Wednesday, 15 July 2015

Lead Free Solder

Lead free solder is being required for the electronics industry, but not yet for the stained glass industry.  However, some people are beginning to use lead-free solders for other reasons.  In general, it is reported that it is harder to get smooth beads.  Some reasons may relate to the physical properties of the material being used.

Lead free solder solidifies at a higher temperature than the common tin/lead solder compositions although the common lead free solders melt at slightly lower temperatures.  For comparison purposes characteristics of some common lead free solders are given with the common tin/lead solders.

Sn = Tin,   Ag = Silver,   Cu = Copper   Pb = Lead
Solidus = solidification temperature.   Liquidus = Melting temperature

96%Sn, 4%Ag which has a Solidus of 221C and Liquidus of 229C
95%Sn, 5%Ag which has a Solidus of 221C and Liquidus of 254C

Slightly less commons is
96.5%Sn, 3.5%Ag which has a Solidus of 221C and Liquidus of 221C, but has poor wetting properties except on stainless steel.

Other solders are available up to 7% silver, but these are increasingly expensive and have much higher liquidus points.

A truly eutectic [reference here] lead free solder can be produced with 95.6%Sn, 3.5%Ag, and 0.9Cu, which has a Solidus and Liquidus temperature of 217C

For comparison:
63%Sn, 37%Pb has a has a Solidus of 183C and Liquidus of 183C
60%Sn, 40%Pb has a has a Solidus of 183C and Liquidus of 188C
50%Sn, 50%Pb has a has a Solidus of 183C and Liquidus of 212C
40%Sn, 60%Pb has a has a Solidus of 183C and Liquidus of 238C

The solidus temperature of lead free solders is almost 40C above the tin/lead solders.  This may be the reason people find the need to turn up the heat of their soldering iron when using lead free solders.  The difference in the Liquidus and Solidus points for 4%Ag is very similar to that for 60%Sn/40%Pb.  So with enough heat should behave similarly.