The Role of Baking in Semiconductor Chip Packaging

In the semiconductor chip packaging manufacturing process, baking is a critical thermal treatment step. Its core purpose is to remove adsorbed moisture within materials, volatilize residual solvents, promote material curing and cross-linking, and stabilize the device structure and performance through controlled heating. Since semiconductor packaging involves various polymer materials (such as epoxy molding compounds, conductive adhesives, and underfill) as well as precision metal/ceramic structures, trace amounts of moisture or stress can lead to delamination, the "popcorn effect," and solder joint failure. Therefore, the baking process directly impacts packaging yield, reliability, and the service life of end products.

Ⅰ.Functions of Baking in Chip Packaging

1. Moisture Removal and Dehumidification: Semiconductor materials—especially molding compounds, PCB substrates, and PI tapes—easily absorb ambient moisture. If not thoroughly removed before packaging, this moisture vaporizes rapidly during high-temperature reflow soldering, causing die/substrate delamination, package cracking (i.e., the "popcorn effect"), or direct circuit damage. Baking reduces the material's moisture content to a safe threshold (typically <0.1%), fundamentally preventing such risks.
2. Curing and Cross-linking: A large volume of thermosetting materials (e.g., epoxy-based molding compounds, conductive silver paste, underfill) are used in packaging. Baking provides the necessary heat to trigger the cross-linking reaction between resins and hardeners, transforming the material from a liquid/semi-solid state into a three-dimensional solid network. This imparts the designed mechanical strength, insulation properties, and temperature resistance.
3. Stress Relief: During packaging, internal stress accumulates due to differences in the Coefficient of Thermal Expansion (CTE) between the chip, substrate, and molding compound. Low-temperature or gradient-heating bakes allow for slow stress release, reducing warpage, deformation, or interface peeling—a factor critical for thin packages (e.g., BGA, CSP).
4. Process Compatibility Optimization: Certain photoresists and polyimide (PI) coatings require baking to remove solvents and densify the film, enhancing stability for subsequent etching and bonding processes. In wafer-level packaging, baking temporary bonding adhesives ensures no displacement or delamination occurs during thinning or TSV processes.

Ⅱ.Process Roles of Baking in Chip Packaging

• Incoming Material Pre-treatment: Pre-baking molding compounds, lead frames, and PCB substrates to remove environmental moisture absorbed during storage and transportation, preventing reliability issues caused by moisture after packaging.
• Mid-process Stages:For example, "soft baking" after wafer coating removes solvents from photoresist to prevent bubbles or pattern distortion during exposure; "pre-curing bake" after dispensing conductive adhesive initially fixes the chip position to avoid offset in subsequent processes.
• Post-packaging Curing: High-temperature post-curing immediately after molding further increases the glass transition temperature (Tg) and cross-link density of the molding compound, enhancing moisture and aging resistance. Underfill rework baking ensures the adhesive completely fills the gap between the chip and substrate, distributing thermal stress.

Ⅲ.Packaging Processes Requiring Baking

• Wafer Fabrication Back-end: Post-thinning bake (stress relief), pre-bonding bake after plasma cleaning (removing surface moisture), soft bake and hard bake after photoresist coating (curing the resist layer).
• Die Attach: After dispensing conductive adhesive/eutectic solder, baking is required for pre-curing to fix the chip position; sometimes low-temperature baking is performed after eutectic welding to eliminate welding stress.
• Wire Bonding: Baking lead frames before bonding in some processes to remove adsorbed water and improve bond strength; baking PI protective layers after bonding.
• Molding: Baking molding compound preforms before molding to dehumidify; immediate entry into a post-cure oven after molding, following a stepped temperature profile (e.g., 125°C → 150°C → 175°C, 2 hours each).
• Underfill: After dispensing and filling the chip, baking is required to allow the adhesive to flow and cure, ensuring void-free filling.
• Final Test & Pre-shipment: Finished chips often undergo high-temperature baking (e.g., 85°C/24h) prior to shipment to screen out early-life failures via simulated harsh environments and to remove moisture adsorbed by packaging materials.

Ⅳ.Applicable Ovens for Baking in Chip Packaging

1. Vacuum oven:Suitable for devices highly sensitive to humidity (e.g., WLP, MEMS sensors). By evacuating the chamber to lower ambient pressure, it accelerates moisture evaporation without high-temperature oxidation. It prevents material degradation or excessive intermetallic compound growth while removing moisture, often used for dehumidification or low-temperature curing of special materials.
2. Clean Room Oven:Equipped with HEPA filters to maintain Class 100~1000 cleanliness. Used for wafer and bare die applications sensitive to particles (e.g., photoresist baking, pre-treatment for bump reflow) to prevent dust contamination that could cause short circuits.
3. Nitrogen Oven:Purged with high-purity nitrogen to create an inert, low-oxygen atmosphere (oxygen content controllable below 10ppm). Suitable for easily oxidized materials (e.g., copper lead frames, silver paste curing) to prevent metal surface oxidation during high temperatures, which would otherwise affect conductivity or soldering quality.
4. Precision Oven:For mass production needs, cart-type large ovens (for bulk molding compound dehumidification) and small precision ovens (for R&D or small batch trials) are utilized.

As a fundamental technology throughout the semiconductor chip packaging process, the baking process is vital from die attach and molding to final shipping. By precisely controlling the temperature profile, duration, and atmospheric environment, baking not only removes moisture and organic impurities from packaging materials and promotes full cross-linking of polymers but also effectively eliminates internal stress, inhibits metal oxidation and whisker growth, and comprehensively enhances the mechanical strength, electrical insulation, and long-term reliability of the package.