What is the Die Attach Process?
Die Attach—also referred to as Die Bonding or Die Mounting in the semiconductor industry—is the process of permanently attaching a silicon die to the die pad of a support structure, such as a leadframe or metal header, within a semiconductor package. As a critical step in backend semiconductor manufacturing, Die Attach directly impacts device reliability, yield, and overall production efficiency. Therefore, the use of high-precision Die Attach equipment is essential to ensure consistent placement accuracy, bonding quality, and process productivity.
Advanced Die Attach / Die Bonder systems—such as PNP 6600 EVO, PNP6600A, PNP 7000M, PNP7000 LA, and other precision Die Attach tools—are designed to accurately pick, align, and place diced dies onto the target substrate with high repeatability and throughput.
After wafer dicing, individual chips are mounted onto the die attach pad located at the center of the substrate (leadframe or header). Depending on device requirements, material characteristics, and application environments, a variety of Die Attach processes may be employed, including:
· Soft Solder Die Attach
· Eutectic Die Attach
· Epoxy Die Attach
· UV Die Attach
· Silver Sintering Die Attach
· Thermocompression Die Bonding
· Flip Chip Die Attach
What is the process of Die Attach?
For the Die Attach / Die Bonding process, the position of the die and the substrate (leadframe) is aligned on the Die Attach / Die Bonding machine. The pick-up tool, or die collet, moves downward (Z motion) either to the die surface or to a position just above the die.
The ejection tooling system, consisting of ejector needles, needle holders, and pepper pots, coordinates the die ejection process together with the pick-up system to ensure that the die is successfully ejected and picked up from the adhesive film. Vacuum pressure holds the die during the transfer from the wafer to the leadframe or header.
Steps in the Die Attach Process Flow
The following steps are included in the semiconductor Die Attach process:
Wafer Expansion
· Die Attach / Die Bonding
· Wafer Expansion
Wafer Expansion:
Wafers (tested and probed) are obtained from wafer manufacturers. These wafers are sent for the wafer expansion process, in which the spacing between each die is enlarged, allowing more clearance between dies. The dies are maintained in their respective positions on the wafer with sufficient clearance for pick-up, optical recognition (wafer mapping), and die bonding.
The use of wafer expansion has been reduced in recent years with the proliferation of smaller and thinner dies.
Die Attach / Die Bonding:
There are various Die Attach methods, which are described as follows:
1) Soft Solder Die Attach
The Soft Solder Die Attach process uses soft solder in wire-roll form as the Die Attach material to mount the die onto the die pad of the leadframe. The die is ejected from the wafer by ejector needles. During the ejection process, a pick-and-place tool—commonly known as a pick-up tool or die collet—retrieves the die from the wafer tape and positions it onto the Die Attach area.
2) Eutectic Die Attach
No soft solder preforms are used in the eutectic Die Attach process. The leadframe in the bonding area is plated with silver (only in selected areas, not on the entire leadframe), while the backside of the die is typically plated with gold. Gold–silicon eutectic bonding is achieved by placing the die onto the Die Attach pad of the leadframe and heating it above the eutectic melting temperature on a eutectic Die Attach machine or die bonder. This process forms a rigid joint between the silicon die and the leadframe. To facilitate die attachment, a scrub motion is applied to initiate eutectic flow and remove any air pockets that may be present during the attachment process.
3) Epoxy Die Attach
The Epoxy Die Attach process is performed by attaching the die to the leadframe using an epoxy adhesive on Epoxy Die Attach equipment or a die bonder. A controlled amount of epoxy adhesive is dispensed onto the package using specialized dispensing tools, and the die is then placed on top of the adhesive. The package is heated to a specified temperature to adequately cure the epoxy, in accordance with the adhesive manufacturer’s specifications.
This epoxy Die Attach process utilizes polyamide, epoxy, and silver-filled glass as Die Bond materials to mount the die onto the die bond pad. After bonding, the mass of epoxy present around the periphery of the die is referred to as the epoxy coverage area.
4) UV Die Attach
UV curing technology is highly effective in controlling the pressure-sensitive adhesive properties of acrylic adhesives, as their mechanical properties, surface tension, and miscibility are significantly altered by UV irradiation. Thin wafers mounted on Die Attach Film (DAF), or die-bonding film used for stacked-die packages, are subjected to an appropriate UV curing process to prevent issues such as die fly-off during dicing, adhesive whiskering, and adhesive attachment or merging after dicing.
Therefore, UV curing is a critical process step that wafers must undergo to ensure the Die Attach Film (DAF) achieves the required level of adhesiveness. Optimization of UV curing process parameters is typically required to achieve the best performance based on production requirements. The UV curing process is commonly performed using wavelengths in the range of 365 nm to 400 nm. Some Die Attach Film adhesives can also be cured in the presence of moisture, heat, or high light intensity. UV curing systems generally require programmable intensity control to generate a defined UV light profile.
5) Silver Sintering Die Attach
Silver sintering is one of the most promising technologies for Die Attach. Due to its superior reliability compared with conventional die attach solder pastes, dedicated reliability flows and physical analyses can be employed for sintering process optimization and durability assessment. Degradation of the sintering layer can be monitored during durability stress testing using scanning acoustic microscopy and electrical measurement of temperature-sensitive electrical parameters.
The sintering layer can be used as an alternative to solder joints for attaching chips to carrier substrates. A reliable and stable bond can be achieved through a suitable combination of time, temperature, and pressure. Sintered joints exhibit excellent thermal conductivity and a very high melting point (approximately 962 °C for silver sintering), and they significantly reduce creep when compared with conventional solder compounds.
6) Thermocompression Die Bonding
The thermocompression process, also known as diffusion bonding, involves the application of heat and controlled mechanical pressure to bring two metal surfaces into atomic contact using die bonding equipment. This atomic interaction causes the interface to adhere, while the combined thermal and mechanical energy promotes metal diffusion, resulting in a metallurgical bond between the two surfaces.
7) Flip Chip Die Attach
In Flip Chip Die Attach, the chip is flipped prior to attachment, and solder or conductive polymer bumps between the chip and the substrate serve as both the electrical and mechanical interconnections using Flip Chip Die Attach equipment or a die bonder. The basic flip-chip interconnection process can be subdivided into three functional areas.
Under Bump Metallization (UBM): UBM is a compatible metal stack that connects the chip metallization (typically aluminum or copper) to the bump metallization. It generally consists of an adhesion layer, a barrier layer, a wetting layer, and an anti-oxidation barrier.
IC bump and bond material: The IC bumps and Die Attach materials consist of a controlled amount of solder per interconnect. These bumps are deposited using methods such as evaporation, electroplating, stencil printing, or solder jetting.
Substrate metallization: For Flip Chip attachment, a higher melting temperature solder is required. The Flip Chip die is mounted onto a laminate substrate together with other Surface Mount Device (SMD) components. In this case, a lower melting temperature solder is typically preferred to enable all board-level interconnects to be formed simultaneously.
