Chip bonding technology: the bridge connecting the future

Chip bonding technology: the core joining process in semiconductor packaging

In the back-end process of semiconductor manufacturing, the packaging process is like putting on a "protective armor" for the chip, and chip bonding technology is the "connection hub" of this armor. The packaging process encompasses key steps such as backside grinding, dicing, chip bonding, wire bonding, and molding, which are not static but can be flexibly adjusted, combined, or even combined according to the iterative needs of packaging technology. In the previous issue, we analyzed in detail the dicing process of cutting wafers into individual chips, and this issue will focus on the core link after dicing - the die bonding process. This technology is like a sophisticated "chip placement" that precisely attaches chips cut from wafers to packaging substrates (lead frames or printed circuit boards), laying the foundation for subsequent electrical connections and protection.

1. Definition and type division of bonding technology

In the field of semiconductor manufacturing, "bonding" is essentially a high-precision connection technology, and its core function is to establish a stable physical connection and electrical channel between the wafer chip and the substrate. Depending on the technological evolution path, the bonding process can be clearly divided into two categories: traditional and advanced:

Traditional bonding technology system

The traditional approach consists of two key links:

Die Bonding: Also known as Die Attach, it is the basic process of physically fixing the chip to the substrate. Wire Bonding: Realize the electrical connection between the chip and the substrate through metal leads, just like building a "wire bridge" for the chip.

Breakthrough in advanced bonding technology

Advanced methods are represented by Flip Chip Bonding, a technology developed by IBM in the late 60s of the 20th century that enables an innovative fusion of chip bonding and wire bonding. The core principle is to pre-form bumps on the chip pads, through which these tiny "connection posts" directly connect the chip to the substrate without the need for additional leads.

If semiconductor packaging is compared to a city, then chip bonding technology is like a "transportation hub" of a city - by firmly attaching semiconductor chips to lead frames or printed circuit boards, the electrical connection channel between the chip and the outside world is constructed. After completing the bonding, this process also needs to meet three core requirements: first, it bears the physical pressure generated after packaging to ensure that the chip does not displace during subsequent processes and use; second, it effectively dissipates the heat generated by the chip when it works, and a test data shows that the high-quality bonding process can increase the heat dissipation efficiency of the chip by more than 30%; The third is to maintain constant conductivity or achieve a high level of insulation according to the needs of the application scenario. As chip sizes continue to evolve towards miniaturization (from millimeters to microns), the precision requirements of bonding technology have increased to the ±1μm level, and its importance in semiconductor manufacturing has become increasingly prominent.

2. Comparison of the process of chip bonding and flip chip bonding

Chip bonding and flip chip bonding are like two different "chip mounting solutions", with significant differences in operating processes and technical characteristics:

Traditional chip bonding adopts a "top-up" installation method, and the dispensing process involves accurately dispensing the adhesive at the predetermined position of the packaging substrate, and the amount of adhesive needs to be controlled within the range of 0.001-0.01mg, with an error of no more than ±5%; Chip placement: Place the chip with the top side facing up (active side up) in the position coated with adhesive, and the positioning accuracy needs to reach ±2μm; Curing Treatment: The assembled unit is fed into a temperature reflow channel to melt and harden the adhesive by precisely controlling the temperature profile (typically 150°C-250°C), ultimately bonding the chip firmly to the substrate.

Innovative path to flip chip bonding

Flip chip bonding adopts a "top-down" flip method, and its process steps are as follows: Bump preparation: small bumps called "solder balls" are preformed on the chip pads, the diameter of the bumps is usually 50-200μm, and the height deviation is ≤3μm; Chip flip mounting: Place the chip face down (active side down) on the substrate to accurately dock the bump with the substrate pad; Reflow Soldering: The bumps are melted through a temperature reflow process (lead-free solder typically requires 240°C-260°C), resulting in a reliable solder joint connection after cooling. Although the operation methods of the two methods are different, they ultimately realize the fixation of the chip and the substrate through the temperature reflow process. Comparative data from a semiconductor packaging factory shows that under the same production conditions, the electrical connection efficiency of flip chip bonding is 40% higher than that of traditional chip bonding, but the requirements for equipment accuracy are also increased by an order of magnitude.

3. The core process of chip bonding

The chip bonding process is like a precise "symphony of micromanipulations", involving key links such as pick-and-place and ejection processes, each of which needs to be controlled with precision at the nanometer level.

