Key factors for successful placement of small chip components
As the size of active components becomes smaller and smaller, the size of passive components is also reduced, and designers can flexibly use them to complete the design of high-density products. 0603 and 0402 components have been widely used for many years and these components enable high assembly yields in volume applications. Recently, 0201/01005 components have been system-in-package (SiP) and have been used in mobile phones, digital cameras, wireless Bluetooth and other products. The 0201 component is approximately one-quarter the size of the 0402 component, while the 01005 component is approximately one-quarter the size of the 0201 component. Smaller component sizes may reduce the robustness of the assembly process. The assembly of such small components is more sensitive than other components in the selection of process materials, 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 mounting small components include the positioning system of the placement machine, the control of the material picking process, the imaging system of the placement machine, and the control of the placement process. In addition to these factors, there are some things that cannot be ignored, such as the accuracy of the feeder, the error of component packaging and the error of the component itself, the material design of the nozzle, etc., which all need to be considered comprehensively before assembly. Let’s discuss the key control points of each link in the placement process.
0201/01005 component placement control
- 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 position accuracy of picking and placement. Many chip placement machines now use variable reluctance motor (VRM) drive systems, which can improve thermal stability, obtain higher acceleration, and achieve high accuracy. Some resolutions have reached 1um. The application of these technologies provides guarantee for the successful placement of small components. Another point worth noting is that the jitter of the single cantilever beam of the placement machine using an arch-type mechanism during the placement process is often overlooked. - Control of the material taking process
Accurate material picking is the first step to successfully achieve placement. In this process, factors that affect correct material picking include differences between components, packaging errors, feeder accuracy, and errors in the drive positioning system of the placement machine. Pressure control in the z-axis direction of the placement head, nozzle material and design, and 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, the placement head is required to automatically sense the changes during this process and take corresponding compensation measures to eliminate sensitivity to component height, thickness, etc. sex. The method adopted is to arrange a pressure sensor on the patch head to prevent excessive pressure from crushing the components during the process, or preventing the material from being obtained. The z-axis resolution of the placement head of a better placement machine can reach 1um, and the pressure sensor can sense deformation of 24um.
Traditional mechanical feeders can no longer meet the high-precision requirements of increasingly smaller components. Small components require motor-driven electronic feeders with higher precision and good anti-static effects. The feeder is installed on the placement machine, and there will be gaps and position errors between them. This error is very small and can be completely ignored when mounting larger devices such as 0603/0805, etc. But for the small 0201 and 01005, the impact will be huge. When picking up components such as 0201/01005, it is difficult to pick up 4 or 7 components at the same time. The reason is this, and there is also the error in component packaging. Therefore, it is safer to pick up 0201 or 01005 for a single piece, which can ensure the reliability of picking materials.
In order to eliminate errors caused by packaging, feeders, etc. and ensure the consistency of picking materials, the placement machine needs to have the ability to dynamically and automatically correct the picking position during the picking process. During the production process, lines and materials need to be changed, and the status of each feeder is different, so the optimal picking position of the components will also change. The machine needs to sensitively capture this change during the process, and automatically find and adjust the suction position to ensure the accuracy and reliability of suction. - Design and maintenance of nozzles
Mounting 0201 and 01005 components requires a thinner 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 must be used. In order to minimize the side-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 on the head of the suction nozzle. Considering that the mounting density is less than 0.25mm, the head of the nozzle must be thin enough, and the holes on it will also be relatively thin. For the 0201 nozzle, the smallest hole diameter will reach 0.127mm, while the nozzle of the 01005 component is even thinner, reaching 0.1mm. This not only makes manufacturing difficult, but also requires increasing the frequency of cleaning and maintenance of these nozzles. The requirements for cleaning and maintenance of suction nozzles are higher than those of other types of suction nozzles, and cleaning solvents and ultrasonic waves are required for cleaning. Since 0201/01005 is very thin, the thickness of the 01005 component is only 0.1mm, which increases the chance of the fine tip coming into contact with the solder paste. It becomes necessary to increase the frequency of cleaning and maintenance. - 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. Defects on the electrical side of components can be detected using a digital camera. But it cannot sense changes in component thickness. For machines with z-axis pressure sensing and pick-up/patch compensation functions, no serious problems will occur. Laser imaging can be used to detect the thickness of components, but defects on the electrical ends of components cannot be detected. In the actual mounting process, the difference in the overlap area between the electrical terminals at both ends of the component and the solder paste will affect the assembly yield after welding is completed. - SMT process control
The key control factors in the placement process include the flat support of the substrate, the control of turning vacuum off to air blowing, the control of placement pressure, and the accuracy and stability of placement.
After the substrate enters the placement machine, the transmission guide rails clamp both sides of the substrate. At the same time, the support platform rises to support the board and continues to rise to the placement height. During this process, the substrate is easily deformed due to external forces. In addition, the possible deformation of the incoming substrate material will seriously affect the quality of the patch. Flat support for the substrate becomes very important. The application of thin substrates is more prone to the “spring bed” effect. The thin plate becomes concave as the placement head presses down, and resumes its deformation as the placement pressure disappears. This repeats, causing components to move on the substrate and causing placement defects. Therefore, a support device needs to be arranged on the support platform to ensure that the substrate is flat and stable during the patching process. This device can use vacuum to suck the substrate, or use a special rubber ejector pin with energy-absorbing effect to eliminate vibration during the patching process and ensure that the substrate is flat.
Patch pressure is another critical factor that needs to be controlled. Improper control of patch pressure can lead to component damage, solder paste collapse, solder beads appearing under the component, and component position deviation. The suitable pressure range for mounting 0201 and 01005 components is 150g – 300g. For substrate deformation, the patch axis must be able to sense changes in pressure corresponding to deformations as small as 25.4um to compensate for substrate deformation.
The impact of patch accuracy on 0201/01005 component assembly
The accuracy of the patch can handle the placement of 0201 and 01005 components very well. Of course, the printing accuracy of the solder paste must also be ensured. A single deviation sometimes will not have a big impact. However, the combined impact of patch deviation and solder paste printing deviation must be controlled. For example, the patching deviation is +50um, while the printing deviation is -50um, and the total deviation reaches 0.1mm. For small components such as 0201 and 01005, this deviation is already very large.
Therefore, we must pay attention to the overlap area between the electrical terminals of small components and the solder paste. The size and difference of the overlap area between the two electrical terminals of small components and the solder paste will have a great impact on the assembly yield. If there is a large difference in the area of contact between the two ends of the component and the solder paste, this asymmetry can easily cause the component to create a “tombstone” in the reflow soldering oven, resulting in a short circuit between the solder beads and the component. The deviation of components in the length direction and width direction produces different defects.
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 errors of PCB and SMT patch stencils, and determine the appropriate “overlap area” to compensate for possible differences. In short, the assembly yield of small components is comprehensively affected by the placement accuracy and solder paste printing accuracy.
The application of small components is becoming more and more widespread, which brings challenges to equipment and processes, because they are more sensitive to various variables. Small changes may cause very significant effects, such as an increase in 0201 placement offset or even less than 0.1mm, and poor performance. The rate will increase by more than 5000ppm! The perfect combination of process materials, machines, methods (processes), personnel, and environment can achieve a robust assembly process and high-quality products. This article only explains the key points of control in the mounting process. The design of the substrate and printing stencil, the selection and printing of solder paste, the control of the printing process and the reflow soldering process are equally important to the entire assembly process.
Send us an inquiry , we will get back to you soon ,thank you!