Pcb manufacturer’s SMT head composition and function analysis

From the concept of the robot, the Pcb manufacturer’s smt head is an intelligent robot, which automatically adjusts the position through program control, 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 head is composed of nozzle, visual alignment system, sensor, and so on.

There are two types of heads: single head and muti-heads. The muti-heads are divided into fixed and rotary. After the suction nozzle of the early single-head placement machine sucks a component, the component is centered by the mechanical centering mechanism and a signal is sent to the feeder, so that the next component enters the suction position. However, this method is very slow, and usually requires 1S for mounting a chip component. In order to increase the  mounting speed, Pcb manufacturer has taken a method of increasing the number of smt heads, that is, using a muti-heads to increase the mounting speed. The multi-head placement machine has been increased from a single head to 3 to 6 smt heads, no longer using mechanical alignment, but improved to various forms of optical alignment. The components are separately sucked during operation, and then placed on the PCB designated location in turn. At present, the placement speed of these models has reached the level of 30,000 components per hour, and these machines are cheaper and can be used in combination. It is also possible to use a rotary multi-head structure, which currently has mounting speed of 4.5 to 50,000 per hour.

Nozzle. At the end of the smt head is a mounting tool controlled by a vacuum pump, that is, a nozzle. Components of different shapes and sizes are often picked up by different nozzles. When the vacuum is generated, the negative pressure of the nozzle sucks the SMD components from the feeding system (bulk silo, tubular hopper, disc-shaped paper tape or tray packaging), and the suction nozzle must reach a certain degree of vacuum when sucking the components. Only can determine whether the picked up components are normal. When the component is sideways or the component "clip" fails to be picked up, the placement machine will give an alarm signal. At the moment when the pick-up nozzle picks up the components and places them on the PCB, they are usually placed in two ways. First, Pcb manufacturer according to the height of the component, that is, the thickness of the component is input in advance, when the head is lowered to this height, the vacuum is released and the component is placed on the pad. This method sometimes causes premature or late posting due to individual differences in components or PCBs. In severe cases, component displacement or flyer defects may occur. Another more advanced method is to realize the soft landing of the placement under the Pcb manufacturer’s action of the pressure sensor according to the reaction of the contact between the component and the PCB, under the action of the pressure sensor, the soft landing of the placement is realized, so the mounting is easy, and the displacement and the flying blade defect are not easy to occur.

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

The nozzle is in contact with the components during high-speed movement, and the wear is very serious, so the material and structure of the nozzle are getting more and more attention. The alloy material was used in the early stage, and then changed to carbon fiber wear-resistant plastic material. The more advanced nozzle uses ceramic material and diamond to make the nozzle more durable.

As the components are miniaturized, and the gap with the surrounding components is also reduced, the structure of the nozzle is adjusted accordingly by Pcb manufacturer. Open a hole in the nozzle to ensure that it is balanced when sucking small components such as 0603, sucking up and placing it without affecting the peripheral components and facilitating placement.

Visual alignment system. As the demand for small, light, thin and high reliability of electronic products continues to increase, only the precise placement of fine pitch components can ensure the reliability of surface mount placement. To accurately mount fine pitch components, the following factors should be considered by Pcb manufacturer.

PCB positioning error. In general, the circuit pattern on the PCB does not always correspond to the machined hole and PCB edge of the PCB mechanical positioning, which will result in mounting errors. In addition, defects such as distortion of the circuit pattern on the PCB, PCB deformation and warpage can cause mounting errors.

Component centering error. The centerline of the component itself does not always correspond to the centerline of all leads, so when the placement system uses mechanical centering jaws to center the component, it does not necessarily ensure alignment of the centerline of all leads of the component. 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 lead loses coplanarity. These problems can cause mounting errors and placement reliability to drop. The surface mount is successful when the component leads are offset from the pad by no more than 25% of the lead width. When the lead pitch is narrow, the allowable deviation is smaller.

The motion error of the machine itself. The mechanical factors affecting the SMT accuracy are: X-Y axis motion accuracy of the SMT head or PCB positioning workbench, precision of the component centering mechanism and mounting precision, and the vision system becomes an important part of the high-precision placement machine.

The vision system of the machine consists of two major parts: visual hardware and visual software. A camera is a sensing component of a vision system image, typically using a solid-state camera . The main part of the solid-state camera is an integrated circuit. The integrated circuit chip is made up of a CCD array composed of many small precision photosensitive components. The electrical signal output by each photosensitive detecting component is proportional to the intensity of the emitted light at the corresponding position on the observed object, and this electrical signal is recorded as the grayscale 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 to a value between 0 and 255 by analog-to-digital conversion and transmitted to the computer. A large amount of information acquired by the camera is micro-processed, and the processing result is displayed by the display. The camera is connected to the microprocessor, the microprocessor and the actuator and the display by a communication cable.

The factors that affect the accuracy of the vision system are mainly the number of pixels of the camera and the optical magnification. The more the number of pixels in the camera, the higher the accuracy; the larger the optical magnification of the image, the higher the accuracy. Because the larger the optical magnification of the image, the more image elements correspond to a given area, and the higher the accuracy. However, when the magnification is increased, it is more difficult for Pcb manufacturer to find the corresponding pattern, and thus the accuracy lowers the placement rate of the placement system, so it is necessary to for Pcb manufacturer to determine the appropriate camera optical magnification according to actual needs.