I'm looking for a type of solder paste which is tested and found pass under 120 deg C operating condition for long hours. Can any one shed some light here?
Your question is oddly worded. Are you asking for a solder that can be used without degradation at up to 120 degrees C? Pretty much any solder that is not pure tin would work fine. (pure tin might grow whiskers at that temperature, especially if there is humidity too).
OK, let me rephrase this, I'm looking for a type of solder paste which can pass at least 500 hrs Hi temp operating life test in a system where the solder joint is subject to 120 degree C. Any recommendation?
Speak to Indium Corp or Multicore Solders. Both have excellent technical departments who will be able to give you advice specific to your application, including selection of a flux to match the combination of process, components, and solder alloy. Depending on volumes both will produce an engineered solder alloy fine-tuned to your requirements, or they may well have an off-the-shelf solution available.
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I've never seen a published "operating temperature" for any solder nor am I aware of something like a UL RTI temperature for solder itself...for components/plastics,etc..sure but never for solder.
Please let us know what you find out.. But yes a call to Indium or Kester would be a great place to start
60-40 lead tin solder will withstand 120 deg C all year long.
I would not want to use it in a structural application to hold any weight, as it will creep more at elevated temperatures. But for electronics, 60-40 has been used in military systems with storage requirements to 120C since Hector was a pup.
You might consider 63-37 tin lead solder, as it is a true eutectic solder mix, and elevated temperatures will not cause any of the metal to precipatate out (which might cause a cold solder joint over time).