Chip bonding: How to achieve "seamless connection" between chips?

Chip bonding: The core process that enables seamless connection between chips

In the back-end process of semiconductor manufacturing, the packaging process is like putting on a "protective armor" for the chip, and chip bonding is the "key hub" connecting the chip to the outside world in this armor. The packaging process includes multiple steps such as backside grinding, dicing, chip bonding, wirebonding, and molding, which are not static, but will be adjusted, fused or even merged according to the innovation of packaging technology. In the previous issue, we detailed the dicing process that cuts wafers into individual chips, and today we focus on the key link after dicing – die bonding. The core role of this process is to accurately and securely attach individual chips cut from the wafer to the packaging substrate (which may be a lead frame or printed circuit board), laying the foundation for subsequent electrical connections and functional implementation.

1. The essence and classification of bonding technology: the "bridge" between the chip and the outside world

In the precision world of semiconductor manufacturing, "bonding" is the core technology that achieves a stable connection between chips and substrates. In simple terms, it is to fix the wafer chip to the substrate through a specific process to establish the physical and electrical connection between the chip and the outside world. According to different stages of technological development and application scenarios, the bonding process can be divided into two types: traditional and advanced, each with its own unique technical path and application advantages.

1. Traditional bonding technology: mature and stable connection solution

Traditional bonding technologies mainly include die bonding (also known as die attach) and wire bonding. Chip bonding focuses on physically attaching the chip to the substrate, while wire bonding enables the electrical connection between the chip pad and the substrate pins through a thin metal wire, such as gold, copper, or nickel wire. These two technologies work together to form a mature, stable and cost-controllable connection solution, which is widely used in consumer electronics, automotive electronics and other fields. Its advantages lie in its high process maturity and strong compatibility, and can adapt to a variety of chip sizes and substrate types, but it has certain limitations in high-density integration scenarios.

2. Advanced Bonding Technology: Flip chips lead the way in high-density connections

A representative of advanced bonding technology is Flip Chip Bonding, which was pioneered by IBM in the late 60s of the 20th century and revolutionized the connection logic of traditional bonding. It innovatively combines the functions of chip bonding with wire bonding: first make tiny metal bumps (such as Solder Balls) on the chip's pads, and then place the chip "upside down" (top side down) on the substrate so that the bumps on the chip are in direct contact with the corresponding pads on the substrate. Through subsequent heating and other processes, the bump melts and forms a strong solder joint with the substrate pad, while achieving physical fixation and electrical connection. The biggest advantage of this technology is that it has a high connection density, which can meet the needs of high-end chips for miniaturization and high speed, and plays an irreplaceable role in smartphone chips, high-performance processors and other fields.

3. The Role of Bonding Technology: Multiple missions that transcend connections

If the engine is the "heart" of the car, then chip bonding technology is the "power source" of semiconductor packaging. It attaches semiconductor chips to lead frames or printed circuit boards to build an electrical path between the chip and the outside world, ensuring smooth transmission of electrical signals. But the mission of bonding is much more than that: it must ensure that the chip can withstand various physical pressures (e.g., vibration, shock) that it may face after packaging, preventing chip detachment or damage; At the same time, it is necessary to efficiently export the heat generated during chip operation to avoid performance attenuation or failure due to overheating. In some specific scenarios, it is also necessary to maintain constant conductivity (e.g., power devices) or achieve high levels of insulation (e.g., high-frequency circuits). As chip size continues to shrink and integration continues to improve, the accuracy and reliability requirements of bonding technology are becoming increasingly stringent, which has become a key factor affecting the performance of semiconductor devices.

2. The core steps of chip bonding: the precise process from dispensing to fixation

Chip bonding is a highly sophisticated system engineering, and each step requires strict control of parameters to ensure the stability and reliability of the connection. Whether it is traditional chip bonding or advanced flip chip bonding, they all follow a set of rigorous operating procedures, but there are differences in details.

