Silver plating on copper plating

A. You did not mention where the blistering occurs: from the Al or from the Cu. If the blisters separate from Al, cleaning and preparation should be reviewed, however, even with good preparation, if the copper thickness is insufficient to prevent near complete diffusion into the silver, blistering will occur. My experience tells me that a minimum of 0.003" of copper is required if any heating process such as soldering etc. is used. If the blisters are from the silver separating from copper, the problem is likely in the silver strike. (too much silver, low cyanide, low amps. contaminated solutions or rinses) chemical replacement deposits of silver on cu have little adhesion. Parts should go into the copper with current on (contact prior to entering the tank.)

A. Hi, Rocky. Acid copper will not adhere to steel and must be preceded by a copper cyanide, copper pyrophosphate, or nickel strike. Good luck.

Regards,

A. You should certainly use a copper cyanide strike when plating onto steel and even more so with Invar. You should also use a live entry to prevent any risk of an immersion deposit. Silver will diffuse into the copper, but the rate will be dependent on the operating temperature. In an ideal world, a barrier layer should be used and historically nickel has been the easiest option, but nowadays other ones, such as white bronze or palladium are also used. You can also use platinum, but this is very expensive at present. Although not relevant here, the use of nickel as a barrier layer is banned in anything that will come into contact with human skin for any prolonged period

A. Rocky

My first point is that it is impractical to advise on a plated finish without knowing what is its purpose.
Is it for corrosion resistance, solderability, a low friction surface? I will guess that you have a contact of some kind.
Ted is quite right, over steel you need a copper cyanide strike but it unusual to quote a thickness for this. It is followed immediately by copper plating and the spec thickness is the total. (It is not usual to quote the strike in the spec, the plater considers this part of the plating prep and may not be needed if the main plating bath is suitable). Remember, copper is soft. If you have a high pressure contact then steel/nickel/silver may be considered.
If a barrier layer is needed to prevent diffusion it will be nickel (forget the exotic suggestions in the real world)
If the part can be electroplated with copper then it can be nickel plated. Electroless nickel is an expensive option and offers no advantage here.
Silver is often used to give a solderable surface or as a "cheap" alternative to gold for connectors (but requires contact pressures to be increased)
Lastly, the term "Flash" is common in the jewelry trade and is taken to mean the minimum thickness which will give an acceptable cosmetic finish i.e., almost nothing! It should never be used in an engineering specification - just state the thickness.
If you use Invar, the question is why plate?

A. Hi Steve. I assume that the purpose of the copper and silver plating is as a break-in coating, rather than as a heat-treating stop-off coating? If so, I doubt that the thickness matters (see Geoff Smith's response above), and that it should be held to a minimum (complete coverage and done). But we'll see if a more knowledgable reader can add anything.

Regards,