In the field of micro-optical display (Mini LED, Micro LED and COB package), the choice of solder paste needs to synthesize the three core dimensions of chip size, encapsulation process and reliability requirements, the specific program is as follows:
1. according to the chip size selection paste particle size: precision guarantee
Mini LED (50-200μm)
Recommended particle size: T6 type (5-15μm), No. 6 powder (5-15μm) or finer No. 7 powder (2-11μm).
Reason: Mini LED chips are small in size and dense in pitch, so ultra-fine particle size solder paste is needed to realize precise printing and avoid bridging (short circuit) or lack of tin (false solder). For example, No. 7 powder (2-11μm) can meet the minimum of about 75μm × 150μm chip soldering needs to ensure that the solder joints are full and no risk of short circuit.
Micro LED (≤50μm)
Recommended particle size: T7 type (2-11μm), No. 8 powder (2-8μm) or finer No. 9 powder (1-5μm).
Reason: Micro LED chips are extremely small in size and require ultra-fine particle size solder paste to form micron-sized solder joints to ensure the reliability of the electrical connection and stability under the impact of hot and cold cycles. For example, No. 8 powder (2-8μm) has been successfully applied to Micro LED direct display screen (such as AR/VR equipment), to achieve high-density integration of reliable welding.
2. according to the packaging process to choose the type of solder paste alloy: temperature management core
COB package (Chip on Board)
Recommended type: low-temperature or medium-temperature no-clean solder paste.
Low temperature alloy (such as Sn42Bi58/Sn42Bi57.6Ag0.4/FL170, melting point 138~143°C): significantly reduce thermal stress, protect the exposed LED chip, gold wire, phosphor and heat-sensitive substrates (such as flexible boards), suitable for high-density, heat-sensitive COB package.
Medium temperature alloys (e.g. Sn64Bi35Ag1, melting point ~172°C): Balance between reducing thermal stress and ensuring solder strength/reliability, Ag element improves mechanical properties, suitable for scenarios with certain requirements on heat resistance.
Typical applications: Low/medium temperature solder pastes (e.g. Fitech™ mLED 1370) designed for COBs can meet the needs of soldering on small pitches (e.g. 0.9mm), ensuring long-term reliability of outdoor displays in extreme environments.
SMD/IMD Packages (Surface Mount/Integrated Matrix Devices)
Recommended type: Medium or high temperature solder paste.
High temperature lead-free alloys (e.g. SAC305/FR209, melting point ~217-220°C): Provide the highest solder strength and thermal fatigue resistance, suitable for applications that require mechanical stress or high temperature operation (e.g. outdoor LED screens).
Medium-temperature alloys: For components with limited heat resistance, lead-free medium-temperature alloys (e.g., SnBiAg series/FL200 (Fitech low-temperature solders)) or leaded alloys (e.g., Sn63Pb37, melting point ~183°C, subject to RoHS restrictions) are available.
Typical applications: SAC305 alloy is widely used for outdoor LED screens in SMD packages to ensure the stability of solder joints in high temperature and high humidity environments.
3. according to the reliability requirements to select the solder paste performance: long-term stability
High reliability applications (such as outdoor screens, vehicle displays)
Recommended performance: high activity, ultra-low void rate (<5%, or even <3%), resistance to environmental corrosion. Such as Fuyinda FR209 high reliability SAC system alloy
Reason: Harsh environments (temperature change, humidity, salt spray) require solder joints with excellent thermal fatigue life and corrosion resistance. Highly active fluxes ensure wettability and ultra-low voids reduce stress concentration points to avoid cracking or failure of the joint.
Typical application: Ultra-fine, low void solder paste for printing tiny apertures (e.g., 80μm) has been used in Mini LED backlight modules for automotive displays to ensure reliability over a wide temperature range of -40°C to 125°C.
High-volume production requirements
Recommended performance: Long working time (>10 hours), excellent mold release (accommodates openings as small as 55μm), resistance to hot and cold slumps, and high stability for continuous printing.
Reason: Ensure stable yield in high volume production, reduce the number of downtime for cleaning, prevent bridging at tiny pitches.
Typical application: FTP-0176 solder paste from Fuyinta supports minimum 70μm stencil opening printing with long-lasting printing life, which meets the high-efficiency demand of Mini LED solid crystal.
Typical Application Recommendations
Mini LED backlight module
Solution: No.7 powder (2-11μm) + Sn64Bi35Ag1/FL200 medium temperature alloy.
Advantage: balancing high-precision printing and sufficient welding strength, meet the requirements of anti-vibration, suitable for TVs, monitors and other consumer electronics.
Micro LED Direct Display (e.g. AR/VR)
Solution: No.9 powder (1-5μm) + Sn42Bi57.6Ag0.4/FL170 low temperature alloy.
Advantage: Extreme precision to protect heat-sensitive chips and ensure the reliability of micro-solder joints, suitable for high-resolution and small-size wearable devices.
4. COB small pitch outdoor display
Solution: No. 8 powder (2-8μm) + Specialized low/medium temperature solder paste (e.g. Fitech™ mLED 1370).
Advantage: Low temperature protection for bare crystals, high reliability soldering at ultra-small pitch (e.g. 0.9mm), resistance to environmental stress, suitable for advertising screens, traffic signs and other scenarios.
5. Summary and trends
Key balance point: need to consider the chip size, packaging, terminal environment, production requirements and costs, find the optimal solution between precision, thermal management, strength, reliability and efficiency.
Suggestions for selection: Give priority to the solder paste brands and models optimized for Mini/Micro LED (such as Fitech, AIM, Heraeus, etc.), whose products have been rigorously verified, which can significantly reduce the risk of selection.
Future Trends: With the further reduction of chip size (<20μm), continued shrinkage of pixel pitch (P0.4 or less) and new substrate (e.g., glass-based) applications, the requirements for solder paste particle size, low-temperature, activity, void rate, etc. will continue to improve, driving continuous innovation in materials technology.
