Chip Moisture Absorption and Baking Conditions

In the electronics manufacturing industry, chips serve as core components, and their storage and handling methods directly impact the reliability and yield of the final product. Many integrated circuits (ICs), Ball Grid Array (BGA) packages, Thin Small Outline Packages (TSOPs), and other devices, once unsealed and exposed to air for an extended period, will absorb moisture from the environment. When a moisture-laden chip enters high-temperature processes such as reflow soldering, the internal moisture rapidly vaporizes and expands, potentially causing fatal defects such as package cracking, delamination, or even bursting. Standardized baking for moisture removal is a critical step in ensuring production yield.

I.Why Chips Absorb Moisture

The essence of chip moisture absorption lies in the absorption and permeation of moisture from the environment by the packaging materials.

● Modern chips widely adopt plastic packaging (such as QFP, QFN, BGA, SOP, etc.). The main packaging material is epoxy molding compound. Epoxy resin itself is somewhat hygroscopic; in high-humidity environments, water molecules gradually penetrate into the packaging material through diffusion and accumulate at the interface between the chip and the lead frame.

● A chip's sensitivity to moisture is related to its Moisture Sensitivity Level (MSL). JEDEC standards classify chips into six levels, from MSL 1 to MSL 6. MSL 1 chips are non-sensitive to moisture and can be stored long-term under ordinary conditions. In contrast, high-level chips such as MSL 5 and MSL 6 are extremely sensitive to moisture; once exposed to a high-humidity environment, they can reach a saturated moisture state within a short period.

● Chip moisture absorption is also related to the storage environment and operational standards. If chips are not sealed and stored in moisture-proof bags, or if the moisture-proof bags are damaged and not replaced in time, or if warehouse humidity control is inadequate during high-humidity seasons (such as the rainy season or in coastal areas), the moisture absorption rate of the chips accelerates. Failure to use chips within the specified time after opening is also a common cause of moisture absorption. In summary, chip moisture absorption is the result of the combined effects of material properties, environmental humidity, and operational management.

II.Hazards of Moisture-Absorbed Chips

If moisture-absorbed chips are subjected to reflow or wave soldering without proper treatment, severe quality defects will occur. The hazards are mainly manifested in the following aspects:

1.Popcorn Effect: This is the most destructive failure mode for moisture-absorbed chips. When a moisture-laden chip enters the high-temperature reflow oven, the moisture accumulated inside the package rapidly vaporizes, causing a sudden volume expansion and generating tremendous steam pressure. When the steam pressure exceeds the mechanical strength limit of the packaging material, it leads to package cracking, delamination, or even bursting. The popcorn effect not only directly causes chip scrap but may also damage PCB pads and surrounding components.

2.Package Delamination: Even without obvious bursting, the steam pressure at high temperatures can cause microscopic delamination between the chip and the packaging material, or between the lead frame and the molding compound. Delamination weakens the mechanical structural integrity of the chip, reduces heat dissipation efficiency, and becomes a pathway for moisture and contaminants during subsequent use, accelerating chip aging and failure.

3.Pin and Pad Oxidation: After moisture absorption, an oxide film tends to form on the surface of the package pins or BGA solder balls. The oxide film increases contact resistance during soldering, leading to defects such as virtual soldering, cold soldering, or poor solder wetting, severely affecting soldering quality and circuit reliability.

4.Electrical Performance Degradation: After moisture intrudes into the chip interior, it may alter the internal dielectric constant, leading to increased leakage current, threshold voltage drift, increased signal transmission delay, and other degradations in electrical performance parameters. For high-precision analog chips and high-speed digital chips, this performance degradation is particularly critical.

5.Decreased Long-Term Reliability: Even if the chip does not exhibit immediate obvious failure after moisture absorption, residual moisture inside will trigger chronic failure mechanisms such as metal migration and corrosion during long-term use, significantly shortening the product's lifespan.

III.Baking Conditions for Moisture-Absorbed Chips

To address the issue of chip moisture absorption, the core purpose of baking is to thoroughly drive out the absorbed moisture from inside the package without damaging the chip, restoring it to a dry state so that it can safely withstand subsequent high-temperature soldering processes.

The selection of baking temperature and time must comprehensively consider the chip's package type, MSL level, degree of moisture absorption, and the chip's tolerance to high temperatures. Standard baking solutions mainly include the following temperature ranges:

1.High-Temperature Baking (125°C): This is the most commonly used and efficient baking temperature, suitable for the vast majority of standard plastic-packaged chips. At 125°C, the moisture evaporation rate inside the chip is fast, and the baking time is relatively short. For lightly moisture-affected chips, baking for 4 to 24 hours is usually sufficient; for severely moisture-affected chips or those with high MSL levels, the baking time may need to be extended to 48 hours or even longer. It should be noted that 125°C high-temperature baking is not suitable for chips with trays, tape-and-reel, or special packaging, as the high temperature may cause packaging material deformation or release harmful substances.

