Die Attach on Ceramic Substrates

die attach on ceramic substrates: what this playbook covers (and who it’s for)

This guide is designed for hardware engineers, procurement leads, and program managers who are transitioning high-power or high-frequency designs from prototype to mass production. Specifically, it addresses the critical process of die attach on ceramic substrates, where semiconductor chips are bonded directly to ceramic carriers (like Alumina or Aluminum Nitride) to maximize thermal performance and reliability.

In this playbook, you will find a structured approach to defining specifications, identifying hidden manufacturing risks, and validating supplier capabilities. We move beyond basic definitions to provide actionable checklists that help you secure a stable supply chain. The focus is on preventing field failures caused by poor thermal management or mechanical stress at the die-substrate interface.

At APTPCB (APTPCB PCB Factory), we often see projects delayed because the initial data package lacked specific die attach parameters. This guide bridges the gap between your design intent and the factory’s manufacturing reality. It ensures that when you request a quote, you are asking for a process that is robust, repeatable, and scalable.

When die attach on ceramic substrates is the right approach (and when it isn’t)

Understanding the specific use case for ceramic substrates is the first step in ensuring project viability, as this technology offers distinct advantages over standard FR4 or metal-core boards but comes with higher costs.

This is the right approach when:

Thermal Loads are Extreme: Your component density or power output generates heat that standard metal core PCBs (MCPCB) cannot dissipate efficiently. Ceramic offers superior thermal conductivity (24–170 W/mK or higher).
CTE Matching is Critical: You are using large bare dies. The Coefficient of Thermal Expansion (CTE) of ceramic (6–7 ppm/°C) closely matches silicon, reducing stress on the bond line during thermal cycling.
Hermeticity is Required: The application operates in harsh environments (aerospace, downhole drilling) where moisture ingress must be zero.
High-Frequency Performance: You require low dielectric loss for RF or microwave applications, where ceramic substrates provide superior signal integrity compared to organic laminates.
High Voltage Isolation: You need breakdown voltages exceeding 5kV in a compact footprint, which ceramic materials naturally provide.

This is NOT the right approach when:

Cost is the Primary Driver: If the thermal budget allows for FR4 with thermal vias or standard aluminum MCPCBs, ceramic is likely over-engineered and too expensive.
Mechanical Flexibility is Needed: Ceramic is brittle. If the assembly will be subjected to significant bending or twisting forces without a rigid housing, it will crack.
Standard SMT Components Only: If you are not attaching bare die (Chip-on-Board) and are only using packaged components that don't require extreme heat dissipation, standard PCB technologies are sufficient.

Specs & requirements (before quoting)

To avoid vague quotes and engineering queries that stall production, you must define the following 12 parameters clearly in your initial documentation package.

Substrate Material Grade: Specify the exact ceramic type. Do not just say "Ceramic." Specify Alumina (96% Al2O3) for standard cost-effective uses, or Aluminum Nitride (AlN) for high-performance thermal dissipation (>170 W/mK).
Metallization Technology: Define how the metal tracks are applied. Options include Direct Bonded Copper (DBC) for high current, Direct Plated Copper (DPC) for fine lines, or Active Metal Brazing (AMB) for extreme thermal cycling reliability.
Surface Finish: Explicitly state the finish required for wire bonding or soldering. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) is often preferred for gold wire bonding reliability.
Die Attach Material: Specify the medium. Are you using Silver (Ag) Sintering paste for highest thermal performance, Gold-Tin (AuSn) eutectic solder for hermetic sealing, or conductive epoxy for lower-cost applications?
Bond Line Thickness (BLT): Define the target thickness of the die attach layer (e.g., 30µm ±10µm). Too thin causes voids; too thick increases thermal resistance.
Shear Strength Requirement: Set a minimum force value (in kg or Newtons) that the die must withstand to pass mechanical validation, typically based on the die size (e.g., MIL-STD-883).
Voiding Percentage: Define the maximum allowable void area in the die attach layer (e.g., <5% total area, with no single void >1%). This is critical for preventing hotspots.
Die Placement Accuracy: Specify the X, Y, and Theta (rotation) tolerance. For optical applications or RF, this might need to be ±10µm; for power electronics, ±50µm might suffice.
Fillet Height and Coverage: Define how far the die attach material should climb up the side of the die (fillet). Usually, 50-75% of die height is required to ensure good adhesion without shorting active areas.
Cleanliness Standards: Specify plasma cleaning requirements before wire bonding. Contamination on the ceramic surface is a leading cause of bond failure.
Thermal Conductivity of the System: Instead of just the material, specify the required thermal resistance (Rth) from junction to case if you have simulation data.
Reworkability: State clearly if the assembly must be reworkable. Many high-reliability ceramic die attach processes (like sintering) are effectively permanent.

