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February 9, 2009

Oxidation on Immersion Silver PCB's

We are processing printed circuit boards that have an Immersion Silver finish and are seeing oxidation that is interfering with solderability. The overall thickness of the boards is 0.32".

Are there specific processes on how to handle Immersion Silver PCB's that would help us avoid oxidation?



R. N.

Experts Comments

Silver naturally tarnishes in open air, especially with chemicals present like in a PWB shop. There are a few recommendations that give to our customers in order to prevent tarnish of immersion silver finishes.

  1. Use an antitarnish bath as a finishing step in the immersion silver process. Several chemical suppliers have this option available, including FCT.
  2. Make sure that final rinses in the immersion silver process are clean DI water. Contaminates in the final rinses promote silver tarnish.
  3. Make sure to thoroughly dry the silver surface, leaving no traces of moisture.
  4. Package the boards as soon as possible after immersion silver coating. Be sure to use sulfur free, air-tight, plastic bags. Exposure to the air in a PWB shop must be minimized. Some customers even use vacuum packaging for immersion silver boards.
  5. Store silver coated boards at room temperature, and in low humidity environments.
  6. Once the package is opened, process the boards through assembly as quickly as possible. Minimize time in open air.
  7. The reflow and wave solder processes will expose the silver to corrosive chemistries (fluxes). This may induce tarnish. It may not be possible to prevent this, with the exception of changing fluxes and/or solder paste chemistries.
  8. If tarnish cannot be prevented, then chemical suppliers offer tarnish removers, including FCT.

These tarnish removers are designed to be used through immersion of the boards. It may not be advisable to use them after components have been soldered in place.

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Mike Scimeca
President
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

To remove oxidation or other contamination from the surface of your Immersion Silver finish, try using a low-pressure gas plasma treatment. Typically, you would use one of the following methods to accomplish this:

  1. Use Argon gas and try to "sputter" (physically remove) the oxide,
  2. Use Hydrogen gas (typically, in concentrations of <10% mixed with Nitrogen, called "forming gas") to chemically reduce the oxide (i.e. Hydrogen in the plasma and Oxygen in the oxide react to form H2O, leaving pure metal behind), or
  3. Use a precise mix of these gasses in order to accomplish simultaneous sputtering and reducing, depending on the thickness and composition of the oxide layer.

Note that the oxide contamination may be a result of something in the Silver plating process itself, or some material migrating through the Silver layer (such as Copper or Nickel) and then oxidizing upon exposure to the air.

Fortunately, the gas plasma treatment mentioned above will remove the oxide/contamination regardless of the underlying source. And once the oxide/contamination is removed, adhesion will be greatly enhanced. Contact gas plasma system suppliers to see if they will run samples for you—most will do so at no charge in order to demonstrate the effectiveness of this gas plasma cleaning process.

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Scott D. Szymanski
Global Marketing Manager
Nordson MARCH
Mr. Szymanski works to expand strategic alliances, strengthen partnerships with equipment suppliers, and develop future product offerings tailored to the semiconductor market.
NOTE: Mr. Szymanski is no longer working at Nordson MARCH
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