Copper sulfate electroplating plays an extremely important role in PCB electroplating. The quality of acid copper electroplating directly affects the quality and related mechanical properties of the electroplated copper layer on PCB board, and has a certain impact on the subsequent processing. Therefore, how to control the quality of acid copper electroplating is an important part of PCB electroplating, and it is also one of the difficult processes in many large factories. According to the author’s experience in electroplating and technical service for many years, the preliminary summary is as follows, hoping to inspire electroplating industry in PCB industry. Common problems of copper sulfate electroplating mainly include the following:
Electroplating roughness;
Electroplating (surface) copper particles;
Electroplating pits;
White or uneven color.
In view of the above problems, some summaries are made, and some solutions and preventive measures are briefly analyzed.
Electroplating roughness:
Generally, the plate angle is rough, most of which is caused by the large electroplating current. You can lower the current and check whether the current display is abnormal with the card meter. The whole board is rough and generally does not appear, but the author once met it at the customer’s office. Later, it was found that the temperature was low in winter and the content of brightener was insufficient. Sometimes, a similar situation will occur when the surface of some reworked stripping boards is not cleaned.
Electroplated copper particles:
There are many factors that cause copper particles on the board surface, including copper deposition and pattern transfer, and electroplating copper on PCB itself is possible. The author met the copper particles on the board caused by copper deposition in a state-owned factory.
1. Copper particles on the board surface caused by copper deposition process may be caused by any copper deposition treatment step.
1) Alkaline degreasing will not only cause the surface roughness but also the hole roughness when the water hardness is high and the drilling dust is too much (especially the double panels are not subjected to glue removal slag) and the filtration is not good, but generally it will only cause the hole roughness, and the slight pitting dirt micro-etching on the surface can also be removed.
2) There are several kinds of micro-etching:
The micro-etching agent hydrogen peroxide or sulfuric acid used is of poor quality or ammonium persulfate (sodium) contains too many impurities, so it is generally recommended that it should be at least CP grade, and industrial grade will cause other quality failures. High copper content or low air temperature in micro-etching groove leads to slow precipitation of copper sulfate crystal; Liquid turbidity, pollution: a. Most of the activation liquid is caused by pollution or improper maintenance, such as leakage of filter pump, low specific gravity of the tank liquid and high copper content (the activation cylinder has been used for too long, more than 3 years), which will produce granular suspended matter or impurity colloid in the tank liquid, which will be adsorbed on the board surface or the wall of the hole. At this time, the roughness in the hole will be produced. B. Degelatinization or acceleration: the bath solution is turbid after too long use, because most of the degumming solutions are prepared with fluoroboric acid, which will attack the glass fiber in FR-4, resulting in the increase of silicate and calcium salt in the bath solution. In addition, the increase of copper content and dissolved tin content in the bath solution will cause copper particles on the board.
3) Copper precipitation tank itself is mainly caused by excessive activity of tank liquid, dust caused by air stirring, and many small particles suspended by solid in tank liquid. It can be effectively solved by adjusting process parameters, adding or replacing air filter elements, and filtering the whole tank.
4) After copper deposition, temporarily store the dilute acid tank of copper deposition. The tank liquid should be kept clean, and it should be replaced in time when the tank liquid is turbid.
5) The storage time of copper deposit should not be too long, otherwise the surface of copper deposit will be easily oxidized, even in acidic solution, and the oxidized film will be more difficult to dispose of, so that copper particles will also be produced on the surface of copper deposit.
The copper particles on the surface of the copper plate deposited in the above-mentioned copper deposition process are generally distributed evenly and regularly on the surface, except those caused by oxidation of the surface, and the pollution generated here will cause the copper particles on the surface of the electroplated copper plate of the PCB board, whether it is conductive or not. Some small test plates can be used for step-by-step treatment and comparison judgment during treatment, and the on-site fault plates can be solved by soft brushing.
2. Graphic transfer process: there is excess glue after development (the extremely thin residual film can be plated and coated during electroplating), or it can’t be cleaned after development, or the board is left for too long after graphic transfer, resulting in different degrees of oxidation of the board surface, especially when the board surface is poorly cleaned or the air pollution in the storage workshop is heavy. The solution is to strengthen water washing, plan and schedule, and strengthen acid degreasing intensity.
