Purpose of Pre-Baking and Moisture Removal for Plastic-Encapsulated Devices

In electronics manufacturing, plastic-encapsulated devices are widely used in consumer electronics, automotive electronics, industrial control, and other fields. Although encapsulation materials (such as epoxy resin) offer excellent electrical insulation and mechanical protection, they are also highly hygroscopic and readily absorb moisture from the air during storage or transportation. If sufficient moisture removal is not performed prior to encapsulation, subsequent high-temperature processes (such as reflow soldering or curing) can cause delamination between the encapsulant and the chip or lead frame, and may even lead to package cracking or "popcorning." Pre-baking for moisture removal has thus become an indispensable step in the manufacturing process of plastic-encapsulated devices.

I. Necessity of Pre-Baking and Moisture Removal

During production, transportation, and storage, plastic-encapsulated devices readily absorb moisture from the environment. This seemingly negligible moisture can rapidly vaporize and expand in subsequent high-temperature processes (e.g., reflow soldering, curing), generating significant internal vapor pressure. This pressure can cause delamination between the encapsulation material and the chip or lead frame, and may even result in package cracking. The presence of moisture can also exacerbate thermal stress, leading to serious defects such as chip cracking or bond wire breakage, ultimately causing electrical failure of the device.

II. Key Objectives of Pre-Baking and Moisture Removal

1.Moisture Elimination and Defect Prevention:​ Moisture may remain in the raw materials or be introduced during manufacturing processes. After encapsulation, especially under high temperatures, this residual moisture can vaporize and expand, causing internal voids, cracks, or even the "popcorn effect," severely compromising the structural integrity and reliability of the device. Pre-baking effectively removes this residual moisture, reducing encapsulation defects caused by moisture expansion.

2.Enhanced Device Stability:​ The presence of moisture can lead to performance instability under varying environmental conditions. For instance, in high-humidity environments, moisture may permeate the package and cause electrochemical corrosion of metal components (e.g., bond wires, pads), leading to increased contact resistance, signal distortion, or even open-circuit failure. Pre-baking ensures thorough removal of moisture from the encapsulation materials, thereby improving device stability across diverse environmental conditions.

3.Reduction of Internal Stress:​ During the manufacturing of plastic-encapsulated devices, differences in the coefficients of thermal expansion (CTE) of various materials can generate internal stress. This stress may cause warpage or cracking after encapsulation. Pre-baking, by elevating temperature and reducing humidity, helps effectively mitigate internal stress, enhancing the overall stability of the device.

4.Material Stabilization and Improved Adhesion:​ The physical and chemical properties of materials used in plastic-encapsulated devices (such as plastics and adhesives) can change under different temperature and humidity conditions. Pre-baking promotes material stabilization, resulting in more uniform and stable material properties. It also enhances the adhesive strength, improving the mechanical robustness and stability of the device.

5.Optimization of Soldering Results:​ For plastic-encapsulated devices that require soldering, pre-baking prevents the formation of voids and cracks caused by moisture vaporization during the soldering process. This not only improves soldering reliability and strength but also reduces soldering defects, thereby enhancing production efficiency.

III. Applicable Oven Types

1.High-temperature ovens:​Commonly used for pre-baking plastic-encapsulated devices, these ovens provide a consistently high-temperature environment with good internal temperature uniformity. They are suitable for devices that require high-temperature treatment, effectively removing moisture and completing curing processes.

2.Oxygen-free ovens:​Utilizing nitrogen purging or vacuum technology, these ovens provide a low-oxygen or oxygen-free environment to prevent oxidation of the chip and components. They are suitable for oxidation-sensitive plastic-encapsulated devices, effectively removing moisture and volatile substances while protecting the device from oxidative damage.

3.Nitrogen Ovens:​By purging with nitrogen to displace oxygen, these ovens prevent oxidation and corrosion. They are particularly suitable for oxidation-sensitive plastic-encapsulated devices, enabling efficient pre-baking while protecting the device. Nitrogen ovens are typically equipped with gas flow control systems to ensure a consistent nitrogen supply and effective oxygen exclusion. The nitrogen concentration can often be adjusted as needed to further optimize the pre-baking process.

4.Hot Air Circulation Ovens:​Featuring a forced internal hot air circulation system, these ovens offer high temperature uniformity. They are suitable for pre-encapsulation moisture removal processes for components like LED lead frames and PCB substrates. Their temperature range generally extends from ambient +10°C to 250°C, meeting the baking requirements for most plastic-encapsulated devices.

5.Precision Ovens:​These ovens provide higher temperature control accuracy and often humidity control capabilities. They are suitable for devices with stringent baking requirements, such as post-encapsulation curing and moisture removal for IGBT modules, and are commonly used in high-reliability product manufacturing.

Pre-baking for moisture removal is a critical process step for ensuring the quality and reliability of plastic-encapsulated devices. By eliminating residual moisture, reducing internal stress, enhancing stability, optimizing soldering results, and helping meet industry standards and environmental requirements, pre-baking significantly improves device performance and service life. In practice, selecting the appropriate oven type and technical parameters based on the device materials, processes, and specific requirements is essential for achieving optimal pre-baking results.