Sn-37Pb Eutectic Solder

Sn-37Pb eutectic solder is a very soft (15 HB, ~14 HV) two-phase alloy with the lowest melting point (183°C) in the Sn-Pb system. The lamellar eutectic microstructure of alternating Sn-rich and Pb-rich phases creates differential polishing challenges, as the Pb-rich phase is softer. Commonly prepared as solder joint cross-sections on PCBs, requiring edge retention and interface preservation.

Use a low-speed precision wafering saw with a thin diamond blade and continuous coolant. The very low melting point (183°C) means heat must be minimized. Cutting speed: 100-200 RPM with minimal feed rate. When sectioning solder joints on PCBs, position the cut to pass through the center of the joint of interest. For BGA (ball grid array) joints, careful alignment is critical. Leave adequate allowance for grinding. The solder will deform easily, so avoid clamping directly on the solder joint.

Cold mounting with castable epoxy is required. Compression mounting temperatures (150-180°C) will approach or exceed the 183°C melting point and will alter the microstructure. Use a low-shrinkage epoxy resin with good edge retention properties. Vacuum impregnation is strongly recommended for solder joints on PCBs to fill gaps around components, under chips, and in via holes. This prevents edge rounding and trapping of grinding debris in gaps during subsequent preparation.

Start at 320-400 grit SiC. Very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. The two-phase eutectic structure means differential polishing is a constant concern; the Pb-rich phase removes faster than Sn-rich. Progress through 600, 800, 1200 grit. Fresh papers at each step. Thorough cleaning between steps to prevent cross-contamination.

Grinding sequence:

• 320 grit: Remove sectioning damage (15-30 seconds). Very light pressure.
• 400 grit: Remove previous scratches (15-30 seconds).
• 600 grit: Refinement (15-30 seconds).
• 800 grit: Prepare for polishing (15-30 seconds).
• 1200 grit: Final grinding (15-30 seconds).

Rotate specimen 90° between steps. For PCB cross-sections, grind carefully to the target plane without overshooting the solder joint of interest.

Use napless or low-nap cloths to minimize relief between Sn-rich and Pb-rich phases.

Diamond polishing sequence:

• 6μm diamond: 2-3 minutes on a napless synthetic pad with light pressure (10-15 N).
• 3μm diamond: 2-3 minutes on a napless pad (10-12 N).
• 1μm diamond: 1-2 minutes on a napless pad (8-12 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft pad, or vibratory polishing for 2-4 hours. Vibratory polishing is strongly recommended for solder joints as it produces excellent surface quality with minimal relief, which is critical for revealing intermetallic layers (Cu6Sn5, Cu3Sn) at the solder/pad interface.

Monitor for differential relief between Sn-rich and Pb-rich phases throughout polishing.

The eutectic structure can often be seen in the as-polished condition under brightfield illumination. Etching enhances phase contrast and reveals grain boundaries within each phase.

5% HCl in Methanol (Chemical Etching) - Primary choice for solder:

• Composition: 5 ml HCl (concentrated), 95 ml methanol
• Application: Immerse for 5-15 seconds or swab gently.
• Reveals: Sn-rich and Pb-rich eutectic phases with excellent contrast. Also reveals intermetallic layers at solder/pad interfaces.
• Rinse: Methanol, then dry with warm air.

2% Nital (Chemical Etching) - Alternative:

• Composition: 2 ml HNO₃, 98 ml ethanol
• Application: Immerse for 3-10 seconds.
• Reveals: General microstructure. Less selective than HCl-methanol.

Safety: Contains lead. All etching waste must be disposed of as hazardous waste. Work in fume hood. Standard PPE required.