PCBA Tech - The composition and function of SMT head

From the perspective of the robot concept, the SMT placement head is an intelligent manipulator. Through program control, it automatically corrects the position, picks up the components as required, and accurately places them on the preset pads to complete the three-dimensional reciprocating motion. It is the most complex and critical part of the placement machine. The placement head is composed of suction nozzle, vision alignment system, sensor and other components.

There are two types of patch heads: single-head and multi-head, and multi-head patch heads are divided into fixed and rotary types. After the suction nozzle of the early single-head placement machine sucked a component, the mechanical centering mechanism was used to realize the component centering and give a signal to the feeder to make the next component enter the suction position. However, the mounting speed in this way is very slow, and it usually takes 1S to place a chip component. In order to increase the patching speed, people adopt the method of increasing the number of patch heads, that is, using multiple patch heads to increase the patching speed. The multi-head placement machine has increased from a single head to 3 to 6 placement heads, instead of using mechanical alignment, it has been improved to a variety of forms of optical alignment. The components are picked up during work, and then placed on the PCB in turn after alignment. On the designated position of the board. At present, the placement speed of this type of machine has reached the level of 30,000 components per hour, and the price of this type of machine is relatively low and can be used in combination. Rotary multi-head structure can also be used. At present, the patch speed of this method has reached 45,000 to 50,000 pieces per hour.

(1) Suction nozzle. At the end of the placement head there is a placement tool controlled by a vacuum pump, that is, a suction nozzle. Components of different shapes and sizes are often picked and placed with different nozzles. After the vacuum is generated, the negative pressure of the suction nozzle sucks the SMD components from the feeding system (bulk silo, tubular hopper, disc-shaped paper tape or tray packaging). The suction nozzle must reach a certain vacuum when sucking the film. Only then can it be judged whether the picked up components are normal. When the component stands on its side or fails to be sucked up due to the component "cartridge", the placement machine will send out an alarm signal. At the moment when the pick-up nozzle picks up the components and places them on the PCB, two methods are usually used for placement. One is based on the height of the component, that is, input the thickness of the component in advance. When the placement head drops to this height, the vacuum is released and the component is placed on the pad. In this method, due to the individual differences of the components or PCB, the phenomenon of early or late placement may occur, and in severe cases, it may cause component displacement or flying chip defects. Another more advanced method is to realize the soft landing of the placement under the action of the pressure sensor based on the instantaneous reaction of the component and the PCB contact, so the placement is easy, and it is not easy to cause displacement and flying chip defects.

The suction nozzle is a component that directly contacts the components. In order to adapt to the placement of different components, many placement machines are still equipped with a device for replacing the suction nozzle. There is also a buffer mechanism for elastic compensation between the suction nozzle and the suction tube to ensure The protection of SMT components during the picking process.

The suction nozzle is in contact with the components during high-speed movement, and its wear is very serious, so the material and structure of the suction nozzle are getting more and more attention. Early adopting alloy materials, later changed to carbon fiber wear-resistant plastic materials, more advanced nozzles use ceramic materials and diamonds to make the nozzles more durable.

With the miniaturization of components and the reduction of the gap with surrounding components, the structure of the suction nozzle has been adjusted accordingly. Open a hole on the suction nozzle to ensure balance when picking up small components like 0603, pick up and place it without affecting the surrounding components, which is convenient for mounting.

(2) Visual alignment system. With the increasing demand for small, light, thin and high reliability of electronic products, only the precise placement of fine-pitch components can ensure the reliability of surface assembly. To accurately mount fine-pitch components, the following factors should generally be considered.

1. PCB positioning error. In general, the PCB circuit pattern does not always correspond to the machining holes and PCB edges of the PCB mechanical positioning, which will cause mounting errors. In addition, defects such as distortion of the circuit pattern on the PCB, distortion and warpage of the PCB will cause mounting errors.

2. The centering error of the components. The centerline of the component itself does not always correspond to the centerline of all leads, so when the placement system uses a mechanical centering claw to center the component, it may not be able to ensure that the centerline of all the leads of the component is aligned. In addition, in the packaging container, or when the centering claw is clamped and centered, the component leads may have defects such as bending, twisting, and overlapping, that is, the leads lose coplanarity. These problems will lead to placement errors and decreased placement reliability. Surface mounting is successful when the lead of the component deviates from the pad by no more than 25% of the lead width. When the lead pitch is narrow, the allowable deviation is smaller.

3. The movement error of the machine itself. The mechanical factors that affect the placement accuracy are: the X-Y axis movement accuracy of the placement head or PCB positioning table, the accuracy of the component centering mechanism and the placement accuracy. The vision system has become an important part of the high-precision placement machine.

The machine's vision system consists of two parts: vision hardware and vision software. The camera is the image sensing component of the vision system, and a solid-state camera is generally used. The main part of a solid-state camera is an integrated circuit, and a CCD array composed of many small and precise photosensitive components is fabricated on the integrated circuit chip. The electrical signal output by each photosensitive detection element is proportional to the intensity of the light emitted from the corresponding position on the observed target, and this electrical signal is recorded as the gray value of this pixel. The pixel coordinates determine the position of the point in the image. The analog electrical signal generated by each pixel is converted into a certain value between 0 and 255 through analog/digital conversion, and then transmitted to the computer. The large amount of information acquired by the camera is processed by the microcomputer, and the processing result is displayed on the monitor. The camera and the microprocessor, the microprocessor, the actuator and the display are connected by a communication cable.

The main factors affecting the accuracy of the vision system are the number of pixels of the camera and the optical magnification. The more pixels of the camera, the higher the accuracy; the greater the optical magnification of the image, the higher the accuracy. Because the greater the optical magnification of the image, the more image elements corresponding to a given area, and thus the higher the accuracy. However, when magnification, it is more difficult to find the corresponding graphics, so the accuracy reduces the placement rate of the placement system, so the appropriate camera optical magnification must be determined according to actual needs.