Which Processes Require Baking in Advanced Packaging?

In advanced packaging, baking is a critical heat treatment technology that permeates multiple core steps, directly impacting the structural integrity, electrical performance, and long-term reliability of the package. From substrate pre-treatment to post-mold curing, and from die attach adhesive curing to underfill protection, the baking process is distributed across multiple nodes in the packaging flow, serving multiple functions such as moisture removal, promoting material cross-linking, and stress relief. Different steps have varying emphases on baking parameters like temperature profiles, time control, and atmospheric environments, and the selection of corresponding Oven equipment must also align with the specific packaging process requirements.

I.Main Process Steps in Advanced Packaging Requiring Baking

1.Substrate Baking:The first step in the assembly flow is often substrate baking. Organic substrates (like laminates used in PGA, BGA, and CSP packages) tend to absorb moisture from the air during storage and transportation. If not thoroughly removed before subsequent high-temperature processes, this moisture can vaporize and expand at interfaces, causing delamination or the "popcorn" effect, which in severe cases can lead to package cracking. The purpose of substrate baking is to completely remove absorbed moisture from the substrate early in the process, providing a dry, stable foundation for subsequent die attachment and interconnection.

2.Die Attach Adhesive Curing:In the die attachment process, whether using conductive silver paste, non-conductive paste, or insulating paste, baking is required for curing. The filler particles in the adhesive provide conductivity or insulation, depending on whether an electrical bias or thermal connection is needed on the chip's backside. In multi-chip and stacked-die applications, die bonding and curing operations may be repeated two to five times, each requiring strict baking and curing to ensure interlayer bonding strength and interface quality.

3.Underfill Adhesive Curing:After flip-chip (FC) soldering, a tiny gap exists between the IC die surface and the substrate, which needs to be filled with a low-viscosity, silica-filled epoxy resin via "underfilling." This underfill material wicks into the gap under the chip via capillary action and is then cured by baking to form a dense filler layer. Post-underfill processing temperatures must not exceed the adhesive's thermal decomposition threshold. Ramped heating is used to avoid thermal shock, oven air flow uniformity directly affects curing consistency, and a vacuum environment can effectively suppress bubble formation.

4.Post-Mold Curing (PMC):In the molding process (transfer molding), thermoset epoxy molding compound is injected into a hot mold, where it undergoes initial curing for 1-2 minutes to form a preliminary encapsulant. However, at this stage, the encapsulant is not fully hardened and may still have unreacted cross-linking points. After removal from the mold, additional baking, called Post-Mold Curing, is essential. PMC not only ensures complete and stable curing of the encapsulant but also helps release internal residual stresses accumulated during molding, enhancing the package's mechanical strength and long-term reliability.

5.Film and Coating Baking:Advanced packaging extensively uses organic dielectric films like polyimide (PI) and benzocyclobutene (BCB) as dielectric layers for redistribution layers. These materials typically require staged baking for curing to form stable, reliable insulating layers. The curing of spin-coated photoresist layers, protective coatings that shield chips from ionic contamination, and potting/sealing compounds for optical elements like LEDs, all rely on baking processes for complete curing. 

6.Moisture Bake-Out under MSL Control:Semiconductor devices can absorb moisture during transportation and storage. The Moisture Sensitivity Level (MSL) system defines the safe handling time for components after removal from their moisture barrier bags. When the exposure time exceeds the specified limit or humidity indicator card readings are exceeded, bake-out for moisture removal is mandatory before surface mounting. This prevents rapid vaporization and expansion of moisture during reflow soldering, which causes package cracking—the "popcorn" effect.

II.Requirements for Baking Processes in Advanced Packaging

1.Precise Matching of Temperature and Time:Different processes have significantly different requirements for baking temperature and time. For die attach adhesive curing, epoxy adhesives typically bake at 90°C for 10 minutes, silver paste at 120°C for 90 minutes, and insulating paste at 120°C for 30 minutes. Post-Mold Curing generally occurs between 125°C and 175°C for 1 to 6 hours. Drying for moisture removal is typically done at 100°C-150°C for 5-20 minutes; curing often occurs between 150°C-180°C for 30-60 minutes. For complex epoxy encapsulants, a staged curing profile may be used—low-temperature pre-curing to initiate preliminary cross-linking and avoid thermal shock to delicate components, followed by higher-temperature post-curing to achieve full cross-linking and form a dense 3D network.

2.Cleanliness and Contamination Control:Chip manufacturing occurs in strictly controlled environments (e.g., Class 1000 cleanrooms). The cleanliness of baking equipment directly affects product yield. Ovens for advanced packaging typically require Class 1000 or even Class 100 cleanliness. They feature high-temperature-resistant stainless steel chambers and high-efficiency filtration systems (HEPA filters with 99.99% efficiency) to ensure no particulate contamination is introduced during baking.

