Analysis of Causes and Solutions for Small Solder Balls Generation in Laser Soldering Process

Analysis of Causes and Solutions for Small Solder Balls Generation in Laser Soldering Process

The formation of small solder balls (or solder splashes) during laser soldering is a common defect in electronic packaging, which not only affects the aesthetics of solder joints but may also lead to reliability issues such as circuit short circuits and degraded electrical performance. The causes involve multiple factors including materials, process parameters, and environmental control, requiring systematic optimization for resolution. Songsheng Optronics provides a detailed analysis of the causes and corresponding solutions:

I. Primary Causes of Small Solder Ball Formation 

1. Improper Temperature Control

Excessive laser energy or rapid heating: Sudden localized temperature spikes cause overly fast melting of solder paste, leading to instant vaporization and expansion of solvents/flux, which explosively eject molten solder (splashing).

Insufficient preheating: Inadequate preheating of substrates or solder paste results in uneven heating during soldering, causing rapid escape of volatile components and "solder explosion."

2. Defects in Flux and Solvent Properties

Mismatched flux activity: Overly active flux generates excessive gas, while insufficient activity fails to remove oxides effectively, causing poor wetting and solder splashing.

Uncontrolled solvent vaporization: Low-boiling-point solvents vaporizing excessively at low temperatures or high solvent content both aggravate solder ejection.

3. Solder Paste Material and Property Issues

Insufficient viscosity: When viscosity is below 60 Pa·s, the solder paste's anti-splashing capability significantly deteriorates, making it more prone to forming large solder balls.

Excessively fine or unevenly distributed solder particles: A high content of micro-powder (<20 μm) tends to agglomerate into balls during melting.

Oxidized or moisture-contaminated solder paste: Exposure to humid environments causes absorbed moisture to vaporize at high temperatures, triggering solder spattering ("solder explosion").

4. Substrate and Contamination Factors

Oxidized or contaminated pads: Contaminants such as oils or dust hinder proper solder wetting, causing molten solder to contract into balls.

Stencil printing defects: Misalignment between the stencil and pads or excessive squeegee pressure leads to solder paste overflow, forming solder balls after reflow.

5. Poor Process Parameter and Equipment Compatibility

Insufficient laser focusing accuracy: Beam misalignment or uneven energy distribution results in localized overheating.

Lack of splash mitigation design: In traditional open soldering setups, small solder balls generated from spattering directly scatter onto the substrate.

II. Systematic Solutions

1. Optimize Soldering Parameters & Temperature Profile

Staged Temperature Control:

Preheating phase: Maintain heating rate at 1~2°C/s to allow gradual solvent evaporation (below 160°C).

Soldering phase: Use constant laser power (e.g., 1W + 300ms duration for Sn58Bi solder paste) to avoid excessive peak temperatures.

Real-Time Temperature Feedback: Integrate infrared thermometry (e.g., PID closed-loop control) to dynamically adjust laser power for stable molten pool temperature.

2. Material Selection & Handling

Solder Paste Optimization:

Use high-viscosity paste (>60 Pa·s) to significantly reduce solder ball formation.

Select high-boiling-point solvents (e.g., rosin-based) and slow-release activators to minimize vaporization shock.

Strict Material Management:

Store solder paste at 40~60% RH, with pre-use thawing & stirring.

Clean and pre-bake substrates (120°C/2h) to remove oxides and moisture before assembly.

3. Process Control

Stepwise Laser Heating: Start with low-power preheating, then gradually ramp to soldering temperature to avoid thermal shock.

Precision Dispensing: Ensure 100% alignment between dispensing valve and pads, with optimized solder volume (uniform coverage as benchmark).

4. Equipment & Structural Improvements

Splash Guard Integration:

Use enclosed nozzle designs (e.g., U-shaped/funnel inlet) to confine molten solder and physically block splashes.

Design tapered inner walls to focus laser beams and collect ejected particles.

High-Precision Positioning:

Combine CCD vision alignment + 6-axis motion stage for <±10μm laser-spot accuracy.

5. Environment & Operational Standards

Workshop Conditions: Maintain 22~28°C temperature and 40~60% RH to minimize environmental variability.

Post-Soldering Inspection: Implement AOI (Automated Optical Inspection) to detect solder balls, followed by localized cleaning/rework if needed.

III. Summary

The issue of small solder balls in laser soldering requires coordinated optimization across four dimensions: materials, processes, equipment, and environment:

Material aspect: Select solder paste with high viscosity and high boiling point solvents, strictly control incoming material status;

Process aspect: Adopt segmented temperature control curves combined with precision solder dispensing technology;

Equipment aspect: Integrate protective shields to intercept splashes, improve positioning and thermal management accuracy through CCD vision and temperature control systems;

Environment aspect: Maintain stable temperature and humidity, enhance post-soldering inspection and cleaning.

Through these measures, solder ball rate can be significantly reduced (actual measurements show <5%), improving micro-solder joint reliability. For high-precision applications, constant-temperature laser soldering equipment (such as integrated infrared feedback + dual Y-axis platforms) is recommended to achieve adaptive parameter adjustment.

Songsheng Optronics Constant-Temperature Laser Soldering Machine Advantages

1. Temperature feedback semiconductor laser welding system: The temperature feedback function enables temperature control during welding,  allowing temperature monitoring in micro-areas with diameters of 0.3-1.5mm; adjustable soldering temperature range of 100-600°C; precise temperature control with accuracy within ±3°C.

2. Multi-station welding system: Based on six-axis high-precision multi-station laser welding system, integrates visual positioning, solder paste dispensing and laser welding operations, improving efficiency by over 20% and significantly enhancing manufacturing capacity.

3. Solder dispensing mechanism: High-precision solder paste dispensing mechanism enables precise solder quantity control through program settings, with dispensing accuracy up to ±0.02g.

4. Vision positioning system: Adopts automatic image capture for custom welding paths, capable of collecting multiple feature points on the same product, greatly improving processing efficiency and accuracy.

5. Coaxial motion system: Laser, CCD, temperature measurement and indicator light share the same optical axis, eliminating complex optical alignment procedures and effectively improving welding efficiency.