Solder paste printing is the first process of the SMT process, and it is a key process to ensure the quality of SMT. It directly affects the soldering quality and reliability of the surface assembly. At present, the most widely used is the automatic printing machine metal mesh solder paste process. The automatic solder paste printing machine is mainly composed of the main body of the equipment, the PCB transfer mechanism, the squeegee device, the printing (work) table, the optical vision system, the computer control system, and the power It is composed of driving device, off-grid mechanism and stencil cleaning mechanism. The basic working principle of the solder paste printer is that after the PCB is sent to the solder paste printer from the conveyor belt, the clamping device fixes it on the XY-θ platform, and the vision system is based on the two (or more) of the PCB and the screen. Multi) Deviation of the coordinate value of the mark recognition point, and correct the deviation. The X-Y-θ platform is adjusted by the drive motor, and the PCB is accurately aligned with the stencil by moving in the three directions of X-Y-θ, and finally the solder paste is printed on the PCB pad through the special opening of the stencil.

 

 

One, the cognition of solder paste printing machine

 

Solder paste printing refers to uniformly printing the solder paste on the PCB pads to ensure that the SMD components and the corresponding pads can be firmly combined. Solder paste printers can be divided into three types: automatic, semi-automatic and manual. Among them, the automatic printing machine has a high degree of automation and can be connected with other SMT equipment, which is suitable for large and medium batch production; the semi-automatic printing machine cannot be connected with other SMT equipment, but uses manual transmission, manual positioning, and automatic printing, which is suitable for medium , Small batch production; the manual printing machine has simple structure and low price. The loading, positioning and printing are all manual, which is suitable for scientific research institutes and teaching use. At present, the solder paste printers configured in the SMT production line are all fully automatic printers, which have comprehensive functions and can automatically complete a series of solder paste printing operations.

The automatic solder paste printing machine mainly includes: mechanical system, optical vision system, PCB positioning system, pneumatic control system, computer control system, automatic cleaning system, etc. With the development of the SMT industry, in order to meet the requirements of fine-pitch, high-density electronic packaging technologies such as QFP, SOP, BGA, CSP, 0603, 01005, etc., the new generation of automatic vision printers produced in my country are synchronized with the international leading technology. The equipment has a high-resolution vision processing system, a high-precision transmission system, a suspended adaptive scraper, precise plate positioning processing and a smart screen frame clamping structure. Its structure is compact, and it has both accuracy and high flexibility. , Provides the functions required for efficient and precise printing. The G3 produced by GKG is a high-precision and high-stability full-automatic vision printing machine. Taking GKG's G3 printing machine as an example, the structure of the full-automatic solder paste printing machine is introduced.

 

1) Mechanical system of solder paste printing machine

 

The mechanical system of the solder paste printer is mainly composed of a conveying mechanism, a printing workbench, a squeegee mechanism, an off-screen mechanism, a stencil clamping mechanism, and an automatic cleaning mechanism.

 

(1) Conveyor track device.

 

The transmission mechanism adopts a motor drive, belt pulley transmission and a transmission method that can automatically adjust the width of the guide rail, as shown in the figure below. In the actual operation, the substrate is first sent from the incoming track to the printing worktable to print the solder paste, and after the printing is finished, it is sent out from the outgoing track. In order to control the sequence of substrate transfer, waiting sensors are installed on the incoming and outgoing tracks. When the substrate passes through the track, it will block or reflect the incident light beam of the photoelectric sensor, realize the detection of the presence of the substrate, and control the positioning of the substrate on the track through the cylinder-type substrate stopper.

 

(2) Clever net plate clamping structure.

 

As shown in the figure below, the highly adaptable screen frame clamping system can realize the printing of various sizes of screen frames, and can quickly change models during the production process.

 

(3) Accurate platform calibration.

 

The X-Y-θ drive mechanism is used to realize the movement in three directions of X-Y-θ to perform platform positioning. The drive mechanism of X-Y-θ is servo motor, used to ensure the accuracy of calibration

 

(4) Scraper head and drive mechanism.

 

The printing head is driven by a suspended self-adjusting stepping motor, which is designed according to the different pressures required by the front and rear scrapers and the stability of the lifting, and the clamping design is designed to prevent the leakage of solder paste and the requirements of the blade to have a certain degree of flexibility. The pressure of the squeegee, the lifting speed, the printing speed, and the printing range can all be set by the software. At the same time, a variety of demolding methods are provided to adapt to PCB boards with different tinning requirements, and a good printing control platform is provided. Figure 1 below shows the print head, and Figure 2 below shows the floating adaptive squeegee.

 

2) Optical vision system

 

The precise optical vision system is shown in the figure below. The system uses ring lights/coaxial lights and other light sources, a unique light path system and omni-directional light source compensation to realize PCB/stencil precise mark point recognition. Both the X and Y directions of the CCD use high-precision servo motors, combined with a high-precision ball screw, to jointly ensure the accuracy of the image obtained, thereby ensuring the accuracy of the machine.

