Causes and Prevention of Lead Frame Discoloration in Semiconductor Packaging

1. Core Causes of Lead Frame Discoloration

Lead frames in semiconductor packaging serve as key components for connection and support. Their surface condition significantly affects device performance and reliability. During the actual production process, lead frames often undergo discoloration due to various factors, which not only affect their appearance but may also damage electrical connections. Below, we analyze the core causes of discoloration and corresponding prevention measures.

(1) Oxidation Reactions: Direct Contact with Oxygen / Moisture

The base materials of lead frames (such as copper, iron-nickel alloys) or surface coatings (such as silver or nickel) are highly susceptible to oxidation when exposed to oxygen, high temperatures, or high humidity, resulting in surface discoloration. This can manifest as:

• Base Material Oxidation: The underlying metal reacts with oxygen and moisture in the air. For example, copper forms copper oxide (black) or cuprous oxide (brick red), causing localized or widespread black or red discoloration. If the nickel coating lacks purity or contains impurities, it may oxidize to nickel oxide, appearing dark green or black in high-temperature environments.
• Coating Oxidation: Silver coatings are particularly sensitive to environmental conditions. In high-temperature settings (such as bonding or mold curing processes), silver is prone to oxidize into silver oxide, resulting in a brown-black surface color.
• Process Protection Deficiencies: If protective gases (such as nitrogen) during packaging processes are not of sufficient purity or flow rate, or if there is excessive oxygen inside the furnace, oxidation will accelerate. Additionally, exposure of lead frames to humid air for prolonged periods during post-treatment can also lead to oxidation discoloration.
• High Temperature Accelerating Oxidation: In bonding (ultrasonic / thermal compression bonding, typically at 200-300°C) or annealing processes, if the protective gas atmosphere (such as nitrogen) contains more than 50ppm of oxygen, the metal surface will quickly oxidize at high temperatures.

(2) Contaminant Residue and Coating Defects: External Impurities and Coating Quality Issues

Lead frames can discolor due to external contaminants or defects in the electroplating layer. This includes:

• Organic Contaminants: Incomplete cleaning before packaging, such as residual photoresist, flux, or cleaning agents, can lead to carbonization at high temperatures, leaving blackish-brown stains on the lead frame surface. Additionally, if lubricants used in stamping or cutting processes are not fully removed, they may carbonize and form black marks when exposed to high temperatures.
• Inorganic Contaminants: Calcium and magnesium ions in cleaning water or dust (metal particles, silicon powder) in the workshop can adhere to the surface. After high-temperature treatment, these contaminants form discoloration spots. Also, residues from mold release agents or encapsulant materials can transfer to the lead frame, leaving brown or black marks.
• Electroplating Defects: If electroplated layers (such as silver or nickel) contain pinholes or cracks, the underlying metal is exposed and susceptible to corrosion or oxidation, causing local discoloration. Inconsistent plating thickness or impurities in the plating solution (such as iron or lead) can lead to rough plating surfaces, which are more likely to undergo oxidation discoloration in high-temperature environments.

(3) Process Parameter Abnormalities: Control Failures in the Packaging Process

If process parameters are outside the acceptable range, they can directly or indirectly cause discoloration in lead frames. Key areas affected include:

• Temperature and Time Control: During die bonding, wire bonding, or molding curing processes, if the furnace temperature exceeds the lead frame or plating layer’s tolerance, the metal surface will oxidize or degrade. Even if the temperature is within the threshold, extended exposure to high temperatures can exacerbate oxidation or thermal aging.
• Atmospheric Conditions: Imbalances in the protective gas atmosphere inside the furnace (such as nitrogen, hydrogen) or the introduction of corrosive gases like hydrogen sulfide or chlorine can lead to metal corrosion and discoloration. Moreover, in the molding process, epoxy resins, curing agents, or additives (such as mold release agents) may release acidic gases (e.g., phenolic substances) that react with the lead frame surface, producing colored compounds.

(4) Material Compatibility Issues: Chemical Reactions and Diffusion Between Materials

Lead frames can discolor due to chemical reactions or metal diffusion with other materials used in the packaging process. Examples include:

• Chemical Reactions: The lead frame may react with materials such as epoxy resins or solder paste. For example, some curing agents in epoxy resins (such as amine-based hardeners) can react with copper lead frames, producing colored compounds and causing discoloration.
• Metal Diffusion: At high temperatures, plated metals can diffuse into the base material. For example, silver plating may form an Ag-Cu alloy with the copper base, leading to color changes on the surface. In bonding or molding processes, silver plating and gold wire bonding may react to form Ag-Au alloys, lightening the surface color.

