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August 2002

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From:
Guenter Grossmann <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 8 Aug 2002 12:56:56 +0200
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Victor

1) All plating systems have the aim of maintaining a surface with oxide layers that can be easily removed during the soldering operation. For that reason copper has to be kept protected from oxygen. One plating is nickel- gold. There are two problems associated with this system:
- Nickel forms a very stable oxide layer
- Gold reacts fast with copper even at room temperature and forms intermetallic layers that are not solderable.
This means, you are protecting the copper with gold but you need a barrier layer of nickel between the gold and the copper.

2) There is no necessity to use gold when you plate copper with a tin- alloy. The flux of the solder materials is designed to break down the tin oxide. Besides, gold dissolver very fast in tin and a gold plating would be eaten away during the tin plating process

3) Actually I am not aware of a reason for that. One reason might be to prevent the formation of tin-copper intermetallics during the plating process. Another reason could be that the copper surface was originally nickel- gold plated and re-tinned later on.

Best regards


Guenter

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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