Hidden risks (root causes & prevention)

Even with perfect specs, the transition to volume manufacturing introduces variables that can cause yield loss; here are the specific risks associated with die attach on ceramic substrates.

Risk: Voiding in the Bond Line
- Why it happens: Outgassing from the die attach paste is trapped under the die during the reflow or curing process.
- Detection: X-Ray inspection is mandatory.
- Prevention: Use vacuum reflow ovens or pressure sintering processes. Optimize the reflow and thermal profile for ceramic to allow volatile solvents to escape before the material hardens.
Risk: Ceramic Substrate Warpage
- Why it happens: Ceramic is rigid, but thin substrates can warp if the copper metallization (DBC) has uneven stress or if the cooling rate is too fast.
- Detection: Laser profilometry or visual inspection during incoming QC.
- Prevention: Ensure balanced copper thickness on top and bottom sides of the ceramic. Control cooling ramps strictly.
Risk: Die Tilt
- Why it happens: Uneven dispensing of die attach material or uneven placement pressure.
- Detection: Cross-sectioning or 3D AOI.
- Prevention: Use specific dispense patterns (e.g., asterisk or cross pattern) rather than a single dot to ensure even spreading.
Risk: Metallization Peeling (Delamination)
- Why it happens: Poor adhesion between the copper layer and the ceramic, often exacerbated by thermal cycling.
- Detection: C-SAM (Scanning Acoustic Microscopy) or peel tests.
- Prevention: Validate the DBC/DPC supplier’s adhesion strength. Use AMB (Active Metal Brazing) for high-stress applications.
Risk: Solder Splash / Sintering Bleed
- Why it happens: Excessive die attach material volume or too much placement force squeezes material onto adjacent pads.
- Detection: Visual inspection / AOI.
- Prevention: Tight control of dispensing volume (mg) and placement force (g). Design solder dams if necessary.
Risk: CTE Mismatch Cracking
- Why it happens: Although ceramic matches silicon well, the interface materials (solder/sinter) must absorb some stress. If the bond line is too thin, the die cracks.
- Detection: Thermal cycling followed by electrical test or CSAM.
- Prevention: Enforce minimum Bond Line Thickness (BLT) controls using spacer beads or precise force control.
Risk: Surface Contamination (Organic Residue)
- Why it happens: Residue from the die attach process migrates to wire bond pads, preventing successful wire bonding later.
- Detection: Non-stick on pad (NSOP) errors during wire bonding.
- Prevention: Plasma cleaning steps immediately prior to wire bonding.
Risk: Inconsistent Thermal Profiles
- Why it happens: Ceramic has high thermal mass. Standard FR4 profiles won't work.
- Detection: Cold solder joints or incomplete sintering.
- Prevention: Develop a custom reflow and thermal profile for ceramic using thermocouples attached directly to the substrate during setup.
Risk: Oxidation of Surface Finish
- Why it happens: Improper storage of substrates before assembly.
- Detection: Discoloration or poor wetting.
- Prevention: Store ceramic substrates in nitrogen dry cabinets or vacuum-sealed bags until use.
Risk: Die Backside Metallization Incompatibility
- Why it happens: The metal on the back of the die (e.g., Gold) doesn't match the die attach material (e.g., certain epoxies), leading to weak bonds.
- Detection: Low shear strength results.
- Prevention: Verify the "Metallization Compatibility Chart" from the paste/solder manufacturer against the die datasheet.

Validation plan (what to test, when, and what “pass” means)

A robust validation plan ensures that the risks identified above are controlled before you ship product to customers.

