High‑Aspect‑Ratio LTCC Via Drilling: Laser Process Optimisation Guide

As LTCC (Low‑Temperature Co‑fired Ceramic) technology advances toward higher layer counts and finer feature sizes, the demand for high‑aspect‑ratio vias has become increasingly common. Aspect ratio—defined as the ratio of via depth (substrate thickness) to via diameter—directly influences electrical performance, manufacturability, and reliability. While a standard via in a 100 μm thick tape with a 100 μm diameter has an aspect ratio of 1:1, advanced designs often require aspect ratios of 3:1, 5:1, or even higher. These high‑aspect‑ratio vias present unique challenges: tapered walls, incomplete drilling, debris entrapment, and inconsistent metallisation.

1. Understanding the Challenge: Why High Aspect Ratio Matters

Geometric constraints: As the via diameter decreases and the substrate thickness increases, the laser beam must penetrate deeper into the material while maintaining a focused spot. However, the beam naturally diverges as it travels through the material, causing the via to taper—wider at the entry and narrower at the exit. Excessive taper can lead to:

• Incomplete via filling (voids or incomplete metallisation)

• Higher electrical resistance

• Reduced mechanical strength at the via neck

Debris evacuation: In high‑aspect‑ratio drilling, the ejected material must travel a longer distance to escape the hole. If debris is not effectively removed, it can re‑deposit on the via walls, obstruct the hole, or contaminate the surrounding surface.

Heat accumulation: Multiple pulses are often required to drill through thick tapes. Without adequate cooling between pulses, heat can accumulate in the via, enlarging the heat‑affected zone (HAZ) and potentially damaging the green tape.

Taper control: Achieving vertical via walls (minimal taper) is critical for consistent via filling and reliable electrical performance. Taper is influenced by focus position, beam quality, pulse energy, and the number of passes.

2. Focus Positioning: The Key to Vertical Via Walls

Focus positioning is arguably the single most important parameter for achieving clean, vertical via walls in high‑aspect‑ratio drilling. The laser beam has a characteristic depth of focus (DOF) —the range over which the beam remains sufficiently focused to ablate material efficiently. If the focus is positioned incorrectly, the via will exhibit excessive taper, poor edge quality, or incomplete penetration.

2.1 Focus Position Relative to Tape Surface

Entry‑side focusing (focus at the top surface):

• The beam is smallest at the entry, producing a clean, well‑defined top edge.

• However, as the beam travels through the material, it diverges, causing the via to taper significantly. The exit diameter may be 50% or less of the entry diameter.

• This approach is suitable for thin tapes or when a tapered via is acceptable.

Exit‑side focusing (focus at the bottom surface):

• The beam converges as it travels through the material, reaching its smallest diameter at the exit.

• The entry diameter is larger, but the exit is clean and well‑defined. Taper is reversed (wider at top, narrower at bottom).

• This approach can produce more vertical walls in thick tapes but may cause excessive entry‑side ablation.

Mid‑depth focusing (focus at the center of the tape thickness):

• The beam is relatively collimated through the middle of the tape, producing more symmetrical vias with minimal taper.

• This is often the optimal choice for high‑aspect‑ratio drilling in LTCC green tape.

• The exact focus position should be determined experimentally for each tape thickness and via diameter.

Note: For a 200 μm thick tape and a 75 μm via (aspect ratio ~2.7:1), start with the focus positioned at 40–60% of the tape thickness from the top surface. Adjust based on taper measurements.

2.2 Beam Quality and Focusing Optics

The achievable via taper is also influenced by the beam quality factor (M²) and the numerical aperture (NA) of the focusing lens.

• Lower M² (closer to 1) produces a tighter focus and longer depth of focus, which is beneficial for high‑aspect‑ratio drilling.

• Higher NA produces a smaller spot size but shorter depth of focus. While a small spot is desirable for fine vias, the short DOF can cause excessive taper in thick tapes.

• Compromise: For high‑aspect‑ratio vias, a moderate NA (e.g., 0.1–0.15) with a long working distance is often preferred. This provides a balance between spot size and depth of focus.

Focal spot size should be approximately 20–30% smaller than the target via diameter to allow for some beam expansion during drilling.

2.3 Dynamic Focus Adjustment

For very high aspect ratios (>5:1), static focus positioning may be insufficient. In such cases, dynamic focus adjustment—moving the focus deeper into the material as the via is drilled—can maintain a more consistent beam diameter throughout the depth.

This can be achieved by:

• Motorised focus stage: The focusing lens is mounted on a motorised axis that moves synchronously with the drilling progress.

• Telescope optics: A pair of motorised lenses adjusts the beam divergence in real time, effectively shifting the focus without moving the workpiece.

Dynamic focus is particularly useful for drilling through multilayer stacks or very thick tapes (>300 μm).

3. Pulse Energy Management: Balancing Speed and Quality

Pulse energy determines how much material is removed per pulse and directly affects the drilling rate, HAZ, and via quality.

3.1 Pulse Energy Selection

Too low:

• Insufficient material removal per pulse → more pulses required → longer drilling time.

• Risk of incomplete penetration if energy is below the ablation threshold for the ceramic.

Too high:

• Excessive material removal → rough via walls, debris, and large HAZ.

• Risk of "hole explosion"—violent ejection of material that can damage the via and surrounding area.

• Potential for micro‑cracks due to thermal stress.

Optimal range for LTCC green tape:

• For nanosecond UV lasers (355 nm, 10–50 ns): 10–50 μJ per pulse is typical, depending on tape thickness and via size.

• For picosecond UV lasers (355 nm, ~10 ps): 5–30 μJ per pulse is typical.

