The SMT process is the core of the electronic assembly process, and the quality of the weld affects the overall quality of the product. For the manufacturing industry, high quality welding quality is the basis of the product, the capital and leverage of the product. The following are the main factors affecting the quality of welding.
Since the material is an important part of SMT, the quality and performance of the material directly affect the quality of reflow soldering, so the following points should be noted:
a. The packaging method of the components should meet the requirements for automatic installation of the machine.
b. The appearance of the components shall comply with the requirements for automatic installation and shall comply with the requirements of standardization and dimensional accuracy.
c. The welding quality of the components and the PCB accessories shall comply with the reflow requirements. The slowest ends and pads of the solder and assembly should be performed prior to contamination oxidation. If wet soldering, soldering, soldering beads and holes occur during reflow soldering. In particular, humidity control sensors and printed circuit boards should be vacuum packed and placed in a storage compartment after use. If necessary, BAK next time.
2. The manufacturability design of the PCB pad:
PCB design quality is an important indicator of the level of surface mount technology and one of the first conditions for surface mount quality. According to HP statistics, 80% of manufacturing defects are directly related to design. For example, the substrate is 40% to 60% (related to the characteristics of the solder paste). When in use, according to the “first in, first out” principle, make a record and ensure that the time to return to the first temperature is greater than 4 hours. Stirring is required before use to give it excellent printing and demolding capabilities.
3. Mold Design:
The main function of the template is to apply the solder paste precisely to the PCB pads. The template is indispensable in the printing process, and its quality directly affects the quality of solder paste printing. Currently, there are three main production methods: chemical etching, laser cutting and electroforming. The main control points for stencil design are as follows:
a. The thickness of the steel plate. To ensure solder paste printing and soldering quality, the surface of the stencil must be smooth and uniform, and the thickness of the steel plate should be determined according to the minimum spacing between the pins on the PCB.
b. Opening design. The opening should be trapezoidal, smooth and burr-free.
4. Solder beads, Mark points, printing direction:
a. Solder bead treatment. For components of 0603 or higher, the template opening should be treated in order to effectively prevent the occurrence of solder beads after reflow soldering. For devices with too large pads, template splitting is recommended to prevent tin.
b. Requirements for making the “MARK” point in the PCB. There should be at least 3 “MARK” points on the B surface of the template. The position of the “MARK” point on the stencil and printed circuit board should be the same. A pair of “MARK” points with the farthest diagonal distance are needed to improve printing accuracy. It is the front and rear half, with clear graphics.
c. Printing direction. The printing direction is also a key control point. When determining the printing direction, care should be taken to avoid the proximity of the spacers too close to the traces, otherwise excessive tinning may result when connected.
5. Printing parameters:
Printing parameters mainly include blade speed, blade pressure, template peeling speed, cleaning method and frequency.
There is a clear limiting relationship between the blade and the template angle and solder paste viscosity. Therefore, only the proper control of these parameters can guarantee the printing quality of the solder paste. In general, the scraper speed is slow, and better print quality can be obtained, but the shape of the solder paste scraper may be blurred, the speed is too slow, and the production efficiency is affected.
If the blade is too fast, the solder paste may not have enough time to fill the hole, resulting in insufficient solder paste. Excessive blade pressure can cause the solder paste in the hole to be pulled out, resulting in less tin, accelerating the wear of the stencil and the squeegee, and too low pressure can result in incomplete solder paste printing. Therefore, when the solder paste can maintain normal rolling, the speed is increased as much as possible, and the blade pressure is adjusted to obtain good print quality. Too fast film removal speeds can result in printed solder paste tip or molding defects, which can affect production efficiency. If the cleaning mode and frequency of the template are not set correctly, the cleaning of the template will not be clean.
6. Equipment accuracy:
The printing accuracy and repeatability of the printing press also affect the stability of solder paste printing when printing high density and narrow pitch products.
7. Component installation
The three factors that ensure the quality of the installation are the right choice, the exact placement and the correct installation pressure for the components. Proper selection of components means that the attached components are consistent with the BOM. Accurate placement means that the mounting coordinates must be correct. At the same time, the accuracy of the mounting equipment should ensure stability so that the material can be accurately pasted into the specified pads. At the same time, attention should be paid to the mounting angle to ensure the correct orientation of the polar device. A suitable mounting pressure is the thickness of the component in the solder paste after installation, which should not be too small or too large.
8. Reflow soldering:
Properly setting the reflow profile is a guarantee of weld quality. A good reflow profile requires good soldering of the various bonding components on the soldered PCB. Solder joints must not only have good appearance quality, but also have good intrinsic quality. If the slope of the temperature rise is too fast, it will cause the components and the PCB to heat up too quickly, which can easily damage the components and cause deformation of the PCB. On the other hand, the solder in the solder paste evaporates too quickly and tends to splash metal parts, resulting in solder balls. The peak temperature is generally set to be higher than the melting point solder paste by about 30 to 40 degrees. If the temperature is too high, the reflow time is too long, which may cause the thermal element to plastically damage the body, resulting in insufficient solder. The solder paste does not form a reliable solder joint. In order to improve the quality of the weld and to avoid oxidation of the component, it is possible to conditionally use a nitrogen reflux. The reflow profile setting is usually based on the following aspects:
a. Set according to the recommended temperature profile of the solder paste used. The composition of the solder paste determines the activation temperature and melting point.
b. Depending on the thermal performance parameters of the thermal and valuable components, the maximum soldering temperature of the particular component should also be considered.
c. According to PCB sheet, size, thickness and weight.
d. Depending on the structure of the reflow oven and the length of the temperature zone, different reflow ovens should be set to different temperature profiles.
The above are important factors affecting the quality of SMT projects. To make high quality and high quality products, please refer to the analysis of the above factors, thank you.