Viscosity - a key factor in determining the quality of solder paste printing

Viscosity - the key determinant of solder paste printing quality Through the analysis of the composition, rheological properties and viscosity measurement of solder paste, it is shown that solder paste viscosity is the key technical index affecting printing quality, and five factors affecting viscosity are elaborated, namely metal alloy powder content, powder size, temperature, rotation speed and printing operation.

Solder paste, also known as solder paste or solder paste, is a paste-like substance made from a system of alloy solder powders that are evenly mixed with a system of flux in specific proportions to form an alloy bond when soldered. In reflow soldering, the substance is used to connect the pins or terminals of surface-mount components to the pads. Solder paste is ideal for reliable soldering in surface mount production and is a product of the high technology of the modern electronics industry. As an electronic material, solder paste is both a traditional and a new material [1]. With the development of computers, communication equipment, household appliances, etc., in the direction of miniaturization, high performance, and multi-purpose, reflow soldering has become an inevitable choice for manufacturing these products, so solder paste came into being. Solder paste application is the first critical process of surface assembly technology, which directly affects the welding quality and reliability of surface assembly components (SMA).

Solder paste is highly malleable and has a wide range of applications, not only for welding many parts of the workpiece that cannot be welded with solid solder, but also for welding metal parts in purely mechanical parts. In the field of electromechanical, solder paste is used in the welding of line and cable components, cable connection stiffeners, etc.; In the electrical industry, it is mainly used in the manufacture of printed circuit boards and components, covering all electronic products.

Composition of solder paste Solder paste usually comes in bottles of 500g per bottle. It is mainly composed of alloy solder powder, flux, activator, viscosity active agent (i.e., thixotropic agent), etc. Metal alloy particles account for about 90% of the total volume, and the proportion of other components is shown in Table 1 below.

Viscosity and Measurement of Solder Paste In the SMT workflow, there is a process of moving, placing, or handling the PCB from the time the paste is printed (or spotted) and the components are applied to the reflow heating process. In this process, in order to ensure that the printed (or spotted) solder paste does not deform and the components that have been attached to the PCB solder paste do not shift, the solder paste needs to have good adhesion and sufficient holding time before the PCB enters the reflow soldering heat, so the viscosity of the solder paste itself must be paid attention to. Solder paste is a kind of paste, which has a certain initial adhesion at room temperature, which is the basis for ensuring that the solder paste does not collapse after printing and does not shift after component mounting, but this viscosity cannot be too large, otherwise it is not easy to succeed when printing the leakage board (template) or the dispensing machine, and the solder paste is difficult to flow out, so it is necessary to choose the solder paste with appropriate viscosity for operation.

The viscosity index (i.e., viscosity) of solder paste is usually expressed in Pa·s, which refers to the measurement of the internal friction between molecules when the liquid is moved by external force, and is a physical quantity to measure the viscosity of the fluid, also known as the viscosity coefficient and dynamic viscosity, which is recorded as μ. Newton's law of viscosity states that the shear stress (or viscous frictional stress) between two adjacent fluid layers in pure shear flow is du/dy, which is the normal velocity gradient in the direction of perpendicular flow. The viscosity value is equal to the shear stress of the fluid under the unit velocity gradient, the relationship between the fluid shear stress τ and the internal friction resistance is shown in Figure 1, and the velocity gradient also represents the angular deformation rate in the fluid motion, so the viscosity also represents the ratio relationship between the shear stress and the angular deformation rate, and the relationship formula is as follows. / If n = 1, then a Newtonian fluid; If n ≠ 1, it is a non-Newtonian fluid. For non-Newtonian fluids, the fluid viscosity μ related to the velocity gradient, as shown in Figure 2 with the solder paste speed viscosity curve. Solder paste is a thixotropic non-Newtonian fluid, and its viscosity decreases with the increase of shear rate, and the ambient temperature, the stability of the solder paste, and the particle size of the solder particles will affect its viscosity.

The solder paste itself is a complex fluid, a high-density suspension with a solid-liquid phase volume ratio of about 1:1, and from a rheological point of view, the state of the solder paste is a dense alloy powder dispersed in a non-Newtonian fluid flux paste carrier. Some defects after solder paste printing are usually caused by the viscosity value and collapse of the solder paste, and establishing the relationship between the printing operation and the rheological value is the key to ensuring the quality of the print. The viscosity of solder paste varies at different shear rates, and as the shear rate increases, its viscosity decreases, a phenomenon known as shear thinning. This feature is suitable for stencil printing, i.e. the solder paste rolls well under the action of the scraper and smoothly fills the stencil window, forming a regular-shaped solder paste pattern on the circuit board.