Pick & Place process

This link is the "handling and positioning" core of chip bonding, all done on a die bonding machine: Chip picking: The process of removing qualified chips one by one from hundreds of chips attached to the cutting tape. The die bonding machine controls the plunger through a high-precision robotic arm, and uses vacuum suction technology (usually -80kPa to -90kPa) to achieve non-destructive pick-up of chips; Chip placement: The picked chip is accurately placed at the predetermined position on the surface of the packaging substrate, and the speed of the placement process can reach 100-300 pieces per minute, with a positioning accuracy of ≤± 1μm. Quality screening: Chips are classified by entering wafer test results (pass/fail) into the mapping table. Once all qualified chips have been bonded, the unremoved non-conforming chips remain on the cutting tape and are uniformly disposed of when the frame is recycled.

The technical parameters of a high-end die bonding machine manufacturer show that the pick-up success rate of its latest model can reach 99.98%, and the placement repeatability reaches ±0.5μm, providing a reliable guarantee for large-scale mass production.

Precise control of the ejection process

After the dicing process is completed, although the chip is divided into independent modules, it is still gently attached to the cutting tape, and it is very easy to cause chip damage when picked up directly. The ejection process acts like a "chip stripper" and achieves non-destructive separation of chips through subtle mechanical control:

Three-dimensional application of force: The ejection device applies a precise physical force (usually 0.1-1N) from the bottom of the chip, while cooperating with the vacuum picker to apply a reverse force from above, forming a slight force difference; Step formation: Through the synergy of this force, the target chip and the surrounding chip form a slight step of 5-10μm, which is convenient for vacuum suction; Tape Flattening Control: During the ejection process, the vacuum picker simultaneously pulls up the bottom of the cutting tape to ensure that the wafer remains flat and avoids chip position shift caused by tape deformation.

The core component of the ejection device is the ejector pin, which typically has a diameter of 50-200μm and a hardness of above HRC55, ensuring no deformation during repeated use. Practice in a packaging factory showed that the chip pickup damage rate was reduced from 0.3% to 0.05% after optimizing the ejection force parameters.

4. Material technology and application characteristics of chip bonding

Chip bonding materials are like "chip-to-substrate adhesives", and their properties directly affect bond strength, heat dissipation efficiency, and electrical properties. Currently, mainstream bonding materials include epoxy resin, solder, metal paste, and wafer bonding film, each with its own unique application scenarios and process requirements.

Epoxy bonding process

In polymer material bonding solutions, silver-containing paste or liquid epoxy resins are widely used due to their ease of use and affordability:

Material properties: Epoxy resin usually contains conductive particles such as silver (or nickel), accounting for 30-50% of the volume, which not only ensures electrical conductivity but also provides sufficient adhesion (shear strength ≥ 20MPa); Dispensing accuracy: Use a high-precision dispenser (minimum dispensing amount of 0.001mg) to dot the epoxy resin on the substrate, the dot diameter is controlled at 50-500μm, and the height deviation is ≤5%; Curing conditions: Reflow or cure at a temperature of 150°C-250°C, the curing time is 1-5 minutes depending on the temperature, ensuring that the epoxy resin is completely hardened;

Key challenge: Uniformity of epoxy thickness is critical, and for every 1μm increase in thickness deviation, the amount of warpage due to the difference in thermal expansion coefficient can increase by 5-10μm, which in turn can cause chip bending or deformation.

Experimental data from a power device manufacturer shows that using epoxy resin with 70% silver content, the thermal resistance of the bonded chip can be controlled below 0.8°C/W to meet the heat dissipation needs of high-power devices.

Wafer bond film (DAF) bonding process

As an advanced bonding method that has emerged in recent years, die attach film (DAF) is gradually becoming the preferred solution for high-precision packaging: Material morphology: DAF is a thin film pre-attached to the bottom of the chip, and the thickness can be controlled at 5-50μm, with a deviation of ≤1μm, which is much better than the thickness control accuracy of epoxy resin. Application scenarios: It is not only suitable for bonding between chips and substrates, but also widely used in stacking bonding between chips and chips, and is a key material for realizing multi-wafer packaging (MCP). Process advantages: No dispensing process is required, simplifying the process flow, and the uniformity of film thickness greatly reduces the risk of warping, and one test showed that the chip warpage of DAF bonding was reduced by 60% compared to epoxy resin; Process challenges: DAF is about 3-5 times more expensive than traditional epoxy resins, and the accuracy of the processing equipment is extremely high (positioning accuracy ≤± 1 μm), while the deformation caused by air penetration through the film needs to be prevented during operation (the bubble rate must be controlled below 0.1%). From a structural point of view, the bottom of the cut chip is supported by DAF, and the cutting tape below pulls the DAF with a weak adhesion (usually 5-10g/cm). During the bonding process, the chip is placed on the substrate immediately after removing the cutting tape and the chip and DAF on it, and the entire process needs to be completed within 1-2 seconds to avoid performance changes caused by DAF exposure to air.

