Solder Paste: Classification, Storage, Printing & Reflow

What is solder paste and how is it classified?

Solder paste is a homogeneous, paste-like mixture of solder alloy powder and a flux/vehicle system. During reflow, the alloy melts and, with the flux removing surface oxides, wets the component leads and PCB pads to form mechanical and electrical connections.

Solder paste is thixotropic: viscosity drops under shear for printing and recovers afterwards, holding components in place until reflow.

How should solder paste be stored and warmed up?

Store solder paste sealed at 5–10°C. Do not freeze it. Cold storage slows flux reaction and solvent loss, helping to maintain viscosity and activity.

Before use, warm the paste at room temperature for 4–6 hours without opening the container, so moisture does not condense on the paste. Then mix in one direction for 1–3 minutes with a dedicated mixer until homogeneous.

Reseal unused paste and keep it at room temperature. Do not return warmed paste to the refrigerator or repeatedly freeze and thaw it.

Warm-up time depends on package size and ambient temperature. Confirm the time for your actual package and shop-floor conditions before mass production.

Key solder paste printing parameters

Printing is sensitive to temperature, humidity and viscosity. Maintain 18–24°C and 40–50% RH at the printer, and avoid direct airflow that can dry the paste locally.

• Stencil printing viscosity: approximately 450,000–700,000 CPS; screen printing: approximately 750,000–1,300,000 CPS.
• Recommended stencil aperture width-to-thickness ratio (W/T): at least 1.5; area ratio: greater than 0.66.
• Stencil materials include metal, stainless steel or brass; squeegees can be 90-durometer rubber or polyurethane.
• Printing speed: 20–65 mm/s; squeegee contact angle: about 45–65°; squeegee pressure: about 10–20 lbs.
• Print evenly and balance the squeegee stroke; adjust parameters based on stencil tension and print results.

These are general process ranges. Validate them against your stencil design, panel layout, component density and paste model.

How to set the reflow profile

Even for the same paste, the temperature-time profile varies with board thickness, assembly density and oven characteristics. The table below is a typical Sn-Pb profile; lead-free paste requires a correspondingly higher peak.

Reflow notes for lead-free paste

Lead-free paste (typical melting point 217–221°C) requires a higher reflow peak. Nitrogen atmosphere is recommended to reduce oxidation. Set the peak and time above liquidus according to the alloy, PCB thickness, component temperature limits and paste datasheet.

For thicker boards and dense assemblies, reduce the ramp rate and extend preheat to improve temperature uniformity.

Do not simply reuse an Sn-Pb profile. The lead-free window is narrower; measure the profile with a profiler and keep records.

How to troubleshoot solder paste defects

Solder balls, bridging, tombstoning, insufficient solder, voids and dull joints are usually traced to paste condition, stencil and printing parameters, placement accuracy, reflow profile and material oxidation. See our technical article “Solder Paste Printing & Reflow Defects: Causes and Solutions” for a detailed troubleshooting table.

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