(5) Corrosive Effects: Chemical / Electrochemical Corrosion by Aggressive Agents

Lead frames can discolor due to chemical or electrochemical corrosion when they come in contact with corrosive substances, including:

• Environmental Corrosion: In post-packaging testing or storage environments, excessive humidity or exposure to sulfur/chlorine gases can cause electrochemical corrosion, resulting in rust or discoloration.
• Process Residue Corrosion: Acidic or basic cleaning agents, flux residues, or electroplating solutions with sulfur/chlorine components can react with metal surfaces. For example, copper reacts with chloride ions to form green copper chloride, while silver reacts with sulfur to form black silver sulfide.
• Sealing Failure Corrosion: If the mold seal is not tight, moisture or contaminants may penetrate and come into contact with the lead frame, causing corrosion and discoloration, which is especially critical in high-reliability devices.

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2. Targeted Prevention Strategies for Lead Frame Discoloration

To prevent discoloration caused by oxidation, contamination, process abnormalities, material incompatibility, and corrosion, various measures can be implemented across process optimization, material control, environmental management, and quality monitoring. These strategies include:

(1) Suppressing Oxidation Reactions: Minimizing Exposure to Oxygen / Moisture

• Optimize Protective Atmosphere Management: During high-temperature processes (such as die bonding, wire bonding, molding curing, annealing), use high-purity protective gases (e.g., nitrogen ≥99.999%, oxygen ≤10ppm) and monitor oxygen content in real-time. Ensure stable gas flow to prevent air ingress.
• Hydrogen Gas for Silver Coatings: For silver-coated lead frames, introduce 1-5% hydrogen during high-temperature processes to reduce silver oxidation.
• Strict Control of Environmental Temperature and Humidity: Maintain low humidity (≤40%) and room temperature (20-25°C) during storage and transfer of lead frames. Use sealed moisture-proof packaging (e.g., vacuum bags with desiccants) for short-term storage to isolate moisture.

(2) Eliminating Contaminant Residues: Cutting Off the Source of Contaminants

• Strengthen Cleaning Process Control: Use a multi-step cleaning process before packaging, starting with neutral cleaners to remove organic contaminants, followed by rinsing with high-purity deionized water (resistivity ≥18.2MΩ·cm), and drying with hot air or vacuum to ensure no residual moisture.
• Increase Workshop Cleanliness: Maintain Class 1000 cleanliness in packaging workshops (especially in cleaning, electroplating, and high-temperature process areas) by using HEPA filters and regularly cleaning equipment surfaces. Operators must wear cleanroom gloves and masks to avoid contaminating lead frames.

(3) Stabilizing Process Parameters: Avoiding Degradation Due to High Temperature / Abnormal Atmosphere

• Precise Control of High-Temperature Parameters: Monitor furnace temperatures with thermocouples or infrared devices to ensure they do not exceed the lead frame and coating tolerance (e.g., copper ≤300°C, silver plating ≤250°C).
• Prevent Corrosive Gas Infiltration: Regularly inspect high-temperature furnaces and related pipelines for leaks. Use low-volatility, non-acidic encapsulants to reduce the release of corrosive gases during high-temperature curing.

(4) Optimizing Material Compatibility: Reducing Chemical Reactions and Diffusion Risks

• Select Compatible Materials: Conduct compatibility testing between lead frames and packaging materials to avoid reactions that can cause discoloration. Use materials that minimize metal diffusion and oxidation, such as palladium-nickel alloy plating instead of pure silver plating.

(5) Preventing Corrosive Effects: Isolating Corrosive Agents

• Control Corrosive Substances in the Environment: Install gas monitors in workshops and storage areas to ensure that levels of corrosive gases like hydrogen sulfide and sulfur dioxide do not exceed 0.1ppm.
• Enhance Sealing Design: Improve mold design in encapsulation processes to ensure tight sealing, and use double protection (e.g., encapsulation + coating) for high-reliability products.

(6) Strengthening Quality Monitoring: Timely Detection and Adjustment of Abnormalities

• Implement Online Monitoring Systems: Use visual inspection stations with optical inspection equipment (AOI) and color recognition algorithms to detect discoloration in real time.
• Conduct Regular Failure Analysis: Regularly perform sample testing (e.g., SEM, EDS analysis) to identify the root causes of discoloration (e.g., oxidation, contamination, corrosion) and improve future processes.