PCB and Chip Sample Preparation

Introduction

The preparation of printed circuit boards (PCBs) and semiconductor chips for metallographic analysis presents unique challenges that require specialized techniques and equipment. Unlike traditional metallographic samples, PCBs and chips consist of multiple layers of different materials—copper traces, solder, epoxy substrates, silicon, and various protective coatings—each with different mechanical properties and removal rates.

Successful preparation of these samples requires precision, patience, and the right equipment. The goal is to reveal cross-sections that show the true structure of interconnects, solder joints, via structures, and layer interfaces without introducing artifacts such as delamination,smearing, or relief between different materials.

Computer chip cross-section showing multi-layer structure. Proper preparation reveals interconnects, via structures, and layer interfaces without artifacts. This demonstrates the precision required for electronics sample preparation.

Growing Demand in Electronics

The explosion of artificial intelligence (AI) and machine learning applications has dramatically increased the demand for PCB and chip analysis. AI chips, GPUs, and specialized processors require rigorous quality control, failure analysis, and process validation. This has led to a massive increase in metallographic work on electronic components.

Why the Demand is Growing

• AI Chip Development: New AI processors require validation of complex multi-layer structures, interconnects, and thermal management features
• Failure Analysis: As devices become more complex, understanding failure modes requires precise cross-sectional analysis
• Quality Control: Manufacturing processes must be validated to ensure reliability in critical applications
• Process Development: New manufacturing techniques require metallographic validation
• Reverse Engineering: Competitive analysis and intellectual property verification
• Reliability Testing: Understanding degradation mechanisms in electronic components

This increased demand has made expertise in PCB and chip preparation more valuable than ever. Laboratories specializing in electronics analysis are seeing unprecedented workloads, and the need for efficient, reliable preparation methods has never been greater.

Unique Challenges in PCB and Chip Preparation

Preparing PCBs and chips for metallographic analysis presents several unique challenges that distinguish this work from traditional metallography:

Multi-Material Systems

PCBs and chips contain multiple materials with vastly different properties:

• Copper: Soft, ductile, prone to smearing
• Solder: Very soft, low melting point, can flow during preparation
• Epoxy/FR4: Brittle, can delaminate, different removal rate than metals
• Silicon: Hard, brittle, can chip or crack
• Protective Coatings: Often very thin, easily removed or damaged
• Dielectric Layers: Soft, can be easily scratched or deformed, requiring careful polishing to avoid artifacts

Delicate Structures

Modern electronics contain extremely fine features:

• Trace widths down to micrometers or even nanometers
• Via diameters of 50-100 μm or smaller
• Layer thicknesses of 10-50 μm
• Solder joints with critical dimensions requiring precise preparation
• Bond wires and interconnects

Precision Requirements

Many applications require precise depth control:

• Targeting specific layers or features
• Removing exact amounts of material (metered removal)
• Preparing samples for specific analysis techniques (SEM, FIB, TEM)
• Maintaining planarity across the entire sample to avoid relief

Sectioning PCBs and Chips

Sectioning is the first critical step in PCB and chip preparation. The goal is to cut through the sample to reveal the cross-section of interest while minimizing damage to delicate structures.

Thin precision cutting blades (0.3-0.5 mm) minimize kerf loss and reduce damage to delicate PCB and chip structures. Precision wafering techniques are ideal for electronics.

Sectioning Considerations

• Low Speed: Use slow cutting speeds (50-150 RPM) to minimize heat generation and mechanical damage
• Thin Blades: Use thin diamond blades or abrasive blades (0.3-0.5 mm) to minimize kerf loss and reduce damage
• Coolant: Adequate coolant is essential to prevent overheating, which can damage solder joints and cause delamination
• Orientation: Plan the cut direction to reveal the features of interest (traces, vias, solder joints)
• Support: Support fragile samples to prevent cracking during cutting

Sectioning Procedure

1. Identify the area of interest and mark the cutting plane
2. Secure the sample in the cutting fixture
3. Use a thin, fine-grit abrasive blade or diamond blade
4. Cut at low speed with steady, light pressure
5. Use continuous coolant flow
6. Allow the blade to do the cutting—avoid forcing
7. After cutting, clean the sample thoroughly to remove cutting debris

Mounting Techniques

Mounting provides edge retention and protects delicate structures during preparation. For PCBs and chips, mounting is critical because the multi-layer structures are prone to delaminationand edge damage.

