How to Use a Tin Indium Silver Alloy Differential Scanning Calorimeter

A tin indium silver alloy differential scanning calorimeter measures how the heat capacity of a solder sample changes with temperature. You use it by preparing a small specimen, typically 5–20 milligrams, sealing it in a pan, and running a controlled heating or cooling program. The instrument records the heat flow into or out of the sample relative to an empty reference pan, allowing you to identify melting points, solidification ranges, and enthalpy changes.

Preparing the Sample and Instrument

Accurate results start with sample preparation. Cut a piece of the tin indium silver alloy that is flat and fits snugly inside the DSC pan. Weigh it on a microbalance to 0.01 mg precision. For most tin-indium-silver solders, a mass between 8 and 15 mg gives a clean signal without overloading the sensor.

Use a standard aluminum pan and lid. Crimp the lid using the manufacturer’s press to create a hermetic seal. A leaky pan will produce erratic heat-flow curves because the sample can oxidize or lose material during heating.

Place an identical empty pan on the reference side. Purge the DSC cell with dry nitrogen at a flow rate of 50 mL/min. Nitrogen prevents oxidation of the solder at high temperature and ensures a stable baseline. Allow the gas to flow for two minutes before starting the run.

Running the Heating Program

Set the temperature program to start at 25 °C and ramp up to 250 °C at a rate of 10 °C per minute. This rate is standard for solder alloys and provides enough resolution to separate overlapping thermal events. A slower rate, such as 5 °C per minute, improves peak separation but takes longer. A faster rate, like 20 °C per minute, may shift peak temperatures and reduce accuracy.

The instrument records heat flow in milliwatts. As the sample heats, you will see a baseline shift when the glass transition occurs (if the alloy is amorphous), followed by a sharp endothermic peak at the solidus temperature. The peak returns to baseline at the liquidus temperature. For a typical tin-indium-silver alloy, the solidus is near 118 °C and the liquidus near 130 °C, depending on the exact composition.

Interpreting the Melting Peak

The area under the endothermic peak corresponds to the enthalpy of fusion. Modern DSC software calculates this area automatically by integrating the heat flow signal between the onset and end of the peak. You can verify the onset temperature by drawing a tangent line along the leading edge of the peak and finding where it crosses the baseline.

Check the shape of the peak. A single sharp peak indicates a eutectic or near-eutectic composition. A broad peak or multiple peaks suggest a non-eutectic alloy with a pasty range. The width of the peak between solidus and liquidus tells you the melting range, which matters for reflow soldering processes.

Cooling and Reheat Cycles

After the heating ramp, hold the temperature at 250 °C for two minutes to ensure complete melting. Then cool the sample at 10 °C per minute back to 25 °C. The cooling curve shows an exothermic peak for solidification. Undercooling is common in tin-indium-silver alloys, so the solidification peak may appear 10–20 °C below the melting onset on the heating curve.

Run a second heating cycle on the same sample. This second melt often gives more reproducible results because the sample has a consistent thermal history. Compare the onset temperatures of the first and second heating cycles. A shift of more than 2 °C may indicate sample degradation or contamination.

Calibration and Baseline Correction

Calibrate the DSC regularly using indium metal, which has a certified melting point of 156.60 °C and an enthalpy of 28.45 J/g National Institute of Standards and Technology. Run a pure indium sample under the same conditions you use for the solder. Adjust the temperature offset in the software if the measured onset differs from the certified value by more than 0.2 °C.

Subtract a baseline run from your sample data. Run an empty pan pair under the same temperature program, then use the instrument software to subtract that baseline from the sample curve. This step removes systematic drift in the heat flow signal.

Troubleshooting Common Artifacts

Several artifacts can distort DSC curves. A sharp spike at the beginning of the melt often comes from a loose pan lid that shifts during heating. Re-crimp the pan and rerun the sample. A wavy baseline before the melt may indicate moisture in the purge gas. Replace the nitrogen cylinder or add a moisture trap.

An exothermic dip immediately after the melting peak suggests recrystallization of a metastable phase. This behavior is more common in alloys with high silver content. If you see this dip, run a slower cooling rate, such as 5 °C/min, to allow equilibrium phases to form. Compare the first and second heating curves to see if the dip disappears after slow cooling.

If the instrument reports a negative heat flow during the baseline portion, the reference pan may be heavier than the sample pan. Balance the pans by adding a small piece of aluminum foil to the lighter side. The baseline should be flat within 0.1 mW over a 100 °C range for reliable integration.

Key Takeaways

• Use a sample mass of 8–15 mg in a hermetically sealed aluminum pan for consistent results.
• Set a heating rate of 10 °C per minute under nitrogen purge at 50 mL/min.
• Identify the solidus and liquidus from the onset and peak return of the endothermic melt curve.
• Run a second heating cycle to confirm thermal stability and reproducibility.
• Calibrate the instrument with indium metal certified by NIST to ensure accurate temperature and enthalpy measurements.