Die Shear Testing (Destructive)
- Objective: Verify mechanical adhesion.
- Method: Apply force to the side of the die until failure (MIL-STD-883, Method 2019).
- Acceptance: Force >1.0x minimum spec; failure mode should be in the material (cohesive), not at the interface (adhesive).
X-Ray Inspection (Non-Destructive)
- Objective: Quantify voiding.
- Method: 100% inline X-Ray or AQL sampling.
- Acceptance: Total voiding <5% (or per spec); no single void >1% of die area; no voids under critical hotspots.
Scanning Acoustic Microscopy (C-SAM)
- Objective: Detect delamination between layers (Die-to-Attach, Attach-to-Substrate, Copper-to-Ceramic).
- Method: Ultrasonic imaging.
- Acceptance: No delamination indications in the active area.
Thermal Shock / Cycling
- Objective: Test reliability under stress.
- Method: -40°C to +125°C (or +150°C) for 500-1000 cycles.
- Acceptance: No increase in thermal resistance; no electrical opens; shear strength remains >50% of initial value.
High Temperature Storage (HTS)
- Objective: Test material aging and intermetallic growth.
- Method: Store at 150°C for 1000 hours.
- Acceptance: Shear strength degradation <20%.
Cross-Section Analysis (Micro-sectioning)
- Objective: Verify Bond Line Thickness (BLT) and fillet geometry.
- Method: Pot, grind, and polish a sample unit; inspect under microscope.
- Acceptance: BLT within tolerance (e.g., 30µm ±5µm); uniform fillet; no micro-cracks in die or ceramic.
Wire Pull Test (if applicable)
- Objective: Ensure die attach process didn't contaminate pads.
- Method: Pull wires to destruction.
- Acceptance: Failure in wire (break), not lift-off from pad.
Thermal Transient Testing (t3Ster)
- Objective: Measure actual thermal resistance (Rth-jc).
- Method: Apply power pulse and measure cooling curve.
- Acceptance: Rth value matches simulation within ±10%.
Visual Inspection (Optical)
- Objective: Check for placement accuracy and epoxy bleed.
- Method: High-magnification microscope or AOI.
- Acceptance: No epoxy on wire bond pads; die rotation <1 degree; no chipping of die edges.
Electrical Functional Test
- Objective: Confirm die was not damaged by heat/pressure.
- Method: Full parametric test.
- Acceptance: 100% Pass.

Supplier checklist (RFQ + audit questions)

Use this checklist to vet potential manufacturing partners. A supplier who cannot answer these questions is a high risk for die attach on ceramic substrates.

RFQ Inputs (You provide these)

Gerber files with separate layers for solder/sinter mask and metallization.
Assembly drawing showing die orientation, X/Y coordinates, and polarity.
Bill of Materials (BOM) specifying the exact die attach paste/preform.
Specification for ceramic substrate (Al2O3 vs AlN, DBC vs DPC).
Defined acceptance criteria for voids (X-Ray) and placement accuracy.
Thermal profile constraints (max temp, max slope).
Wire bonding diagram (if APTPCB is performing wire bonding).
Packaging requirements (Gel-Pak, Tape & Reel, Waffle Pack).

Capability Proof (Supplier provides)

Do they have in-house vacuum reflow or pressure sintering capability?
Can they handle the specific reflow and thermal profile for ceramic (higher heat capacity)?
What is their minimum and maximum die size capability?
Do they have automated die bonders with pattern recognition?
Can they process the specific wafer size (6", 8", 12") or waffle packs you use?
Do they have in-house plasma cleaning equipment?
What is their standard Bond Line Thickness (BLT) control method?
Do they have experience with LED MCPCB assembly and reflow (often a good proxy for thermal competence)?

Quality System & Traceability

Is X-Ray inspection 100% or sampling? (For NPI, demand 100%).
Can they trace a specific PCB serial number to the Die Wafer Lot number?
Do they perform shear testing on every setup/lot start?
Is there a dedicated dry storage area for ceramic substrates and moisture-sensitive dies?
Do they have ISO 13485 (Medical) or IATF 16949 (Automotive) if your industry requires it?
How do they manage the shelf life of sintering pastes or epoxies (FIFO)?

Change Control & Delivery

Will they notify you before changing the die attach material brand?
Do they have a "Frozen Process" agreement for mass production?
What is the procedure for handling "die drop" or pick-up errors?
Can they support buffer stock for long-lead-time ceramic substrates?
What is their disaster recovery plan for the cleanroom environment?
Do they offer failure analysis (FA) services if field returns occur?

Decision guidance (trade-offs you can actually choose)

Engineering is about compromise. Here are the trade-offs you will face when specifying this process.