3.Oxygen-Free/Low-Oxygen Environment:Many packaging materials are prone to oxidation at high temperatures, leading to performance degradation. Nitrogen/Inert Gas Ovens continuously purge with nitrogen, maintaining oxygen levels below 100 ppm or even 10 ppm, preventing metal surface oxidation and material discoloration. A nitrogen atmosphere can also accelerate solvent evaporation and improve curing uniformity.

4.Temperature Uniformity and Control Precision:Uniform temperature distribution within the oven is critical for process consistency. Due to varying heat diffusion rates in different areas of complex multi-layer products, equipment must use forced convection circulation and precise PID temperature control systems to ensure minimal temperature variation across the workspace. Temperature control accuracy is generally required within ±0.5°C, with uniformity controlled within ±1.5%.

5.Strict Control of Moisture Content:For moisture bake-out under MSL control, internal package moisture content must be reduced below 0.1%. Vacuum ovens can rapidly remove deep-seated moisture from micro-pores and gaps at lower temperatures, effectively preventing package cracking.

6.Electrostatic Discharge (ESD) Protection:ESD protection for electronic components is also important. Airflow friction during baking can generate static electricity. Advanced ovens are equipped with ionization neutralization systems to control electrostatic voltage within safe limits, preventing ESD damage to sensitive devices.

III.Oven Types Suitable for Advanced Packaging

Based on different process characteristics and production capacity needs, various oven types can be selected for advanced packaging. Different models within the same oven category can also be flexibly configured in size and power to suit various scenarios from R&D to mass production.

1.Cleanroom Ovens:Cleanroom ovens (also called clean ovens) incorporate air purification systems. HEPA filter units continuously filter the circulating air, ensuring the workspace meets Class 1000 or even Class 100 cleanliness standards. Chambers are constructed with fully welded and polished interiors to avoid dust accumulation. Airflow is typically vertical laminar flow, carrying particles away from the workspace for exhaust. Clean ovens are primarily used for PI/BCB adhesive curing, BPO baking, and the clean drying of precision electronic components, especially in wafer-level packaging stages highly sensitive to surface cleanliness.

2.Nitrogen Ovens/Oxygen-Free Ovens:Nitrogen/oxygen-free ovens continuously purge with nitrogen, strictly suppressing oxygen content within the chamber to below 100 ppm or even 10 ppm, completing curing and baking in an oxidation-free environment. They are frequently used in processes requiring both cleanliness and oxidation prevention, such as in backlight unit and photoresist curing. Advanced nitrogen ovens also feature nitrogen throttling technology to effectively reduce nitrogen consumption, balancing process quality with operational cost.

3.Vacuum Ovens:Vacuum ovens evacuate the chamber while heating, lowering the boiling point of water to achieve rapid, low-temperature dehydration and deaeration. For residual moisture in microscopic voids within wafer-level packages, vacuum ovens can thoroughly remove it, minimizing the "popcorn" effect. The vacuum environment is also used for deaeration processes, effectively suppressing bubble formation within encapsulation compounds, ensuring good adhesion between filler materials and underlying components without voids.

4.Tunnel Ovens (Continuous Ovens):Tunnel ovens are suitable for high-volume, continuous production. Materials for curing pass through multiple temperature zones with different functions via a conveyor belt, completing stages like solvent removal, gelation, and full curing step-by-step. Tunnel ovens support multi-zone gradient heating, allowing a gradual temperature rise from ambient to the target, preventing issues like surface skinning caused by overly rapid heating leading to violent internal solvent evaporation. They can also feature strong exhaust systems to quickly remove volatile organic compounds (VOCs), preventing condensation and re-contamination. This continuous operation significantly improves production efficiency and process consistency.

5.Automated Smart Ovens:Smart ovens integrate automation concepts, featuring precise temperature control and clean/oxygen-free environments. They can seamlessly integrate with AGVs, robotic arms, and MES systems, enabling barcode binding, automatic recipe retrieval, and full-process data traceability. This significantly enhances automation levels in high-precision, large-scale advanced packaging production.

Baking technology permeates the entire advanced packaging process, from substrate treatment and die attachment to underfilling, mold curing, and final product drying. It is a fundamental safeguard ensuring structural density, freedom from delamination and cracking, and stable electrical performance of the package. Different steps rely on different baking characteristics: substrate pre-baking focuses on moisture removal and "popcorn" prevention; die attach and post-mold curing emphasize material cross-linking and stress relief; film and coating baking demand extremely high standards for oxidation prevention and cleanliness; and MSL-controlled moisture bake-out is directly linked to device reliability grades.