PCB Technical - Prevention of copper surface oxidation during PCB production

Prevention of copper surface oxidation during PCB production

At present, during the production process of double-sided circuit boards and multi-layer PCBs, during the operation cycle of copper sinking, plating of the whole board and pattern transfer, the oxidation of the copper layer in the board surface and the hole (from the small hole) seriously affects the pattern transfer And the production quality of pattern plating; In addition, the number of false points in AOI scanning caused by oxidation of the inner layer board has seriously affected the test efficiency of AOI. Such incidents have always been a headache in the industry, and now we are going to solve this problem and use it professionally. Do some research on the copper surface antioxidant.

1. The method and status quo of copper surface oxidation in the current PCB production process

1. Immersion copper of the circuit board factory anti-oxidation after electroplating the whole board

Generally, the boards after copper immersion and electroplating of the whole board will go through: 1. 1-3% dilute sulfuric acid treatment; 2. 75-85 degree Celsius high-temperature drying; 3. Then insert the rack or stack the board and wait for the dry film or the wet film to be printed. Do graphics transfer; 4. In this process, the board needs to be placed for at least 2-3 days, and as many as 5-7 days; 5. At this time, the PCB surface and the copper layer in the hole have long been oxidized to "black" (Figure 1 below).

In the pre-processing of graphics transfer, the copper layer on the board surface is usually processed in the form of "3% dilute sulfuric acid + brushing". However, the inside of the hole can only be treated by pickling, and it is difficult for the small holes to achieve the desired effect during the previous drying process; therefore, the small holes are often not completely dried and contain water, and the degree of oxidation is also higher. The board surface is much more serious, and the stubborn oxide layer cannot be removed by only pickling. This may cause the PCB board to be scrapped due to no copper in the hole after pattern plating and etching.

2. Anti-oxidation of inner layer of PCB multilayer board

Usually after the inner layer circuit is completed, it is developed, etched, stripped and treated with 3% dilute sulfuric acid. Then it is stored and transported by means of septa and waiting for AOI scanning and testing; although during this process, the operation and transport will be very careful and careful, but it is inevitable that there will be fingerprints, stains, oxidation spots, etc. on the surface of the board. Defects; there will be a large number of false points generated during AOI scanning, and AOI testing is carried out based on the scanned data, that is, all scanned points (including false points) AOI must be tested, which leads to the efficiency of AOI testing Very low.

2. Some discussion on the introduction of anti-oxidant on copper surface

At present, the potion suppliers of many PCB multilayer circuit board manufacturers have introduced different copper surface antioxidants for production purposes; our company currently also has a similar product, which is different from the final copper surface protection (OSP), It is suitable for the copper surface anti-oxidation potion in the PCB production process; the main working principle of the potion is: the use of organic acids and copper atoms to form covalent bonds and coordination bonds, and replace each other into chain polymers, forming multiple layers on the copper surface The protective film prevents oxidation-reduction reactions on the surface of the copper, and no hydrogen gas, thus playing the role of anti-oxidation. According to our use and understanding in actual production, the copper surface antioxidant generally has the following advantages:

A, the process is simple, the scope of application is wide, and it is easy to operate and maintain;

B, water-soluble technology, free of halide and chromate, which is good for environmental protection;

C. The removal of the formed anti-oxidation protective film is simple, only the conventional "pickling + brushing" process;

D. The generated anti-oxidation protective film does not affect the welding performance of the copper layer and hardly changes the contact resistance.

1. The application of circuit board in immersion copper-anti-oxidation after electroplating of the whole board

During the treatment process after copper sinking and electroplating of the whole board, the "dilute sulfuric acid" is changed to the professional "copper surface anti-oxidant", and the other operation methods such as drying and subsequent inserting or stacking are unchanged; in this treatment process Among them, a thin and uniform anti-oxidation protective film is formed on the surface of the PCB board and the copper layer in the hole, which can completely isolate the surface of the copper layer from the air, prevent the sulfide in the air from contacting the copper surface, and oxidize the copper layer. Turns black; under normal circumstances, the effective storage period of the anti-oxidation protective film can reach 6-8 days.

2. The application of anti-oxidation in the inner layer of PCB multilayer board

The procedure is the same as the conventional treatment, just change the "3% dilute sulfuric acid" in the horizontal production line to a professional "copper surface antioxidant". Other operations such as drying, storage, and transportation remain unchanged; after this treatment, a thin and uniform anti-oxidation protective film will also be formed on the surface of the board, which completely isolates the surface of the copper layer from the air, so that the surface of the board is not oxidized. At the same time, it also prevents fingerprints and stains from directly contacting the board surface, reducing false points in the AOI scanning process, thereby improving the efficiency of AOI testing.

3. Comparison of AOI scanning and testing of inner laminates treated with dilute sulfuric acid and copper surface antioxidant

The following are the inner layer boards of the same model and batch number treated with dilute sulfuric acid and copper surface antioxidant, and the results of AOI scanning and testing for each 10PNLS are compared.

Note: According to the above test data:

A, the AOI scanning false points of the inner layer board treated with copper surface anti-oxidant treatment is less than 9% of the AOI scanning false points of the inner layer board treated with dilute sulfuric acid;

B. The AOI test oxidation point of the inner layer board treated with copper surface antioxidant is: 0; and the AOI test oxidation point of the inner layer board treated with dilute sulfuric acid is: 90.

4. Summary

In short, with the development of the circuit board industry, the product grades are improved; the small holes caused by oxidation and the low efficiency of inner and outer AOI testing due to small holes The emergence and application of oxidants have provided very good help in solving such problems. It is believed that in the future PCB production process, the use of copper surface antioxidants will become more and more popular.