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The Main Factors Affect the Quality of SMT Soldering and Its Solutions

by: Oct 10,2018 131 Views 0 Comments Posted in PCB Assembly

Reflow Welding 3. Scraper Stencil Solder paste SMT

Abstract:

SMT process as the core of the electronic assembly process, the welding quality affects the overall quality of the products. For SMT industry, high quality welding quality is the basis of products, and the capital and leverage for products to compete with others. The following are some main factors affecting SMT welding quality.

Materials:

As one of the important components of SMT, the quality and performance of materials directly affect the quality of backflow welding, so that the following points need to be noted:

A.     The packaging method of components should meet the requirements of automatic mounting of the SMT machine.

B.     The appearance of components shall meet the requirements of automatic mounting and shall be standardized, having good dimensional accuracy.

C.     The coating quality of component solderable end and PCB pad shall meet the requirements of reflow welding, and the component sloderable end and pad shall be free of pollution or oxidation. If the solder ends as wetting, false soldering, soldering beads and holes may 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. If necessary, baking should be done next time.

Manufacturability design of PCB pad:

PCB design quality is an imortant mark to measure the level of surface mount r=technology, and is one of the first conditions of surface mount quality. According to statistics from HP, 80% of manufacturing defects are directly related to design. For example, substrate 40% to 60% (related to the characteristics of solder paste). When using, according to the principle of "first in first out", making record and ensuring the time of back to the first temperature greater than 4 hours. And it need to be fully stir before using which can make it have the excellent abilities of printing and demoulding.

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2.      Design of Stencil:

The main function of the stencil is to apply the solder paste accurately on the PCB pad. Stencil is indispensable in printing process, and its quality directly affects the quality of solder paste printing. At present, there are mainly three production methods: chemical corrosion, laser cutting and electroforming. The main control points of steel mesh design are as follows:

a)      The thickness of the sheet steel. To ensure solder paste printing and welding quality, the surface of the stencil must be smooth and uniform, and the thickness of the steel sheet shall be determined according to the minimum spacing between the pins on the PCB.

b)     Design of opening. The opening shall be a trapezoid cut face, smooth, without burrs.

The aspect ratio=the opening width/the thickness of stencil

The area ratio=base area of the opening/the area of the opening hole wall

As shown in the following figure:

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a)      Soldering tin bead treatment. For the components of 0603 or above, in order to effectively prevent the occurrence of soldering tin beads after backflow welding, the stencil opening shall be treated. For the device with too large solder pad, it is suggested to use stencil division to prevent the amount of tin.

b)     The requirements of making “MARK” points in PCB. At least 3 “MARK” points shall be made on the B surface of the stencil. The position of “MARK” points on the stencil and the printed board shall be the same. A pair of “MARK” points with the farthest diagonal distance is required to improve printing accuracy. It is made in front and back half and the figure is clear.

c)      Printing direction. Printing direction is also a key control point. When determining the printing direction, care should be taken to avoid the close spacing device being too close to the trace, otherwise, it may cause too much tin when connected.

3.      Scraper:

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. If there are through-hole components recommended to use a 45-degree scraper can increase the amount of tin pupil components.

4.      Printing parameters:

Printing parameters mainly include blade speed, blade pressure, stripping speed of stencil, cleaning mode and frequency, etc. There is a definite restrictive relationship between the blade and the angle of the stencil and the viscosity of solder paste. Therefore, only proper control of these parameters can guarantee the printing quality of solder paste. Generally speaking, the scraper is slow, which can get better printing quality, but may cause the shape of the scraper of the solder paste to be blurred, the speed is too slow and will 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. Too high blade pressure will cause the solder paste inside the hole to be pulled out, resulting in less tin, will speed up the stencil and scraper wear, too low pressure will lead to the solder paste printing incomplete. Therefore, 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. Too fast film removal speed may cause printing solder paste tip or forming defects, too slow speed will affect production efficiency. If the cleaning mode and frequency of the stencil are not properly set, the cleaning of the stencil will not be clean.

5.      Equipment Accuracy:

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.

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6.      PCB Support:

PCB support is an important debugging content of solder paste printing. If the PCB lacks effective support or is unreasonable in the printing process, the printing solder paste will be too thick or uneven. The PCB support should be flat and even, ensuring that the stencil can be attached to the PCB during printing. A thimble template or support vehicle can be made with a strip.

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Components Mounting:

The three factors to ensure the mounting quality are the correct selection, the accurate placement and the proper mounting pressure of components. The correct selection of components means that the pasted components are consistent with the BOM, and the accurate placement means that the mounting coordinate must be correct, and in the meanwhile, the precision of the mounting equipment should ensure the stability so that the materials can be accurately pasted to be the specified welding pad. At the same time, attention should be paid 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, which cannot be too small or too large. 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.

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Reflow Welding:

Proper setting of temperature curve of reflow welding is the guarantee of welding quality. A good reflow curve requires good soldering for all kinds of pasted components on the PCB board to be soldered. The welding spots should not only have good appearance quality, but also have good internal quality. If the slope of temperature rise is too fast, on one hand, it will cause the components and PCB to be heated too fast which can easily damage the components and cause PCB deformation. On the other hand, 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, such as the temperature is too high, back to the flow of time is too long can 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 backflow curve setting is generally 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 the thermal sensitive elements and valuable elements, the maximum welding temperature of the special elements should also be considered.

c)      According to the PCB sheet, size, thickness and weight.

d)     According to the structure of reflux furnace and the length of temperature zone, different reflux furnace should set different temperature curves.

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