Surface Mount Technology (SMT) mainly includes: solder paste printing, precise placement, reflow soldering.
Among them, the quality of solder paste printing has a great impact on the quality of surface mount products. According to industry evaluation and analysis, about 60% of reworked circuit boards are caused by poor solder paste printing.
In solder paste printing, there are three important parts: solder paste, stencil and printing equipment. If selected correctly, good printing results can be obtained. The brief discussion is as follows:
1. Solder paste Solder paste is a kind of slurry made by mixing solder powder and paste flux (rosin, thinner, stabilizer, etc.) with soldering function. The solder paste is viscous. During printing, it is subjected to the thrust of the scraper, and its viscosity decreases. When it reaches the opening hole of the stencil, the viscosity reaches the lowest level, so it can smoothly pass through the template hole and settle on the PCB pad. With the stop of the external force, the viscosity of the solder paste quickly rises again, so that there will be no slump and overflow of the printing molding, and a good printing effect will be obtained.
Viscosity is an important characteristic of solder paste. Dynamically, in the printing stroke, the lower the viscosity, the better the fluidity, and it is easy to flow into the stencil hole; statically, after printing, the solder paste stays in the stencil hole, and its viscosity is high, it maintains its filled shape. , without collapsing down. Factors Affecting Solder Paste Viscosity:
1. The effect of solder paste alloy powder content on viscosity – the increase of alloy powder in solder paste causes an increase in viscosity;
2.2. The influence of the particle size of solder paste alloy powder on the viscosity – the viscosity will decrease when the particle size increases; Ø Fine particle solder paste has better printability, especially for high-density, narrow-pitch products, due to the small opening size of the stencil , must take small particle alloy powder, otherwise it will affect the printing and demoulding. Ø Advantages of small particle alloy powder: good printability and high definition of printed graphics. Ø Disadvantages of small particle alloy powder: easy to collapse, large surface area easy to be oxidized. The relationship between the lead pitch and solder particles of SMT components Lead pitch (mm) 0.8 or more 0.65 0.5 0.4 Particle diameter (um) 75 or less 60 or less 50 or less 40 or less
3.3. The influence of temperature on the viscosity of solder paste: the temperature increases, the viscosity decreases, and the best ambient temperature for printing is 23±3℃;
4.4. The effect of shear rate on the viscosity of solder paste: the viscosity decreases with the increase of shear rate. Validity period and storage and use environment of solder paste: Ø Generally, solder paste can be stored for 6 months under the condition of 0-10℃ without opening the lid, and it should be used up as soon as possible after opening; Ø The use environment of solder paste is: requirements The temperature of the SMT room is 20-26℃ and the humidity is 40-60%;
Ø If the solder paste is not opened, the storage time under ambient temperature and humidity conditions is ≤ 48 hours;
Ø After the lid is opened, the placement time of the solder paste under ambient temperature and humidity conditions is ≤18 hours;
Ø Use time on steel mesh ≤ 12 hours;
Ø After printing, the solder paste stays on the line for ≤2 hours;
Ø The time from opening the can to before reflow soldering is less than or equal to 18 hours.
Defects caused by solder paste: 1. No dip soldering Ø Flux activity is not good; Ø Metal particles are oxidized very much; 2. No rolling during printing Ø Inappropriate flow, such as: viscosity, thixotropy index is not suitable; 3. Bridging Ø Solder paste collapsed; 4. Insufficient solder Ø Due to large alloy powder particles, incorrect shape or unprintability, solder paste blocked stencil holes; 5. Solder balls Ø Solder paste collapsed; Ø In Solvent splashing during reflow; Ø Oxidation of metal particles.
Second, the stencil template The main function of the stencil is to accurately apply the solder paste to the pads that need to be coated with solder paste on the PCB. In the solder paste printing process, the processing quality of the stencil stencil directly affects the printing quality of the solder paste. The thickness of the stencil and the size of the opening determine the printing volume of the solder paste, and the stainless steel laser stencil needs to be processed through outsourcing. Therefore, parameters such as template thickness and design opening size must be confirmed before outsourcing processing to ensure the printing quality of solder paste.
Usually, there are components with a spacing of more than 1.27mm and narrow spacing components on a PCB. Components with a spacing of more than 1.27mm need a stainless steel plate of 0.2mm thickness, and components with a narrow spacing need a stainless steel plate of 0.15-0.10mm thick. At the same time, the thickness of the stainless steel plate can be determined according to the situation of most components on the PCB, and then the amount of solder paste leakage can be adjusted by expanding or reducing the size of the pad openings of individual components.
If the components on the same PCB require a large disparity in the amount of solder paste, the template at the narrow-pitch components can be partially thinned, but the processing cost of the thinning process is higher. Therefore, a compromise method can be adopted, and the thickness of the stainless steel plate can be taken as an intermediate value. For example, some components on the same PCB require a thickness of 0.20mm, and other components require a thickness of 0.15-0.12mm. At this time, the thickness of the stainless steel plate can be selected as 0.18 mm. The opening size can be 1:1 for general components, and the opening area of large Chip components and PLCCs that require a large amount of solder paste should be enlarged by 10%. For devices such as QFPs with pin pitches of 0.5mm and 0.65mm, the opening area should be reduced by 10%.
Appropriate opening shape can improve the placement effect, for example: when the chip component size is smaller than 1005, 0603, due to the small distance between the two pads, the solder paste on the pads at both ends is easy to be placed at the bottom of the component during placement Sticking, bridging and beading on the bottom of the component are easy to occur after reflow soldering. Therefore, when processing the template, the inner side of the opening of a pair of rectangular pads (Figure 1) can be modified into a sharp corner or an arcuate shape (Figure 2, the shape of the chip component opening) to reduce the amount of solder paste at the bottom of the component, which can improve the placement process. The solder paste on the bottom of the component sticks.
3. Printing equipment The printing machine is the equipment that prints the solder paste on the PCB board, and it is the equipment that has the greatest impact on the process and quality. At present, printing machines are mainly divided into: manual printing machines, semi-automatic printing machines and fully automatic printing machines.
Ø Semi-automatic printing machine: simple operation, fast printing speed, simple structure, the disadvantages are: less controllable points of printing process parameters, low printing centering accuracy, poor solder paste release, generally suitable for components above 0603, pin spacing PCB printing process larger than 1.27mm.
Ø Fully automatic printing machine: high printing accuracy, good solder paste release effect, stable printing process, suitable for printing dense components and narrow spacing components; disadvantages are: high maintenance costs, high requirements for the knowledge level of operators