Talking about key technologies of placement equipment.

Today’s semiconductor industry development and application trends include smart mobile devices, big data, artificial intelligence, 5G communication networks, high-performance computers, the Internet of Things, smart cars, Industry 4.0, cloud computing, etc. These applications have given rise to the rapid development of electronic devices. Chips require higher computing speeds, smaller size, and larger bandwidth. They also require low power consumption, low heat generation and large storage capacity. This requires the manufacturing and packaging of chips to meet high-performance requirements. In the era known as the post-Moore’s Law, chip packaging has received more and more attention, and the placement machine is an important equipment in the chip packaging process. Mounter machines can be divided into SMT mounter machines and advanced packaging mounter machines according to application types. The latter is mainly used in the wire bonding process and flip chip process that have developed rapidly in recent years. Among the interconnection technologies for IC chips, the traditional three-level packaging: mainly chip-level packaging, substrate level packaging and master packaging. This packaging method has gradually been replaced by system-level packaging SIP. No matter how the packaging method evolves, an important process is inseparable from the chip packaging process, namely the mounting process. The mounting process has gone through from in-line, SMT surface mount, to today’s advanced packaging processes such as wire bonding and flip chip bonding. The mounter is also accompanied by the development of technology, confirming the changes of a generation of technology and equipment. Nowadays, in order to achieve refined mounting and meet the large-scale and low-cost production characteristics of electronic products, high-precision and high-yield performance indicators have been proposed for mounters. In this article, we want to discuss more about the chip equipment and technology. 

Firstly, let‘s discuss the SMT mounter machines, also called pick and place machines. Mounter is a key equipment in the semiconductor back-end process. We can divide it into two categories according to the type of mounting: The first is SMT mounter: It belongs to the key equipment of the surface mount process production line. It is mainly used to mount packaged chips, electronic components such as resistors, capacitors, etc. on the PCB board. The mounter is characterized by fast mounting speed, which can reach 20000 CPH, and sometimes it can even reach 150000 CPH. The mounting accuracy is not high, generally between 20 and 40μm; the second is the advanced packaging and mounter: It is mainly used for the mounting of bare chips or microelectronic components. It mounts chips to lead frames, heat sinks, substrates or directly to PCB boards. It can generally be divided into wire bonding and flip chip chips. It is currently the mainstream of semiconductor packaging. connection technology. Wire bonding first completes the stacked packaging of chips through a placement machine, and then connects the pad points on the front side of the chip to the frame or substrate pads through a wire bonding machine. The current process is relatively mature. Flip-chip mounting is a method of placing solder on the solder pad on the chip surface and directly soldering it to the corresponding solder ball on the substrate after flipping. Compared with wire bonding, it can achieve higher package density, shorter line interconnection, reduced interference, reduced capacitive impedance, and more stable and reliable connections. 

Another point is that advanced packaging and chip equipment is mainly used in the micro-assembly of logic devices, memories, MEMS, LEDs, Optoelectronic, RF, LD and other devices. The assembly process includes C2C, C2W and W2W and 2.5D/3D packages. Among them, high-density 3D packaging is the future development trend. Through through-silicon technology, stacked chip interconnection is realized. The most obvious feature of high-density 3D packaging is that it can reduce the size and quality of the product to 1/5~1/10 of the original. The main mounting technologies used include reflow soldering, hot pressing bonding, eutectic bonding, adhesive process, ultrasonic bonding, ultraviolet curing, conductive adhesive process, etc.

Secondly, we want to focus on the key technology of SMT pick and place machines. The development of advanced packaging and mounting machine equipment involves multidisciplinary systems engineering. The main performance indicators of the equipment are mounting accuracy and mounting yield. Currently, most mounting machines either meet high-precision mounting or high-yield mounting. Meeting both indicators is the current challenge. The main performance indicators of the mounter are affected by the following key technologies, such as accurate visual alignment system, reasonable structural layout, precise motion control and complete system software. 

The first key technology we want to discuss is visual counterpoint system. The alignment system of the mounter has gone through the process from the earliest mechanical alignment, laser alignment to visual alignment, and the alignment accuracy has gradually improved. The visual alignment system generally includes lighting sources, imaging lenses, photoelectric conversion cameras, acquisition cards and processing software for data transmission and processing. At present, the alignment of the position of the chip and the target patch is mainly carried out through visual alignment.

