Physical Properties at Room Temperature
◆Solder paste has physical properties beyond room temperature
Due to the different tin and lead compositions in solder, its physical properties also differ, as shown in the table below. Because tin and lead have different densities, the melting temperature, density, conductivity, and mechanical properties of the solder also vary. For example, solders containing Sn37Pb have very high tensile and shear strengths and excellent mechanical properties. From an electrical perspective, the conductivity improves with increasing tin content.
Physical properties of tin-lead solders
| Sn (%) | Pb (%) | Melting Point (Liquidus) (°C) | Melting Point (Solidus) (°C) | Density (g/cm³) | Tensile Strength (kg/mm²) | Yield Strength (kg/mm²) | Elongation (%) | Brinell Hardness | Electrical Conductivity IACS (%) | Resistivity ×10⁻⁶ Ω·cm |
|---|---|---|---|---|---|---|---|---|---|---|
| 100 | 0 | 232 | 232 | 7.29 | 1.49 | 2.02 | 55 | - | 13.9 | 10 |
| 70 | 30 | 183 | 186 | 8.17 | 4.48 | 3.52 | 20.0 | 17.0 | 12.5 | 13.79 |
| 63 | 37 | 183 | 183 | 8.46 | 4.41 | 3.80 | 28~30 | 17.0 | 11.5 | 14.99 |
| 60 | 40 | 183 | 188 | 8.52 | 4.34 | 3.94 | 27~40 | 14.0 | 11.5 | 14.99 |
| 50 | 50 | 183 | 214 | 8.90 | 4.36 | 3.66 | 38~98 | 14.0 | 10.9 | 14.82 |
| 40 | 60 | 183 | 238 | 9.28 | 3.80 | 3.77 | 39~115 | 12.0 | 10.1 | 17.07 |
| 20 | 80 | 183 | 277 | 10.04 | 3.37 | 2.95 | 22.0 | 11.0 | 8.7 | 20.50 |
| 0 | 100 | 327 | 327 | 11.34 | 1.39 | - | - | - | - | - |
The table below lists the coefficients of linear expansion and thermal conductivity of tin-lead solder. These data show that the change in thermal conductivity follows a simple functional relationship as the solder composition changes from pure tin to pure lead. Data for a specific intermediate point can be obtained using extrapolation. Understanding these data is important for soldering ceramic-glass substrates and silicon wafers.
The coefficient of linear expansion and thermal conductivity of tin-lead alloy solders
| Sn (%) | Pb (%) | Linear Expansion Coefficient (×10⁻⁶/°C) | Thermal Conductivity (W/(m·K) = cal/(cm·s·°C)) | Specific Heat Capacity (cal/(g·°C)) |
|---|---|---|---|---|
| 100 | 0 | 23.0 (0–100°C) | - | 0.157 |
| 70 | 30 | 21.6 (15–110°C) | - | - |
| 63 | 37 | 24.7 (15–100°C) | - | 0.121 |
| 50 | 50 | 23.6 (15–110°C) | - | 0.111 |
| 20 | 80 | 26.5 (15–110°C) | - | 0.089 |
| 5 | 95 | 28.7 (13–110°C) | - | 0.085 |
| - | 100 | 29.3 (17–100°C) | - | 0.083 |
The hardness of tin-lead solders varies with the tin-to-lead ratio, temperature, production method, and cooling method during production, unlike their tensile strength. Hardness decreases sharply with increasing temperature. The surface tension and viscosity of tin-lead solders are directly related to their alloy composition, as shown in the table below. None of these properties have any maximum values within the composition range listed in the table.
Surface tension and viscosity of tin-lead solders
| Pb (%) | 0 | 20 | 37 | 50 | 58 | 70 | 80 | 100 |
|---|---|---|---|---|---|---|---|---|
| Sn (%) | 100 | 80 | 63 | 50 | 42 | 30 | 20 | 0 |
| Temperature (°C) | 290 | ~280 | 280 | 280 | 280 | 280 | 280 | 350 |
| Surface Tension (dyn/cm) | 545 | 514 | 490 | 476 | 474 | 470 | 467 | 439 |
| Wettability (Contact Angle θ, °) | 0.0165 | 0.0192 | 0.0197 | 0.0219 | 0.0229 | 0.0245 | 0.0272 | 0.0244 |
