Good morning Techs,

 

Looking at the time above liquidus (TAL) is a common reflow parameter
normally evaluated and discussed in J-STD-020 for Sn-Pb eutectic solder.
Some of our CMs use Sn62Pb36Ag02 solder paste, which is a semi-eutectic
solder that has a liquidus/pasty range of 179 to 189C (vs. eutectic
temperature of 183). There seems to be differences of opinion on what
the correct starting temperature should be for evaluating the Time Above
Liquidus (TAL) when evaluating a reflow profile. For example, is the
correct starting temperature 179, 183 or 189C.  With Sn63 it is easy,
183 C but with a paste that has a melting range it is less clear to me
and no one has presented a clear explanation of what is the correct
starting temperature and why.

 

The other related question I have is at what temperature is the standard
20 - 25 C added to ensure good wetting, etc..  Is the 25 C added to 179
or 189 C or some temperature in-between for Sn62?  Which is more correct
and why????

 

Would anyone like to share there opinions on these subjects.  I may be
splitting hairs but the devil is always in the details. Thanks and happy
new year.

 

 

-Joe 

 


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