Practical recommendation: Start with a pulse energy that removes approximately 10–20% of the tape thickness per pulse. This ensures controlled material removal and allows the debris to evacuate between pulses.

3.2 Repetition Rate and Pulse Overlap

The repetition rate (pulse frequency) determines how quickly pulses are delivered. Higher repetition rates increase throughput but also increase heat accumulation.

For high‑aspect‑ratio drilling:

• Use a moderate repetition rate (e.g., 50–150 kHz for nanosecond, 200–500 kHz for picosecond) to allow some cooling between pulses.

• Ensure that the pulse overlap (spatial overlap between consecutive pulses) is sufficient to produce a continuous cut but not so high that excessive heat accumulates.

4. Scan Strategies: Optimising the Drilling Trajectory

The scan strategy—how the laser beam moves relative to the workpiece—has a profound impact on via quality and drilling efficiency.

4.1 Single‑Pass vs. Multi‑Pass Drilling

Single‑pass drilling:

• The laser drills through the entire tape thickness in one continuous motion.

• Suitable for thin tapes and low aspect ratios (<2:1).

• Risk of taper and debris entrapment in high‑aspect‑ratio vias.

Multi‑pass drilling:

• The laser makes multiple passes over the via location, gradually increasing depth.

• Allows debris to evacuate between passes.

• Reduces heat accumulation and taper.

• Recommended for high‑aspect‑ratio vias (>3:1).

Multi‑pass strategy step-by-step:

1. First pass: Drill to ~30% of the depth.

2. Second pass: Drill to ~60% of the depth.

3. Third pass: Drill through the remaining thickness.

4. Optional fourth pass: Clean‑up pass at reduced power to smooth the walls.

4.2 Trepanning vs. Percussion Drilling

Percussion drilling: The laser remains stationary (or moves slightly) while multiple pulses drill the via. Fast for small vias but prone to taper and debris in high‑aspect‑ratio applications.

Trepanning drilling: The laser beam moves in a circular path to cut the via perimeter. Produces cleaner, more vertical walls. Recommended for vias >75 μm diameter with aspect ratio >3:1.

Trepanning parameters:

• Circle diameter: Slightly smaller than the target via diameter (e.g., 80% of target).

• Number of revolutions: 2–5, depending on thickness.

• Overlap between successive revolutions: 30–50%.

4.3 Helical Drilling

Helical drilling combines percussion and trepanning: the beam moves in a spiral path, gradually descending into the material. This approach distributes heat more evenly, allows debris to escape along the spiral path, and produces excellent wall quality in high‑aspect‑ratio vias (>5:1).

Helical parameters:

• Spiral pitch: 10–30 μm per revolution.

• Number of revolutions: Determined by tape thickness and pitch.

• Scan speed: 500–1000 mm/s.

4.4 Skip‑Drilling for Thermal Management

When drilling multiple vias in close proximity, heat can accumulate in the region, causing excessive HAZ and potential delamination. Skip‑drilling—drilling vias in a non‑sequential order (e.g., alternating between distant locations)—allows heat to dissipate between drilling operations. This strategy is especially important for high‑density via arrays.

5. Cleaning Steps: Ensuring Debris‑Free Vias

Even with optimised drilling parameters, some debris may remain on the via walls or surrounding surface. Proper cleaning is essential for subsequent via filling and lamination.

5.1 In‑Process Cleaning

Gas assist: A coaxial or side‑jet gas stream (typically compressed air or nitrogen) helps eject debris from the via during drilling. For high‑aspect‑ratio vias, a pulsed gas jet synchronised with the laser pulses can improve debris removal.

Vacuum extraction: A vacuum nozzle positioned near the drilling site can capture airborne debris before it settles on the surface.

5.2 Post‑Process Cleaning

Ultrasonic cleaning: Immerse the drilled green tape in a suitable solvent (e.g., isopropyl alcohol or a proprietary green tape cleaner). Apply ultrasonic agitation for 2–5 minutes. Rinse and dry with filtered air or nitrogen.

Plasma cleaning: Use an oxygen plasma to remove organic contaminants and oxidise any residual carbon. Effective for preparing via walls for metallisation.

Micro‑ablation clean‑up pass: After drilling, a low‑energy laser pass (below the ablation threshold for the ceramic) can be used to gently remove any loosely adhered debris from the via walls.

5.3 Inspection and Quality Control

After cleaning, vias should be inspected for:

• Wall quality: Use SEM or optical microscopy to check for debris, recast, or cracks.

• Taper: Measure entry and exit diameters to calculate taper angle.

• Roundness: Check for deviation from circularity.

• HAZ: Inspect for discolouration or binder degradation around the via.

Acceptance criteria (typical for LTCC):

• Taper angle: <10° for aspect ratios up to 5:1.

• Roundness deviation: <5% of via diameter.

• No visible debris or recast on via walls.

• HAZ: <20 μm (nanosecond) or <5 μm (picosecond).

6. Troubleshooting Guide for High‑Aspect‑Ratio Via Drilling

Conclusion: Chanxan Laser Solutions for High‑Aspect‑Ratio LTCC Via Drilling

High‑aspect‑ratio via drilling in LTCC green tape demands precise control over focus positioning, pulse energy, scan strategies, and cleaning protocols. By implementing the optimisation techniques described in this article, manufacturers can achieve clean, vertical via walls with minimal taper, reduced HAZ, and consistent electrical performance—even at aspect ratios exceeding 5:1.

Chanxan Laser offers advanced UV nanosecond and picosecond laser systems specifically engineered for LTCC green tape processing. Whether you are developing next‑generation 5G RF modules, high‑density interposers, or automotive radar substrates, Chanxan Laser provides the technology and expertise to meet your high‑aspect‑ratio via drilling challenges.