Preface

 

 

 

The solder paste printer is equipped with an optical vision system (or optical alignment system), which uses a camera to identify the alignment marks (mark/fiducial) on the PCB substrate and stencil to achieve steel plate openings and PCB pads. Automatic alignment, the X and Y directions of the CCD camera use high-precision servo motors, combined with high-precision ball screws (ballscrew), to jointly ensure the accuracy of captured images, thus ensuring the accuracy of the machine. The printing machine has high repeatability and can achieve fully automatic operation after being equipped with a PCB automatic loading system.

 

 

 

If the printing machine is not equipped with a vision system, it will be difficult to find the parallel plane of the stencil and PCB. Manual identification marks will help to set the stencil horizontally. The identification mark can be of any shape. Generally speaking, a 2.54mm (0.100in) square is enough. There are two identification marks on the diagonal corners of the stencil, just like the PCB. When using it, just place the PCB in the printing machine and align the identification marks of the PCB and the steel plate. This identification mark can also be used as a process control tool. For example, applying solder paste or adhesive on the identification mark can be used to check the flatness, thickness and boundaries, rather than inspecting the entire board.

 

 

 

Be familiar with the structure and principles of the optical alignment system of solder paste printers, and be able to set up and maintain the optical alignment system of printers.

 

 

 

1. Structure of optical vision system

 

 

 

The printing machine vision system is mainly used to identify the mark points (mark/fiducial) on the PCB substrate and the mark points (mark/fiducial) on the stencil. It positions the camera by moving in the X-axis and Y-axis directions, and can achieve the following functions.

 

 

 

(1) The substrate stops moving.

 

 

 

(2) Capture of mark points (for aligning the stencil and substrate).

 

 

 

(3) Position capture (used for two-dimensional pattern detection before and after printing solder paste). The positioning of the camera is performed under the joint action of the X-axis and Y-axis drive mechanisms. There are two types of drive mechanisms: linear servo motor system and rotary servo motor system. As shown in Figure 1 below is the rotary servo motor system, and the structure description of the camera rotation servo motor system is shown in Table 1 below; as shown in Figure 2 below is the linear servo motor system, and the structure description of the camera linear servo motor system is as shown in Table 2 below Show. The camera device consists of the following three parts.

 

Figure 1 Rotary servo motor system structure

 

 

 

Table 1 Structural description of camera rotation servo motor system

 

 

 

 

 

Figure 2 Linear servo motor system structure diagram

 

 

 

Table 2 Camera linear servo motor system structure description

 

 

 

 

 

(1) Camera mounted board stop.

 

 

(2) Board stops sensor (boardatstopsensor).

 

 

 

(3) Boardstop extended sensor (boardstopextendedsensor).

 

 

 

Camera mounted board stop: It is a pneumatic drive unit. When the substrate moves to a predetermined position, the cylinder drops to stop the movement of the substrate. The clamper clamps the substrate and starts visual calibration.

 

 

 

Board stop sensor (boardat stop sensor): The board stops blocking the light source. The board stop sensor senses photoelectrons formed due to background suppression, thereby detecting when the board reaches the board stop (boardstop). When it is detected that the substrate is at a predetermined position, a timer is started to control when to stop the conveyor belt and the substrate is clamped with a clamper.

 

 

 

Boardstop extended sensor (boardstopextendedsensor): The boardstop extended sensor is configured to ensure that the cameramountedboardstop must be lifted when the camera carrier needs to be moved.

 

 

 

The camera position is relative to the camera home position. Each main axis of the camera only sets the reference position during initialization so that it can be used to diagnose faults when the power is disconnected or the system is exited.

 

 

 

The camera position adjustment and setting methods are as follows.

 

 

 

(1) Camera origin position X: Since the positions of the sensor and blade are fixed, the camera origin position X does not need to be adjusted.

 

 

 

(2) Camera origin position Y: Since the positions of the sensor and blade are fixed, the camera origin position Y does not need to be adjusted.

 

 

 

(3) X-axis parallelism: X-axis parallelism is set by the manufacturer and usually does not require adjustment.

 

 

 

(4) Y-axis parallelism: The Y-axis parallelism used for rotation and linear motor drive is also set by the manufacturer and usually does not require adjustment.

 

 

 

The CCD camera (chargecoupled device, charge-coupled camera) captures the mark point image of the substrate or screen, converts it into a digital signal, and then uses computer hardware and software technology to process the digital signal of the image to obtain various target images required. Feature values, and on this basis, the four parts of pattern recognition, coordinate calculation, data conversion, and data output are implemented. Image acquisition consists of a CCD camera, light source, video image acquisition card and computer; image processing mainly completes image noise removal, geometric transformation and positioning through software programming; data conversion is to convert the collected digital image data into mark point feature values; The data output is mainly the data output obtained by calculating the converted data and displaying the mark points. Driven by the servo motor, the camera makes corresponding movements along the X and Y axis ball screws. The boardatstop sensor in Figure 2 above is a cylinder with a stopper. When the camera illuminates the substrate, the cylinder stopper drops to block the substrate and complete the positioning of the substrate.

 

 

 

The camera carrier also provides a driving mechanism for the cleaner under the screen.

 

 

 

The PCB is fed into the solder paste printing machine along the conveyor belt, and the machine automatically finds the main edges of the PCB and positions them. The Z-shaped frame moves upward to the position of the vacuum plate, adds vacuum, and firmly fixes the PCB in the predetermined position. After the camera slowly moves to the first target mark (reference point) of the PCB, the machine can move the stencil to align it with the PCB, and the machine can move the stencil in the X and Y axis directions and rotate in the θ axis direction, such as As shown in Figure 2-14. Once the stencil and PCB are aligned, the Z-shaped frame will move upward, driving the PCB to contact the underside of the stencil to prepare for solder paste printing.

