Question: We have acquired a customer who manufactures small baskets from brass wire mesh. Initially a small series was sampled. Unfortunately, we failed to provide detailed pictures or an initial sample test report. The customer was very satisfied with the sampling and sent us further orders, which did not indicate any problems or cause for complaint. This has now changed. The reason was gray solder joints, which we suspect were not treated with nickel. We very rarely have to deal with soft solder. The parts are alkaline and electrolytically degreased, activated in a mixed acid solution and then high-gloss nickel-plated. The rejected parts were electrolytically nickel-plated, blasted and nickel-plated again - without success. We also applied various solders (leaded, non-leaded) to a hull cell sheet and attempted to nickel plate it in the hull cell. The high current density areas were more or less plated, while the low current density areas did not accept anything. Both solders could not be electroplated properly. The problems with the customer parts mainly occur in the low current density range. Do you have any advice on how to electroplate soldered brass properly and reliably?
Answer: Your situation, where initial sampling and first orders work very well, but difficulties arise later, unfortunately occurs frequently in electroplating technology. There are several reasons for this.
Problematic initial sampling
Particularly during initial sampling, all process steps are carried out very thoroughly; after all, the aim is to make the technical feasibility a success. Problems occur later, mainly for four reasons:
- Sloppiness,
- cost-cutting measures,
- shorter dwell times,
- changed conditions due to larger batch sizes.
These points not only affect electroplating, but also all the steps that take place beforehand. In relation to the initial state, point 1 can cause problems from several sources. For example, the use of too little flux, too much solder or simply insufficient time.
Point 2 can lead to a cheaper, perhaps unsuitable solder being used. Lead and many other alloys should not cause problems, but it is known that bismuth and especially indium can lead to problems.
The third point is very well known from the coating of steel - but not only there. In the past, manufactured parts were often kept in storage for several weeks before being transported to an electroplating shop. These parts were often much easier to coat than freshly manufactured parts. It is quite conceivable that such effects also play a role here. This will be discussed in more detail in a moment.
Sampling is often carried out in a manual electroplating shop, as are smaller batch sizes. From a certain number of pieces, a switch is made to a system. Here, it is often not only the condition of the baths that is different, but also the suspension, resistors, anodes and their distribution and, of course, the overall surface. According to the photo (available to the editors), the correct surface area does not appear to be easy to calculate. Inaccuracies do not have the same effect with, for example, three parts as with thirty parts.
Solder to brass connection
Generally speaking, the electrical conditions will be different for solder than for the base material. This is of course due to the alloys on the one hand and the connection on the other. Experience has shown that soldered joints improve when they are aged. The lower the current density, the greater the difference between the base material and the solder during electroplating. This is exactly the effect you are experiencing. You would have to test how strong this effect is in your case on the parts themselves. To do this, carry out current flow and resistance measurements on individual parts and on the frame. How much current reaches the solder and how much reaches the brass just before it? Depending on how strong the differences are, this can also have an influence on the electrolytic degreasing. This can result in dark, wipeable layers on the solder joints.
To improve the connection and thus the current flow, you should age the parts in an oven. It is best to start with 70 °C and a dwell time of four to eight hours. You can then repeat your measurements and electroplate again if there is an improvement. If necessary, the surface should be briefly activated in a hydrochloric acid solution (1:1) before electroplating.
Once you have found the optimum time at 70 °C, you can carry out another test at 80 °C and half the time. According to the RGT rule, this should lead to a similar result [1].
Pre-coatings
The deep scattering of high-gloss nickel baths is known to be comparatively poor. If the conductivity does not improve as a result of the described ageing process, you should - if possible - try to coat the solder joints sufficiently before applying the high-gloss nickel. It is difficult to say how well you can compare the Hull cell sheets with the actual parts. However, they may be suitable for initial preliminary tests in the laboratory. There are several possibilities:
- Matt nickel before bright nickel. The slightly better depth scattering may already be sufficient to nucleate the solder joints and ultimately to pre-nickel them sufficiently with a layer of approx. 3-5 µm.
- Nickel strike or nickel strike. Such baths have a much more even metal distribution [2]. A corresponding electrolyte would have the following composition:
200 g/L nickel chloride (NiCl2 - 6 H2O)
100 g/L nickel sulphate (NiSO4 - 7 H2O)
40 g/L boric acid (H3BO3)
0.2 - 0.8 g/L sodium lauryl sulphate (NaC12H25SO4)
as wetting agent
Temperature: 50 - 60 °C
Current density: 2 - 10 A/dm2
pH value: 2.5 - 4 - Cyanide copper plating. If you already have a suitable electrolyte in use, we recommend starting the experiments with it. All experience from technical papers assumes cyanide copper plating for the electroplating of soft solder.
- If this does not work, a trial with a bronze electrolyte would be advisable.
Matt pickling
Occasionally, a matt pickle is also recommended, which is applied before electroplating. The parts or test sheets are treated in this chromate-based pickle for 2-5 minutes at room temperature. The parts are then briefly immersed in a 1:1 hydrochloric acid solution to remove the passive film.
Two-part problem
It would be better to divide the problem into two parts: rework and new jobs.
The surface condition changes depending on what has already been done to the parts that are the subject of the complaint. Among other things, the surface becomes larger due to sandblasting, which leads to a lower current density with the same amount of current. This will even exacerbate your problem.
If the rework is a reasonable amount, it would really be worth considering saving it with cyanide copper plating - even if you are not currently using this in-house. As the parts appear to be relatively small, a fairly small volume would be sufficient.
For new orders, however, the question arises of a generally improved process. Although the initial sampling has shown that coating is technically possible, practical experience has shown that this process does not work 100% reliably for you.
You will therefore not be able to avoid the listed preliminary tests and further coating tests on the customer parts. In principle, your initial sampling starts all over again.
LINKS
[1] https://www.galvanotechnik-for-you.de/galvano-rechner/?seite=alterungs-rechner
[2] https://www.galvanotechnik-for-you.de/uebersicht-kurse/die-galvanische-vernickelung/