At present, in the electronic product SMT manufacturing industry, solder paste suppliers and large solder paste users commonly use two viscometers to detect solder paste viscosity, one is the Brookfield viscometer in the United States, and the other is the Malcom viscometer in Japan, as shown in Figure 4. These two types of viscometers are widely used in the development, production and application of solder paste, but Malcom viscometers are more commonly used by Japanese and Taiwanese manufacturers.

The working principle of the Brookfield viscometer is that a calibrated beryllium-copper alloy spring drives a rotor to rotate continuously in the fluid, and the degree of torsion of the spring measured by the rotation torque sensor is the torque, which is proportional to the resistance of the rotor immersed in the sample by viscous dragging, so the torque is also proportional to the viscosity of the liquid.

The Malcom Solder Paste Viscometer uses a double cylindrical, screw pump method with an inner cylinder with spiral grooves, resembling screws, connected to a torque sensor, and an outer cylinder with a spade input and output. It rotates at a constant speed, because the geometry is two cylinders, the length of the groove is fixed, so the speed and time of trimming are fixed, after knowing these two values and torque, you can get parameters such as viscosity and adhesion, so as to obtain the effect of successful screen printing. Both viscosity testing methods have their own advantages and have different characterization of the properties of the solder paste.

An economical and practical way to determine that solder paste has the correct viscosity: stir the solder paste for 30s, pick up some solder paste so that it is three or four inches above the container, let the solder paste drip on its own, if it slides off like a thick syrup at the beginning, and then breaks off in sections and falls back into the container, it is good, otherwise the viscosity is poor.

Viscosity The viscosity of solder paste is one of the key factors affecting the printing quality, although its viscosity is mainly related to the powder content, powder size, and flux viscosity, but the viscosity requirements are not as high as possible, but the selection of the appropriate is appropriate, and the viscosity requirements vary depending on the application method. There are three main methods of solder paste application: template missing print, screen plate missing print, and distributor injection, and these three methods have high and low viscosity requirements, as shown in Table 2. If the choice is wrong, the viscosity will be relatively high or low, and when the viscosity is high, the solder paste is not easy to coat and form, and it is easy to draw and tail, and pollute the template, needle and other fixtures; When the viscosity is low, the solder paste is easy to demold, but it is difficult to form and collapse; The viscosity is insufficient to provide sufficient initial adhesion for the components, and the components are displaced due to movement, handling and other actions after mounting, and eventually there are welding defects such as open circuit, short circuit, monument, side stand, virtual welding, and missing welding.

1.Factors affecting the viscosity of solder paste 1) Effect of solder powder content on viscosity The increase of solder powder in solder paste will lead to an increase in viscosity, which can also effectively prevent collapse after printing and preheating stage, which helps to improve the quality of soldering and make the solder joint full. This is also the reason why solder paste with a high solder powder content is often selected and printed with a metal stencil.

2. Effect of solder powder particle size on viscosity The particle size of solder paste usually refers to the diameter of the solder powder, which is generally expressed in mesh. The so-called "mesh" is the number of holes in the 1-inch (25.4 mm) wide screen in which more than 80% of the solder powder passes through the screen at one time. When the metal powder content and flux components in the solder paste are exactly the same, the size of the solder particle size will affect the viscosity. When the particle size increases, the viscosity decreases, which is the same as the shear stress increases due to the decrease in the particle size of the solder.

3.The influence of temperature on viscosity Temperature has a great influence on the viscosity of solder paste, and as the temperature increases, the viscosity decreases significantly. Therefore, whether testing solder paste viscosity or printing solder paste, it is important to pay attention to the ambient temperature between operations. In general, the optimal printing environment temperature of solder paste is (23±3)°C, and a constant temperature system should be used to ensure the temperature of the printing machine when printing fine pitch.

4.Effect of rotational speed on solder paste viscosity As the rotational speed increases, the solder paste viscosity decreases, which is evident when measuring the solder paste viscosity with a rotational viscometer. Therefore, in the early stage of solder paste preparation for printing, it is necessary to pay attention to the rotation speed when using a blender or hand mixing, and try to be as uniform as possible and the size is moderate. This will effectively improve the performance of solder paste and increase the reliability of electronic products.

5.The influence of printing operation on the viscosity of solder paste The viscosity of solder paste will change with the movement of the scraper on the metal template, when the scraper pushes the solder paste to roll forward, its viscosity decreases, and when the scraper reaches the template window, the viscosity reaches the lowest, so the solder paste can settle smoothly through the window to the PCB pad, with the release of external force, the viscosity of the solder paste rises rapidly, so that the solder paste will not appear the phenomenon of collapse and diffuse flow of printing patterns, so as to obtain a good printing effect.

