SMT manufacturing process - solder paste control

In the SMT production process, solder paste is as important as the top ingredient in a top-notch meal. Only the combination of high-quality ingredients and superb cooking techniques can make delicious food.

SMT product defects caused by solder paste account for 60% to 70% of all defects in the SMT production process, so in the SMT production process, high-quality solder paste and reasonable use of solder paste are extremely important. In the reflow soldering process of surface mounts, solder paste is used to connect the leads or endpoints of surface mount components to the pads on the printed board, and the high-quality solder paste ensures sufficient mechanical strength, good electrical contact, and a clean and clean appearance of the solder.

Composition and characteristics of solder paste Solder paste is a homogeneous mixture mixed with alloy solder powder, flux and some additives, which has a certain viscosity and good thixotropy, good printing performance and reflow soldering performance, and is stable during storage, presenting as a paste. The use of this paste state is determined by the SMT manufacturing process, which requires the paste to be easily printed on the PCB through the holes in the stencil.

Solder paste under magnifying glass Alloy solder powder is the main component of solder paste, accounting for about 85% - 90% of the weight of solder paste. Commonly used alloy solder powders are as follows: tin-lead (Sn-Pb), tin-lead-silver (Sn-Pb-Ag), tin-lead-bismuth (Sn-Pb-Bi), tin-silver-copper (Sn-Ag-Cu), etc., the most commonly used alloy composition is Sn63Pb37 (leaded solder paste), which has a melting point of 183 °C. With the increasing environmental problems, lead-free solder paste is now used, and its alloy composition is 95.4Sn/3.1Ag/1.5Cu, that is, tin-silver-copper alloy with a melting point of 217°C.

The shape of alloy solder powder can be divided into spherical and oval (amorphous), and its shape and particle size will affect the surface oxidation and fluidity, which in turn will have a great impact on the performance of the solder paste. Typically, the size and shape of the solder powder particles are determined by the size of the opening of the printed steel or stencil or the diameter of the syringe. Different pad sizes and component pins should be selected with different particle sizes, and small particles should not be used all of them, because the surface area of small particles is much larger, which will increase the burden of oxidation on the surface of flux treatment.

Fluxes are composed of: active agents, film formers and adhesives, wetting agents, thixotropic agents, solvents and thickeners, and various other additives. In solder paste, the flux is the carrier of the alloy solder powder, and its main function is to remove the surface oxides of the weldment and the alloy solder powder, so that the solder can quickly diffuse and adhere to the surface of the soldered metal.

The activity of flux: the activity of the flux must be controlled, too little active dose may affect the welding effect due to poor activity, but too much active dose will lead to an increase in the residual amount, and even enhance corrosion, especially the halogen content in the flux needs to be strictly controlled.

In fact, depending on the performance requirements, the weight ratio of the flux can be increased to 8 to 20 percent. The flux composition and content in the solder paste have a great influence on things like slump, viscosity and thixotropy.

When the metal content is high (greater than 90%), the slump of the solder paste can be improved, which is conducive to the formation of full solder joints, and because the solder dose is relatively small, the flux residue can be reduced, and the solder ball can be effectively prevented, but the printing and soldering process requirements are more stringent;

When the metal content is low (less than 85%), the solder paste has good printability, is not easy to stick to the scraper, the steel mesh has a long service life, good wettability, and is easy to process, but it is easy to collapse, and it is easy to have defects such as solder balls and bridging.

The classification of solder paste can be according to the following methods: according to the melting point: the melting point of high-temperature solder paste is greater than 250 °C, the melting point of low-temperature solder paste is less than 150 °C, the melting point of commonly used solder paste is 179 °C - 225 °C, and the composition is Sn63Pb37, Sn62Pb36Ag2 and 95.4Sn3.1Ag1.5Cu. According to flux activity: it can be divided into inactive (R), medium active (RMA) and active (RA) solder paste, and the commonly used is medium active solder paste.

We call objects that can change shape or divide arbitrarily as fluids, and the science that studies the laws and characteristics of fluid deformation and flow behavior caused by external forces is called rheology. However, the concept of viscosity is used in engineering to characterize the viscosity of a fluid.

