PCB Tech - Prevent copper surface oxidation during PCB production

This article mainly discusses the preventive measures against copper surface oxidation in the PCB production process, and discusses the use of a new type of copper surface antioxidant.

At present, during the operation cycle from copper sinking to pattern transfer after electroplating of the whole board in the production process of double-sided and multilayer PCBs, the oxidation of the copper layer in the board surface and the hole (from the small hole) seriously affects the pattern transfer and pattern The production quality of electroplating; 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, etc.; such incidents have always been compared in the industry, and now the solution to this problem and the use of professional copper surface Anti-oxidants do some discussion.

1. Current methods and status quo of copper surface oxidation in PCB production

1.1 Immersion copper-anti-oxidation of the whole board after electroplating

Generally, the boards after copper immersion and electroplating of the whole board will go through:

(1) 1-3% dilute sulfuric acid treatment;

(2) High temperature drying at 75-85 degree Celsius;

(3) Then insert the rack or stack the board and wait for the dry film to be attached or the wet film to be printed for graphic transfer;

(4) During this process, the board needs to be placed for at least 2-3 days, and at most 5-7 days;

(5) At this time, the board surface and the copper layer in the hole have long been oxidized to become "black".

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 in the previous drying process; therefore, the small holes are often incompletely dried and contain water, and the degree of oxidation is also higher. The surface of the board is more serious, and the stubborn oxide layer cannot be relied on by pickling. This may cause the board to be scrapped due to no copper in the hole after pattern plating and etching.

1.2 Anti-oxidation of the inner layer of the 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 a film, and waiting for AOI scanning and testing; although during this process, the operation and transport will be very careful and careful, but the board surface is still unavoidable such as fingerprints, stains, oxidation spots and other defects; A large number of false points are generated during AOI scanning, and the AOI test is carried out based on the scanned data, that is, all scanned points (including false points) AOI must be tested, which leads to a very low efficiency of AOI testing.

2. Discussion on the introduction of anti-oxidant on copper surface

At present, many PCB suppliers have introduced different copper surface antioxidants for production; the main working principle is: use organic acids and copper atoms to form covalent bonds and coordination bonds, and replace each other into chain polymers., A multilayer protective film is formed on the surface of the copper, so that no oxidation-reduction reaction occurs on the surface of the copper, and no hydrogen gas is generated, 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 conducive to environmental protection;

c. The removal of the anti-oxidation protective film produced is simple, and only the conventional "pickling + brushing" process is required;

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

2.1 Application of anti-oxidation after plating of copper-plating to the whole board

During the process of copper sinking and electroplating of the whole board, change the "dilute sulfuric acid" to the professional "copper surface anti-oxidant", and the other operation methods such as drying and subsequent inserting or stacking are unchanged; in this process A thin and uniform anti-oxidation protective film is formed on the surface of the 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 and change the copper layer. Black; under normal circumstances, the effective storage period of the anti-oxidation protective film can reach 6-8 days, which can fully meet the operating cycle of a general factory.

In the pre-processing of the pattern transfer, only the usual "3% dilute sulfuric acid + brushing" method can be used to quickly and completely remove the anti-oxidation protective film on the board surface and the hole without any influence on the subsequent processes.

2.2 Application of anti-oxidation in the inner layer of 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 is a comparison of the AOI scan and test results of the same model and batch number of inner laminates treated with dilute sulfuric acid and copper surface antioxidant.

Note: It can be seen from the above test data:

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

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

4. Summary

In short, with the development of the circuit board industry, product grades have improved; the small holes caused by oxidation and the low copper-free test efficiency of the inner and outer layers of AOI test efficiency need to be solved vigorously in the PCB production process; and the copper surface protection The emergence and application of oxidants have provided very good help in solving such problems. I believe that in the future PCB production process, the use of copper surface antioxidants will become more and more popular.