cro3.400 g/ L chromic anhydride

Sulfuric acid H2SO4:350 g/ L

Temperature: 50-60℃

Time: 10-15min

According to this method, to desmear and pitting, and then the holes were metallized. The metallographic analysis and thermal stress test were carried out, and the results were in full compliance with GJB962A-32 standard. Therefore, chromic acid method is also suitable for the rigid-flex printed circuit board to remove dirt and pitting, for small enterprises, this method is indeed very suitable, easy to operate, and more importantly, the cost, but the only regret of this method is the presence of toxic chromium anhydride.

(3) Alkaline potassium permanganate method:

At present, a lot of PCB manufacturers because of a lack of professional technology, still follow rigid multilayer printed circuit board is to drill the dirt and pitting technology – alkaline potassium permanganate to handle just – flexible printed circuit board, the method to remove resin after drilling sewage, at the same time can make its surface etching resin surface produce small bumpy small holes, in order to improve the adhesion strength of coating and the substrate of hole wall, under high temperature and high alkali environment, swelling of the resin can be removed using potassium permanganate oxidation drilling sewage, the system is work for the general rigid laminated, but for just – flexible printed circuit board does not adapt, Because rigid-flex printed circuit board main insulation substrate polyimide is not alkali resistant, in alkaline solution to swell and even a small part of the solution to dissolve, let alone the high temperature and alkali environment. If this method is adopted, even if the rigid-flex printed circuit board is not scrapped at that time, it will greatly reduce the reliability of equipment using the rigor – torsion printed circuit board.

3. Neutralization.

After oxidation treatment, the substrate must be cleaned to prevent contamination of the activated solution after the process, so it must go through the neutralization reduction process, according to the different oxidation way to choose different neutralization reduction solution.

At present, the popular dry method at home and abroad is the plasma cleaning and etching technique. Plasma is used in the production of rigid – flexible printed circuit board, which is mainly used to clean the hole wall and modify the hole wall surface. The reaction can be seen as the gas and solid phase chemical reaction between highly activated plasma and pore wall macromolecules and glass fibers, and the process in which the generated gas products and some non-reactive particles are evacuated by vacuum pump, which is a dynamic chemical reaction equilibrium process. N2, O2 and CF4 gases are usually used as the original gases for the polymer materials used in rigid – flexible printed circuit boards. N2 plays the role of cleaning vacuum and preheating.

The plasma chemical reaction of O2+CF4 mixed gas is as follows:

O2 + CF4O + OF + CO + COF + F + e- + …

[Plasma]

Due to the acceleration of the electric field, it becomes a highly active particle and collidles O and F particles to produce highly active oxygen free radicals and fluorine free radicals, which react with the polymer materials as follows:

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

The reaction between plasma and glass fiber is as follows:

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

So far, the plasma treatment of rigid – flexible PCB is realized.

It is worth noting that the carbonylation of Atomic O with C-H and C=C results in the addition of polar groups on the polymer bond, which improves the hydrophilicity of the polymer material surface.

The rigid-flexible printed circuit board treated by O2+CF4 plasma, and then treated by O2 plasma, can not only improve the wettability (hydrophilicity) of hole wall, but also remove the reaction. After the end of the sediment and reaction of incomplete halfway products.

After treating rigid – flexible printed circuit board with plasma technology to remove fouling and concave corrosion and directly electroplating, metallographic analysis and thermal stress experiment on metallized hole were carried out, and the results fully accord with GJB962A-32 standard.

No matter dry method or wet method, if appropriate method is chosen according to the characteristics of the main material of the system, the purpose of to desmear and pitting of rigid-flexible interconnected motherboards can be achieved.