 

 

 

2. Common faults, diagnosis and treatment methods

 

 

 

Common faults, diagnosis and treatment methods of optical alignment systems are shown in the table below.

 

 

 

 

 

3. Settings of optical vision system

 

 

 

The main interface for startup operation is as shown below (the interfaces of various types of printing presses are different, so they will not be introduced in detail here)

 

 

 

 

 

 

 

4. Comments on common parameters set

 

 

 

Board1 Fiducial Type: The type of Fiducial point of PCB board 1, including Circle, Rectangle, Diamond, Triangle, DoubleSquare, Cross, Videomodel and other types.

 

 

 

Screen1FiducialType: The type of Fiducial point of screen 1, including Circle, Rectangle, Diamond, Triangle, DoubleSquare, Cross, Videomodel and other types.

 

 

 

Fiducial1Xcoordinate: X coordinate value of Fiducial1, range is 0~508mm, increment is 0.1mm.

 

 

 

Fiducial1Ycoordinate: Y coordinate value of Fiducial1, range is 0~508mm, increment is 0.1mm.

 

 

 

ForwardXoffset: When the scraper scrapes from back to front, the printing offset on the PCB board. When the offset is +, the printing position moves to the right relative to the PCB board. The range is -1.0~+1.0mm, and the increment is 0.004mm.

 

 

 

ForwardYoffset: When the scraper scrapes from back to front, the printing offset on the PCB board. When the offset is +, the printing position moves backward relative to the PCB board. The range is -1.0~+1.0mm, and the increment is 0.004mm.

 

 

 

Forwardθoffset: When the scraper scrapes from back to front, the printing offset on the PCB board. When the offset is +, the printing position moves clockwise relative to the PCB board. The range is -1000~+1000arcseconds, and the increment is 2arcseconds.

 

 

 

ReversXoffset: When the scraper scrapes from front to back, the printing offset on the PCB board. When the offset is +, the printing position moves to the right relative to the PCB board. The range is -1.0~+1.0mm, and the increment is 0.004mm.

 

 

 

ReversYoffset: When the scraper scrapes from front to back, the printing offset on the PCB board. When the offset is +, the printing position moves backward relative to the PCB board. The range is -1.0~+1.0mm, and the increment is 0.004mm.

 

 

 

AlignmentWeighting: Pattern for two FID points. It is a weight value used to set the degree of deviation that FID2 can accept (for 2 points), and the setting range is 0~100%.

 

 

 

X Alignment: The weight of X in 3 FID point mode. Y Alignment: Y weight in 3 FID point mode. AlignmentMode: FID point selection mode.

 

 

 

Tooling Type: The support mode of the PCB board set by the program. Including AUTOFLEX, VACUUM, MAGNETIC, PILLARS (box type) and other 4 modes.

 

 

 

BoardStopX: The distance from the machine centerline to the PCB board stop position. The range is 50~254mm. BoardStopY: The distance from the machine rail to the PCB board stop position. The range is 25 (board width-20) mm.

 

 

 

RightFeedDelay: The extension time allowed by BoardStop when an irregular board enters the board from the right.

 

 

 

Adjust relevant parameters according to mark point quality

 

 

 

1) Shape of mark point (type selection)

 

 

 

The mark point types that the printing machine can accept can be Circle, Rectangle, Diamond, Triangle, DoubleSquare, Cross, Video Model, etc.

 

 

 

Select the corresponding mark point type according to the shape of the mark points on the PCB board and stencil, and give the corresponding identification point type parameters. When the quality of the identification points on the PCB board is poor or there are no identification points and the pads are used as identification points, VideoModel mode needs to be selected, but printing deviation is prone to occur when using this mode.

 

 

 

2) Adjustment of light

 

 

 

(1) Background selection and target score setting. Select Dark/Light based on the mark point itself and the surrounding background.

 

 

 

AcceptanceScore: The minimum score set by the machine that can be accepted for each type of mark point (comparison of the set and actual optical recognition quality). The better the actual value matches the set value, the higher the score. The AcceptanceScore score is given by the operator, and the optical recognition system accepts all mark points higher than this score.

 

 

 

TargetScore: Setting TargetScore tells the optical recognition system that the machine will accept the first point it searches for that is higher than this score. When no point with a score higher than this is found, the machine will remember all points with a score exceeding the AcceptanceScore and select the best one.

 

 

 

Under normal circumstances, the AcceptanceScore score is set to 500 and the TargetScore score is set to 700. If the score setting is inappropriate, for example, the target score is too small, when searching for an identification point, if the machine first finds other points that meet the score requirements, it will misjudge the point as an identification point, and some accidental deviations will occur.

 

 

 

(2) The corresponding parameters of the circular mark point are as shown in the figure below.

 

 

 

 

 

Among the three circles in the figure, the middle circle represents the shape of the recognition point, the outer circle represents the background range considered when analyzing the recognition point image, and the area between the inner circle and the recognition point is the range of the recognition point image analysis. For areas outside the above analysis range If there is any interference point, the machine will not consider it. However, if the inner ring is too large and the outer ring is too small, printing deviation will be more likely to occur.

 

 

 

(3) Adjustment of luminosity, as shown in the figure below.

 

 

 

 

 

Adjustment principle: There are two peaks in the histogram, which represent the graphics quality of Dark and Light areas respectively. The difference between the peaks can be changed by changing the level of illumination light. The adjustment result is that the higher the two peaks in the picture above are, the larger the distance is, the better the photographic quality will be. On the contrary, the photographic quality will be poor and the possibility of deviation will be greater. According to the current experience of using equipment, printing deviation will generally not occur if the score exceeds 700 points. However, if there is another similar point near the identification point, deviation may also occur.