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Very few of or customers have even heard of an Intermetallic Formation even though it is believed to be the most common cause for solder-ability failure. This is an issue that affects not only the assembly houses, but brokers, franchised distributors and even component manufacturers. 

Component leads using tin alloy in their plating with copper or nickel alloy substrate have a usable shelf life before the tin loses its solder-ability. This is caused by the tin (Sn) and the substrate copper (Cu) or nickel (Ni) undergoing a chemical reaction and reforming most commonly into Cu3Sn or Cu6Sn5 for copper substrates and NiSn3 or NI3Sn4 for nickel substrates. When these new compounds reach the surface and oxidize, they loose solder-ability.

Elevated heat accelerates the formation/oxidation of these inter-metallics and the determining factor for shelf life is how thick the tin plating is as well as the composition of the plating. The latest information about lead free solders can be found here => http://www.boulder.nist.gov/div853/lead%20free/solders.html .

Generally, when we are sent parts for baking, we bake without question. We have to believe you know what you are doing. Recently we have been asked to warranty the solder-ability of components we bake. Since we don’t know if components are solderable when we receive them, we can’t warrant it afterwards. Questions like this demonstrate that this is becoming a more commonly acknowledged problem. Unfortunately, the solution is not always cost effective and verifying a component is solderable should be done BEFORE you pay for them.

Components with date codes 2 years old and older probably have their outer tin plating depleted, or are close to it. If the intermetallic formation has not leached to the surface and oxidized, you can probably retin old components. If simple retinning won’t work, I would suggest contacting Kester Solder for a suggestion.

As a rule, we can certify the temperature and length of time we bake components for and that they are repackaged in accordance with IPC / JEDEC J-STD-033.

We have also discovered that with fine pitch leads that are 3 – 4 years old in need of lead refurbishing have an unusual challenge – the leads will spring back towards their original position in a day or so after straightening! This spring back is caused by the plate-like structures formed by the intermetallic layer. Leads with this condition can be straightened, but it takes several sessions since over correcting the lead can cause lead fractures.

Not long ago we refurbished some old QFP’s. Most of them were 5 years old and had to be reworked 6 or 7 times for them to stop springing back. Out of 42 we lost to lead breakage. The point being even components with an old oxidized intermetallic formation can be reworked and used, but preparation is expensive.

When possible, use new components. If you have to use old components, be careful. Get somebody from you QA or Inspection team to give you guidelines if necessary.

This gets ugly because a 60/40 Sn/Pb plating on a nickel substrate found in older components has a very low rate of intermetallic formation and could last for years. However, since these are being phased out as we go to Lead Free plating, buying older components becomes more risky.

From what we’ve seen in the real world, the typical manufacturer plates components with a 1 year shelf life in mind. This varies down to 6 months and up to 2 years. Best thing to do is check the manufacturers spec.

Our goal is to make your life simpler. That’s as we determine issues that affect our customers, even though they don’t really concern us, we share that knowledge with you. Your success is our success.

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Last modified: 06/21/05