Delamination in Leadframe IC Packaging: Mechanism, Prevention, and QFN Package Reliability

1. Introduction

Molded plastic packaging has long dominated the semiconductor packaging industry due to its cost-effectiveness, mature process, and scalability. However, as chip integration continues to advance, package dimensions shrink (with QFN packages demanding higher dimensional accuracy), and lead-free soldering processes impose stricter requirements on thermal resistance, plastic-encapsulated devices face three major challenges:

1. Reduced mechanical strength: Smaller package volumes weaken structural robustness, decreasing resistance to mechanical and thermal stresses.
2. High-temperature reflow stress: During reflow soldering, molding compounds (EMC) must endure elevated temperature shocks, significantly raising the demand for interfacial adhesion strength. Insufficient adhesion easily causes delamination.
3. CTE mismatch among materials: Chips, leadframes, and EMCs have mismatched coefficients of thermal expansion (CTE), leading to stress concentration and interfacial cracks.

Ultimately, these problems manifest as delamination, a critical bottleneck for QFN packages and other advanced packaging types. Therefore, analyzing the mechanisms of delamination and formulating targeted prevention strategies is of significant engineering value for improving yield and extending device reliability.

2. Overview and Classification of Delamination

Delamination is essentially the separation or peeling at material interfaces inside plastic-encapsulated devices under external or internal stress, caused when interfacial adhesion strength cannot withstand applied stresses. Depending on the location, delamination can be classified into four types:

1. Chip-to-molding compound interface delamination: Caused mainly by residual organic contaminants (e.g., photoresist, flux) or oxide layers on the chip surface. Plasma cleaning and vacuum baking before encapsulation can effectively reduce this risk. In QFN packages, with higher interface area ratios, stricter control is needed.
2. Wire bond area delamination: Often originates from excessive bonding stress or uneven/defective leadframe surfaces. In QFN packages, with denser bond pads, such delamination may cause signal transmission issues, requiring precise bonding parameter control.
3. Chip-to-plating layer (Ag/Cu) delamination: Caused by uneven plating thickness, unstable electroplating quality, or poor adhesion to chip passivation layers. In QFN packages, where plating precision directly affects electrical conduction, this issue must be strictly managed.
4. Leadframe-to-molding compound delamination: The most common and complex type. Leadframe oxidation is a key contributor but not the only factor. Even after rigorous cleaning, adhesion risks remain. For QFN packages, where leadframes tightly bond with molding compounds, this delamination critically impacts reliability.

From a failure mechanism perspective, delamination arises mainly from thermal stress damage and moisture-induced damage.

2.1 Thermal Stress Mechanism

In thermal cycling or high-temperature operation, CTE mismatches among molding compound, chip, and leadframe (and QFN substrates) generate localized stresses. When stress exceeds adhesion strength or material yield strength, cracks or separation occur.
Notably, EMC properties change dramatically near its glass transition temperature (Tg)—its modulus drops sharply and CTE increases significantly. This amplifies thermal stresses, making QFN packages that undergo reflow soldering (temperatures beyond Tg) particularly vulnerable to delamination.

2.2 Moisture-Induced Mechanism

Epoxy molding compounds are not fully dense, allowing moisture to enter by:

1. Diffusion through molecular gaps in the EMC bulk, and
2. Penetration along micro-voids at material interfaces.

Moisture reaching the chip surface can form conductive water films at passivation defects, dissolving ionic residues (Na⁺, Cl⁻) and inducing electrochemical corrosion of aluminum interconnects, eventually causing open circuits. Residues from plating or cleaning steps aggravate corrosion and weaken adhesion, indirectly causing delamination.
For QFN packages, with higher surface-to-volume ratios, moisture penetration is faster, requiring strict preventive measures.

3. Key Factors and Improvement Strategies

This study focuses on the leadframe-to-molding compound delamination, analyzing three main influencing factors—molding process parameters, material systems, and leadframe oxidation—with specific solutions proposed for QFN packages.