Epoxy castable mounting resin provides excellent edge retention for delicate PCB structures.

Mounting equipment and accessories for both compression and castable mounting.

Mounting Material Selection

Epoxy Resin: Preferred for PCBs and chips because:

• Excellent edge retention
• Low shrinkage minimizes stress on delicate structures
• Transparent options allow viewing of sample orientation
• Good adhesion to various materials
• Can be used at room temperature (castable) or elevated temperature (compression)

Mounting Procedure

1. Clean the sample thoroughly to remove cutting fluid and debris
2. If using castable epoxy, mix according to manufacturer's instructions
3. Place sample in mounting cup with cut surface facing up
4. Pour epoxy carefully to avoid bubbles
5. Allow to cure completely (typically 4-8 hours for room temperature cure)
6. For compression mounting, use low pressure (1000-2000 psi) and moderate temperature (120-150°C)

Grinding and Initial Preparation

Grinding removes sectioning damage and prepares the surface for controlled removal polishing. For PCBs and chips, grinding must be done carefully to avoid delamination and maintain the integrity of multi-layer structures.

Silicon carbide (SiC) abrasive grinding papers in fine grit sizes (400, 600, 800, 1200) for progressive grinding. Use light pressure to avoid delamination of delicate PCB layers.

Grinding Sequence

Use a progressive grinding sequence with fine grits:

1. 400 grit: Remove sectioning damage (30-60 seconds, light pressure)
2. 600 grit: Remove previous scratches (30-60 seconds)
3. 800 grit: Further refinement (30-60 seconds)
4. 1200 grit: Final grinding step (30-60 seconds)

Grinding Best Practices

• Light Pressure: Use minimal pressure to avoid delamination
• Water Lubrication: Continuous water flow prevents overheating
• Rotation: Rotate sample 90° between grits to ensure complete scratch removal
• Time Control: Don't over-grind—just remove previous scratches
• Inspection: Check frequently under low magnification to monitor progress

Controlled Removal (ATTO) - The Critical Technique

Controlled removal, also known as metered removal, is the most important technique for PCB and chip preparation. This method allows for precise material removal in micrometer-level increments, making it essential for applications requiring exact depth control or targeting specific features.

How Controlled Removal Works

Controlled removal systems, such as the ATTO-1000S, use precision measurement and feedback to remove material in exact increments. The system monitors removal in real-time with micrometer-level resolution, allowing operators to:

• Remove precise amounts of material (e.g., 5 μm, 10 μm, 50 μm)
• Target specific layers or features
• Maintain consistent removal rates across the sample
• Prepare multiple samples to the same depth
• Achieve planarity across multi-material systems

The ATTO-1000S Controlled Removal Polisher provides micrometer-level precision essential for PCB and chip preparation. Real-time monitoring and feedback ensure precise material removal to target specific features or depths.

Key Features of the ATTO-1000S

• Micrometer-Adjustable Control: Pitch and roll control for exacting results
• Real-Time Monitoring: 0.2 micron resolution removal monitoring
• Precise Load Control: Sample load control from 0-300 grams
• Variable Speed: Optimal material removal rates for different materials
• Touchscreen Interface: Easy operation and parameter adjustment
• Planarity Control: Maintains flatness across multi-material samples

Controlled Removal Procedure

1. Initial Setup:
   • Mount sample securely in holder
   • Set initial load (typically 50-150 grams for PCBs)
   • Select appropriate polishing pad and compound
   • Set polishing speed (typically 50-100 RPM)
2. Calibration:
   • Calibrate the system according to manufacturer's instructions
   • Verify removal rate with a test sample if needed
3. Polishing:
   • Start with coarse diamond (9 μm or 6 μm) for initial material removal
   • Monitor removal in real-time on the display
   • Adjust parameters as needed to maintain desired removal rate
   • Continue until target depth is reached or sectioning damage is removed
4. Progressive Refinement:
   • Move to finer diamond sizes (3 μm, 1 μm) for surface refinement
   • Continue monitoring removal to ensure consistent progress
   • Adjust load and speed for optimal results
5. Final Polish:
   • Use 0.05 μm colloidal silica for final polish
   • Minimal removal at this stage—focus on surface finish
   • Verify final depth and surface quality

