TABLE OF CONTENTS Among the numerous lead-free solder options available, the following families are of particular interest and are the prevailing choices of industry: eutectic Sn-Ag, eutectic Sn-Cu, Sn-Ag-Bi, Sn-Ag-Bi-In, Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Ag-Cu-In, Sn-Ag-Cu-Sb, Sn-Zn, and Sn-Zn-Bi. Their characteristics and potential performance in electronic applications follow.
Eutectic Sn-Ag, 96.5Sn-3.5Ag, has been used in hybrid applications for many years. Some physical properties of this alloy, as well as 99.3Sn-0.7Cu and 63Sn-37Pb, are shown in Table 13.1.1 3
| Property | 96.5Sn-3.5Ag | 99.3Sn-0.7Cu | 63Sn-37Pb |
|---|---|---|---|
| Melting temperature ( C) | 221 | 227 | 183 |
| Surface tension (dyne/cm) at 260 C | 460 at 260 C;431 at 271 C (air);493 at 271 C (nitrogen) | 491 at 277 C (air); 461 at 277 C (nitrogen) | 380 at 260 C;417 at 233 C (air);464 at 233 C (nitrogen) |
| Density (gm/cm 3) | 7.36 | 7.31 | 8.36 |
| Electrical resistivity ( ? ?-cm) 10 | 10.8 | 10 15 | 15.0 |
| Thermal conductivity (W/cm C) | 0.33 at 85 C |
| 0.509 at 30 C; 0.50 at 85 C |
| Coefficient of thermal expansion (CTE) at 20 C (ppm/K) | 30 |
| 25 |
| Hardness | 16.5 [Vickers hardness (VH)];40 (Brinell) |
| 12.8 (HV);17 (Brinell) |
The melting temperature of 96.5Sn-3.5Ag is 38 C higher than eutectic Sn-Pb, suggesting that a considerable thermal stress can be introduced into the devices being assembled during soldering. The high surface tension of the eutectic Sn-Ag system predicts a higher contact angle during soldering, hence a greater difficulty in solder spreading. The lower density and lower electrical resistivity of eutectic Sn-Ag versus eutectic...