In manual and semi-automatic patch equipment, the alignment is carried out directly through image overlap. Fully automatic patch equipment mainly uses multi-dimensional visual image detection. Indirect alignment, which includes at least two independent imaging systems. The camera collects images, extracts image edges, and identifies the center position of the image through image algorithms. Generally, upper and lower field of view cameras are arranged to respectively obtain feature points on the chip or chip shape, and feature points related to the position of the target patch, thereby establishing the coordinate relationship between the chip and the target position points. In the process of establishing coordinate positions, the alignment methods used for the target patch position (substrate or wafer) are divided into global alignment and local alignment according to different patch accuracies. The global alignment efficiency is high. The prerequisite for positioning the target bit coordinates in one alignment is that the surface area accuracy of the substrate or wafer is high. The local alignment can adapt to the deviation of different array positions. Each patch position is individually identified and positioned. It is suitable for high-precision patches, but due to frequent alignment, the yield is relatively low. Most image recognition processes are in a static state. The dynamic recognition developed in recent years is mainly to improve productivity and reduce motion waiting time. It is called flight vision. Flight vision means dynamic photography. The flight vision system of the mounter needs to complete the mounting. When the mounting head moves above the vision camera at a certain speed, it collects images of the components to be mounted *ed by the suction nozzle, and at the same time, high-speed vision processing technology is used to complete the task of vision computing.

Flight vision technology is of great significance to improving the working efficiency of the entire machine. Flying photography requires high-speed image acquisition, and the positioning accuracy is affected by camera exposure time, communication time, etc. Mounter using this method to take pictures is mainly used in low-precision surface mounting equipment, such as mounting accuracy between 20 and 50μm. The accuracy of the system is directly related to the resolution of the camera and lens, as well as the image recognition algorithm. Improving the NA of the lens can effectively improve the resolution of the lens, while reducing the field of view of the lens, requiring balanced selection. Also, for cameras, increasing camera resolution also improves image recognition capabilities. The negative impact is to increase the data processing amount of a single image, increase the image acquisition and processing time, and cause yield impact. Image recognition algorithms are greatly influenced by the process. Using different algorithms to extract mark edge features can increase the adaptability of the vision system, thereby generate lower errors and improve alignment accuracy. 

The second key technology we want to discuss is the structural design of SMT pick and place machines. In addition to the accurate visual alignment system, the mounter must also ensure a reasonable structural layout, accurate motion mechanism and parallel motion design to improve productivity. At the same time, it must ensure the stability of the system and small environmental interference errors. Looking at the evolution process of the mounter, it can be roughly divided into four types according to the working methods of the mounter: boom type, turntable type, composite type and large parallel system.

The first is the moving arm type. This kind of structural mounter has high flexibility and high mounting accuracy. It is generally arranged on a marble or cast gantry, and is equipped with mounting arms that move back and forth. It is the main structure of most mounters. However, compared with several other structures, patch yields are relatively low, and our Nectec customers generally use two arms to improve yields. The second is the turntable type, which installs the chip head on a rotating spindle. While a single chip head absorbs chips, the chip heads at other stations can perform actions such as alignment and mounting, which greatly improves the productivity. Due to the long transmission link and complex structure, the mounting accuracy brought by this structure is lower than that of the movable arm type. It is mainly used in SMT mounters, and advanced packaging and mounters still use the movable arm type structure as the main part. The third type is a composite structure, which can transfer a large number of chips at one time and concentrate on suctioning and pasting. It combines the advantages of a movable arm type and a turntable type, but the structure is relatively complex, high development costs, and lacks flexibility. The fourth type is that large-scale parallel systems adopt modular design, and multiple sets of chip transfer or mounting components are set up according to the specific bottleneck stations of the production line to meet the batch packaging needs of large-scale production lines. 

