Flip chip technology is one of the key technologies in chip packaging and is widely used in various types of advanced packaging. Due to its special structure, the solder joint reliability is relatively low in practical applications. To address this problem, the underfill process has emerged as a solution. This process is mainly used to improve the reliability of the connecting bumps. During implementation, defects such as voids and adhesive dosage mismatch will degrade the filling quality, resulting in product quality failing to meet technical standards.In the process of operation, it is essential to fully master the technological process of flip chip underfill adhesive to maximize its practical application value.

The underfill process flow is as follows

A rigorous and complete underfill process generally includes steps such as pre-baking, plasma cleaning, dispensing, curing and inspection during implementation. Each step serves a distinct purpose; strict control over process parameters and operational standards is required throughout the implementation, otherwise additional defects may be introduced, and the expected dispensing effect will not be achieved.

The Role of Plasma Cleaning in the Underfill Process of Flip Chip Packaging

Plasma cleaning is a process in which a cleaning device excites gases such as argon or hydrogen into a plasma state via its internal electromagnetic field. The plasma then bombards the surface of the product to be cleaned, achieving the cleaning purpose through physical and chemical reactions occurring during the bombardment.

Among these, the physical reaction mechanism is that active particles bombard the surface to be cleaned, dislodging contaminants from the surface, which are then finally removed by a vacuum pump. The chemical reaction mechanism is that various active particles react with contaminants on the surface to form volatile substances, which are subsequently extracted by the vacuum pump.

The primary purposes of plasma cleaning are to improve the surface roughness of the substrate, remove contaminants and impurities, and provide a clean environment for the underfill process. Meanwhile, as the surface roughness of the substrate is improved, the fluidity of the adhesive will be enhanced.

The Effect of Plasma Cleaning on Substrate Wettability

To verify the surface wettability of ceramic substrates treated by different surface processes, contact angle measurement tests were conducted separately on ceramic substrates with and without plasma cleaning. Figure 1 shows the contact angle images of water and underfill adhesive on the surface of ceramic substrates at the moment of contact, both with and without plasma cleaning. It can be observed that the contact angles of water and underfill adhesive on the surface of non-plasma-cleaned ceramic substrates are 81.1° and 45.8° respectively; after plasma cleaning, the contact angles of both water and underfill adhesive on the ceramic substrate surface are reduced to 26.8° and 21.1° respectively.

This is attributed to two factors: on the one hand, plasma cleaning increases the number of hydrophilic groups and reduces hydrophobic groups on the ceramic substrate surface; on the other hand, it reduces residual contaminants on the ceramic surface to a certain extent. Together, these effects ultimately enhance the wettability of water and underfill adhesive on the ceramic surface.

Figure 1. Comparison of Underfill Adhesive Contact Angles on Ceramic Substrates Before and After Plasma Cleaning

In the flip chip packaging process, the fluidity of underfill adhesive determines filling efficiency, which in turn affects production efficiency and costs. After plasma cleaning, the wettability of both water and underfill adhesive on the ceramic substrate surface is improved; plasma cleaning can also promote the flow of underfill adhesive between the chip and ceramic substrate, thereby effectively reducing the flow time of underfill adhesive, improving filling efficiency, and further boosting production efficiency while cutting production costs.