How to control the placement quality of SMT pick and place machine
From: Author:Mark Hardy Publish time:2021-09-10 15:09 Clicks:0
1. The influence of placement machine parameters
Different types of placement machines have their own different structural characteristics, but in general, the main factors that affect placement quality are placement height, placement pressure, vacuum suction and blowing, etc., which will be introduced below.
1. Patch height
The effect of patch height on placement is mainly due to the fact that the placement position that is too high or too low will affect the placement pressure (described later), thereby affecting the placement quality. The following lists the situations that may change or need to change the height of the patch for reference:
① The thickness of the component exceeds the range of the placement machine;
② The mounting axis is loose;
③ Special-shaped components or special-shaped nozzles are used.
I.C.T is a manufacturer of SMT machines. It mainly provides customers with SMT production lines including SMT Stencil Printers, Pick and place machines, Reflow Oven, AOI Machine, Wave Soldering Machine and PCB Handling Machine etc. I.C.T has more than 25 researches on SMT and DIP technology, for the world Customers provide SMT total solutions. There are successful cases of SMT technICal team in Asia, Europe, AmerICa, AfrICa, and Australia.
More details, please contact us:
Tel: +86 13670124230 (WhatsApp/Skype/WeChat)
2. Patch pressure
Patch pressure is another key factor that needs to be controlled. For chip components and leaded ICs, improper placement pressure control can cause component damage, solder paste collapse, solder beads under the component, and may cause component position shift. For example, for mounting 0201 and 01005 components, the appropriate pressure range is 150-300g. For special-shaped plug-in components (multi-function machines), too little pressure will cause the components to be unable to be embedded in the positioning holes, such as the placement and production of mobile phone shielding covers and computer motherboard connectors. Under special circumstances, the maximum pressure required may reach 2.5~ 5.0kg. In addition, in the case of PCB deformation, the patch axis must be able to sense the change in pressure corresponding to a deformation as small as 25.4μm to compensate for the deformation of the PCB. Excessive pressure will cause a horizontal force on the component during the downward pressing process (because the PCB is deformed and bent downward), which will cause the component to slide sideways.
In addition, excessive pressure will squeeze the solder paste at the bottom of the component to form a solder ball, or cause a short circuit between adjacent components, as shown in the figure below.
3. Vacuum suction
The vast majority of placement machines use a vacuum generator to generate negative pressure to absorb components in the suction of chip components and ICs. The figure below is a schematic diagram of the component's force on the placement head during the placement process.
The ellipse in the above figure represents the suction position. It can be seen from the above figure that for the placement system, the vacuum suction force is not only greater than the gravity of the component, but also meets the movement (acceleration) requirements of the placement head. The suction is too small. After the component is recognized by the camera, it is likely that the component will shift when the placement head moves to the placement point. This will directly cause the placement of the placement, and it is easy to cause defects such as tombstones after soldering. The vacuum suction formula is
In the formula, P is the negative pressure generated by the vacuum valve, and S is the effective contact area of the nozzle. The normal negative pressure of the vacuum valve depends on the air pressure and atmospheric pressure of the system air source (related to the altitude). At present, most of the placement machines require air pressure between 60 and 90 PSI, which depends on the characteristics of the equipment. related. In addition, the effective contact area of the suction nozzle is determined by the type of the suction nozzle, and it is also directly related to the degree of wear of the suction surface. If the effective contact area is too small due to the wrong nozzle model, this will directly reduce the vacuum suction power. If the suction surface wears too much to cause gas leakage, the P value will be severely reduced, and the vacuum suction will also be affected.
Therefore, when users suspect that there is a problem of suction or sliding components on the placement head, the vacuum degree and factors related to the vacuum degree should be checked first, or these items should be checked regularly during the production process to control the placement quality.
In the final stage of the component movement process of the placement head, when the placement head puts the component to a given height, the vacuum will be turned off, and then there will be a short blowing action. The purpose of this process is to completely eliminate the nozzle duct In order to ensure that the component will not stick to the suction nozzle at the moment when the suction nozzle is raised, it will not cause the component to be taken away or shifted on the solder paste. If the action process does not happen in time, the impact is predictable. In addition, the blowing time should not be too long, otherwise it may affect (blow off) adjacent components or blow away the solder paste.
Second, the influence of the structural parts of the placement machine
The structural parts of the placement machine will also have a serious impact on the placement quality, especially for placement equipment without component height inspection. Of course, even if there is a device for component height inspection, it will have an impact. The following will explain from both the feeder and the suction nozzle.
