Copper electroplating with copper sulfate solution plays an extremely important role in PCB manufacturing. The quality of copper electroplating directly affects the quality of the copper layer and related mechanical properties, and has certain influence on subsequent processing.
Therefore, how to control the quality of copper electroplating is an important part of the PCB plating technique, which is one of the difficult processes even in many veteran PCB factories.
The Common Problems in Copper Electroplating
1. Rough Copper Electroplating Surface
Generally, the rough board angle is mainly caused by the large electroplating current, which can be diminished by reducing the current. But the situation that the whole the copper electroplating surface is rough is rare.
2. Copper Particles
The copper particles could be generated by many factors, it is possible in each process such as PTH, pattern transfer, even copper electroplating itself.
The copper particles on the surface might be caused by each step of PTH.
- It will not only cause rough surface but also cause a rough wall of the holes when alkaline degreaser has a high water hardness while there remains a lot of drilling dust.
- In micro-etching, bad quality hydrogen peroxide, sulfuric acid or the ammonium persulfate (sodium) containing too many impurities may cause the issue, at least CP-level generally recommended. In addition to this, the industrial-grade agent will cause other quality failures.
- The copper content in the micro-etching tank is too high, producing granular suspension or impurity colloid in the solution, which will adsorb on the surface of the PCB or the wall of the hole, while leading to a rough hole wall.
- The solution will be turbid after long-time use, because most of the debonding solution is now prepared with fluoroboric acid, which will attack the glass fiber in FR-4, causing the increasing of silicates, calcium salts. In addition, the increase in copper content and the amount of dissolved tin in the solution will cause copper particles on the surface.
- In the PTH process, the activity of the solution liquid is too strong, the dust in the air and the small particles suspended in the solution are the causes of copper particles on the PCB surface.
We can adjust the parameters in the PTH process and employee an air filter to eliminate it. After the PTH process, the dilute acid tank for temporarily storing the boards should be clean, the solution liquid should be replaced when it is turbid. The storage time of the bare boards should not be too long, otherwise, the surface will be easily oxidized, and the copper particles get generated.
In the pattern transfer process, the residual film glue, the uncomplete cleaning after development, or the long-term placement after the pattern transfer, may cause surface oxidized, which lead to the copper particles. The solution is to strengthen the washing and the manufacturing scheduling, and strengthen the acid degreasing.
The reasons of copper particles in the copper electroplating are probably summarized into several aspects: the maintenance of the solution, the production operation, and the processing maintenance.
- In the maintenance of solution: Including high sulfuric acid content, low copper content, low or high temperature, especially in plants without a temperature-controlled system, a conventional operation may generate the copper powder in the solution
- In terms of production operation: The current is too large, the fixtures are defective, then the copper powder might get generated, drop into the solution and cause copper particles.
- In terms of processing maintenance: Many factories have poor handling of anode cleaning and anode bag cleaning. The anode bag should be immersed in sulfuric acid hydrogen peroxide and lye, and be cleaned. Especially for the anode bag, a 5-10 micron gap PP filter bag should be used.
3. Plating Pit
There are many reasons for this defect, from PTH, pattern transfer, to pre-plating treatment, copper electroplating, and tin plating.
- The contaminated liquid containing palladium-copper will drip from the hanging kits on the surface of the board in micro-etching, causing pollution, if the hanging kits do not get cleaned for a long time.
- The equipment is not maintained and the cleaning after development is poor.
- Some plants employed low-quality hang fixtures. After a long time use the bonding adhesive on the fixtures will dissolve and diffuse in the tank night, polluting the solution.
All of these non-conductive particles that adsorb on the board surface may cause different degrees of plating pits in the subsequent plating.
4. Whitish or Uneven Color Surface
In the copper electroplating, the causes of this problem may be that the air tube has deviated from the original position and the air is not evenly stirred, or the filter pump leaks, or the liquid inlet is close to the air tube to take in air. Another possibility is using an inferior cotton core, the anti-static treatment agent used in the cotton core manufacturing process pollutes the solution liquid, causing incomplete plating.
Some of the common problems in the copper electroplating process summarized in this article actually happened in the many PCB plants, which have practical reference value and hope it help.