As the size of active components becomes smaller and smaller, the size of passive components is also reduced, and designers can use them flexibly to complete the design of high-density products. 0603 and 0402 components have been in widespread use for many years and these components are capable of high assembly yields in volume applications. Recently, 0201/01005 components have been implemented in system assembly (SiP) and have been applied in mobile phones, digital cameras, wireless Bluetooth and other products. 0201 components are approximately one quarter the size of 0402 components, and 01005 components are approximately one quarter the size of 0201 components. Smaller size components may reduce the robustness of the assembly process. The assembly of such small components is more sensitive than other components in terms of process material selection, design, and process control. Due to its very small size, its dimensional tolerance will also have a very significant impact on the assembly process. Therefore, the assembly process of small components is different from other components and requires more precise control.

Key factors affecting the successful placement of small components

The key factors for placing small components include the positioning system of the placement machine, the control of the reclaiming process, the imaging system of the placement machine, and the control of the placement process. In addition to these factors, there are some points that cannot be ignored, such as the accuracy of the feeder, the error of the component packaging and the error of the component itself, the material design of the suction nozzle, etc., all need to be considered comprehensively before assembly. Let’s discuss the key control points of each link in the placement process.

Placement control of 0201/01005 components

1. Positioning system of placement machine

For the placement of small components, the drive positioning system is required to adopt closed-loop control on all drive axes to ensure the positional accuracy of picking and placement. Many placement machines now use a variable reluctance motor (VRM) drive system, which can improve thermal stability, obtain higher acceleration, and high precision, and some resolutions have reached 1um. The application of these technologies provides a guarantee for the successful placement of small components. Another point worth noting is that the vibration of the single cantilever beam of the placement machine with the arch mechanism during the placement process is often easily overlooked.

2. Control of the reclaiming process

Accurate retrieving is the first step to successful placement. Factors that affect correct retrieving in the process include differences between components, packaging errors, feeder accuracy, placement machine drive positioning system errors, The pressure control in the z-axis direction of the placement head, the material and design of the nozzle, and the control of static electricity during the picking process.

Since small differences between small components will have a significant impact on the picking and placement process, it is required that the placement head can automatically sense its changes during this process and take corresponding compensation measures to eliminate sensitivity to component height, thickness, etc. sex. The method used is to arrange a pressure sensor on the patch head to prevent excessive pressure from crushing the component during the process, or not getting the material. The z-axis resolution of the placement head of a better placement machine can reach 1um, and the pressure sensor can sense the deformation of 24um.

Traditional mechanical feeders can no longer meet the requirements of increasingly smaller components for high precision. Small components require motor-driven electronic feeders with higher precision and good antistatic effects. The feeder is installed on the placement machine, and there will be a gap and position error between them. This error is very small, and it can be completely ignored when mounting larger devices such as 0603/0805. But for the small 0201 and 01005, the impact will be great. When picking up components such as 0201/01005, it is difficult to pick up 4 or 7 components at the same time, because of this, and there are also errors in component packaging. Therefore, it is safer to pick up 0201 or 01005 by a single piece, which can ensure the reliability of picking

In order to eliminate errors caused by packaging, feeders, etc., and ensure the consistency of retrieving materials, it is necessary for the placement machine to have the ability to dynamically and automatically correct the retrieving position during the retrieving process. During the production process, the line needs to be changed, the material needs to be changed, and the status of each feeder is different, so the optimal pick-up position of the component will also change. The machine needs to sensitively capture this change in the process, and automatically find and adjust the suction position to ensure the accuracy and reliability of the suction.

3. The design and maintenance of the suction nozzle

Mounting 0201 and 01005 components requires a thinner suction nozzle. At the same time, in order to prevent static electricity from damaging the components and taking away other components during the picking process, the material of the thin nozzle needs to be antistatic, so ESD materials should be used. In order to minimize the lateral standing of components during the suction process and ensure sufficient vacuum and balance after the components are sucked up, 2 or 3 holes need to be designed in the head of the suction nozzle. Considering that the mounting density is less than 0.25mm, the head of the nozzle should be thin enough, and the holes on it will also be relatively thin. For the nozzle of 0201, the minimum aperture will reach 0.127mm, while the nozzle of 01005 component is thinner, reaching 0.1mm. This not only brings difficulty to manufacture, but also needs to increase the cleaning and maintenance frequency of these nozzles. The requirements for cleaning and maintenance of the nozzle are higher than other types of nozzles, which need to be cleaned with cleaning solvents and ultrasonic waves. Due to the thinness of 0201/01005, the thickness of 01005 components is only 0.1mm, which increases the chance of the thin nozzle contacting the solder paste. Increasing the frequency of cleaning and maintenance becomes necessary.

