SAC305 Lead-Free Solder

SAC305 is a soft (15 HB, ~15 HV) lead-free solder alloy with a near-eutectic melting range of 217-220°C. The microstructure consists of beta-tin dendrites with Ag3Sn needles and Cu6Sn5 intermetallic particles in the eutectic regions. The hardness contrast between the soft tin matrix and harder intermetallics creates relief during polishing. Commonly prepared as solder joint cross-sections for reliability analysis.

Use a low-speed precision wafering saw with a thin diamond blade and continuous coolant. Cutting speed: 100-200 RPM with minimal feed rate. The melting range (217-220°C) is higher than eutectic Sn-Pb but still low enough to require temperature awareness. When sectioning solder joints on PCBs, position the cut through the center of the joint. For BGA joints, careful alignment is critical for revealing the full joint cross-section. Leave adequate allowance for grinding.

Cold mounting with castable epoxy is required. The melting range (217-220°C) is close enough to compression mounting temperatures (150-180°C) that softening and microstructural changes are possible. Use a low-shrinkage epoxy with good edge retention. Vacuum impregnation is strongly recommended for solder joints on PCBs. Edge-retaining mounting compounds are essential when examining intermetallic layers at the solder/pad interface, as these layers are typically only 1-5 μm thick.

Start at 320-400 grit SiC. Very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. The Ag3Sn and Cu6Sn5 intermetallics are harder than the tin matrix, so excessive pressure preferentially removes the matrix and leaves intermetallics standing proud. Progress through 600, 800, 1200 grit.

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).

Thorough cleaning between steps.

Use napless or low-nap cloths to minimize relief between the tin matrix and harder intermetallics.

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). Monitor for relief around Ag3Sn needles.
• 1μm diamond: 1-2 minutes on a napless pad (8-12 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes or vibratory polishing 2-4 hours. Vibratory polishing is strongly recommended for revealing the thin intermetallic layers (Cu6Sn5, Cu3Sn) at the solder/pad interface without relief artifacts.

SAC305 microstructure is often analyzed in the as-polished condition, especially for intermetallic layer measurements. Etching reveals beta-tin dendrite boundaries and enhances intermetallic contrast.

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

• Composition: 5 ml HCl (concentrated), 95 ml methanol
• Application: Immerse for 5-15 seconds or swab gently.
• Reveals: Beta-tin dendrites, Ag3Sn particles (appear as bright needles/plates), Cu6Sn5 scallops at interfaces. Excellent for revealing the overall solder microstructure.
• 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 and grain boundaries.

Safety: Work in fume hood. Standard PPE required.