How to obtain high-quality solder paste printing

2022-08-26
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How to obtain high-quality solder paste printing

1 introduction

with the rapid development of surface mount technology, in its production process, the influence and role of solder paste printing on the whole production process has been paid more and more attention by production technologists and process engineers. Solder paste printing technology is to use the prepared board, use certain methods to make the wire directly contact with the printing machine, and make the solder paste flow evenly on the board, The hole is injected by the mask pattern. When the wire pulls away the printed board, the solder paste falls off the hole and onto the corresponding pad pattern of the printed board in the shape of a mask pattern, thus completing the printing of the solder paste on the printed board, as shown in Figure 1. The equipment used to complete this printing process is the silk printing machine. Solder paste printing is one of the most critical processes in the SMT production process. The quality of printing will directly affect the quality and efficiency of SMD assembly. According to statistics, 60% - 70% of welding defects are caused by poor solder paste printing results. Therefore, to improve the quality of solder paste printing and minimize the printing defects as far as possible, to achieve high-quality repeated printing, the characteristics of solder paste, the production of board, and the setting of printing process parameters are very critical, Next, we will discuss these points one by one

2 characteristics and selection of solder paste

solder paste is much more complex than pure tin lead alloy. Its main components are: solder alloy particles, flux, rheology regulator/viscosity controller, solvent, etc. Different types of solder pastes have different compositions and different application ranges, so you should be extra careful when choosing solder pastes, and really grasp the relevant factors to ensure good quality. Generally, when choosing solder pastes, you should pay attention to the following factors:

2.1 good printing performance

the viscosity of solder paste is an important factor affecting printing performance. The viscosity is too large, the solder paste is not easy to pass through the opening of the board, the printed lines are incomplete, and the viscosity is too low, It is easy to flow and collapse, affecting the printing resolution and flatness of lines. The viscosity of commonly used solder paste is 0.5 × Between 1.2kpa · s, the optimum viscosity of solder paste is 0.8kpa · s when steel mold printing. The viscosity of solder paste can be measured with an accurate viscometer, but the following methods can be used in actual work: stir the solder paste with a scraper for about 30min, and then pick up a little solder paste with a scraper to let the solder paste fall naturally. If the solder paste slowly falls paragraph by paragraph, it indicates that the viscosity is moderate; If the essence of the solder paste does not slip, it means that the viscosity is too large. If the solder paste keeps sliding at a faster speed, it means that the solder paste is too thin and the viscosity is too small

the shape, diameter and uniformity of solder particles of solder paste also affect its printing performance. Generally, the diameter of solder particles is about 1/5 of the size of plate opening. For pads with a fine spacing of 0.5mm, the size of plate opening is 0.25mm, and the maximum diameter of solder particles does not exceed 0.05mm, otherwise it is easy to cause blockage during printing. The specific relationship between pin spacing and solder particles is shown in Figure 2. Generally, the solder paste with small particles will have better solder paste strip definition, but it is easy to produce edge collapse, and the degree and opportunity of oxidation are also high. Generally, the pin spacing is taken as one of the important selection factors, taking into account both performance and price

2.2 good adhesion

the adhesion of solder paste refers to the ability of solder paste to stick together, which mainly depends on the composition of the welding aid system in the solder paste and the proportion of other additives (such as adhesives, solvents, thixotropic agents, etc.). If the solder paste itself has a strong ability to stick together, it is conducive to the mold release of the solder paste, and can well fix the components on it, reduce the flying or falling pieces during the component mounting, and can withstand the vibration or turbulence during the mounting and transmission process

2.3 melting point of solder paste

solder pastes with different melting points are selected according to the process requirements and the temperature that components can withstand. The melting point of solder paste is determined by the alloy composition. For SMT production, 63Sn-37Pb or 62sn-36pb-2ag are generally selected, with melting points of 183 ℃ and 179 ℃ respectively. These types of solder pastes not only have low melting points, but also have high solder joint strength, which can better meet the welding requirements, Solder pastes with different melting points are often used in the production of double-sided paste printing plates. The melting point of the solder paste on the first side is required to be dozens of degrees higher than that on the second side, so as to prevent the components on the first side from falling off when welding the components on the second side

2.4 type of flux

the functions of flux in solder paste are: 1) remove the oxide layer of PCB pad; 2) Protect the surface of the pad from oxidation; 3) Reduce the surface tension of solder in welding and promote the flow and dispersion of solder. Since the solder powder will accelerate oxidation during reflow soldering, the flux must have enough activity to remove these oxides. Another consideration is whether the plate should be clear after welding. If it is non cleaning, it must choose non cleaning flux with no corrosion and low residue. The flux in the solder paste includes RSA (strong activation type), RA (activation type), RMA (weak activation type), R (non activation type). Generally, RMA type is more suitable

2.5 metal content of solder paste

the metal content of solder paste determines the thickness of solder after welding. With the increase of metal content, the thickness of solder also increases (see Table 1), but under a given viscosity, with the increase of metal content, the bridging tendency of solder also increases correspondingly

after reflow soldering, the device pins are required to be welded firmly, the welding volume is full and smooth, and there is a 1//3 height climb in the direction of the end height of the device (resistance capacitance device). It can be seen from the above table that with the reduction of metal content, the thickness of solder after reflow welding decreases. In order to meet the requirements of solder paste quantity of solder joints, solder paste with 85% - 92% metal content is usually selected. The manufacturer of solder paste generally controls the metal content at 89% or 90%, and the use effect is better