3.1 Optimization of Molding Parameters

Molding parameters directly determine interfacial adhesion, especially critical in miniaturized QFN packages:

• Mold temperature: Should be slightly above the Tg of EMC (typically 160–180 °C) to ensure flowability for filling QFN cavities while minimizing internal stresses. Excessive temperature accelerates curing and stress concentration; too low leads to voids and weak adhesion. Uniform mold temperature (±2 °C) is essential.
• Injection pressure and speed: Must match EMC gel time and viscosity. Too low causes incomplete filling; too high risks wire deformation or flash. Filling must complete before gelation ends to ensure uniform adhesion.
• Clamping pressure: Must be dynamically adjusted to avoid lead deformation or poor sealing.

Additionally, EMC pellets should be protected from moisture absorption, molds kept clean, and environmental humidity controlled (40–60% RH) to minimize process variations.

3.2 Material System Selection and Matching

Materials are fundamental to delamination control, especially for QFN packages:

• Moisture resistance: Use high-purity EMCs (Na⁺, Cl⁻ < 50 ppm) with strong resin–filler bonding. Ensure high-quality chip passivation (SiNx, SiO₂) to block moisture ingress.
• CTE matching: Select low-CTE EMCs to reduce mismatch with Si chips (~3 ppm/°C) and Cu leadframes (~17 ppm/°C). Optimize filler (e.g., spherical Al₂O₃, 70–85% loading) to balance CTE and thermal conductivity for QFN heat dissipation.
• Interfacial adhesion: Add coupling agents (e.g., silanes) to improve EMC–metal bonding, enhancing stability in tightly bonded QFN structures.

3.3 Leadframe Oxidation Layer Control

Copper leadframes oxidize during heating steps, forming Cu₂O and CuO. While thick CuO layers (>10 nm) reduce adhesion, thin, dense Cu₂O layers can enhance roughness and chemical bonding with EMC.

• Oxidation control: Adjust preheat temperature (120–150 °C) and time (5–10 min) to form beneficial Cu₂O while limiting CuO. For QFN packages, N₂ protection during molding preheat can further optimize oxide layer composition.
• Impact mechanism: Oxide layers alter surface chemistry and energy, affecting EMC wetting and adhesion. Properly engineered, oxidation layers can be transformed from a “defect source” into a “controllable adhesion enhancer.”

3.4 Post-Molding (Deflash) Process Control

Chemical deflash can damage leadframe–molding compound interfaces, especially critical for fine QFN edges.

• Solution temperature: Use low-temperature chemicals (40–60 °C) to minimize thermal stress.
• Solution composition: Use high-molecular-weight solvents with low penetration capability to avoid interface weakening.
• Oxide protection: Ensure beneficial Cu₂O layers are preserved.
• Reaction intensity: Control solution concentration and exposure time to prevent over-etching or edge damage in QFN packages.

4. Conclusion

Delamination in plastic IC packaging is a multifaceted challenge involving materials science, process engineering, and interfacial chemistry. For QFN packages, with miniaturized structures and higher performance demands, prevention is even more difficult. Solutions require a comprehensive, systematic approach:

1. Process optimization: Precisely control molding parameters (temperature, pressure, speed) and manage variations to meet QFN requirements.
2. Material innovation: Develop low-stress, low-ion, high-adhesion EMCs with matched CTE and balanced thermal properties.
3. Interface engineering: Reframe leadframe oxidation as a controllable factor for enhancing adhesion rather than solely a defect risk.
4. Full-process control: Manage all steps from die attach and wire bonding to molding and deflash, identifying and eliminating potential delamination risks.

Through coordinated improvements in these areas, delamination risks can be fundamentally reduced, reliability of plastic packages (including QFN packages) significantly enhanced, and long-term performance of high-density electronic packaging ensured.

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