 

3) PCB positioning system

 

The stable and reliable PCB positioning system is shown in the figure below, including the round belt PCB transmission guide, the original PCB board pressing and elastic side pressing device, and the bottom vacuum nozzle and magnetic thimble ensure that the PCB board is stable and reliable after positioning. The board surface is flat, which completely solves the problem that the automatic printing machine is difficult to locate the double-sided board, the deformed board and the thin board below 0.6mm and other special-shaped boards.

 

4) Electrical separation control system

 

The electrical separation control structure that is easy to maintain is shown in the figure below. The printing press needs not only electric drive, but also pneumatic drive. When the printing press is working, various electrical appliances and motors need to be driven by a power supply voltage; while the squeegee drive, substrate clamping and screen clamping, and the lifting and lowering of the pallet on the worktable need to be driven by air pressure. By using an electrically separated control structure, the electrical control part, air control part, industrial control part and conversion card of the printing press are concentrated on the front and rear doors. This structure not only saves space, but also is simple, safe and convenient to maintain.

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5) Screen cleaning system

 

The automatic and effective screen cleaning system has three cleaning methods: dry, wet, and vacuum. The structure shown in Figure 2-10 has an automatic and effective screen cleaning function, and the computer can be used to set the cleaning cycle, time, speed and other parameters to remove the residual solder paste in the mesh to ensure the printing quality.

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6) Computer control system

 

The computer control interface is shown in the figure below. The operator uses it to understand the status of the printing press, perform file management and all machine operations. The mouse is used to move the cursor on the screen, and each button (SELECT, NEXT, EXIT) on the mouse is used to set parameters and operate the printing press.

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Second, the preparation before the operation of the solder paste printing machine

 

Make preparations before solder paste printing, craftsmen and technicians should do the following:

①Familiar with the process requirements of the product;

②According to the product process documents, receive the PCB that has passed the inspection;

③Choose the correct solder paste material, printing machine, screen and squeegee, etc.;

④ Determine the reliable process, such as good positioning, cleaning and wiping, etc.;

⑤ Make visual images of mark points, etc.

 

And while understanding the product structure process characteristics in depth, the best design plan should be optimized and selected. For different products, set the corresponding printing parameters in the printing program, such as working temperature, squeegee pressure and speed, automatic cleaning cycle of the screen, etc. At the same time, it is necessary to formulate strict process management and process procedures to ensure solder paste printing Finished smoothly and well.

 

1. Process analysis and plan formulation

 

1) Develop process regulations for solder paste printing

 

In order to achieve a good printing effect, the operator should fully understand the technological characteristics of the product structure according to the inspection technical requirements of the electronic products to be processed and the finished product, and formulate the best plan to ensure the smooth completion of the printing work. The semi-automatic production line of the semi-automatic printing machine can be used for the products to be processed in small batches and multiple varieties. When the solder paste printing of a fully automatic, large-scale production line may cause a bottleneck that affects production efficiency, a semi-automatic printing machine can be arranged next to the fully automatic production line to improve production efficiency. The principle, printing process, and operation principle of the semi-automatic printing machine and the automatic printing machine are basically the same. The difference is that the PCB clamping process of the semi-automatic printing machine is manually placed.

 

Make the preparations before operation, refer to the following aspects to formulate the solder paste printing process regulations.

 

(1) Follow the principle of solder paste selection, use and store solder paste correctly. In the production process, 100% inspection of the solder paste printing quality is required, that is, to check whether the solder paste pattern is complete, whether the thickness is uniform, whether there is solder paste sharpening, etc.

 

(2) Set printing programming parameters, according to process requirements, set squeegee pressure, squeegee speed, squeegee angle, off-screen speed, screen cleaning mode and cleaning cycle, as well as screen and PCB mark viewing angle image production.

 

(3) The adjustment of the printing condition setting, confirm the printing effect through trial printing, and check and correct the relevant parameters to achieve the best effect.

 

2) Formulate the solder paste printing process management process

 

The success of solder paste printing depends on three key elements in the printing process: solder paste rolling, filling, and demolding. The printing effect of PCB directly affects the quality of the product. In actual production, about 60% of the PCBs that cannot pass the inspection and need to be reworked are caused by poor solder paste printing. Therefore, an accurate grasp of solder paste, squeegee, and stencil is the key to ensuring stable printing quality.

 

 

According to the process requirements and the types of solder paste and printing machines, various operating instructions are formulated. Such as solder paste storage and use operation instructions, XX printing machine operation instructions, solder paste automatic printing process operation instructions, inspection operation instructions, etc. Strictly standardize the operation process to ensure the quality of solder paste printing.