Other bonding material technologies

In addition to the above two mainstream materials, there are also a variety of special bonding materials suitable for different scenarios: Metal alloys: Alloys made of gold, silver, or nickel, suitable for large sealed packages, with bond strength of more than 50MPa, can withstand extreme environmental conditions; Solder: Including tin-lead solder (Sn63Pb37) and lead-free solder (such as Sn96.5Ag3.5), bonded by fusion soldering, with excellent electrical conductivity (resistivity ≤ 10μΩ・cm); Polymer-polyimide: Excellent insulation properties (volumetric resistivity ≥ 10¹⁴Ω・cm) and suitable for applications where insulation is required, but low thermal conductivity (typically ≤ 0.5 W/m・K).

5. Coordinated development of substrate type and bonding technology

The type of packaging substrate is like the "mounting foundation of the chip", which directly determines the operation mode and process parameters of chip bonding. Currently, the mainstream substrate types include lead frames and printed circuit boards (PCBs), which have significant differences in application scenarios and bonding requirements.

PCB-based substrate bonding PCB substrates are widely used in small-sized mass production packages due to their flexible design and affordable cost:

Process Features: Suitable for bonding with epoxy resin or DAF, the bonding temperature is usually controlled at 150°C-200°C to avoid damage to the PCB substrate due to high temperature. Accuracy requirements: The positioning accuracy is generally ±5-10μm, which is lower than the requirements of the lead frame, and is suitable for cost-sensitive fields such as consumer electronics; Development trend: With the development of high-density PCB technology, its application in mid-to-high-end packaging is gradually increasing, and data from a PCB manufacturer shows that the bonding yield of its high-density interconnect (HDI) PCBs has reached 99.5%.

Substrate bonding based on lead frames

As a traditional packaging substrate, lead frames still occupy an important position in the field of high reliability: Material properties: Copper or iron-nickel alloys are often used, and the thermal conductivity is excellent (the thermal conductivity of copper lead frames ≥ 380 W/m・). K), suitable for high-power devices; Bonding requirements: usually solder or metal alloy bonding, the bonding temperature is high (200°C-250°C), and the positioning accuracy needs to reach ±2-5μm; Process advantages: It has good mechanical strength and electrical properties, and is widely used in automotive electronics, industrial control and other fields. With the increasing diversification of bonding technologies, the temperature profile of drying adhesives is also being optimized. In the case of epoxy curing, the traditional single-stage heating curve has been gradually replaced by a multi-stage curve: preheating (80°C-120°C, 2 minutes), curing (180°C-200°C, 5 minutes), and cooling (natural cooling to room temperature), which can improve the curing uniformity of the adhesive by 40%. At the same time, emerging methods such as heat bonding and ultrasonic bonding are also being used in specific areas – ultrasonic bonding enables metal-to-metal bonding through ultrasonic vibration at 20-40kHz without the need for high temperatures, making it particularly suitable for temperature-sensitive chips.

6. Development trend and future prospects of bonding technology

Driven by the continuous improvement of semiconductor integration technology, the packaging process is rapidly developing in the direction of ultra-thinning (chip thickness from 500μm to less than 50μm) and high-density (reducing the bond pitch from 100μm to less than 20μm), which brings new challenges and opportunities to chip bondingtechnology.

The process accuracy continues to improve

The positioning accuracy of bonding equipment has been improved from the traditional ±5μm to ±1μm, and some high-end models have even reached ±0.5μm, while the bonding speed is also increasing (currently up to 500 pieces per minute) to meet the needs of large-scale mass production.

Material innovation continues to make breakthroughs

The thermal conductivity of the new DAF material has exceeded 10W/m·K, which is close to the thermal conductivity level of metals. At the same time, bonding materials with self-healing functions are also being developed, which is expected to solve the problem of microcracks in the bonding process.

Emerging technologies are integrated and applied

Artificial intelligence technology has begun to be applied to real-time monitoring and parameter optimization of the bonding process, and through machine learning algorithms, the equipment can automatically adjust parameters such as ejection force and placement speed, so that the bonding yield is stable at more than 99.9%.

From traditional epoxy bonding to advanced DAF bonding, from single-chip bonding to multi-chip stack bonding, every advancement in the chip bonding process is driving the upgrade of semiconductor packaging technology. Today, as Moore's Law gradually slows down, bonding technology, as a key support in the era of "beyond Moore", is becoming a core breakthrough in improving chip performance and integration, injecting new impetus into the sustainable development of the semiconductor industry.