Anti-Tarnish Products

In the July 2000 column, a reader was asking for information on a product called Pasa-Jell 107. I was not familiar with that particular product but took a guess as to what it might be. I asked anybody familiar with it to send me more information. I would like to thank the readership from the aerospace community for a very solid response. Pasa-Jell 107 is a “blend of mineral acids, activators and inhibitors.” There are two versions of it. One is a gel allowing it to be placed onto localized areas, and the other is a liquid that will allow immersion of whole parts. Its purpose is to provide an etch to titanium alloys prior to bonding. Pretreatment should include alkaline cleaning and fine, dry aluminum oxide blast (preferred) or sanding with a fine grit aluminum oxide sandpaper. Many thanks to the readers for pointing this out to me.

Anti-Tarnish Products

Question:

How do we prevent tarnish from forming on stamped copper lead frames? The lead frames are stamped out of oxygen-free copper and shipped to a molding company, which molds the stampings into a plastic part, which is then shipped to the assembling plant. The stamper swears parts are tarnish-free when shipped. The molder says parts come in tarnished and they have to sort them. The molder swears the parts are good when shipped. Incoming plant inspection says the parts come in tarnished and they have to sort them. The plant has to ultrasonically weld conductors to copper lead frames. How can the finger pointing be stopped? What are the proper ways to ship parts? Are there anti-tarnish products that could be placed inside the shipping carton boxes? P.K.

Answer:

You do not mention that the parts are cleaned. Some lubricants may cause tarnishing if left on the parts over time, especially if they contain sulfur or sulfonated additives. The first step would be to clean the parts in a mildly alkaline detergent, rinse, then provide an anti-tarnish treatment to the parts. There are specially formulated process chemicals that minimize or eliminate tarnish on copper and copper alloys. All are based on triazoles (usually benzotriazole and/or tolytriazole). Ask your chemical supplier for such an inhibitor.

Another approach would be to place an anti-tarnish product in your shipping container as you mentioned. These products are called vapor phase inhibitors (VPI) or vapor corrosion inhibitors (VCI) and come in several different forms. There are companies that make various packaging products that are impregnated with a vapor phase inhibitor. They may come in the form of papers, plastics, corrugated cardboard, sponges and other devices that can be placed in the container. It is important that the inhibitor be placed in an airtight enclosure in order to make it effective (the inhibitor will vaporize and drift outside the container and be ineffective otherwise). Here again, it is important that the parts be cleaned prior to packing and exposure to the inhibitor. I found a web site, www.packagingbusiness.com, that may assist you in finding suppliers. Perform a search using the keyword “corrosion.”

Both methods are very effective and adhere to the surface only a few molecules thick. This extremely thin layer will not interfere with the ultrasonic welding later in the process.