How to Properly Reheat and Stir Lead-Free Solder Paste?

Properly reheating and mixing lead-free solder paste is a critical step in ensuring soldering quality, requiring attention to uniformity while preventing oxidation and bubble formation. Below are detailed steps and precautions:  

I. Pre-Mixing Preparation

Environmental Control

Temperature: Solder paste must be brought to room temperature (typically 20-25°C) beforehand to prevent component separation caused by temperature inconsistencies.

Humidity: Maintain operating humidity between 40-60% to prevent moisture absorption that could impair soldering performance.

Cleanliness: Ensure mixing tools (e.g., manual stirrers or automatic mixers) are free of contaminants to avoid solder paste contamination.

Solder Paste Condition Check

Inspect the solder paste for separation, oil separation, clumping, or drying. If separation occurs, gently squeeze the paste container to achieve preliminary mixing.

II. Mixing Methods

Method 1: Manual Mixing (Suitable for Small Batches or Temporary Use)

Tool Selection: Use clean, static-free plastic or metal mixing rods (Avoid direct metal-to-paste contact to prevent static buildup).

Stirring Direction: Stir slowly in a consistent direction (clockwise or counterclockwise) to prevent frequent direction changes that create bubbles.

Force and Duration:

Stir gently to avoid excessive force causing paste heating and oxidation.

Stir for approximately 2-3 minutes until the paste color is uniform and free of gritty texture.

Settling: Let the paste rest for 5-10 minutes after stirring to allow bubbles to dissipate naturally.

Method 2: Automatic Mixer (Suitable for High-Volume Production)

Equipment Selection: Use a dedicated solder paste mixer set to low speed (≤50 rpm recommended) and short duration (1-2 minutes).

Parameter Settings:

Avoid high-speed mixing (>100 rpm) to prevent paste heating or shear stress generation.

Consider staged mixing (e.g., 1 minute low speed, 1-minute pause, then 1 minute low speed).

Settling: After mixing, let the paste settle for 5-10 minutes to ensure complete bubble release.

III. Critical Precautions

Avoid Over-Mixing

Excessive mixing raises solder paste temperature, accelerating flux evaporation and compromising soldering performance.

Prolonged mixing may introduce air bubbles, causing voids, print defects, or placement failures during printing or dispensing.

Prevent Oxidation

Minimize air exposure during mixing by covering with plastic wrap or using sealed mixing containers.

Seal immediately after mixing to prevent flux evaporation.

Addressing Separation

If slight separation persists after mixing, gently stir again or use a spatula to scrape along the container walls.

Severe separation (e.g., metal powder settling) requires contacting the supplier to replace the solder paste.

Pre-Use Inspection

After mixing, examine a small amount of solder paste:

Color: Should be uniform with no color variation.

Viscosity: When lifted with a spatula, the paste should flow slowly without lumps or separation.

Thixotropy: Should return to original shape after light pressure, without collapse or excessive hardness.

IV. Storage and Reuse

Unused Paste

If not fully used after mixing, store sealed in a refrigerator (2-10°C) to avoid repeated temperature fluctuations.

Before reuse, allow it to return to room temperature and stir again.

Disposal

Solder paste exceeding its shelf life or exhibiting degraded performance must be disposed of according to environmental regulations; avoid casual discarding.

V. Troubleshooting

Particles remain after stirring: May indicate metal powder agglomeration. Replace the solder paste or contact the supplier.

Voids appear after printing: May result from uneven mixing or trapped air bubbles. Extend settling time or adjust mixing parameters.

Poor soldering (e.g., cold solder joints): May indicate flux evaporation. Review mixing duration or storage conditions.

Standardizing the mixing process ensures uniformity and stability of lead-free solder paste, thereby improving soldering yield. Adjust specific parameters according to the manufacturer's Technical Specifications.