Reasons and Solutions for Pores in Solder Joints in Laser Soldering

Reasons and Solutions for Pores in Solder Joints in Laser Soldering

Laser soldering offers many advantages, such as high efficiency and speed. However, during the laser soldering process, various factors may lead to the formation of pores in the solder joints. Songsheng Optoelectronics will introduce the causes of pores in laser-soldered joints and the corresponding solutions. Let’s take a look.

Causes of Pores in Laser Soldering Joints

(1) Workpiece Factors

Volatile Substances on the Workpiece Surface

• Oil and Moisture: If the workpiece surface is contaminated with oil or moisture, these substances rapidly vaporize during laser heating. For example, if the pins of electronic components are not thoroughly cleaned before soldering, trace amounts of oil or moisture on the surface will turn into gas under laser irradiation. Since laser soldering is a relatively fast process, these gases become trapped in the solder before they can escape, resulting in pores.

• Organic Residues: Some workpieces may retain organic residues from the manufacturing process, such as plastic mold release agents. When heated by the laser, these organic compounds decompose and release gas, which can also easily form pores in the solder joints.

Characteristics of the Workpiece Material

Porous Materials: If the workpiece is made of a porous material (e.g., certain powder metallurgy products), the internal pores may trap air or other gases. During laser soldering, these gases escape as the solder melts and become trapped within the molten solder, forming pores.

(2) Solder Material Factors

High Gas Content in Solder

• Moisture-Absorbed Solder: Solder materials may contain moisture if improperly stored. For example, solder paste can absorb humidity from the air when storage conditions are inadequate. During laser soldering, this moisture vaporizes into steam, leading to pore formation in the solder joints.

• Intrinsic Gas in Solder: Some solder materials may retain trapped gas from the manufacturing process. Insufficient degassing during mixing or casting can leave microscopic bubbles within the solder. When melted during laser soldering, these bubbles expand and become trapped in the joint, resulting in pores.

(3) Solder Fluidity and Viscosity Issues

Poor Fluidity:
The fluidity of solder significantly affects gas escape during the melting process. When solder exhibits poor fluidity (e.g., due to incorrect composition or insufficient temperature), its high viscosity hinders gas movement, trapping bubbles that solidify into pores. For instance, off-spec solder formulations or inadequate heating can increase viscosity, preventing gas evacuation.

Viscosity Fluctuations:
During soldering, viscosity may shift abruptly due to temperature variations or compositional changes (e.g., oxidation). A sudden viscosity spike can arrest gas expulsion mid-process, creating pores. Oxidized solder, for example, often experiences increased viscosity, exacerbating gas entrapment.

((3) Welding Parameters and Process Factors

Improper Laser Power and Welding Time

Excessive Laser Power:

• When laser power is set too high, the solder melts too rapidly, causing violent vaporization and generating excessive steam within the molten solder.

• If the welding time is too short, this steam cannot escape in time, resulting in trapped pores.

• Example: When soldering small electronic components, excessively high laser power produces a large volume of steam almost instantly, while the solder lacks sufficient time to flow and release the gas.

Insufficient Welding Time:

• A too-short welding duration prevents adequate gas venting from the molten solder.

• Example: Larger solder joints require extended melting time to allow gas bubbles to rise and escape. Inadequate time leaves gases trapped, forming pores.

Excessive Welding Speed:

• Overly fast welding reduces dwell time at each solder point, analogous to how a gust of wind blowing over water doesn’t allow bubbles to surface.

• The rapid movement denies gases enough time to evacuate, leading to porosity in the solidified joint.

Improper Use of Shielding Gas

Excessive Shielding Gas Flow Rate:
The primary function of shielding gas is to prevent solder oxidation during the welding process. However, when the gas flow rate is too high, it generates strong turbulent flow in the welding zone. This turbulent flow can interfere with the normal melting and solidification processes of the solder, making it more likely for gas bubbles to become trapped within the solder and form pores.

Impure Shielding Gas:
When the shielding gas contains impurities such as oxygen or moisture, these contaminant gases may either react chemically with the solder or directly form bubbles within the molten solder. Both scenarios can lead to the formation of pores in the solder joints.

Solutions to Address Pore Formation in Solder Joints

(1) Workpiece Preparation

Surface Cleaning:

• Thoroughly clean the workpiece surface before soldering using appropriate solvents (e.g., alcohol, acetone) to remove oil, moisture, and organic residues.

• Example: For electronic component leads, carefully wipe with alcohol-soaked cotton swabs to ensure cleanliness. For plastic parts with mold release agents, use specialized cleaning solutions.

Porous Material Treatment:

• For porous materials (e.g., powder metallurgy components):
       a) Preheating: Preheat the workpiece to allow trapped gases to escape prior to soldering.
       b) Vacuum Soldering: Employ vacuum soldering techniques to minimize gas entrapment effects.

(2) Solder Material Management

Ensure Solder Quality:

• Select high-quality, dry solder materials.

• For solder paste:
       • Store in dry conditions to prevent moisture absorption
       • Check for moisture contamination before use (e.g., thinning or condensation)
       • Discard if moisture damage is detected

Optimize Solder Fluidity:

• Improve flow characteristics by:
       • Selecting appropriate solder alloy compositions
       • Adding suitable flux agents
       • Incorporating surface tension-reducing additives

• Maintain  proper soldering temperature range to ensure optimal fluidity

(3) Process Parameter Optimization

Laser Power and Duration Adjustment:

• Customize parameters based on:
       • Workpiece material and size
       • Solder type

• For porosity-prone applications:
       • Reduce laser power
       • Extend soldering duration

• Conduct DOE (Design of Experiments) to identify optimal settings

Welding Speed Control:

• Moderate speed to:
       • Allow sufficient gas venting time
       • Maintain production efficiency

• Example: When soldering multi-pin ICs, reduce speed to enable proper gas escape

Protective Gas Application:

• Use high-purity shielding gas (e.g., argon for precious metals)

• Optimize flow rate through testing to:
       • Prevent oxidation
       • Minimize flow-induced disturbances

• Implementation method:

1. Start with low flow rate

2. Gradually increase while monitoring results

3. Establish optimal flow range