Introduction
In SMT production, solder paste is the core welding material. Proper storage and stirring are critical to maintaining its activity. Insufficient stirring causes flux and alloy powder to separate, leading to printing and welding defects.
Purpose of Stirring
Solder paste is a thixotropic fluid composed of alloy powder and flux. During storage, gravity causes the denser alloy powder to sink and the flux to rise, resulting in stratification. Stirring restores uniformity for stable printing and welding performance.
Stirring Tools
Manual stirring knife: low cost, suitable for small batch production. Automatic stirring machine: controllable speed and time, uniform mixing, suitable for mass production.
Standard Stirring Procedure
Step 1: Pre-treatment
Remove from cold storage and thaw thoroughly at room temperature. Avoid direct heating. Check for condensation before opening.
Step 2: Loading
Transfer paste to a clean container. Do not overfill. Secure symmetrically in the machine chuck.
Step 3: Setting Parameters
Adjust time based on paste type. Set speed and tilt angle per manual. Use forward and reverse alternating modes.
Step 4: Starting and Monitoring
Observe the paste. It should gradually become uniform. Stop if paste becomes too thin or generates many bubbles.
Step 5: Finishing
Let rest briefly after stopping. Lift with scraper. Paste should flow in continuous ribbon, uniform in color.
Precautions
- Control environment temperature and humidity
- Avoid cross-contamination between paste models
- If paste hardens, it may be over-stirred or abnormal temperature
- If excessive bubbles, lower speed and extend rest time
Troubleshooting
Paste collapses after printing: shorten stirring time, reduce speed. Solder beads after welding: extend stirring time. Low printing volume: extend stirring time, check thawing.
