Rigid-flex PCB wet de-drilling and etchback technology involves three steps:
1. Bulking (also called swelling treatment). Alcohol ether leavening liquid is used to soften the pore wall substrate, disrupt the polymer structure, and increase the surface area for oxidation, facilitating the oxidation process. Typically, butyl carbitol is employed to swell the pore wall substrate.
2. Oxidation. This step cleans the hole wall and adjusts its charge. In China, three traditional methods are used:
(1) Concentrated sulfuric acid method: Concentrated sulfuric acid, with its strong oxidizing properties and water absorption, carbonizes most of the resin and forms water-soluble alkyl sulfonates that are removed. The reaction formula is as follows: CmH2nOn + H2SO4 → mC + nH2O. The effectiveness of resin drilling on the hole wall depends on the sulfuric acid concentration, processing time, and solution temperature. The concentration of sulfuric acid should not be less than 86%, and processing should be done for 20-40 seconds at room temperature.
1. If etchback is required, increase the temperature of the solution and prolong the treatment time. Concentrated sulfuric acid only affects the resin and is ineffective on the glass fiber. After etching the hole wall with concentrated sulfuric acid, the glass fiber ends will protrude from the hole wall and need treatment with fluoride (such as ammonium bifluoride or hydrofluoric acid). When using fluoride, control process conditions to prevent over-corrosion and the wicking effect on the glass fiber. The general process is as follows:
2. H2SO4: 10%
NH4HF2: 5-10 g/l
Temperature: 30°C Time: 3-5 minutes
3. Using this method, the punched rigid-flex printed circuit board was drilled, etched, and the hole was metalized. Metallographic analysis revealed that the inner layer was not thoroughly drilled, leading to low adhesion between the copper layer and the hole wall. As a result, during thermal stress experiments (288°C, 10±1 seconds), the copper layer on the hole wall detached, and the inner layer broke.
4. Moreover, ammonium bifluoride or hydrofluoric acid are extremely toxic and challenging to treat in wastewater. Additionally, polyimide is inert in concentrated sulfuric acid, making this method unsuitable for de-drilling and etchback of rigid-flex printed circuit boards.
5. (2) Chromic acid method: Chromic acid’s strong oxidizing properties and etching ability can break down the long chains of pore wall polymer material, causing oxidation and sulfonation, which generates more hydrophilic groups on the surface, such as carbonyl groups (-C=O), hydroxyl groups (-OH), and sulfonic acid groups (-SO3H). This improves hydrophilicity, adjusts the charge of the hole wall, and achieves the ultimate goal of etchback. The general process formula is as follows:
6. Chromic anhydride CrO3: 400 g/l
Sulfuric acid H2SO4: 350 g/l
Temperature: 50-60°C Time: 10-15 minutes
7. Using this method, the punched rigid-flex printed circuit board was de-drilled and etched, and the holes were metallized. Metallographic analysis and thermal stress experiments on the metallized holes met the GJB962A-32 standard.
8. Therefore, the chromic acid method is suitable for the de-drilling and etchback of rigid-flex printed circuit boards. For small businesses, this method is simple, easy to operate, and cost-effective, though it has the drawback of using the toxic substance chromic anhydride.
9. (3) Alkaline potassium permanganate method: Due to a lack of professional technology, many PCB manufacturers still use alkaline potassium permanganate technology for rigid multi-layer boards. This method removes resin drilling dirt and etches the resin surface, creating small pits to enhance the bonding force of the hole wall plating layer and substrate. Potassium permanganate oxidizes and removes swollen resin contamination in a high-temperature, high-alkali environment. Although effective for rigid multi-layer boards, it is unsuitable for rigid-flex boards because polyimide, the main insulating material, is not alkali-resistant and will swell or dissolve in alkaline solutions. This method risks significantly reducing the future reliability of equipment using rigid-flex boards.
10. Neutralization. After oxidation treatment, the substrate must be cleaned to avoid contamination of the activation solution in subsequent processes. Thus, a neutralization and reduction process is necessary, with different solutions chosen based on the oxidation method used.
11. Currently, the popular dry method both domestically and internationally is plasma decontamination and etchback technology. Plasma is used in rigid-flex printed circuit board production to de-drill and modify the hole wall surface. The reaction involves a gas and solid-phase chemical interaction between the highly activated plasma, polymer material, and glass fiber. Generated gases and unreacted particles are removed by a vacuum pump. The reaction uses gases like N2, O2, and CF4, with N2 assisting in vacuum cleaning and preheating.
12. The schematic formula for the plasma chemical reaction of O2+CF4 mixed gas is:
O2+CF4 → O+OF+CO+COF+F+e–+…….
13. Due to the electric field’s acceleration, highly reactive particles collide with O and F particles to generate highly reactive oxygen and fluorine radicals, reacting with polymer materials as follows:
[C, H, O, N]+[O+OF+CF3+CO+F+…] → CO2+HF+H2O+NO2+……
14. The plasma reaction with glass fiber is:
SiO2+[O+OF+CF3+CO+F+…] → SiF4+CO2+CaL
15. Plasma treatment for rigid-flex printed circuit boards is thus realized. Notably, the atomic carbonylation reaction of O with C-H and C=C adds polar groups to the polymer bond, enhancing surface hydrophilicity.
16. Rigid-flex printed circuit boards treated with O2+CF4 plasma and then with O2 plasma can improve wettability (hydrophilicity) and remove reaction sediment and incomplete halfway products. After plasma treatment, direct electroplating, metallographic analysis, and thermal stress experiments on metallized holes meet the GJB962A-32 standard.
17. In summary, selecting a method suited to the main material characteristics of the system, whether dry or wet, achieves the goal of de-drilling and recess etching for rigid-flex interconnection motherboards.
