SMT pcb assembly factors
2023-06-02
SMT process is the core of electronic assembly process, weld quality affects the overall quality of the product. For the manufacturing industry,
high quality welding quality is the foundation of the product, is the capital and leverage of the product. The following are the main factors that
affect the welding quality.
1. Material:
As materials are an important part of SMT, the quality and performance of materials directly affect the quality of reflow soldering, so the following
As materials are an important part of SMT, the quality and performance of materials directly affect the quality of reflow soldering, so the following
points should be paid attention to:
a. The packaging of components shall meet the requirements of automatic installation of the machine.
b. The appearance of the parts shall meet the requirements of automatic installation and shall meet the requirements of standardization and
dimensional accuracy.
c. The soldering quality of components and PCB accessories shall meet the requirements of reflow soldering. The slowest end of solder and
assembly and pad should be carried out before fouling oxidation. If wet weld, weld, bead and hole occur during reflow welding. In particular,
humidity control sensors and printed circuit boards should be vacuum-packed and placed in a storage room after use. BAK next time if necessary.
2. Manufacturability design of PCB pads:
PCB design quality is an important indicator of surface mount technology level, and also one of the primary conditions for surface mount quality.
PCB design quality is an important indicator of surface mount technology level, and also one of the primary conditions for surface mount quality.
According to HP, 80% of manufacturing defects are directly related to design. For example, the substrate is 40%~60% (related to the characteristics
of solder paste). When using, according to the principle of "first in, first out", make a good record to ensure that the return temperature time is
greater than 4 hours. It needs to be stirred before use, so that it has excellent printing and demoulding ability.
3. Mold design:
The main function of the template is to accurately apply the solder paste to the PCB pad. The template is indispensable in the printing process,
The main function of the template is to accurately apply the solder paste to the PCB pad. 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 of template design are as follows:
a. Thickness of steel plate. To ensure the quality of solder paste printing and welding, the surface of the steel mesh must be smooth and uniform,
and the thickness of the steel plate should be determined according to the minimum spacing of the pins on the PCB.
b. Open design. The opening should be trapezoidal, smooth and burr free.
4. Welding bead, marking point and printing direction:
a. Welding bead processing. For COMPONENTS 0603 and ABOVE, template OPENINGS should BE treated to effectively prevent bead formation after
a. Welding bead processing. For COMPONENTS 0603 and ABOVE, template OPENINGS should BE treated to effectively prevent bead formation after
reflow soldering. For large pad devices, it is recommended to split the template to prevent tin.
b. Requirements for making "MARK" points on PCB. There should be at least 3 "MARK" points on the B side of the template. The position of the "MARK"
point on the steel mesh and the printed circuit board shall be the same. A pair of "MARK" points with the furthest diagonal distance are needed to
improve the printing accuracy. It is the first half and the second half, with clear graphics.
c. Printing direction. Printing direction is also a critical control point. When determining the printing direction, care should be taken to avoid the gasket
being too close to the line, otherwise it may lead to excessive tin plating when connecting.
5. Printing parameters:
Printing parameters mainly include scraper speed, scraper pressure, template stripping speed, cleaning method and frequency.
There is an obvious limiting relationship between blade and template Angle and paste viscosity. Therefore, only proper control of these parameters can
guarantee the printing quality of solder paste. Generally speaking, the scraper speed is slow, and better printing quality can be obtained, but the shape
of the solder paste scraper may be blurred, and the speed is too slow, affecting the production efficiency.
If the blade is too fast, the solder paste may not have enough time to fill the hole, resulting in insufficient solder paste. If the scraper pressure is too
high, the solder paste in the hole will be pulled out, resulting in less tin and accelerating the wear of the steel mesh and the scraper. If the pressure is too
low, the solder paste printing will be incomplete. Therefore, under the condition that the solder paste can maintain normal rolling, increase the speed as
much as possible and adjust the scraper pressure to obtain good printing quality. Too fast film removal can lead to defects in the printed solder paste tip
or molding, thus affecting production efficiency. If the cleaning method 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, narrow
The printing accuracy and repeatability of the printing press also affect the stability of solder paste printing when printing high density, narrow
spacing products.
7. Component installation
The three factors that ensure installation quality are correct selection of components, accurate placement, and correct installation pressure. The correct
The three factors that ensure installation quality are correct selection of components, accurate placement, and correct installation pressure. The correct
selection of components means that the add-on is consistent with the BOM. Accurate placement means that the installation coordinates must be correct.
At the same time, the accuracy of the mounting equipment should ensure the stability, so that the material can be accurately pasted to the specified pad.
At the same time, attention should be paid to the installation Angle to ensure the correct orientation of the polar equipment. A suitable mounting pressure
is the thickness of the element in the paste after mounting, which should not be too small or too large.
8. Reflow welding:
Correct setting of reflow curve is the guarantee of welding quality. A good reflux curve requires good soldering of all bonding elements on the soldered
Correct setting of reflow curve is the guarantee of welding quality. A good reflux curve requires good soldering of all bonding elements on the soldered
PCB. Solder joints should not only have good appearance quality, but also have good internal quality. If the temperature rise slope is too fast, the components
and PCB will heat up too fast, which is easy to damage the components and cause PCB deformation. On the other hand, the solder in the solder paste evaporates
too quickly, and it is easy to splash out metal parts and produce welding balls. The peak temperature is generally set to be about 30 ~ 40 degrees higher than
the melting point of solder paste. If the temperature is too high and the reflux time is too long, it may lead to plastic damage to the body of the thermal sensing
element, resulting in insufficient solder. Solder paste does not form reliable solder joints. In order to improve welding quality and avoid component oxidation,
nitrogen reflux can be conditionally used. Reflux curve setting is usually based on the following aspects:
a. Set according to the recommended temperature curve of the solder paste used. The composition of the solder paste determines the activation temperature
and melting point.
b. According to the thermal performance parameters of thermal elements and valuable elements, the maximum welding temperature of specific elements
shall also be considered.
c. According to PCB board, size, thickness and weight.
d. According to the structure of the reflow furnace and the length of the temperature zone, different reflow furnace should set different temperature curves.