|Ask the Experts|
September 10, 2020
BGA Solder Ball Shelf Life
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?
|Expert Panel Responses|
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.
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.
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.
Technical Support Engineer
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.
Senior Manufacturing Engineer
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.
Process Engineering Manager - Electronics
Altech UEC, South Africa
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