Applications Requiring Controlled Removal

• IC Flip Chip Preparation: Precise removal to reveal bump structures and interfaces
• Via Analysis: Targeting specific via layers or depths
• Layer Thickness Measurement: Removing exact amounts to measure layer thicknesses
• Failure Analysis: Preparing samples to reveal failure sites at specific depths
• SEM/FIB Preparation: Preparing samples for electron microscopy at precise depths
• Multi-Sample Consistency: Preparing multiple samples to the same depth for comparison
• Planarity Requirements: Maintaining flatness across multi-material systems

Polishing Techniques

After controlled removal, final polishing prepares the surface for high-magnification analysis. The goal is to achieve a mirror-like finish without introducing artifacts or removing too much material.

Diamond polishing compounds in various particle sizes for removing grinding scratches.

Various polishing pads and cloths for different polishing stages.

Diamond Polishing Sequence

If not using controlled removal for all steps, use this sequence:

1. 9 μm diamond: 3-5 minutes on hard cloth (Texmet or similar)
2. 3 μm diamond: 3-5 minutes on medium-hard cloth
3. 1 μm diamond: 2-3 minutes on soft cloth

Final Polishing

Final polish with colloidal silica:

Colloidal silica for oxide polishing produces excellent surface finishes.

1. 0.05 μm colloidal silica: 1-2 minutes on soft cloth
2. Use light pressure to minimize material removal
3. Rinse thoroughly with water
4. Dry with compressed air (avoid touching the surface)

Polishing Considerations for PCBs and Chips

• Light Pressure: Always use minimal pressure to avoid delamination
• Short Times: Don't over-polish—just remove scratches from previous step
• Frequent Inspection: Check progress frequently under microscope
• Clean Between Steps: Thoroughly clean sample and change cloths between steps
• Material-Specific Rates: Be aware that different materials polish at different rates, which can create relief

Specific Applications

Flip Chip Analysis

Flip chip packages require precise preparation to reveal bump structures, underfill, and interfaces. Controlled removal is essential to target specific bump rows or reveal the chip-to-substrate interface.

Computer chip cross-section showing flip chip structure. Proper preparation with controlled removal reveals bump structures, underfill, and layer interfaces critical for failure analysis.

Via and Through-Hole Analysis

Vias and through-holes are critical features in PCBs. Preparation must reveal:

• Via wall quality and plating thickness
• Via fill material (if present)
• Connection quality at layer interfaces
• Void detection in via structures

Solder Joint Analysis

Solder joints are prone to defects and require careful preparation to reveal:

• Intermetallic compound formation
• Void content and distribution
• Crack initiation and propagation
• Wetting quality

Layer Thickness Measurement

Precise layer thickness measurements require controlled removal to:

• Remove exact amounts of material
• Reveal layer interfaces clearly
• Maintain planarity for accurate measurements
• Prepare multiple samples consistently

Failure Analysis

Failure analysis often requires targeting specific failure sites:

• Precise depth control to reach failure location
• Minimal damage to preserve failure evidence
• Consistent preparation for comparison samples
• Preparation for SEM/FIB analysis

Troubleshooting Common Issues

Delamination

Symptoms: Layers separating, especially at interfaces

Solutions:

• Reduce polishing pressure
• Use shorter polishing times
• Ensure adequate mounting material penetration
• Consider vacuum impregnation during mounting
• Use controlled removal for more consistent results

Relief Between Materials

Symptoms: Height differences between copper, solder, and substrate

Solutions:

• Use controlled removal to maintain planarity
• Adjust polishing parameters for different materials
• Use appropriate polishing pads (harder pads for soft materials)
• Minimize final polish time to reduce relief

Smearing

Symptoms: Copper or solder smeared across surface

Solutions:

• Use harder polishing pads
• Reduce polishing pressure
• Use appropriate diamond size (not too fine too early)
• Ensure adequate lubrication
• Clean sample thoroughly between steps

Scratches

Symptoms: Visible scratches in final polish

Solutions:

• Ensure complete scratch removal at each grinding step
• Rotate sample 90° between grits
• Use fresh polishing compounds
• Clean polishing pads/cloths between uses
• Don't skip grit sizes in the grinding sequence

Over-Removal

Symptoms: Target feature removed or too much material removed

Solutions:

• Use controlled removal for precise depth control
• Monitor removal progress frequently
• Reduce polishing time
• Use lighter pressure
• Document parameters for future reference