PCB Tech - Common problems of copper sulfate electroplating process

Copper electroplating is the most widely used pre-plating layer to improve the adhesion of the coating. The copper coating is an important protective decorative coating component of the copper/nickel/chromium system. The flexible copper coating with low porosity is useful for improving the The adhesion and corrosion resistance between the coatings play an important role. Copper plating is also used for local anti-carburization, printed board hole metallization, and as the surface layer of the printing roller. The colored copper layer after chemical treatment is coated with organic film and can also be used for decoration. In this article, we will introduce the common problems encountered in the PCB process of electroplating copper technology and their solutions.

1. Common problems of acid copper electroplating

Copper sulfate electroplating occupies an extremely important position in PCB electroplating. The quality of acid copper electroplating directly affects the quality of the electroplated copper layer and related mechanical properties, and has a certain impact on subsequent processing. Therefore, how to control the quality of acid copper electroplating is An important part of PCB electroplating is also one of the difficult processes for many large factories to control the process. The common problems of acid copper electroplating mainly include the following: 1. Rough electroplating; 2. Electroplating (plate surface) copper particles; 3. Electroplating pits; 4. Whitening or uneven color of the plate surface. In response to the above problems, some conclusions were made, and some brief analysis solutions and preventive measures were carried out.

1. Rough plating

Generally, the board angle is rough, most of which is caused by the electroplating current being too large. You can lower the current and check the current display with a card meter for abnormalities; the whole board is rough and usually does not appear, but the author has encountered it once at the customer’s place and checked it later.

Tomorrow, the temperature in winter was low and the content of brightener was insufficient; and sometimes the surface of some reworked faded sheets was not cleaned, and similar conditions would occur.

2. Electroplating plate surface copper particles

There are many factors that cause copper particles on the board surface. From the copper sinking to the entire process of pattern transfer, copper electroplating itself is possible. The author met in a large state-owned factory, copper particles on the plate surface caused by copper sinking.

Copper particles on the board surface caused by the copper immersion process may be caused by any copper immersion treatment step. Alkaline degreasing will not only cause roughness on the board surface, but also roughness in the holes when the water hardness is high and the drilling dust is too much (especially the double-sided board is not de-smeared). The internal roughness and slight stains on the surface of the board can also be removed; there are mainly several conditions for microetching: the quality of the microetching agent hydrogen peroxide or sulfuric acid is too poor, or the ammonium persulfate (sodium) contains too much impurities, generally It is recommended that it should be at least CP grade. In addition to industrial grade, it will cause other quality failures; excessively high copper content or low temperature in the micro-etching tank causes slow precipitation of copper sulfate crystals; the tank liquid is turbid and polluted.

Most of the activation solution is caused by pollution or improper maintenance. For example, the filter pump leaks, the bath liquid has a low specific gravity, and the copper content is too high (the activation tank has been used for too long, more than 3 years), which will produce particulate suspended matter in the bath. Or impurity colloid, adsorbed on the plate surface or hole wall, this time will be accompanied by the roughness in the hole. Dissolving or accelerating: the bath solution is too long to appear turbid, because most of the dissolving solution is prepared with fluoroboric acid, so that it will attack the glass fiber in FR-4, causing the silicate and calcium salt in the bath to rise. In addition, the increase of the copper content and the amount of dissolved tin in the bath will cause the production of copper particles on the board surface.

The copper sinking tank itself is mainly caused by the excessive activity of the tank liquid, the dust in the air stirring, and the large amount of suspended solid particles in the tank liquid. You can adjust the process parameters, increase or replace the air filter element, filter the whole tank, etc. Effective solution. The dilute acid tank for temporarily storing the copper plate after the copper is deposited, the tank liquid should be kept clean, and the tank liquid should be replaced in time when it is turbid. The storage time of copper immersion board should not be too long, otherwise the board surface will be easily oxidized, even in acid solution, and the oxide film will be more difficult to deal with after oxidation, so that copper particles will also be produced on the board surface.

The copper particles on the surface of the board caused by the copper sinking process mentioned above, except for the surface oxidation, are generally distributed on the board surface more uniformly and with strong regularity, and the pollution generated here will cause no matter whether it is conductive or not. The production of copper particles on the surface of the electroplated copper plate can be treated with some small test boards step by step and individually processed for comparison and judgment. For the on-site faulty board, it can be solved with a soft brush and light brush; graphics transfer process: there is excess glue in the development (very thin residual film It can also be plated and coated during electroplating), or it is not cleaned after development, or the plate is placed for too long after the pattern is transferred, which causes the plate surface to be oxidized to different degrees, especially when the plate surface is poorly cleaned or stored When the air pollution in the workshop is heavy. The solution is to strengthen the water washing, strengthen the plan and arrange the schedule, and strengthen the acid degreasing intensity.

3. Electroplating pits

There are also many processes caused by this defect, from copper sinking, pattern transfer, to electroplating pretreatment, copper plating and tin plating. The main cause of copper sinking is the poor cleaning of the sinking copper hanging basket for a long time. During microetching, the pollution liquid containing palladium copper will drip from the hanging basket on the surface of the board, causing pollution. Pits. The graphics transfer process is mainly caused by poor equipment maintenance and developing cleaning. There are many reasons: the brush roller suction stick of the brushing machine contaminates the glue stains, the internal organs of the air knife fan in the drying section are dried, there is oily dust, etc., the board surface is filmed or the dust is removed before printing. Improper, the developing machine is not clean, the washing after development is not good, the defoamer containing silicon contaminates the board surface, etc.

Pre-treatment for electroplating, because the main component of the bath liquid is sulfuric acid, whether it is acidic degreasing agent, micro-etching, prepreg, and the bath solution. Therefore, when the water hardness is high, it will appear turbid and pollute the board surface; in addition, some companies have poor encapsulation of hangers. For a long time, it will be found that the encapsulant will dissolve and diffuse in the tank at night, contaminating the tank liquid; these non-conductive particles are adsorbed on the surface of the board, which may cause electroplating pits of different degrees for subsequent electroplating.

4. The PCB surface is whitish or uneven in color

The acid copper electroplating tank itself may have the following aspects: the air blast tube deviates from the original position, and the air is not uniformly stirred; the filter pump leaks or the liquid inlet is close to the air blast tube to inhale air, generating fine air bubbles, which are adsorbed on the PCB surface or the edge, Especially at the side of the line and the corners of the line; in addition, there may be inferior cotton cores used, and the treatment is not thorough. The anti-static treatment agent used in the cotton core manufacturing process contaminates the bath solution and causes plating leakage. This situation can be Increase the aeration and clean up the liquid surface foam in time. After the cotton core is soaked in acid and alkali, the color of the board surface is white or uneven: mainly due to polishing agent or maintenance problems, and sometimes it may be cleaning problems after acid degreasing., Micro-etching problem. Misalignment of the polisher of the copper cylinder, serious organic pollution, and excessively high bath temperature may all be caused.