Alumina (Al2O3) vs. Aluminum Nitride (AlN)
- Trade-off: AlN has 7x the thermal conductivity of Alumina but costs 2-3x more.
- Guidance: If your power density is <50 W/cm², stick to Alumina. If >100 W/cm², you must use AlN.
Solder vs. Silver Sintering
- Trade-off: Sintering offers superior thermal and reliability performance (no fatigue) but is a slower, more expensive process requiring specialized equipment.
- Guidance: For standard industrial temps (-40 to 125°C), solder is fine. For EV traction inverters or temps >175°C, choose sintering.
DBC (Direct Bonded Copper) vs. DPC (Direct Plated Copper)
- Trade-off: DBC handles massive current (thick copper) but has poor fine-line resolution. DPC offers precision features but thinner copper.
- Guidance: Use DBC for power modules (IGBTs). Use DPC for high-density LED arrays or sensors.
Vacuum Reflow vs. Standard Reflow
- Trade-off: Vacuum reflow virtually eliminates voids but increases cycle time and cost.
- Guidance: If voiding >10% is acceptable, standard reflow works. For high-reliability power parts (voiding <5%), vacuum is non-negotiable.
Dispensing vs. Stencil Printing
- Trade-off: Stencil printing is faster for volume but less flexible for varying die heights. Dispensing is slower but adapts to cavities.
- Guidance: Use printing for flat, high-volume arrays. Use dispensing for complex, multi-chip modules with cavities.

FAQ

Q: Can I rework a die attached to a ceramic substrate? A: It depends on the material. Solder is reworkable using a hot plate, but Silver Sintering is permanent. Attempting to remove a sintered die usually destroys the substrate metallization.

Q: What is the typical shelf life of ceramic substrates? A: Typically 12 months if stored in a nitrogen environment or vacuum sealed. The risk is oxidation of the Ag or Cu surface, which degrades solder wetting.

Q: How does the reflow profile differ for ceramic vs. FR4? A: Ceramic has a higher heat capacity and thermal conductivity. The reflow and thermal profile for ceramic requires a slower ramp-up to prevent thermal shock cracking and a longer soak time to ensure the entire mass reaches reflow temperature.

Q: Why is plasma cleaning recommended before wire bonding? A: Die attach epoxy often "bleeds" or outgasses invisible organic residues onto the wire bond pads. Plasma cleaning removes this atomic-level contamination to ensure strong wire bonds.

Q: Is underfill required for die attach on ceramic? A: Usually no. Because the CTE of ceramic matches the die closely, the stress on the bumps or bond line is low, unlike Flip-Chip on FR4 where underfill is mandatory.

Q: What is the difference between "Soft Solder" and "Hard Solder"? A: Soft solders (high lead or tin-lead) are more ductile and absorb stress but fatigue over time. Hard solders (AuSn) are brittle but do not fatigue, making them ideal for rigid ceramic substrates in hermetic packages.

Q: Can APTPCB handle bare die sourcing? A: Yes, but we prefer the customer to consign the die (wafer or waffle pack) to ensure the exact silicon revision is used. We can source the ceramic substrates and attach materials.

Q: What is the minimum die size for automatic placement? A: typically 0.2mm x 0.2mm (8 mil x 8 mil). Smaller dies may require specialized equipment.

Ceramic PCB Manufacturing
- Why this helps: Deep dive into the substrate material properties (Al2O3 vs AlN) and metallization types (DBC/DPC) available for your die attach project.
Turnkey PCB Assembly
- Why this helps: Explains how we integrate substrate fabrication, component sourcing, and die attach into a single seamless workflow.
High Thermal PCB Solutions
- Why this helps: Explores alternative thermal management strategies if ceramic proves to be over-engineered for your specific application.
DFM Guidelines
- Why this helps: Provides general design rules that help you prepare your data package to minimize engineering questions and delays.
X-Ray Inspection Services
- Why this helps: Details the validation technology used to detect voiding in the die attach layer, a critical quality metric.

Request a quote

Ready to move forward? Request a Quote here. When you submit your data, our engineering team will perform a DFM review to check for die placement feasibility and thermal profile requirements.

For the fastest response, please include:

Gerber files (including paste/mask layers).
Assembly drawing with die coordinates and orientation.
Datasheets for the Die and the desired Substrate.
Specific requirements for voiding % and shear strength.
Estimated annual volume (EAU).

Conclusion

Successfully implementing die attach on ceramic substrates requires more than just buying the right glue; it demands a holistic view of materials, thermal profiling, and rigorous validation. By controlling the bond line thickness, managing voiding through vacuum processes, and selecting the correct metallization, you can achieve a product that withstands the harshest thermal environments. APTPCB is ready to support your transition from design to high-reliability production with the precision this technology demands.