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Intermediate Growth The tin plating can react with the underplating or substrate to form an intermetallic that grows as a function of time and temperature. An intermetallic forms when the plating and the underplating (or substrate) interdiffuse. The materials mix and react chemically to form new compounds -- principally Cu3Sn and Cu6Sn5 for a tin-copper interface and Ni3Sn4 and NiSn3 for a tin nickel interface. The intermetallic grows by continued diffusion and reaction. Copper-tin intermetallics grow in a "lumpish" manner, with high peaks pushing upward. Nickel-tin intermetallics grow as tendril-like platelets, with each tendril snaking upward toward the surface.
Eventually an intermetallic grows to the surface of the plating, where it oxidizes. An oxidized intermetallic layer will not solder.
Intermetallics from more rapidly with a copper substrate than with a nickel underplating. They also form faster at higher temperatures. At low temperatures, lead added to the tin slows intermetallic growth. At high temperatures, lead can actually accelerate growth with a copper substrate. At 170 degrees C, for example, a mixture of 40% lead/60% tin exhibits faster intermetallic growth than pure tin.
For a nickel underplating, intermetallic growth is reduced by 10% lead and essentially eliminated by 40% lead in the tin.
The characteristics of intermetallic growth suggest that growth can be minimized by:
- Using tin-lead plating with a 60:40 tin-to-lead ratio
- Using a nickel underplating between the copper and tin
- Storing components at room temperature
Surface Contamination Oxides and other contaminants formed at room temperatures are easily removed by flux and do not affect solderability. Elevated temperatures, high humidity, and corrosive gasses can lead to excessive surface films that degrade solderability. Again storage conditions are important.
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Intermetallic growth, a leading cause of solderability failures, depends both on the composition of the tin plating and on storage temperatures. |
Carbon Accumulation Excessive carbon from brightness used in the plating bath also leads to solder failures. Accumulating at the intermetallic boundary, the carbon causes dewetting on nonwetting when the part is soldered. However, careful control of the brightener levels allows bright tin plating to offer the same solderability as unbrightened matte plating.
To prevent soldering failures, storage time and temperatures must be carefully considered, as does the plating type or thickness. Avoid elevated temperatures; room temperatures suffice to achieve maximum shelf life.
Be sure the plating is thick enough (see chart) for expected storage times. What's important is minimum plating thickness, not average thickness. Intermetallics will reach the surface faster and oxidize into unsolderable spots faster through a thin point in the plating.
Getting maximum shelf life from tin-plated components is easy to achieve once you understand the influencing factors.
Use these telephone and FAX numbers to get more information.
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