4. Image centering of components

There are two ways to determine the center of the component, one is to use a digital camera, and the other is to use a laser (laser). Both methods have advantages and disadvantages. A digital camera can be used to check for defects in the electrical terminals of components. But it cannot sense thickness variations of elements. For machines with z-axis pressure sensing and pick/placement compensation, there will be no serious problems. The thickness of the component can be detected by the method of laser imaging, but the defects in the electrical terminal of the component cannot be detected. In the actual mounting process, the difference in the area where the electrical terminals at both ends of the component overlap with the solder paste will affect the assembly yield after soldering.

5. SMT process control

The key control factors in the placement process are the flat support of the substrate, the control of switching from vacuum to blowing, the control of placement pressure, and the accuracy and stability of placement.

After the substrate enters the placement machine, the transmission guide rail clamps both sides of the substrate, and at the same time the support platform rises to support the board and continues to rise to the placement height. During this process, due to the action of external force, it is easy to cause the deformation of the substrate, and the possible deformation of the incoming substrate will seriously affect the quality of the patch. Support for flat substrates becomes very important. The application of thin substrates is more prone to the “spring bed” effect. The thin plate is concave with the pressure of the placement head, and recovers and deforms with the disappearance of the placement pressure. This repeats, causing the component to move on the substrate, and the placement defect occurs. Therefore, a support device needs to be arranged on the support platform to ensure that the substrate is flat and stable during the placement process. This device can use vacuum to suck the substrate, and can also use a special rubber thimble with energy absorption to eliminate vibration during the placement process and ensure that the substrate is flat.

Patch pressure is another critical factor that needs to be controlled. Improper control of the patch pressure will lead to component damage, solder paste collapse, solder beads appearing under the component, and may cause the component position to shift. The suitable pressure range for mounting 0201 and 01005 components is 150g – 300g. For the case of substrate deformation, the patch axis must be able to sense changes in pressure corresponding to deformation as small as 25.4um to compensate for substrate deformation.

Influence of placement accuracy on assembly of 0201/01005 components

The precision of the patch can handle the placement of 0201 and 01005 components very well. Of course, the printing accuracy of the solder paste must be guaranteed, and a single deviation sometimes does not have a great impact. But the combined effect of placement deviation and solder paste printing deviation must be controlled. For example, the patch deviation is +50um, while the printing deviation is -50um, and the overall deviation reaches 0.1mm, which is already very large for small components such as 0201 and 01005.

Therefore, we must pay attention to the overlapping area between the electrical terminal of the small component and the solder paste. The size and difference of the overlapping area between the two electrical terminals of the small component and the solder paste will have a great impact on the assembly yield. If there is a large difference in the contact area between the two ends of the component and the solder paste, this asymmetry can easily lead to “tombstoning” of the component in the reflow soldering furnace, solder balls and short circuits between components. The deflection of the component in the length direction and the width direction is not the same.

When designing PCB and printed patch stencils, it is necessary to consider the accuracy of the placement machine and printing machine, as well as the manufacturing error of PCB and SMT patch stencils, and determine the appropriate “overlapping area” to compensate for possible differences. In short, the assembly yield of small components is affected by the combination of placement accuracy and solder paste printing accuracy.

Small components are more and more widely used, which brings challenges to equipment and processes, because they are more sensitive to various variables, and small changes may cause very significant effects, such as the increase in 0201 mounting offset is even less than 0.1mm, bad The rate will increase by more than 5000ppm! The perfect combination of process materials, machines, methods (processes), personnel, and environment can achieve robust assembly processes and high-quality products. This time, we only elaborate on the key points of control in the mounting process, the design of substrate and printing stencil, the selection and printing of solder paste, the control of printing process and reflow soldering process are equally important to the entire assembly process.

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