Considering the stability of the structure and the influence of ambient temperature, in the design of the structural frame of the mounter, try to select materials with better specific rigidity, that is, the ratio of the elastic modulus to the density of the material. These materials have good rigidity and light weight, such as marble frames and cast-iron frames. High-precision mounting equipment adds a passive or active vibration damping system to the bottom of the frame to reduce the interference of foundation vibration. From the perspective of error size chain analysis, the thermal expansion coefficient of the material must also be taken into account. The smaller the coefficient, the less the measurement system will be affected by the ambient temperature. Thanks to the maturity of modern computer simulation technology, the impact of environmental factors on the above structural design can be optimized through finite element simulation analysis combined with actual test data, such as static simulation, modal simulation, dynamic simulation, thermodynamic simulation, etc. In terms of productivity improvement, the structural design tries to minimize the relationship between material supply and target patch locations, shorten the path, and reduce material transfer time, because about 70% of the cycle time of a single patch is used for material handling. In structural design, the chip head is a key component in the structural design. In order to adapt to the chip placement process, in addition to meeting the basic negative pressure adsorption of the chip, it also needs to meet the multi-freedom leveling to ensure the tight and uniform fit of the chip and the substrate during the chip placement process. Some processes also require pressure and heating to meet the eutectic chip placement process. 

The third key technology we want to discuss is precision motion control. Since there are two types of motion system, we will explain each one at a time. The first motion system is sports table system. In the application of advanced packaging and placement machines, in order to coordinate the transfer and placement of chips, multi-axis displacement platforms are laid out inside the equipment. These motion platforms include the movements of the X, Y, Z, and Rz axes of the chip carrier, as well as the multi-dimensional movement of the chip head. In recent years, the transmission mechanism has gradually been improved from a ball screw structure driven by servo and stepping motors to a direct-drive motor structure. For the bearing table with heavy loads, an air float guide rail or a maglev guide rail is used instead of the transmission rolling guide rail, which reduces the mechanical transmission. Wear, reduce motion errors, and at the same time increase the speed, acceleration of the moving platform, thereby improving the productivity of the system. While increasing the speed of the chip head movement, the entire system often introduces impact. In the design of the mechanism, some manufacturers use methods such as increasing the rigidity of the frame or increasing weights and attracting gravity to buffer the reaction force of movement and achieve dynamic balance of the system. The traditional semi-closed-loop system, such as the encoder feedback position accuracy, is gradually replaced by full-closed-loop servo feedback grating ruler measurement system, directly bringing the chip accuracy from tens of microns to micron or even sub-micron mounting accuracy.

During the driving process of the motion table, the X and Y axes stacked driving method is generally used. Due to the heavy load on the Y axis in the lower layer, the double guide rail and double beam driving technology can increase the movement speed of the Y axis and reduce left and right shaking. At this time, the left and right drive shafts require strict synchronization and require synchronous motion control. Then, the second motion system is control system. The control system is divided into control hardware and control software. The hardware architecture depends on the main control module. There are generally the following types: single-chip microcomputer system, professional sports PLC system, and PC plus professional sports control card. Among them, single-chip microcomputer and PLC are mainly used in equipment with simple motion structures and fixed motion trajectories, while PC plus professional sports cards can realize complex curve motions and complex motion algorithms. For fully automatic complex control systems, a PC plus professional sports card can also be replaced by a server plus professional sports controller. The system software is divided into upper computer main control program, human-computer interactive interface software and lower computer multi-axis motion control, image acquisition and analysis, I/O control, analog quantity acquisition, and system accuracy calibration software. Part of the accuracy improvement of the mounter is improved through alignment compensation of the vision system. The upper computer is usually an industrial computer or server, which completes human-computer interaction, image display, task division management and communication functions.

The lower computer is usually an independent motion control module, microprocessor, PLC, etc., which requires high real-time performance and coordinates various motion axes, sensors, image acquisition, I/O control and other actions. For links with high requirements for real-time actions, hard trigger methods are generally used to reduce code execution time and improve productivity. 

To conclude, as IC chips develop towards high density, high reliability and low cost in the integrated circuit industry, higher requirements are put forward for the key equipment mounter in the packaging field, and the mounting accuracy and mounting yield are increasing year by year. With the continuous investment in the integrated circuit industry in recent years, equipment suppliers will also face new opportunities and challenges. We feel that in the future, advanced packaging and chip equipment needs to have multi-functional, modular, flexible, and intelligent characteristics. Only by continuously investing in research and development of key technologies can we be unique in market competition.