2.6 working life and storage period

working life refers to the time allowed from the printing of the solder paste to the placement of components. If the solvent contained in the solder paste is too volatile, it is easy to dry the solder paste and make it difficult to operate, and it is easy to lose the adhesion to components. The solder paste with at least 4H effective working time should be selected, otherwise it will cause trouble to the batch production

storage life refers to the storage life of solder paste from delivery to serious reduction of performance layout under the specified storage. The following items are shared after careful consideration. Generally, they are specified as months, and there are also one year. Because there are chemical additives in solder paste, they are easy to change due to temperature and time, and lose their original functions, so the storage life and the use of devices need to be paid attention to

3 precautions for the use and storage of solder paste

3.1 registration and storage of solder paste

after the arrival of solder paste, the arrival time, shelf life, model and number of each pot of solder paste should be registered, and the principle of "first in, first out" should be followed when using. The solder paste should be stored in a refrigerator with constant temperature and humidity in a sealed form. The temperature is about ℃. If the temperature is too high, the solder flux and alloy solder powder react chemically, so that the viscosity increases and affects its printability, If the temperature is too low (below 0 ℃), rosin in the flux will produce crystallization, which will deteriorate the shape of the solder paste, and will endanger the rheological characteristics of the solder paste when thawing

3.2 precautions for the use of solder paste

pay attention to the following points during the use of solder paste:

(1) when using, take it out of the refrigerator at least 4H in advance, write down the time, number, user and applied product, seal it and place it at room temperature, and open the bottle cap when the solder paste reaches room temperature. If it is opened at low temperature, it is easy to absorb water vapor, and it is easy to produce tin beads during reflow soldering. Note: the solder paste cannot be placed next to the hot air heater, air conditioner, etc. to accelerate its temperature rise

(2) after unsealing, stir with a mixer for at least 30s or manually for 5min to make the components in the solder paste uniform and reduce the viscosity of the solder paste

(3) after the first printed board or equipment on duty is adjusted, the solder paste thickness tester should be used to measure the solder paste printing thickness. The test points should be selected at the top and bottom, left and right, and middle of the test surface of the printed board. Record the value. The solder paste thickness is required to be between -10% and + 15% of the board thickness

(4) if it is not used after being placed on the plate for more than 30min, it should be mixed with the mixing function of the screen printing machine before use. If the interval is long (more than 1h), the solder paste with greater risk of holding mode should be put back into the tank and the bottle cap should be tightly closed. After the solder paste is used again and opened, it should be stirred at least with a mixer or manually to make the components in the solder paste uniform

(5) the number of board surface and solder joints of the printed board determines the amount of solder paste added to the board for the first time. Generally, add g for the first time, and add a little after printing for a period of time to ensure that the solder paste rolls clockwise along the direction of the scraper during printing, and the thickness is about 1/2 to 3/4 the height of the metal scraper

(6) after printing the solder paste on the board, it should be pasted as soon as possible to prevent the volatilization of solvents such as flux. In principle, it should not exceed 8h. After the time is exceeded, the solder paste should be cleaned and reprinted

(7) after opening, in principle, it should be used up at one time within the same day, and the solder paste beyond the service life of the time must not be used. The solder paste scraped back from the board should also be sealed and refrigerated

(8) the best temperature for printing time is 25 ℃ ± 3 ℃, and the relative temperature is 45% - 65%. If the temperature is too high, the solder paste is easy to absorb water vapor and produce tin beads during reflow soldering

(9) do not put fresh solder paste and used solder paste into the same bottle. When the solder paste is to be collected from the board, replace it with another empty bottle to prevent the fresh solder paste from being polluted by the old solder paste

(10) it is recommended that when new and old solder pastes are mixed, 1/4 of the old solder paste and 3/4 of the fresh solder paste should be mixed evenly to keep the new and old solder pastes in the best state when mixed together

(11) during the production process, 100% inspection shall be carried out on the printing quality of solder paste, mainly including whether the solder paste pattern is complete, whether the thickness is uniform, and whether there is solder paste sharpening

(12) clean the board according to the process requirements after the completion of the work on duty

(13) after the printing experiment or printing failure, the solder paste on the printed board is required to be thoroughly cleaned with ultrasonic cleaning equipment and dried to prevent solder balls after reflow soldering caused by residual solder paste on the board during reuse

4 board production

4.1 the key to board production

board is the key component of solder paste printing, which is composed of frame, wire and mask graphics. Generally, the mask pattern is made on the wire in a proper way, which corresponds to the SMT pad of the solder paste to be missed on the PCB one by one, and the wire is stretched on the frame, as shown in Figure 3. At the same time, we should pay attention to several key points of board making:

(1) box. The function of the frame is to support and tighten the wire so that the wire is parallel to the workbench of the PCB clamping mechanism. Generally, hollow aluminum alloy profiles are used to meet the strength requirements and facilitate the printing operation

(2) silk, which is tight on the frame, is the carrier of mask graphics and an important tool to control the amount of solder paste printing. It can determine the accuracy and quality of solder paste printing. 1. They can store data or print out quality with a printer. Silk can be made of different materials, of which stainless steel wire is the most suitable for solder paste printing

(3) plate opening type. Generally, there are three methods: chemical corrosion, laser cutting and laser cutting and electroplating:

1) the chemical corrosion hole wall is rough and can only be used for printing with a spacing of more than 0.65mm, but the cost is lower than that of other steels (see Figure 5)

2) laser cutting adopts conical opening, which is conducive to demoulding. It can be processed with Gerber files, with less error and higher accuracy (see Figure 6)

3) laser cutting and electroplating, the hole wall is smooth and shrinkable

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