BGA Solder Ball Shelf Life

 

August 24, 2015

I believe BGA Solder Balls are a homogeneous mass of metals, why is the shelf life only two years? With proper handling and storage, can the shelf life be extended?

V.P.

Experts Comments

 

The solder alloy itself should not change composition significantly over a period of several years. The main potential issue is oxidation of the surface of the BGA solder ball. Even when stored properly air will eventually make its way to the solder surface and oxidation will occur. As the oxide layer grows the balls will become more difficult to solder. The rate of oxidation is slow enough that the balls will be usable for at least two years, but may be difficult to use after that time.

Tony Lentz Field Applications FCT Assembly Mr. Lentz has worked for FCT Companies for 14 years as a Lab Manager and Facility Manager. Over the last 2 years he has worked in Field Application for FCT Assembly. He holds BS and MBS degrees in Chemistry.

The shelf life, as defined by the manufacturer, takes into account all aspects of the package, but let's stick to the solder ball characteristics. You are correct that the balls are a homogeneous alloy. So what might degrade after two years? Assuming that the parts are stored in MBBs compliant to J-STD-033, moisture will not play a role; the MBB and desiccant will protect against moisture ingress for approximately three years. The MBB does not, however, protect against oxygen, and the surface of the balls will oxidize over time. It is very likely that, after two years properly stored, solderability will be quite acceptable. It's just not guaranteed. Can we extend this time? The answer is, certainly we can, if we exclude oxygen. For long-term storage of parts for aerospace applications, where a program needs to be supported for a decade or more, we use nitrogen inerted cabinets, which both exclude oxygen and moisture. This eliminates surface degradation of parts. It does not, however, eliminate growth of intermetallic compounds, which will occur over time. IMC growth only impacts solderability when it grows through to the surface, however, which is something you won't see on BGAs. Bear in mind that any steps you take to extend solderability still don't affect the warranted period for solderability, only the practical life.

Fritz Byle Process Engineer Astronautics Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Unfortunately solder suppliers have no control how their products are handled once they leave the factory. Also, the products are shipped to a wide variety of locations and climates including humid areas as well as locations near salt water. With this knowledge, the solder has to protect against returns involving old and oxidized product. For this reason these forms of solder are assigned a shelf life. With that said, the assigned shelf life is often conservative, and the product, if stored and handled properly and used with a flux, are usable well beyond the stated shelf life. I often encourage the end user to establish an internal protocol to recertify or lengthen the shelf life of the product. This would typically involve simple wetting tests with fluxes and substrates representative of what is used in their actual products. Some solder suppliers offer testing to "recertify" the product. However, the fee for so doing, is often near or close to the cost of just buying new product.

Eric Bastow Senior Technical Support Engineer Indium Corporation Eric is an SMTA-certified process engineer (CSMTPE) and has earned his Six Sigma Green Belt from the Thayer School of Engineering at Dartmouth College. He is also a certified IPC-A-600 and 610D Specialist. He has an associate's degree in Engineering Science from the State University of New York and has authored several technical papers and articles.

Many BGA manufacturers have a standard 2-year shelf life for their products.  I have also seen others which recommend just one year. Proper handling and storage may extend the life of the BGA but it is not a predictor of its performance and reliability.  One way of predicting shelf life would be to expose the components to some type of accelerated aging process to see if there is a change in component's physical properties along with its performance within the circuit.

Edithel Marietti Senior Manufacturing Engineer iDirect Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

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