SMT process is the core of the electronic assembly process, and the welding quality affects the overall quality of the products.
PCB design quality is very imortant to measure the level of surface mount , and is one of the first conditions of SMT quality. According to statistics from HP, 80% of manufacturing defects are directly related to design. When using, according to the principle of "first in first out", making record and ensuring the time of back to the first temperature more than 4 hours. And it need to be fully stir before using which can make it have the excellent abilities of printing and demoulding.
As one of the important elements of SMT, the quality of materials affect the quality of backflow welding directly, and the following factors should be important:
A. The footprint of components should meet the requirements of automatic mounting and should be standardized, having good dimensional accuracy
B. The packaging way of component should be fit for the automatic mounting of the SMT machine.
C. The solderable side of component and PCB pad shall meet the requirements of reflow welding, and the solderable side of component and pad shall be not with pollution or oxidation. If the solder ends is wetting, false soldering, soldering beads and holes would occur during reflow soldering. Especially for the control of humidity sensor and PCB, vacuum packing should be done in time after use and put into dry cabinet for storage.
Printing parameters mainly include blade speed, blade pressure, stripping cleaning mode and so on. There is a definite restrictive relationship between the blade and the angle of the stencil and the viscosity of solder paste. Only proper control of these parameters can guarantee the printing quality of solder paste. If the scraper is slow that can do better printing quality, but may cause the shape of the scraper of the solder paste to be blurred. If the speed is too slow, it should affect the production efficiency. If the scraper is too fast, the solder paste may not have enough time to fill the hole, resulting in insufficient solder paste. When the solder paste is able to keep rolling normally, increase the speed as much as possible, and adjust the blade pressure to achieve good printing quality. If the cleaning mode and frequency of the stencil are not properly set, the cleaning of the stencil will not be clean.
Different hardness of the material and shape of the scraper has a certain impact on the printing quality, generally use nickel plated steel scraper, and 60-degree scraper is more common.
The main function of the stencil is to put the solder paste on the PCB pad accurately. Stencil is necessary in printing process, and the quality affects the quality of solder paste printing directly . At present, there are mainly three production methods: chemical corrosion, laser cutting and electroforming.
The pasted components should be consistent with the BOM, and the pick and place file should be accurate. The precision of the mounting equipment should ensure the stability so that the materials can be accurately pasted to be the specified welding pad. What’s more, the workers should be pay attention to the mounting angle to ensure the correct direction of the polar device. Suitable mounting pressure refers to the thickness of the components into the solder paste after mounting. The pasting pressure can be controlled by setting the PCB thickness, component package thickness, SMB nozzle pressure and adjusting the z-axis height of pasting tip in the production process.
The printing accuracy and repeat printing accuracy of the printing press will also affect the stability of solder paste printing when printing high-density and narrow spacing products.
The temperature curve of reflow welding is the guarantee of welding quality. A good reflow curve could make good soldering for all kinds of pasted components on the PCB board to be soldered. If the slope of temperature rise is too fast, it will cause the components and PCB to be heated too fast which can easily damage the components and cause PCB deformation. What’s more, the flux in the solder paste evaporates too quickly and tends to spatter out the metal components, resulting in solder balls. Peak temperature generally set higher than the melting point solder paste is about 30 to 40 degree. If the temperature is too high and back to the flow of time is too long could cause thermal sensitive element plastic damage of ontology, and leads to insufficient solder paste melting cannot form a reliable welding spot. In order to enhance welding quality and avoid component oxidation problems, nitrogen reflux can be used conditionally. The following aspects should be considered when we set the backflow curve:
a) 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 structure of reflux furnace and the length of temperature zone, different reflux furnace should set different temperature curves.
c) According to the thermal performance parameters of the thermal sensitive elements and valuable elements, we should also conseder the maximum welding temperature of the special elements.