QUESTION:

BGA Solder Ball Shelf Life I believe BGA Solder Balls are a homogeneous mass of metals, why is the shelf life only one year? With proper handling and storage, can the shelf life be extended?

Anonymous

Answers:

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 it's 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

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

Answers:

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.

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There are no special storage conditions required for solder spheres other than a clean, dry environment (dry meaning that the spheres should be regards as a MSL device). Typical room temperature changes do not affect the quality of the parts.

The product should be stored in sealed containers and should not be refrigerated. Probing of spheres in jars with fingers or other implements can damage the material by changing its shape or scoring the sphere surface or by contaminating the material. This could result in sphere placement difficulties. Shelf life of all solder spheres, stored in sealed ESD jars that are not agitated, is 1 year. Most manufactures specify these in the datasheet.