Wednesday, 19 July 2017

Lead Free Solders

Lead free solders have been created in response to concerns about lead, especially in the electronics industry. The following tables present a selection of available solder compositions.  The characteristics of these lead free solders can be compared to the common lead bearing solders in the last table.

Abbreviations for the metals of the compositions:
Ag=Silver; Bi=Bismuth; Cu=Copper; Ge=Germanium; In=Indium;
Sb=Antimony; Sn=Tin; Zn=Zinc



Melting Temperatures of Lead-Free Solders

Alloy  %                     Melting Temperature    Comments
Range (ÂșC)
Sn 65, Ag 25                         233           High strength; patented by Motorola (“Alloy J”)
Sn 99.3, Cu 0.7                     227           Eutectic
Sn 96.5, Ag 3.5                     221           Eutectic. Excellent strength and wetting
Sn 98, Ag 2                          221 – 226
Sn 77.2, Ag 2.8, In 20           175 – 186
Sn 95, Sb5                           232 – 240 Good high-temperature shear strength
Sn 42, Bi 58                         138           Well established; expensive
Sn 91, Zn 9                          199   Eutectic. Corrodes easily; high dross
Sn 95.5, Ag 0.5, Cu 4            217 – 350 Lead-free plumbing solder
Sn 97.25, Ag 2, Cu 0.75        217 – 219
Sn 91.8 Ag 3.2, Cu 0.5          217 – 218
Sn 95.5, Ag 3.8, Cu .07         217 – 220
Sn 95.5, Ag 4, Cu 0.5            217 – 225
Sn 95, Ag 4, Cu 1                 217 – 220
Sn 94.6, Ag 4.7, Cu 1.7         217 – 244
Sn 89, Zn 8, Bi 3                   192 – 197
Sn 97, Ag 0.2, Cu 2, Sb 0.8    287 – 218  High melting range; “Aquabond”
Sn 96.2, Ag 2.5, Cu 0.8, Sb 0.5      217 – 225
Sn 90.5, Ag 2, Bi 7.5             190 – 216
Sn-91.8, Ag 3.4, Bi 4.8          201 – 205
Sn 93.5, Ag 3.5, Bi 3             208 – 217
Sn 94.25, Ag 2, Bi 3, Cu 0.75   205 – 217
Sn90.7, Ag3.5, Bi 5, Cu 0.7     198 – 213
Sn 93.4, Ag 2, Bi 4, Cu 0.5, Ge 0.1         202 – 217
Sn 42.9, Bi 57, Ag 0.1           138 – 140
Sn 48, In 52                         118           Eutectic. Lowest melting point. Expensive

Source:



Liquidus Temperatures (°C) of Candidate Lead-Free Solder Alloys for Replacing Eutectic Tin-Lead Solder

Alloy Composition%     Liquidus             Melting Range
98Sn-2Ag                                             221-226
96.5Sn-3.5Ag              221                    221
99.3Sn-0.7Cu              227                    227
96.3Sn-3.2Ag-0.5Cu     218                   217-218
95.5Sn-3.8Ag-0.7Cu     210                   217-210
95.5Sn-4.0Ag-0.5Cu                             217-219
95Sn-5Sb                                            232-240
42Sn-58Bi                   138                   138
89Sn-3Bi-8Zn                                      189-199

Where there is a single temperature in the melting range column, the solder is eutectic.

Based on:
V. Solberg, “No-Lead Solder for CSP: The Impact of Higher Temperature SMT Assembly Processing,” Proc. NEPCON West 2000 Conf. (Feb. 28 - Mar. 2, 2000) Anaheim, CA (Source: Indium Corp.) # N.-C. Lee, “Lead-Free Chip-Scale Soldering of Packages,” Chip Scale Review, March-April 2000.
Source:




Solidus and Liquidus Temperatures of Some Leadfree Alloys on Copper

Alloy  %                             Solidus (°C)        Liquidus (°C)
98Sn-1Ag-1Sb                      222                   232 
89Sn-4Ag-7Sb                      230                   230
91.2Sn-2Ag-0.8Cu-6Zn          217                   217
89.2Sn-2Ag-0.8Cu-8Zn          215                   215
89.2Sn-10Bi-0.8Cu               185                    217
85Sn-10Bi-5Sb                     193                   232
52Sn-45Bi-3Sb                     145                   178
42Sn-58Bi                            138                   138

Based on:
M.E. Loomans, S. Vaynman, G.Ghosh and M.E. Fine, “Investigation of Multi-component Lead-free Solders,” J. Elect. Matls. 23(8), 741 (1994)
Source:



Eutectic Composition of Solders

Most solders and especially tin-lead alloys have a melting (or pasty) range between which the metal has moved from a proper solid (solidus) to a completely liquid (liquidus) state.  Wide melting ranges are ideal for plumbers, they are not for electronics, or stained glass.  It is much easier to run a nice bead with a narrow range of melting (pasty) temperatures.

Some alloys of solder have what is known as an eutectic characteristic.  This is where the liquidus and solidus states occur at the same temperature.  A composition of 61.9% tin and 38.1% lead is both eutectic and the melting occurs at a minimum temperature.

For comparison with lead free solder characteristics the following % compositions of Tin (Sn), Lead (Pb) and Silver (Ag) solders are given.

Element % of solders  Melting point        Comment
Sn 62, Pb 36, Ag 2       179                    Eutectic; traces of antimony
Sn 63, Pb 37               183                    Eutectic; traces of antimony
Sn 60, Pb 40               183-191             Traces of antimony
Sn 96.3, Ag 3.7           221                    High melting point. Eutectic
Sn 10, Pb 90               275-302
Sn 3, Pb 97                275-320
Sn 5, Pb 93.5, Ag 1.5   296-301

Source:
http://en.wikipedia.org/wiki/Solder#Lead-free_solder



Conclusions

Most of the alternative solders contain tin as it assists in the formation of bonds with a wide variety of metals.  These solders are also mechanically weaker than tin-lead solders.  Lastly, they are much more expensive than tin-lead solders.  Even within the tin-lead solders there is a variation in price, as tin is much more expensive than lead. If high temperatures were not a problem, you could use a high lead content solder.  However, that also raises the liquidus temperature and increases the pasty range.

The choice in lead free solders is between the high liquidus temperatures of the less expensive compositions and the high price of the eutectic, or nearly so, ones.  The lowest eutectic composition is the Tin-Bismuth solder, but it is also among the most expensive to buy.  You should also note that the inclusion of copper in the composition prolongs the life of the solder bit, as low lead content of the solder leads to the incorporation of small amounts of copper from the tip into the solder joint.