The placement machine is one of the key equipment of the SMT production line, and it is one of the key factors that determine the degree of automation, assembly accuracy and production efficiency of the assembly system. With the further miniaturization and fine pitch of SMC/SMD, and the introduction of lead-free projects, the performance (precision, speed, component detection ability, etc.) requirements of the placement machine have greatly increased.
3.3.1 Patch quality analysis
The placement machine is a typical integrated equipment with a high degree of integration of light, mechanics, electricity and gas, and its manufacturing requirements are relatively high. This is also the main reason why there is no high-end placement machine with independent intellectual property rights in my country. Compared with other SMT process links, the placement quality depends more on the performance of the placement machine, and the human factor is relatively small. The key factors affecting the quality of placement include: placement force, placement speed/acceleration, placement accuracy of placement machine, components, solder paste and PCB.
Mounting equipment
1. Patch force
In the actual placement process, based on the alignment principle of the placement head, the placement head can generally be divided into two types: non-centering claw placement heads and centering claw placement heads. For placement heads without centering jaws, since the placement head has only a vacuum suction nozzle, the mechanical damage to the components is small. For placement heads with centering claws, a mechanical centering claw is installed on the placement head, so that during the placement process, due to the clamping effect of the centering claws, a greater clamping force is generated on the components. It is very likely to cause damage to smaller or leaded components. ‘
The placement of the chip is to release the components after the placement head moves to a certain height directly above the position to be placed, and the components are placed to the specified position under the action of gravity. However, the size of the components mounted on the same placement head is different, and the weight varies greatly. Only relying on gravity will cause a large difference in placement quality. Therefore, in the current advanced placement machines, there is also the intelligent management of the airflow of the placement head, which enables the components to be placed quickly and has a small impact on the solder paste and components. Another important factor that affects the placement force is the height of the patch. The height of the patch is the height of the nozzle from the PCB surface when the component is released. Without additional force, the height of the patch will directly affect the pair of components. The impact force of PCB, the higher the height, the greater the impact force, resulting in solder paste collapse and so on. In the production process, the height of the component is often input into the program in advance, and the component is released when it drops to this height. However, due to the difference in the height of the components and the large height error, it is easy to cause early or late posting, which results in component displacement or even damage to the components. The general placement machine is equipped with multiple photoelectric sensors to detect the height of different components. Some advanced placement machines install pressure sensors on the placement head to release the components at the moment they contact the PCB surface, so as to achieve “soft The effect of “landing” is also called Z-axis soft landing. This effectively solves all kinds of defects caused by the impact of the patch.
2. Patch speed and acceleration
The speed of placement is not only a direct cause of productivity, but it also has a great impact on placement quality. In terms of the speed of the placement machine, the speed of the knife W has an impact on the placement quality, X, Y and. The speed and acceleration of the shaft transmission components are the key influencing factors, and they seriously affect the quality of the patch. Common transmission methods are: ①PCB bearing platform translation; ②Placement head rotation/PCB bearing platform translation; ③Orthogonal translation of the XY axis of the placement head. If the PCB workbench is used to move, the PCB workbench must move quickly during the working process, which requires the PCB carrier to accelerate and decelerate quickly. At this time, for those large and heavier components, the greater the acceleration, the greater the impact of the component, and the greater the mass of the component, the greater the inertia. In this way, for those that have been mounted In terms of components, it is possible to cause displacement due to large inertia, which will reduce the placement accuracy. Therefore, when mounting large and heavy components, the speed and acceleration of the PCB bearing table should be reduced.
3. Components
With the miniaturization and fine pitch of components, the influence of components on the quality of the placement is increasing. The smaller the components, the higher the precision requirements for the placement. A small rotation error or translation error will cause the components to be biased or even completely deviated from the soldering pad. For fine-pitch components, extremely small rotation errors may cause the components to completely deviate and cause bridging. Therefore, when mounting small, fine-pitch components, such as 0402.0201 and 01005 in the development stage, high-precision placement machines with small rotation and translation errors must be used. In the actual production line, there are generally at least two placement machines, namely high-speed placement machines and high-precision placement machines. High-speed placement machines mainly mount chip resistors, chip capacitors and other chip components, while high-precision placement machines mainly mount large, multi-pin integrated circuit devices such as PLCC and QFP. When mounting a device like a cylindrical metal end, because the device is prone to rolling, the viscosity of the solder paste should be relatively high during the mounting process, and the PCB should preferably not move or the moving speed and acceleration are small, otherwise It is easy to produce defects.
4. Solder paste
Solder paste provides the expected electrical and mechanical connections after soldering. It also requires sufficient adhesion to fix the components during the placement process. During the mounting process of components, if the PCB bearing platform is moving, the moving bearing platform will have an impact on the mounted components. If the components are to be kept in the mounting position, there must be sufficient adhesion. , This is very important when mounting large components. Therefore, this issue should be considered when selecting or evaluating solder paste. The other is the aging time of solder paste. If the life of the solder paste template is relatively short, the mounting process should be performed immediately after the solder paste is printed. Otherwise, the viscosity will decrease due to the rapid evaporation of the flux.
