Microelectronic solder paste printing quality control

[Abstract] Solder paste printing is a key process in current electronic manufacturing process. The quality of solder paste printing directly affects the performance of the product. This article from the leakage template, printed circuit board, solder paste, printing presses to proceed with the four aspects, to explore the quality control of solder paste printing.
Keywords: Solder paste leaky template printed circuit board printing machine I. INTRODUCTION With the rapid development of the electronics manufacturing industry, the current electronic assembly process is surface mounting technology SMT (Surface Mount Technology), and a new printing process emerges. - Solder paste printing technology. Although its printing method is a kind of screen printing - the stencil printing, but the printing medium is not ink, but the electronic functional material - solder paste. The purpose of solder paste printing is not to obtain a pleasing tone and level, but to accurately dispense the solder paste to obtain the best solder paste deposition thickness, which will lay the foundation for the next step of component hot melt welding. Below, first briefly introduce the relevant knowledge of hot-melt welding, and then explore the quality control of solder paste printing.
Second, hot melt welding In surface mounting technology, hot melt welding is the surface mounting components and printed circuit board fixed together. The first procedure is to apply the solder paste and adhere the component, that is, to print the solder paste on the component placement point of the printed circuit board. Due to the sticky effect of the solder paste, the component is temporarily fixed. The second process is to perform hot melt welding (abbreviated in Figure 1), usually in the gas phase or in the infrared furnace. The solder paste is liquefied after heating to the melting point, and is spread under the action of gravity and surface tension. After cooling, the components are Printed circuit boards are connected together. The third step is to remove the flux and clean it.
Third, the quality control of solder paste printing Many factors affect the quality of the solder paste printing process, including the leakage of the template, printed circuit board, solder paste, printing press is composed of four basic contents of solder paste printing, these four aspects of mutual contact, Interactions determine the quality of the entire printing process.
1. Optimized Leakage Template Leakage stencils are the basic tool for solder paste printing and are used to limit the location of solder paste on printed circuit board surface pads. It is a key factor affecting the printing quality. It is important to design and make a good leaky template.
1) The material of the leaky template The influence of the material of the leaked template on the printing quality is mainly that the material is easily deformed when the blade applies pressure, so that the distribution of the solder paste is deviated. When making the leaky template, try to use a material with small deformation, usually made of stainless steel or brass plate.
2) The thickness of the drain plate The thickness of the drain plate determines the amount of solder paste deposited on the printed circuit board. Good solder paste printing must have a suitable amount of solder paste deposited. Excessive thickness of the drain plate can cause excessive deposition of the solder paste. The consequence of this is the short circuit and bridging between the fine pitch components (abbreviated in Figure 2); too thin a drain plate can result in insufficient solder paste deposition, resulting in soldering strength. Not enough, prone to failure (see Figure 3 omitted). Table 1 shows the values ​​of the drain template thickness for different lead spacings.
3) Opening size of the drain plate The opening size of the drain plate is one of the main factors that affect the quality of the solder print. The size of the opening of the drain template has a certain relationship with the pad size, and is usually slightly smaller than the pad size by 10% to 20%. If the opening is too large, short-circuiting and bridging between components can easily occur; if the opening is too small, welding strength tends to be insufficient. Table 2 shows the size of the die plate opening for different pad widths.
4) The shape of the opening of the drain template and the shape of the opening of the hole wall with a good finish and a smooth hole wall will make the solder paste print well. A good process (such as laser cutting) should be used as far as possible to make the leakage template, so that the opening shape is small and large. In this way, solder paste can easily escape from the drain plate during printing. In addition, the open hole wall should be as smooth as possible to reduce the adhesion between the solder paste and the hole wall, making it easy to escape from the drain plate.
2. Printed circuit board In the solder paste printing process, the printed circuit board is the substrate, which also has a great influence on the printing quality.
1) Flatness of Printed Circuit Board Warpage of the printed circuit board will degrade the printing quality, resulting in less than 0.3% deflection of the printed circuit board. When an uneven situation occurs, it can be corrected with a support needle or a vacuum suction cup.
2) Pad finish The pad surface should be clean and smooth, so that the solder paste and pad adhesion ...
3. Solder Paste 1) Paste Viscosity No matter the viscosity of the solder paste is too large, the solder paste will have poor fluidity and can easily stick to the wall of the drain plate or the scraper; if the viscosity of the solder paste is too small, it is difficult to control the solder paste. Deposited shape, the solder paste easily collapsed, resulting in bridging. The viscosity of general solder paste is controlled at 800~1100Pa·S (measured by Brookfield viscometer).
2) Size and Uniformity of Solder Paste Printed solder paste generally requires moderate and uniform particle size. If the particles are too large or irregular in shape, it is difficult to allow the solder paste to pass through small openings, resulting in insufficient deposition amount. Some particles will also produce solder balls. If the solder paste particles are too small, the solder paste will easily collapse. Or bridging. It is generally required that the smallest drain template opening can simultaneously pass through 4 solder paste particles.
4. Solder Paste Printer 1) Accurate Positioning Capability The typical edge of the die plate opening is only 0.03 to 0.05 mm from the edge of the pad. Therefore, the printer must be capable of repeatable and accurate positioning. Otherwise, the solder paste will be deposited outside the edge of the pad and bridging can easily occur.
2) Printing pressure and speed Printing pressure is an important factor. If the printing pressure is too low, the solder paste may not be effectively deposited on the pad through the opening of the drain plate; if the printing pressure is too high, the blade will be deformed, the solder paste in the opening of the drain plate will be scraped off, resulting in concave deposition. If it is serious, it will damage the leaked template. A suitable printing pressure is 0.2-0.4 N/mm.
Printing speed is also very important. If the speed is too fast, the heat generated by the friction will change the viscosity of the solder paste, which will make the printing effect worse. If the speed is too slow, the solder paste will flow poorly, which will result in irregular deposit shape. General printing speed takes 12.7 ~ 50mm/s, the smaller the distance, the lower the speed.
3) Scraper blades are mostly made of metal or polyester materials, each suitable for different occasions. The metal knife is hard and the printing effect is generally good. The polyester knife is soft and easy to be deformed. It is suitable for the case where the opening of the die plate is small. The operating angle of the doctor blade is generally 60° to 65°. If it is too large, the solder paste cannot be effectively pushed. If it is too small, sufficient force cannot be applied to form a prescribed amount of deposited solder paste. In addition, the bottom edge of the scraper should be kept horizontal, so that the pressure everywhere is consistent.
IV. Summary Solder paste printing is a highly technical process. In addition to some of the above factors, there are many other factors such as environmental temperature and humidity, solder paste storage, wear of the drain plate, and other factors that affect the quality of solder paste printing. In actual production, we must be good at observing and conscientiously analyzing, and put forward solutions in light of actual conditions to obtain high-quality products.

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