Higher metal content enhances joint strength but may reduce paste workability.
Balanced metal ratios prevent weak joints and void formation.
Flux Composition and Oxidation Prevention
Flux removes oxide layers from surfaces, enabling strong bonds.
Phosphorus-based fluxes offer superior oxidation resistance.
Flux types:
No-clean: Minimal residue, ideal for hard-to-clean assemblies.
Water-soluble: Effective oxidation control with post-cleaning required.
Factors Affecting Solder Paste Quality
Storage Conditions
Storage Condition
Temperature
Shelf Life
Notes
Long-term
0–10°C (32–50°F)
6 months
Refrigerate
Room temperature
19–25°C (66–77°F)
2 weeks
Short-term use only
High temperature
>29°C (84°F)
Reduced
Avoid exposure
Allow paste to equilibrate to room temperature before use to prevent condensation.
Application Methods and Stencil Techniques
Stencil printing best practices:
Pressure: 7 kgf
Speed: 20 mm/s
Clogged or damaged stencils cause inconsistent paste deposition.
Environmental Factors
Condition
Impact on Solder Paste
47% RH
Normal performance
80% RH
Paste slumping, bridging
Temperature surge
Blurred prints, solder balling
Handling Practices
Contamination risks and prevention:
Contaminant
Effect
Prevention
Dust
Uneven joints
Clean tools/workspace
Moisture
Viscosity changes
Sealed storage
Skin oils
Weakened bonds
Gloves required
Defects Caused by Poor-Quality Solder Paste
Solder Bridging
Electrical shorts and mechanical weakness.
Caused by:
Excessive paste deposition
Inconsistent viscosity
High humidity environments
Impact Type
Description
Electrical Shorts
Device failure or component damage
Thermal Hot Spots
Reduced component lifespan
Insufficient Solder Joints
Weak connections from low metal content or improper application.
Solder alloy performance:
Alloy
Small BGAs
Large Components
Innolot
High reliability
Less effective
SAC305
Less reliable
Ideal for PBGA1156
Voiding
Air gaps in joints affecting thermal and electrical performance.
Leads to overheating and signal degradation.
Tombstoning and Misalignment
Tombstoning: Component lifting due to uneven heating or paste deposition.
Misalignment: Caused by excessive paste or stencil design flaws.
Ensuring Solder Paste Performance
Material Selection
Evaluate based on:
Alloy type (e.g., SAC305, Innolot)
Flux activity level
Particle size distribution
Storage and Handling
Follow IPC-7527 and J-STD-004 standards for:
Refrigeration at 0–10°C
Thawing procedures before use
Contamination control
Quality Testing
Key tests:
Standard
Focus Area
IPC-J-STD-002
Soldering materials
IPC-J-STD-006
Surface mount components
IPC-SM-817
Assembly processes
Reflow Parameter Optimization
Adjust heating profiles to match paste specifications:
Peak temperature: 15–25°C above solder melting point
Cooling rate: 1–4°C/sec
Thermal profiling tools ensure uniform heating across PCBs.
FAQ
What is solder paste, and why does it matter?Solder paste is a metal-flux mixture essential for bonding components to PCBs. High-quality paste ensures reliable connections and reduces failure rates.
How should solder paste be stored?Store at 0–10°C in sealed containers. Allow 4–6 hours to warm to room temperature before use to prevent moisture condensation.
What causes solder bridging?Bridging results from excessive paste, uneven printing, or high humidity. Proper viscosity control and environmental management mitigate this.
How to select the right solder paste?Consider alloy type (e.g., lead-free SAC305), flux type (no-clean/water-soluble), and particle size based on component density and PCB complexity.
Do environmental factors affect solder paste?Yes—optimal conditions are 22–26°C and 45%±5% humidity. Deviations cause paste slumping, balling, or viscosity changes.