5. The degree of meal of the placement machine
The placement machine is actually a high-precision, high-automation numerical control equipment, which is an automated equipment composed of light, electricity, gas, precision machinery, and computers. The main performance parameters of the placement machine include: production rate, functional range, placement quality, placement accuracy, reliability (long-term and short-term), flexibility or flexibility, maintenance and repairability, operability and error prevention. The overall performance of the placement machine greatly affects the efficiency of the entire SMT production line.
The placement accuracy mainly includes the accuracy of the substrate, the positioning accuracy of the substrate, the positioning accuracy of the placement head, the positioning accuracy of the components and the repeat positioning accuracy of the placement head. The positioning accuracy generally refers to the deviation between the actual placement position of the placement machine and the set position, and the repeat positioning accuracy refers to the ability of the placement head to repeatedly return to the set point. The placement accuracy of the placement machine is mainly affected by the following aspects: the X-Y axis movement accuracy of the PCB positioning table, the accuracy of the component centering mechanism and the placement head (Z axis up and down movement, rotation) accuracy. The placement accuracy of components is generally affected by the three parameters X, Y, and 6. Among the above three parameters, 6 has the greatest impact, especially for those large and multi-pin components. Said, choose an XY axis placement accuracy is lower. A higher-precision placement machine has higher precision than choosing an XY-axis placement machine. A low-precision placement machine can improve the placement accuracy of components. Therefore, it is absolutely necessary to select a placement machine with appropriate axis accuracy according to the package form and size of the components.
(1) Substrate accuracy
The accuracy of the substrate includes the accuracy of substrate drawing, the manufacturing accuracy of the substrate, and the influence of the material quality of the substrate. If the accuracy of the substrate drawing is not high, even if the placement accuracy is high, the components will be offset; the manufacturing accuracy includes the accuracy between the pads, the accuracy of the pads to the positioning holes, and the accuracy of the pads to the edge of the PCB; Material quality mainly refers to mechanical stability and thermal stability. Mechanical stability refers to the ability of PCB to resist impact, while thermal stability refers to changes in PCB hardness and ductility when the temperature of the PCB increases. As the mounting speed increases, the temperature rise problem is becoming more and more serious.
(2) Substrate positioning accuracy
The substrate positioning is mainly for fixing the substrate and determining the accurate position of the PCB. The positioning of the PCB generally has two methods: hole positioning and edge positioning. Hole positioning can reduce the warpage of the PCB, and it does not require the reference point of the substrate; edge positioning is generally used in the case of thick substrates, small substrate warpage requirements, ceramic substrates, etc., this positioning method must have a reference point. Generally speaking, the accuracy of the positioning hole or positioning edge of the substrate pad is not ideal, and the reference point is usually used. The reference point is completed in the same process and in the same process as the pad during the manufacturing process, so it usually provides very accurate pad accuracy. By using an optical vision system, the position of the pad can be accurately measured.
(3) XY axis translation error
For the X-Y axis guide rail, its positioning accuracy mainly has three parts: step accuracy, linearity of the guide rail movement, and verticality of the x-y axis guide rail movement. In the actual transmission system, the non-linearity of the xy-axis motion trajectory is caused by many reasons, including: the change of the screw pitch, the change of the tooth pitch, the non-linearity of the rotary encoder and the non-linearity of the linear synchronous motor . As the patch speed increases, the movement speed of the x-y transmission mechanism increases correspondingly, which causes the x-y transmission mechanism to generate heat due to excessive movement. Generally speaking, the linear expansion coefficient of aluminum is 1.5 times that of steel. The ball screw is the main heat source. As the temperature changes, the placement accuracy will be affected. In the latest xy transmission system, the guide rail is installed inside The cooling system effectively reduces mounting errors caused by temperature.
(4) Z-axis motion error
Through the up and down movement of the Z axis, the pickup and placement of components are realized. Another important parameter of the Z axis is 0. The rotation accuracy largely determines the placement accuracy of the entire placement machine. The placement head usually moves to the designated position according to the program to pick up and place the components. However, due to the inconsistency of the height of the components and the inaccurate positioning of the position of the components in the feeding belt, it is easy to cause the center of the component and the center of the suction nozzle to not coincide. For placement heads with centering claws, you can use The mechanical claw aligns the center of the component with the center of the suction nozzle. For placement heads without centering claws, a detection camera should be installed on the placement head to detect the center of the component and the center of the placement head in real time, and feedback the offset to the servo motor to correct the deviation of the Z axis.
(5) Component positioning accuracy
For components, according to the actual placement accuracy requirements, mechanical centering jaws, centering workbenches or optical alignment systems are used for component centering, and mechanical centering jaws have simultaneous centering and sequential centering. The two alignment methods, which are the earliest positioning technology, are generally considered to damage the components, and the quality of the patch is relatively poor. Generally speaking, the accuracy of the centering platform is higher than that of the mechanical centering jaw. The disadvantage is that it may slow down the mounting speed. Optical CCD centering can obtain the most accurate centering. It is a necessity for fine pitch and FC/CSP patch technology. The accuracy level can reach 0.03mm~0.05mm (3a level). At present, this centering method has been widely used in high-tech applications. Precision placement machine. The latest developments include laser projection centering and single-line scanning centering, both of which are non-contact, high-speed centering methods.