1. The basic process of traditional chip bonding

The steps of traditional chip bonding can be summarized as three major links: dispensing-placement-curing. First, the appropriate amount of adhesive (such as epoxy resin) needs to be applied at the precise point of the predetermined position of the packaging substrate, and the amount and distribution of the adhesive directly affect the stability of the subsequent connection. The chip is then placed face up on the adhesive-coated substrate through a high-precision equipment to ensure that the alignment accuracy of the chip to the substrate is controlled at the micron level. After placement, the assembled unit is fed into a Temperature Reflow channel, where the temperature is precisely adjusted over time according to a preset Temperature Profile, allowing the adhesive to gradually melt and cure over a temperature range of 150°C to 250°C. After the temperature cools, the adhesive hardens, firmly bonding the chip to the substrate, forming a stable physical connection.

2. Unique process for flip chip bonding

The process of flip chip bonding has been revolutionized on the basis of traditional processes, and the core difference is "bump making" and "chip inversion". First, in the later process of chip manufacturing, metal bumps (solder balls) are made on the chip pads through evaporation, electroplating and other processes, and the material, size and spacing of the bumps are accurately designed according to the chip requirements. Then, place the chip face down (pad facing the substrate) on the substrate so that the bumps on the chip are precisely aligned with the corresponding pads on the substrate. Next, it also enters the temperature reflow channel, where the bump (solder ball) is melted by heating to form an alloy solder joint with the substrate pad. After cooling, the solder joints solidify, which not only realizes the physical fixation of the chip and the substrate, but also directly establishes an electrical connection, eliminating the wire connection link of traditional wire bonding, and greatly improving the connection density and signal transmission speed.

3. Chip pick-up and placement: precise operation before bonding

Before chip bonding, there is also a critical preparation – transferring the cut chip from the cutting tape to the substrate, a process known as "Pick & Place" that is the first hurdle to ensure bonding accuracy.

1. Pickup: Separate the qualified chips from the cutting tape

After the dicing process is complete, the wafer is divided into hundreds of individual chips, which remain gently attached to the cutting tape and await further processing. "Pickup" is the process of separating qualified chips from the cutting tape one by one. The device will accurately identify the position of the qualified chip based on the mapping table generated by the wafer test results (pass/fail). A special plunger or vacuum nozzle is then used to apply the appropriate force to the qualified chip to separate it from the cutting tape. For unqualified chips, they will be left on the cutting tape and discarded uniformly in the subsequent frame recycling process to avoid unqualified products flowing into the subsequent process.

2. Placement: Precise positioning of the chip onto the substrate

"Placement" is the step immediately following pick-up, which refers to the precise placement of the picked qualified chip in a predetermined position on the packaging substrate. This operation is done on a dedicated die bonder, which uses a high-precision robotic arm and vision positioning system to ensure chip placement accuracy of ±10 microns or more. During placement, it is not only necessary to ensure the alignment of the chip and the substrate, but also to control the pressure and speed during placement to avoid physical damage to the chip. After all qualified chips have been placed, the entire substrate can proceed to subsequent curing or reflow processes to complete the final bonding.

4. Chip ejection process: an innovative solution to solve the pick-up problem

The chips after slicing are flat on the cutting tape, and the distance between them is extremely small, which is not only difficult to pick up directly, but also easy to cause damage to the chip (such as chipping, cracks) due to uneven force. To solve this problem, the "ejection process" has emerged, which makes it easier to separate the chip from the tape through clever mechanical design.

The core principle of the ejection process is to apply a slight physical force from the bottom of the chip through an ejection device, creating a small height difference (usually a few microns to tens of microns) between the target chip and the surrounding chip. This height difference breaks the uniform adhesion between the chip and the cutting tape, allowing the vacuum picker to easily pull the chip up from above. During the ejection process, the device will simultaneously pull up the bottom of the cutting tape through the vacuum picker to keep the wafer flat and avoid displacement or damage to other chips due to local stress. The force, action time and position of the ejection device are accurately calculated to ensure that the chip can be separated smoothly without causing damage to the chip structure, which is a key auxiliary technology for high-precision chip pickup.

5. Bonding adhesive materials: the core factors affecting the quality of the connection

The bonding material between the chip and the substrate is the core factor that determines the bond strength, thermal conductivity, electrical conductivity and even reliability. At present, there are two main types of mainstream adhesive materials: epoxy resin (Epoxy) and die attach film (DAF), each with its own unique performance characteristics and applicable scenarios.

