Hi Wee Mei! Well, this is going to be way more information than you wanted 
but you asked a real metallurgy question and I haven't gotten to work on 
any real metallurgy yet this week! There are specific definitions for 
reaction points on phase diagrams that describe the change of  state: 

1) Eutectic: when a metal alloy transforms from a liquid state into a 
single solid state

2) Peritectic: when a metal alloy transforms from a liquid plus solid 
state into single solid state

3) Eutectoid: when a metal alloy transforms from a single solid state into 
two solid states

The reason for having the specific definitions is that is allows materials 
engineers/scientists to characterize an alloy system changes in terms of 
temperature and the formation of different phase reactions. Material 
engineers favor the use of eutectic alloy systems because the alloy 
changes from a liquid to a solid at one temperature which is very 
advantageous in a manufacturing process. The interesting but mostly 
useless knowledge fact (a good Trivial Pursuit question to driving Doug 
Pauls crazy!)  is that the electronics industry has called Sn63Pb37 the 
tin/lead eutectic alloy for many years but the true tin/lead eutectic is 
the Sn61.9Pb38.1 alloy. In practice the Sn63Pb37 is so close to the true 
eutectic point that it doesn't really matter.  One of the "interesting" 
issues that the Pbfree solder push has thrown at the industry is that the 
popular SAC solder alloy family does not have a eutectic alloy in the 
temperature range we desire and we don't have the advantage of changing 
form a liquid to a solid at one temperature. As for your course, 
technically the eutectic alloy for the SnAg alloy system is Sn96.5Ag3.5 
but in a similar fashion to the Sn63Pb37 alloy, most folks call the 
eutectic alloy Sn96Ag4. 

Dave Hillman
Rockwell Collins
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Lum Wee Mei <[log in to unmask]> 
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05/22/2007 07:08 PM
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[TN] Definition of Eutectic Solder






Hello,
 
What is actually eutectic solder? All along I have this impression that 
when we talk of eutectic solder, it is refer to Sn63. However, yesterday, 
I attended a course and was told that Sn96 (4% silver) is also eutectic. 
Can someone enlightenment me?
 
Thanks and regards,
Wee Mei

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