3. Acid copper electroplating bath itself, at this time, its pretreatment will generally not cause copper particles on the board surface, because non-conductive particles will cause plating leakage or pits on the board surface at most.
4. The causes of copper particles on the surface of copper tank can be summarized into several aspects: maintenance of bath parameters, production operation, materials and process maintenance: maintenance of bath parameters includes too high sulfuric acid content, too low copper content, low or too high bath temperature, especially in factories without temperature control cooling system, which will cause the current density range of bath liquid to decrease. According to normal production process, copper powder may be produced in bath liquid and mixed into bath liquid; In the production operation, excessive current, bad clamping plate, empty clamping point, plate falling from the tank and dissolving against the anode, etc. will also cause excessive current of some plates, resulting in copper powder falling into the tank liquid and gradually causing copper particle failure; The material is mainly the problem of phosphorus content and distribution uniformity of phosphorus. In terms of production maintenance, it is mainly the big treatment, when the copper corner is added, it falls into the tank, mainly during the big treatment, anode cleaning and anode bag cleaning, which are not handled well in many factories, and there are some hidden dangers. Copper ball treatment means that the surface should be cleaned, and the fresh copper surface should be etched with hydrogen peroxide. The anode bag should be soaked in hydrogen peroxide sulfate and alkali liquor successively, and cleaned, especially the anode bag should be PP filter bag with a gap of 5-10 microns.
Electroplating pits:
There are many processes caused by this defect, from copper deposition, pattern transfer, to pretreatment of electroplating, copper plating and tin plating.
1. The main cause of copper deposition is the poor cleaning of the copper-deposited hanging basket for a long time. During micro-etching, the polluted liquid containing palladium and copper will drip from the hanging basket on the board surface, causing pollution, which will lead to spot leakage plating, that is, pits, after the copper-deposited plate is electrified.
2. The graphic transfer process is mainly caused by poor equipment maintenance and development and cleaning. There are many reasons: glue stains are polluted by the brush roller suction stick of the brush machine, internal organs of the air knife fan in the drying section are blown dry, there are oily dust, etc., improper film pasting or dust removal before printing on the board surface, poor development of the developing machine, poor washing after development, and silicon-containing defoamer pollutes the board surface.
3. Pretreatment before electroplating, because the main component of bath solution is sulfuric acid, whether it is acid degreaser, micro-etching, pre-immersion, so when the water quality is high in hardness, it will be cloudy and pollute the board surface; In addition, some companies’ hangers are poorly coated, and it will be found that the coated rubber dissolves and spreads in the night of the bath, polluting the bath liquid; These non-conductive particles are adsorbed on the surface of the plate, which may cause plating pits in different degrees for subsequent plating.
4. The copper electroplating bath itself may be in the following aspects: the air tube deviates from the original position, and the air is unevenly stirred; When the pump leaks air or the liquid inlet is close to the air drum, fine air bubbles are generated, which are adsorbed on the board surface or the line edge, especially at the horizontal line edge and the line corner; In addition, there may be some problems, such as the use of inferior cotton core, incomplete treatment, the anti-static treatment agent used in the manufacturing process of cotton core pollutes the bath solution, resulting in plating leakage. In this case, it is possible to increase air blowing and clean the foam on the liquid surface in time. After the cotton core is soaked in acid and alkali, the color of the board surface is white or uneven: it is mainly due to the problem of brightener or maintenance, and sometimes it may be the problem of cleaning after acid degreasing and micro-etching.
5. The imbalance of brightener in copper cylinder, serious organic pollution and high temperature of bath solution may all be caused. Generally, acid degreasing will not cause cleaning problems; however, if the PH value of water quality is acidic and there are many organic substances, especially recycling water washing, it may cause poor cleaning and uneven micro-etching. Micro-etching mainly considers that the content of micro-etching agent is too low, the content of copper in micro-etching solution is too high, and the temperature of bath solution is too low, which will also cause uneven micro-etching on the board. In addition, the water quality of the cleaning water is poor, the washing time is a little longer or the pre-soaking acid is polluted, and the treated board surface may be slightly oxidized. When electroplating in the copper bath, because it is acidic oxidation and the board is charged into the bath, the oxide is difficult to remove, which will also cause uneven color of the board surface. In addition, such defects can also be caused by the contact of the plate surface with the anode bag, uneven anode conductivity, anode passivation and so on.
