PCB Tech - Common problems and solutions of PCB copper plating technology

Common problems and solutions of PCB copper plating technology

PCB copper electroplating is the most widely used pre-plating layer to improve the bonding force of the plating. The copper plating layer is an important part of the protective decorative plating copper/nickel/chromium system. The flexible copper plating layer with low porosity is suitable for It plays an important role in improving the adhesion and corrosion resistance between coatings. Copper plating is also used for local anti-carburization, printed circuit 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 by copper electroplating technology in PCB production and their solutions.

Acid copper electroplating common problems

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 more difficult processes for process control in many circuit board factories. Common problems in acid copper electroplating mainly include the following:

1. Rough plating;

2. Plating (board surface) copper particles;

3. Electroplating pits;

4. The PCB surface is white or uneven in color.

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 corners of the PCB board are rough, and most of them are caused by the high plating current. You can reduce the current and check whether the current display is abnormal with the card meter; the whole board is rough and usually does not appear, but the author has encountered it once in the customer. At the time of the investigation, the temperature was low in winter and the content of brightener was insufficient; and sometimes some reworked faded boards were not cleanly treated, and similar conditions occurred.

2. PCB plating copper particles

There are many factors that cause the production of copper particles on the PCB 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, PCB board surface copper particles caused by copper sinking.

The 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 in 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 situations 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, other quality failures may be caused; excessively high copper content in the micro-etching bath or low temperature may cause slow precipitation of copper sulfate crystals; and the bath 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 dispose of after oxidation, so that copper particles will be produced on the board surface. The copper particles on the PCB surface caused by the circuit board copper sinking process mentioned above, except for the oxidation of the PCB surface, are generally distributed on the PCB surface more evenly, with strong regularity, and the pollution generated here is no matter if it is conductive. Whether or not, it will cause the production of copper particles on the surface of the electroplated copper plate. During processing, some small test boards can be used to process separately for comparison and judgment. For on-site faulty boards, a soft brush can be used to solve the problem; graphics transfer process: there is excess glue in the development ( Very thin residual film can also be plated and coated during electroplating), or it is not cleaned after development, or the board is placed for too long after the graphics is transferred, resulting in varying degrees of oxidation on the board surface, especially the PCB board surface Under poor cleaning conditions or when the air pollution in the storage workshop is heavy. The solution is to strengthen the water washing, strengthen the plan and arrange the schedule, and strengthen the acid degreasing intensity.

The acid copper electroplating bath itself, at this time, its pre-treatment generally does not cause copper particles on the board surface, because non-conductive particles can at most cause leakage or pits on the board surface. The reasons for the copper particles on the plate surface caused by the copper cylinder can be summarized into several aspects: the maintenance of bath parameters, the production and operation, the material and the process maintenance. The maintenance of bath parameters includes too high sulfuric acid content, too low copper content, low or too high bath temperature, especially for circuit board factories without a temperature-controlled cooling system. At this time, the current density range of the bath will decrease, according to normal The operation of the production process may produce copper powder in the bath, which is mixed into the bath.

In terms of production operation, excessive current, poor splint, empty pinch points, and the plate dropped in the tank against the anode to dissolve, etc. will also cause excessive current in some plates, resulting in copper powder, falling into the tank liquid, and gradually causing copper particle failure ; The material aspect is mainly the phosphorus content of the phosphor copper corner and the uniformity of the phosphorus distribution; the production and maintenance aspect is mainly large processing, and the copper corner falls into the tank when adding, mainly for large processing, anode cleaning and anode bag cleaning, many circuits The board factories are not handling well, and there are some hidden dangers. For copper ball treatment, the surface should be cleaned, and the fresh copper surface should be micro-etched with hydrogen peroxide. The anode bag should be soaked with sulfuric acid hydrogen peroxide and lye successively to clean, especially the anode bag should use a 5-10 micron gap PP filter bag. .

3. Plating pit

There are 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 of the brushing machine is contaminated with 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 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 of electroplating, because the main components of the bath liquid are sulfuric acid, whether it is acidic degreasing agent, micro-etching, pre-soaking, and the bath liquid. Therefore, when the water hardness is high, it will appear turbid and contaminate the board surface; in addition, some companies have poor glue encapsulation. For a long time, it will be found that the encapsulation 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 in different degrees for subsequent electroplating.