6.The influence of viscosity on printing parameters The printing parameters of solder paste mainly include printing speed, scraper pressure, scraper angle and scraper pushing angle, scraper hardness, cleaning method and frequency, printing gap, off-grid speed, etc., solder paste viscosity is more or less related to these parameters, and these connections will determine the final printing quality of solder paste, and the qualified specification diagram under the appropriate viscosity selection is shown in Figure 5. Too much solder paste viscosity may mean that the solder paste metal powder particles are smaller, the content is higher, and the flux viscosity is higher, although the outside temperature, printing behavior, storage, etc. may also affect it, and vice versa. Due to excessive viscosity, the printing speed will be affected by irregularities and decrease, which will eventually affect the yield; The squeegee pressure may need to be greater, otherwise the solder paste is not easy to fall off, even so, due to excessive pressure, it will also lead to the shortening of the life of the template, the solder paste printing pattern is blurred, collapse, etc.; The scraper angle and push angle may need to be smaller, which is conducive to decomposition to a greater injection force (F 2) and thus facilitate demoulding, and the scraper hardness may need to be higher, only in this way can the amount of solder paste accepted by the pad be reduced due to scraper deformation; The cleaning method and frequency will also change, when the viscosity is low, it may be enough to simply dry clean or wet cleaning, because the viscosity becomes high, the solder paste is not easy to remove, so a variety of methods are required to cooperate with cleaning, and the cleaning frequency increases; The printing gap and off-screen speed will also increase due to the increase in viscosity, otherwise the solder paste will not easily fall off, and the drawing trail will be formed. The above printing parameters will even change due to the change of viscosity, of course, the viscosity will also change in the opposite direction, and the parameter setting will not be in place, which will eventually affect the printing quality. Controlling the viscosity of the solder paste is the key to ensuring the quality of the print.

In the actual production process, in order to ensure that the viscosity of the solder paste is in the right range, many companies have adopted strict quality control measures. For example, in some large electronic manufacturing enterprises, a solder paste management department will be set up to monitor the procurement, storage, and use of solder paste. According to the above-mentioned situation that the viscosity of solder paste is affected by a variety of factors, when purchasing solder paste, they will have a detailed understanding of the composition of solder paste, especially the metal alloy powder content, powder size, etc., because these factors are directly related to the viscosity characteristics of solder paste (as mentioned earlier, the viscosity will increase when the solder powder content increases, and the viscosity decreases when the particle size increases).

7. In the storage process, because the temperature has a great impact on the viscosity of the solder paste (the viscosity will decrease significantly when the temperature increases), these companies will use constant temperature storage equipment to ensure that the viscosity of the solder paste is not affected by temperature fluctuations during the storage process. In the use process, the operator will operate in strict accordance with the specified process parameters. For the solder paste application method, whether it is stencil missing print, screen plate missing print or distributor injection, the solder paste with appropriate viscosity is selected according to the different viscosity requirements of each method (e.g., the viscosity requirement for stencil missing printing is relatively high, and the dispenser injection is relatively low).

8. In the process of printing operation, the operator will pay close attention to the relationship between printing parameters such as printing speed, scraper pressure, scraper angle and pushing angle, scraper hardness, cleaning method and frequency, printing gap, off-grid speed and solder paste viscosity. For example, when they find that there is a phenomenon such as stringing and tailing in the printing process, they analyze it based on the relationship between viscosity and printing parameters mentioned earlier. If it is judged that the solder paste is caused by excessive viscosity (when the viscosity is too high, the solder paste may not be easy to fall off, and the pressure of the scraper needs to increase), the relevant parameters will be adjusted in time or whether the storage conditions of the solder paste are suitable to ensure that the viscosity of the solder paste is restored to the appropriate range, so as to ensure the printing quality.

In addition, with the continuous development of the electronics industry, the quality requirements for electronic products are getting higher and higher, and the control of solder paste viscosity will also face more challenges. For example, as electronics move toward smaller sizes and higher densities, solder paste printing is becoming more accurate, requiring more precise control of solder paste viscosity. In the future, more advanced viscosity measurement and control technologies are likely to emerge to accommodate this trend.

Summary The viscosity of solder paste is one of the most important technical indicators of solder paste, which is not only controlled by its own factors, but also by many external factors. From the above, it can be seen that controlling the viscosity of solder paste is the basis for achieving the best printing quality in the SMT manufacturing process of electronic products. Choosing the right solder paste, designing a well-designed process solution, having it operated by highly skilled technicians, using highly accurate coating equipment, and having good inspection methods are the keys to ensuring a controlled and appropriate viscosity that we must do in order to achieve high-quality electronic products.