Precautions against copper surface oxidation during PCB production

This paper mainly discusses the prevention measures for copper surface oxidation in the PCB production process, and discusses the case of citing a new type of copper surface antioxidant.

  At present, in the operation cycle of copper plating, plate plating and pattern transfer in the production process of double-sided and multi-layer PCB, the oxidation problem of the copper layer in the board surface and the hole (within the small hole) seriously affects the graphics transfer and graphics. The production quality of electroplating; the increase of AOI scanning false spots caused by oxidation of the inner layer plate seriously affects the testing efficiency of AOI; such incidents have always been a headache in the industry. Now solve this problem and use professional copper. Some anti-oxidants are discussed.

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

  1.1 Copper-plated - anti-oxidation after plating

  Generally, most of the plates after copper plating and plate plating will pass through:

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

  (2) drying at a high temperature of 75-85 ° C;

  (3) Then insert the stack or stack, wait for the dry film or print the wet film for graphic transfer;

  (4) In the process, the board needs to be placed for 2-3 days, and then 5-7 days;

  (5) At this time, the copper layer in the plate surface and the hole is already oxidized to "black".

  In the pre-processing of the pattern transfer, the copper layer of the board is usually treated in the form of "3% dilute sulfuric acid + brush". However, the pores only have the effect of pickling, and the pores are difficult to achieve the desired effect in the front drying process; therefore, the pores are often incompletely dried and contain moisture, and the degree of oxidation is also higher than that in the pores. The board surface is much more serious, and it is impossible to remove its stubborn oxide layer by only pickling. This may cause the board to be scrapped after being electroplated and etched, resulting in no copper in the hole.

  1.2 Anti-oxidation of the inner layer of the multilayer board

  Usually, after the inner layer is completed, it is developed, etched, filmed, and treated with 3% dilute sulfuric acid. Then it is stored and transported by means of film separation and waiting for AOI scanning and testing; although in this process, operations, transshipment, etc. will be particularly careful and careful, but the board surface is inevitably such as fingerprints, stains, oxidation points, etc.; There are a lot of false points generated during AOI scanning, and the AOI test is based on the scanned data, that is, all scanning points (including false points) AOI are tested, which results in very low AOI testing efficiency.

  2. Discussion on the introduction of copper surface antioxidants

  At present, many PCB syrup suppliers have introduced different copper anti-oxidants for production. The main working principle of the syrup is to use organic acids to form covalent bonds and complex bonds with copper atoms. The polymer forms a multilayer protective film on the surface of the copper, so that the surface of the copper does not undergo a redox reaction, and hydrogen does not occur, thereby preventing oxidation. According to our use and understanding in actual production, the copper surface antioxidant generally has the following advantages:

  a. Simple process, wide application range, easy to operate and maintain;

  b. Water-soluble process, halogen-free and chromate, which is good for environmental protection;

  c. The resulting oxidation protection film is easy to remove, only the conventional "pickling + brushing" process;

  d. The resulting oxidation protection film does not affect the solderability of the copper layer and hardly changes the contact resistance.

  2.1 Application of anti-oxidation after copper-plating

  In the process of copper-to-plate plating, the “dilute sulfuric acid” is changed to the professional “copper surface antioxidant”, and other operations such as drying and subsequent inserting or stacking are unchanged; In the middle, the surface of the copper layer on the plate surface and the hole will form a thin and uniform anti-oxidation protective film, 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; normally, the effective storage period of the anti-oxidation protective film can reach 6-8 days, which can fully meet the operation cycle of a general factory.

  In the pre-processing of the pattern transfer, the anti-oxidation protective film in the board surface and the hole can be quickly and completely removed by using the usual "3% dilute sulfuric acid + scrubbing" method, and has no effect on the subsequent process.

  2.2 Application of anti-oxidation in the inner layer of multi-layer board

  As with the conventional processing procedure, it is only necessary to change "3% dilute sulfuric acid" in the horizontal production line to "copper surface antioxidant" for professional use. Other operations such as drying, storage, and transfer are unchanged; after this treatment, a very thin and uniform anti-oxidation protective film is formed on the surface of the board to completely isolate the surface of the copper layer from the air so that the surface is not oxidized. At the same time, fingerprints and stains are prevented from directly contacting the surface of the board, and false points in the AOI scanning process are reduced, thereby improving the testing efficiency of the AOI.

  3. Comparison of AOI scanning and testing of inner layer boards treated with dilute sulfuric acid and copper surface anti-oxidants

  The following are the same model and the same batch of inner layer plates treated with dilute sulfuric acid and copper surface anti-oxidant respectively, and the results of each 10PNLS in AOI scanning and testing are compared.

  Note: From the above test data, we can know:

  a. The AOI scan false points of the inner layer treated with the copper surface antioxidant are less than 9% of the AOI scan false points of the inner layer treated with dilute sulfuric acid;

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

  4. Summary

  In short, with the development of the circuit board industry, the product grades are improved; the copper-free scrap of small holes due to oxidation and the low efficiency of AOI testing of the inner and outer layers are all required to be solved in the PCB production process; The emergence and application of oxidants have helped to solve such problems. It is believed that the use of copper anti-oxidants will become more and more popular in the future PCB production process.