What Is “Cold Processing”? Pulse Duration, HAZ, and Microcracks Explained

Why Pulse Duration Matters More Than Laser Power

A common misconception in laser processing is that lower power automatically means less heat. In reality, pulse duration is the dominant factor governing thermal effects.

Pulse Duration vs. Thermal Diffusion Time

Every material has a characteristic thermal diffusion time—the time required for absorbed energy to spread from the interaction zone into the surrounding lattice.

• If the laser pulse duration is longer than the diffusion time → heat spreads and melting occurs
• If the laser pulse duration is shorter than the diffusion time → material removal happens before heat transfer

Nanosecond lasers typically operate within or beyond the thermal diffusion window. Picosecond and femtosecond lasers operate well below it, fundamentally changing how energy interacts with matter.

From Nanosecond to Picosecond: Where the Heat Actually Goes

Nanosecond Lasers: Heat Accumulation and Melting

With nanosecond pulse durations:

• Energy couples into the lattice as heat
• Local melting occurs
• Molten material resolidifies after the pulse

This melt–recast cycle produces:

• Heat-affected zones
• Residual stress
• Microcracks initiated at resolidified boundaries

For ductile metals, this may be acceptable. For brittle materials such as glass and sapphire, it often leads to premature failure.

Picosecond Lasers: Ablation Before Heat Diffusion

With picosecond pulses:

• Energy is first absorbed by electrons
• The lattice does not have time to respond thermally
• Material is removed via direct ablation rather than melting

Because heat transfer is suppressed:

• HAZ is dramatically reduced
• Edge geometry remains sharp and consistent
• Subsurface damage is minimized

This is the physical basis of cold processing laser machining.

HAZ Explained: Why It Matters in Brittle Materials

The heat-affected zone (HAZ) is the region where material properties change due to thermal exposure rather than direct material removal.

In brittle materials:

• Thermal expansion is poorly accommodated
• Localized heating creates internal stress
• Stress concentrations become crack initiation points

Even when edge geometry appears acceptable, a hidden HAZ can:

• Reduce edge strength
• Lower drop-test performance
• Cause delayed failure during assembly or thermal cycling

Controlling HAZ is therefore essential for maintaining glass fracture strength and long-term reliability.

Microcracks: The Hidden Failure Mode

Microcracks are often invisible under standard inspection, yet they are one of the most common root causes of failure in glass and sapphire components.

How Microcracks Form

• Melting and resolidification introduce non-uniform cooling
• Residual tensile stress develops at the edge
• Cracks initiate at microscopic defects

Once formed, microcracks can propagate during:

• Mechanical loading
• Thermal cycling
• Chemical exposure

Why Ultrafast Lasers Reduce Microcracks

By avoiding melt-based material removal, ultrafast lasers:

• Limit stress concentration
• Reduce crack nucleation sites
• Preserve edge integrity at the microscopic level

This directly improves edge strength in glass cutting and reduces downstream yield loss.

Cold Processing in Real Manufacturing Scenarios

Cold processing is not an academic concept—it solves real production problems.

Cover Glass Cutting

Ultrafast lasers enable clean, chip-free edges with minimal post-processing, making them ideal for consumer electronics and optical components.

Thick Glass Cutting and Splitting

In thick glass cutting and splitting, controlled internal modification followed by separation produces stronger edges than mechanical scoring, especially in high-strength applications.

Sapphire and Optical Windows

Sapphire’s extreme hardness and brittleness make it highly sensitive to thermal stress. Cold ablation is often the only viable method for achieving acceptable quality.

When Is Nanosecond Laser Processing Still Acceptable?

Cold processing is not always required.

Nanosecond lasers may still be suitable when:

• Edge quality requirements are low
• Post-processing (grinding or polishing) is planned
• Cost sensitivity outweighs yield concerns

Recognizing these boundaries is part of sound engineering decision-making.

How to Choose: Nanosecond or Picosecond?

A simplified decision framework:

• Is the material brittle or transparent? → Picosecond
• Is edge strength critical? → Picosecond
• Is post-processing acceptable? → Nanosecond may suffice
• Is yield loss expensive? → Picosecond

In many modern glass and optical applications, cold processing is no longer optional—it is required.

Conclusion

Cold processing is not defined by the absence of heat, but by control over heat transfer. By shortening pulse duration below the thermal diffusion threshold, ultrafast lasers fundamentally change how materials respond to energy input.

For brittle materials, this enables:

• Minimal HAZ
• Reduced microcrack formation
• Higher edge strength and manufacturing yield

Understanding these mechanisms is essential when selecting laser technology for precision manufacturing.

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