While the placement machine pays attention to accuracy, it must also emphasize high speed and efficiency. During the suction process, the placement machine does not use the camera to determine the center point of the component in the feeder, because this is very time-consuming, so a feeder is required. The components are parked at a very high-precision reclaiming position, which depends on the accuracy of the mechanical position of the feeder on the feeding station, as well as the accurate correction of the feeder and the accuracy of the paper tape pitch (for the reel Language) to guarantee. Ideally, the component should be located within the designed position accuracy range so that its effective suction area coincides with the center of the nozzle. Taking the 0201 component as an example, the dark area in the left picture in Figure 1 below represents its effective suction area , If the nozzle sucks in the center of the cross coordinate
2. The influence of the suction nozzle
The suction nozzle is a part directly in contact with the component, and its influence on the quality of the patch is unquestionable. It is mainly reflected in the design of the nozzle, the correct configuration of the nozzle and the maintenance of the nozzle, which are explained separately below.
(1) The design of the nozzle
The nozzle design should mainly consider the material, the effective suction shape and area, and the number and symmetry of the suction hole. For materials, it is necessary to consider and weigh whether there are anti-static requirements, whether it is abrasion resistance and reflective strength. Regarding the number and symmetry of the suction holes, for the regular rectangular parallelepiped small magnetic chip components, the suction nozzle usually uses a larger diameter hole in the center, and a plurality of smaller diameter holes are symmetrically equipped around the nozzle.
Figure 2 A model 0201 nozzle (b)
① Magnetic chip component suction nozzle: the above figure 1 and figure 2 are a certain model 0201 suction nozzle, its suction surface has two small holes symmetrically located on both sides of the central large hole, and the shape of the suction surface is designed as a rectangle; the following picture is general The suction nozzle of the magnetic chip element adopts a structure with a large central hole and 4 small holes evenly arranged at 90°.
②Small-size IC component nozzle: The figure below shows a single-hole structure, and the larger-size IC component nozzle should be smaller.
③Large size IC component nozzle: a structure with a large middle hole and a rubber sealing ring
For the design of the nozzle for special-shaped components, in addition to the above design factors, the position of the center of gravity of the special-shaped parts should also be considered. Otherwise, during the acceleration and deceleration of the placement head, it is easy to cause the special-shaped components to slide on the suction nozzle, resulting in sticking. The film is bad.
④ MELF component suction nozzle: Since the shape of MELF component (such as chip diode) is cylindrical, a special suction nozzle with a concave cylindrical surface should be used, as shown in the figure below.
(2) The correct configuration of the nozzle
Different equipment manufacturers have their own matching nozzles, and each nozzle has its applicable suction range. Therefore, the equipment user should configure the nozzle according to the equipment manufacturer's definition. For example, a large-caliber suction nozzle is not suitable for sucking small components 0201, and a small-caliber suction nozzle is also not suitable for sucking large IC materials (due to the small effective area for suction, the suction power may be insufficient).
(3) Suction nozzle maintenance
The use frequency of the suction nozzle is very high, it is easy to wear, and it is also easy to be contaminated or blocked. Therefore, it should be cleaned and maintained at an appropriate time. Usually, it can be cleaned with a cleaning solvent. For example, a nozzle with a rubber sealing ring structure (as shown in Figure 5.58), the rubber sealing ring is easy to wear, and needs to be inspected regularly to ensure that it is replaced regularly. With the miniaturization trend of chip components, thin components like 0201/01005, 01005 components are even thinner to 0.1mm. They not only bring difficulties to manufacturing, but also increase the contact between the suction nozzle and solder paste or glue. It is necessary to increase the frequency of cleaning and maintenance of such nozzles. The requirements for cleaning and maintenance of such nozzles are higher than other types of nozzles, and cleaning solvents and ultrasonic waves are required for cleaning.
Third, the influence of PCB performance parameters
Regarding the performance parameters of the PCB, it is closely related to the manufacturing process of the PCB and the design requirements of the PCB designer. Among the many PCB parameters, the main influence on the placement process is its geometrical size parameters, which mainly include the flatness of the entire PCB, PCB manufacturing tolerances and PCB surface treatment, etc., which will be discussed separately below.
According to the characteristics and manufacturing process of PCB, we know that it is formed by layer by layer bonding and crimping, stress will be generated during the manufacturing process, and the material and thickness of the pad layer and the middle electrical isolation layer are different. The coefficient of thermal expansion is also different. Therefore, during packaging and transportation, the PCB may undergo plastic deformation (Bow) or twist (Twist). As shown in the figure below, the degree of plastic deformation is related to the thickness and area of the PCB and whether its layer design is symmetrical or not. .