2.Medium-Temperature Baking (90°C): Suitable for chips that are sensitive to high temperatures, or components with plastic trays, carrier tapes, or other high-temperature-intolerant packaging materials. Although 90°C baking takes longer (usually 24 to 72 hours), it effectively prevents packaging material deformation and excessive pin oxidation, balancing dehumidification effectiveness with device safety.

3.Low-Temperature Baking (40°C): Mainly used for precision components that are extremely sensitive to temperature, or chips with special coatings or labels. 40°C low-temperature baking takes the longest, often requiring several days to over a week, but it maximizes the protection of the chip's physical and electrical characteristics from thermal damage.

In actual operation, it is essential to confirm the chip's MSL level and maximum allowable baking time before baking. Some high-level chips have strict limits on cumulative baking time; excessive baking may cause pin oxidation, changes in solder ball alloy composition, or aging of packaging materials. After baking is completed, the chips should be cooled in a dry environment and put into production as soon as possible, or re-vacuum sealed to prevent re-moistening.

IV.Selection of Baking Ovens for Moisture-Absorbed Chips

Chip baking requires high performance from the oven, necessitating not only precise temperature control and uniform temperature distribution but also good cleanliness and atmosphere control capabilities. Based on working principles and structural characteristics, commonly used chip baking ovens are mainly divided into the following types:

1.Hot Air Circulating Oven: This is the most widely used baking equipment in the electronics manufacturing industry. Its working principle is to use a built-in fan to force hot air to circulate within the chamber, ensuring uniform heat transfer to the chip surface and interior. High-quality hot air circulating ovens are equipped with PID temperature control systems, keeping temperature fluctuations within ±1°C. Hot air circulating ovens feature a simple structure, moderate cost, and large capacity, making them suitable for batch baking of standard plastic-packaged chips. However, their cleanliness is relatively low, making them unsuitable for precision chips with extremely high particulate matter requirements.

2.Vacuum oven:A vacuum oven reduces the air pressure inside the chamber by pumping out air, significantly lowering the boiling point of water and enabling efficient dehumidification at lower temperatures. For example, under vacuum conditions, water evaporates abundantly at 40–60°C. The advantage of vacuum baking is low-temperature efficiency, making it especially suitable for chips sensitive to temperature but requiring rapid dehumidification. The vacuum environment also effectively prevents chip oxidation at high temperatures. The drawbacks of vacuum ovens are higher equipment costs, limited single-batch processing capacity, and relatively complex operation.

3.Nitrogen Oven (Oxygen-free oven):A nitrogen oven continuously introduces high-purity nitrogen into the chamber during the baking process, creating a low-oxygen or even oxygen-free environment. A nitrogen atmosphere effectively prevents oxidation of chip pins and solder balls at high temperatures, while the thermal conductivity of nitrogen helps improve temperature uniformity. Nitrogen ovens are widely used for baking chips with precision solder balls, such as BGA and CSP, as well as components with precious metal pins that are extremely sensitive to oxidation. Nitrogen ovens are usually combined with hot air circulation systems, offering the dual advantages of temperature uniformity and anti-oxidation protection.

4.Clean Oven (Dust-Free Oven): Clean ovens are equipped with high-efficiency filtration systems (such as HEPA filters), capable of controlling the concentration of particulate matter in the air within the chamber to cleanliness levels of Class 100 or even Class 10. Clean ovens are suitable for baking chips in semiconductor front-end processes or high-end packaging that have extremely high cleanliness requirements. They effectively prevent dust particles from adhering to the chip surface during baking, which could cause subsequent soldering defects or circuit short circuits.

Chip moisture absorption is a quality risk in the electronics manufacturing process that cannot be ignored. Its root cause lies in the hygroscopic nature of packaging materials, while its hazards are concentrated in physical damage and performance failure during high-temperature soldering. Standardized baking is an effective means to salvage moisture-absorbed chips and ensure soldering yield. In actual operation, temperature and time parameters must be set strictly according to the chip's moisture sensitivity level. Selecting appropriate oven equipment and combining them with standardized storage and handling management can reduce the risk of moisture absorption from the source, ensuring that chips deliver stable and reliable performance in electronic products.