Subjecting boards that have been assembled with Sn/Pb to a typical Pb Free thermal profile should, in principle, not result in questionable solder joints. But there are a number of considerations:- It is assumed that this was an assembly process using solder paste. The flux incorporated in Sn/Pb solder pastes is formulated to follow a particular thermal profile that causes certain process steps to occur sequentially, namely: evaporation of vehicle, activation of flux, ramp up to and soak at soldering temperature for a given period of time prior to cool-down.
The question has to be asked as to whether this Sn/Pb paste formulation would work the same waywhen subjected to the Pb Free reflow thermal profile. Probably - especially since "the assemblies appear fine"- but that is somewhat subjective.
- Apart from flux considerations, the Sn/Pb alloy would certainly not dissolve as much copper from the substrate at the higher temperatureas would the Pb Free alloy which is much richer in Sn.
Consequently, to the extent that it is important, the creation of Cu/Sn intermetallic compounds would be reduced as compared to Pb Free.
- What is the effect of the elevated thermal profile on the assembly? If it can satisfactorily be reflowed using Pb Free solder paste and thermal profile without damaging the board or components then there should be no concern that those with Sn/Pb solder will suffer.
The reflow oven performs the same way regardless of the solder alloy that passes through it.
As regards testing for long term reliability, the same tests as those used on the Pb Free assemblies should be valid for the Sn/Pb assemblies.
Would I scrap them? It would depend on their value and how confident I am of the tests carried out.
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Harold Hyman
Consultant
VJ Electronix
Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.
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