The allowable value of PCB bending and twisting is 0.75% of the PCB length L, that is (L×0.75%) mm, but the maximum value cannot exceed 3.175mm.
If the flatness exceeds the allowable range, the shape of the reference point in the camera will change. For example, the shape of the standard circle reference point in the camera will become an ellipse, which will cause the center distance reading between the reference points to change, which will affect the position of the component pad on the PCB relative to the reference point, which may directly cause Components are biased. In addition, due to the warpage of the PCB, the component will slide relative to the solder paste on the pad during the pressing process, or the solder paste at the bottom of the component will be squeezed away, thereby forming solder beads or causing a short circuit between adjacent components.
2. The influence of manufacturing tolerances
PCB manufacturing tolerances will also have an impact on the placement quality, which is mainly reflected in the profile and position deviation of the reference point (Fiducial). As for other errors on the PCB, such as the manufacturing tolerances of the vias, it will directly affect the electrical performance after soldering, but this is not an impact on the placement machine.
3. Surface treatment
The impact of surface treatment on placement is mainly reflected in the coverage of fiducials. It can mainly affect whether the video system can accurately and quickly identify fiducials. The important principle is that the solder mask cannot cover the reference point and the auxiliary resolution area around it.
Fourth, the influence of PCB pad graphic design
Regarding the overall requirements of the PCB graphic design, please refer to the definition of the IPC-782 file, which defines the geometric tolerances of the ends/pins and PCB pads of various components. The following is a discussion on several points of general concern in the placement process. PCB design engineers should be familiar with these industry standards and understand related manufacturing processes in order to reduce quality risks. Here we mainly introduce the reference points, pads and board margins. Design etc.
PCB fiducial point (Fiducial, also known as Fiducial Point or Fiducial Mark, or Mark for short) is the basis for the placement machine to locate other components on the PCB. The shape and position design of the fiducial point and its manufacturing accuracy will directly affect the placement quality. The description of the reference point can be divided into 4 types according to the definition of IPC "SMEMA FIDUCIAL MARK STANDARD 3.1", and its layout on the PCB is shown in Figure 1 and Figure 2.
Figure 1 Reference point (a)
Figure 2 Reference point (b)
The single board shown in Figure 1 above has a global fiducial (Global Fiducial) and a local fiducial (Local Fiducial). The local fiducial is usually used when components have higher position accuracy requirements, such as high-density pin ICs and BGAs. Etc.; the above figure 2 shows the multi-joint board, which has Panel Fiducial and Image Fiducial. Of course, local fiducial can also be placed in the sub-board. The following table is the reference point related requirements defined by IPC.
In the definition of IPC, it is recommended to use a circular reference point, but in practical applications, PCB design engineers may use a variety of geometric shapes, and chip equipment manufacturers have also increased some compatible shapes accordingly. For example, the reference point shape and size that Universal Instruments is compatible with
2. Pad design
The impact of pad design on the placement machine is mainly the impact of the placement of the components on the PCB on the nozzle of the placement head. When laying out PCB components, design engineers should consider the difference between the minimum spacing between components relative to the thickness of the components. The principle is to increase the spacing between components with large thickness differences as far as possible to avoid the nozzle When attaching a thin component, it touches a thicker component, which causes the thicker component to move laterally, and finally causes welding defects.
3. The board margin from the PCB component to the feeding direction
Regarding the transfer board margin, there are two common standards in the SMT industry, namely 3mm margin and 5mm margin. At present, 3mm margins are widely used, so PCB design engineers should pay special attention to 3mm margins at least, otherwise it will be restricted during patch production. Please refer to the figure below for the description of the board margins.
LED Non-Standard Automatic Assembly Line SMT, Full Hot Air Reflow Oven, Double Head DIP Soldering Machine, Pick And Place Automation System, Hanwha Sm482, Fuji Nxt SMT Machine Price, Vision Pick And Place, Asm SMT Machine, Hanwha Pick And Place Machine, Siemens Pick And Place Machine, SMT Stencil Cleaning, Vaccume Cleaner PCB, Inline PCB Cleaning Machine, SMT Mounting Suction Nozzle Cleaning Machine, SMT Nozzle Clean, Handheld Automatic Screw Feeder Robot, Automatic Screw Locking Robot Vibrating Feeder, Automatic Screw Locking Robot With Feeder