 

2. Process preparation for solder paste printing

 

1) Process flow The process flow of solder paste printing is shown in the figure below.

 

2) Preparation before operation

 

(1) Equipment status inspection. Before printing, all switches of the equipment must be in the off state; turn on the power of the air compressor, discharge accumulated water before starting up, open all air valves, and confirm that the air pressure of each part meets the requirements of the printing machine; check whether the air filter has accumulated water, If there is any, it will be discharged; install the wiping paper, check the amount of alcohol in the stencil cleaner container, and add it when it is less than 1/3 of the total capacity; check whether the mechanical parts of the machine are normal and whether it affects the normality of the machine The running device is placed in the machine, if any problems are found, they should be dealt with in time until the problem is solved.

 

(2) Check the screen and scraper. The stencil should be intact, and the slippage should be complete and not clogged; the scraper blade and the stencil should be clean, and prepare brushes, cleaning agents, rags, etc. for cleaning.

 

(3) Prepare solder paste. Select the solder paste according to the regulations of the product process documents, check whether the solder paste or red (yellow) glue is ready to be warmed up, and the container lid can be opened only after the temperature is completely warmed up. Before printing, use a stainless steel stirring rod to fully stir the solder paste in one direction before use.

 

3) Boot operation

 

(1) Make sure that there are no foreign objects inside the machine before it can be turned on.

 

(2) Open the air valve of the air supply pipe to ensure that the air pressure meets the requirements of the printing press.

 

(3) Turn on the power switch of the printing press.

 

(4) Press the green "MACHINE ON" switch on the control panel on the front of the machine, and the "MACHINEON" light on the control panel will light up. The machine starts to initialize and enters the main interface.

 

4) Install the screen and scraper

 

(1) Install the screen first, and then install the scraper.

 

(2) When installing the stencil, insert the stencil into the stencil rail, push it in place and clamp it tightly.

 

(3) When installing the scraper, distinguish the front and rear scrapers. Install the rear scraper first, and then install the front scraper, as shown in the figure below.

 

5) Principle of process operation

 

The PCB is sent to the solder paste printing machine along the conveyor belt, and the machine automatically finds and locates the main side of the PCB. The Z-shaped frame moves up to the position of the vacuum plate, adds vacuum, and then firmly fixes the PCB in a specific position. After the vision axis (lens) slowly moves to the first target mark (reference point) of the PCB, the machine can move the screen to align it with the PCB, and the machine can move the screen in the X and Y axis directions and on the θ axis Direction rotation. Once the stencil is aligned with the PCB, the Z-frame will move upward, driving the PCB to contact the bottom of the stencil.

 

When moved in place, the scraper will push the tin paste to roll on the screen and print it on the PCB's PAD position through the holes in the screen. The tin paste fills the holes in the screen and piles up on the PCB, as shown in the following figure. After the printing is completed, the Z-shaped frame moves downward to drive the PCB and the screen to separate (ie demoulding). At this time, the machine will perform 2D and 3D automatic optical inspection (checking the solder paste coverage) at an interval of 21 points per second (minimum 0.0012 inches). After the PCB passes the regular inspection, the machine will send the PCB to the next process. Then the printer will ask the next PCB to be printed to go through the same process, but use the second squeegee to print in the opposite direction. After the PCB is sent out, the machine will automatically clean the screen to ensure the printing quality.

 

When the squeegee moves forward at a certain speed and angle, it will generate a certain pressure on the solder paste, thereby injecting the solder paste into the mesh (that is, the opening of the screen). The shear force is generated at the board junction, and the solder paste is smoothly injected into the mesh. The extent to which the solder paste fills the opening of the screen depends on the amount of solder paste; the completeness of demolding depends on the amount of solder paste leakage and the integrity of the solder paste pattern; the completeness of demolding also determines the success of solder paste printing . Since solder paste printing is a key process to ensure the quality of SMT assembly, the quality of solder paste printing must be strictly controlled.

 

Third, the spot inspection and maintenance before the operation of the solder paste printing machine

 

1. Checking the electricity and gas system

 

The inspection work items of the electric and gas system are confirmed by the operator, and the method is shown in the table below.

 

2. Checking the transmission system

 

The check operation items and confirmation methods of the conveyor system are shown in the table below.

 

3. Checking the printing workbench

 

The check work items and confirmation methods of the print table are shown in the table below.

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4. Checking the visual system

 

The inspection work items and confirmation methods of the vision system are shown in the table below.

 

5. Scraper check operation

 

The inspection items and confirmation methods of the squeegee are shown in the table below.

 

6. Checking the cleaning system

 

The check operation items and confirmation methods of the cleaning system are shown in the table below.

 

7. Checking the safety protection system

 

The check operation items and confirmation methods of the safety protection system are shown in the table below.