1. Epoxy: A traditional and flexible bonding option

Epoxy resin is a commonly used bonding material in chip bonding, especially silver-containing paste or liquid epoxy resin, which is widely used in medium and high-power device packaging due to its ease of use, moderate cost, and good conductivity. When used, a very small amount of epoxy resin is precisely coated onto the substrate by a dispensing device, and then the chip is placed and heat-cured (150°C to 250°C) to form a strong connection.

However, there are also certain challenges in the use of epoxy resin: if the thickness of epoxy resin is uneven, it will cause stress during the curing process due to the difference in the coefficient of thermal expansion between the chip and the substrate, leading to warpage, which in turn will cause the chip to bend or solder joint failure. Therefore, although epoxy resin is flexible to use, it requires extremely high coating accuracy and thickness control, otherwise it is easy to affect the bond quality.

2. Wafer Bond Film (DAF): An advanced high-precision bonding solution

In recent years, wafer bond films (DAFs) have gradually become the preferred adhesive material in the field of high-end packaging. DAF is a thin-film adhesive that is pre-attached to the bottom of the chip, and its biggest advantage over liquid epoxy resin is its excellent thickness uniformity – it can be controlled within a very small and constant range (typically a few microns to tens of microns) and effectively avoids warping problems caused by uneven thickness.

DAF has a wide range of application scenarios, not only for chip-to-substrate bonding, but also for chip-to-chip stacking bonding, providing an ideal bonding solution for multi-wafer packaging (MCP). In terms of process, the DAF is attached to the back of the wafer before the chip is cut, and after cutting, it comes into contact with the cutting tape along with the chip. For bonding, the chip (along with the DAF) is simply removed from the cutting tape and placed directly onto the substrate, eliminating the need for traditional dispensing steps and simplifying the process while increasing productivity and consistency.

However, DAFs also have some limitations: they are relatively expensive and require high precision from the handling equipment – if not done properly, they can cause air to penetrate the film and cause deformation, affecting bond quality. However, overall, the advantages of DAF in terms of thickness uniformity, process simplification, and reliability have led to its increasing use in high-end packaging.

6. Diversified development of bonding technology: adapt to the needs of different scenarios

The development of chip bonding technology has always been closely linked to the needs of the semiconductor industry. With the improvement of chip integration, the reduction of size and the diversification of application scenarios, bonding technology is also constantly innovating, deriving a variety of process solutions to meet different needs.

1. The influence of substrate type on the bond direction

Different substrates used to place chips (lead frames or printed circuit boards) can vary significantly in chip bonding orientation and process parameters. As a traditional substrate form, the lead frame is suitable for mass production, and the chip is usually top-facing when bonding. Printed circuit boards (PCBs) are more suitable for small-batch, multi-variety packaging scenarios, and the bonding direction can be flexibly adjusted according to the circuit design. With the development of packaging technology, the emergence of new substrates (such as ceramic substrates and organic substrates) has further promoted the diversification of bonding processes.

2. Innovation in temperature curves and bonding methods

Drying of adhesives or melting of solderballs requires precise temperature control, so the design of the temperature profile is crucial. Different adhesive materials (epoxy, DAF, solder balls) and substrate types correspond to different temperature profiles – including ramp-up rate, peak temperature, holding time, and cooling rate, all of which need to be precisely set according to the material properties.

In addition to traditional heating bonding, ultrasonic bonding has also become an important bonding method. This method plastically deforms the metal surface through the energy generated by ultrasonic vibration, forming a strong metallurgical bond without the need for high-temperature heating, and is especially suitable for temperature-sensitive chips or substrates.

3. The development trend of packaging process

With the continuous improvement of integration technology, semiconductor packaging is developing in the direction of ultra-thin, miniaturized and high-density. The rise of advanced technologies such as 3D stack packaging and system-in-package (SiP) has placed higher demands on chip bonding – not only higher connection density, but also better thermal conductivity, electrical conductivity, and reliability. In the future, bonding technology will be deeply integrated with materials science and precision manufacturing technology, continuously breaking through the limits of performance and providing core support for the continuous innovation of the semiconductor industry.

In the next issue, we will continue to delve into the field of semiconductor packaging, introducing another key technology - wire bonding, and exploring how it cooperates with chip bonding to build a complete electrical connection between the chip and the outside world.