Electroless Nickel Plating on Aluminium Alloy - Blistering

A customer of ours has electroless nickel plated onto an aluminium alloy extrusion (6063 - 0.5 % Si and 0.7 % Mg). The plating contractor is reported to use a zincate treatment and is now trying a double-zincate dip.

The EN blisters during a subsequent "solder reflow" process, and a post-plating heat treatment of 200 to 250 degrees C does not help. The blistering is more prevalent on - but not confined to - machined surfaces than on as-extruded. Any advice on how to avoid the blistering? I might add that I analysed, using EDX (x-ray energy spectrum), the surface of an EN foil that had been in contact with the substrate aluminium and had blistered off. No zinc was detected, nor was zinc found on the corresponding aluminium substrate area. Is this absence of zinc to be expected?

Thanks in advance.

Here some possible causes that produce blistering:

1-Contaminated plating bath.

2-High metal turn over.(M.T.O.)

3-Improper pretreatment

Actions:
1-Avoid zinc contamination using a Alkaline Stricke electroless before Plating.
2-Spent your EN bath at 3 MTO .
3- Use double zincate and a correct etching step, (fluoride -based)