Rheological behavior of solder paste: The solder paste is mixed with a certain amount of thixotropic agent, which has pseudoplastic fluid properties. When printing, the solder paste is affected by the thrust of the scraper, and its viscosity decreases, and when it reaches the opening of the steel mesh, the viscosity reaches the lowest, so it can settle smoothly through the window to the pad of the PCB, and with the external force stops, the viscosity of the solder paste rises rapidly, so that there will be no collapse and diffuse flow of the printed pattern, so as to obtain a good printing effect.

Factors affecting the viscosity of solder paste include: solder powder content, which is caused by an increase in solder powder in the solder paste; The size of the solder powder particles, the viscosity will decrease when the solder powder particles increase; Ambient temperature, the viscosity decreases as the temperature increases, and the optimal ambient temperature for printing is 23 ± 3 °C; The printing rate, shear rate increases, and the viscosity decreases.

Solder paste testing items Whether the technical indicators of solder paste meet the production standards must be strictly tested, and only by ensuring the quality of solder paste can the production quality of SMT be ensured. The main test items of solder paste are as follows: solder paste appearance, solder weight percentage, flux acid number determination, solder paste printability, solder composition determination, flux halide determination, solder paste viscosity test, solder particle size distribution, flux water soluble conductivity determination, solder paste slump, solder powder shape, flux copper mirror corrosivity test, solder paste residue dryness after hot melting, flux insulation resistance determination, solder ball test of solder paste, solder paste wettability expansion test.

Solder paste testing items A good solder paste should have the following characteristics:

1. It has a long storage life, and can be stored at 0-10 °C for 0-6 months. There are no chemical changes during storage, no separation of solder powder and flux, and no viscosity and adhesion remain unchanged.

2. It has a long working life, and it is usually required to be placed at room temperature for 12-24 hours after printing or dropping, and the performance remains unchanged.

3. After printing or coating and during the reflow soldering preheating process, the solder paste should maintain its original shape and size without clogging.

4. It has good wetting performance. The active agent and wetting agent components in the flux should be selected correctly to meet the wetting performance requirements.

5. No solder spatter. This mainly depends on the water absorption of the solder paste, the type of solvent in the solder paste, the boiling point and the amount used, and the type and amount of impurities in the solder powder.

6. It has good welding strength to ensure that the components will not fall off due to vibration and other factors. 7. The stability of the residue after welding is good, no corrosion, high insulation resistance, and good cleaning.

Performance requirements of solder paste The following items should be paid attention to when choosing solder paste:

 1. It has excellent storage stability.

 2. It has good printability (fluidity, off-plate, continuous printability).

3. After printing, it has a certain adhesion to SMD within a certain period of time.

 4. After welding, a good joint state can be achieved, and the solder joints are required to be smooth and clean.

 5. The composition of the flux should have high insulation and low corrosion.

 6. It has good cleaning ability for the flux residue after welding, and no residue composition can be left after cleaning.

Good tin printing effect

Precautions for the use and storage of printed solder paste

1. Solder paste should be stored in a refrigerator with constant temperature and humidity at a temperature of about (2 - 10) °C.

2. The principle of first-in-first-out should be followed when using solder paste, which should be taken out of the refrigerator at least 4 hours in advance, sealed and placed at room temperature, and then open the cap when the solder paste reaches room temperature. If it is opened at a low temperature, it is easy to absorb water vapor, and it is easy to produce solder beads during reflow soldering. Do not place the solder paste next to the hot air heater, air conditioner, etc. to accelerate the heating, which will cause the chemical reaction between the flux in the solder paste and the solder powder in advance, which will affect the subsequent soldering effect.

 3. Before the solder paste is opened, a solder paste mixer must be used to stir the warmed solder paste to evenly distribute the components of the solder paste and reduce the viscosity of the solder paste. After the solder paste is opened, in principle, it should be used up within 24 hours, and the solder paste that exceeds the time should be scrapped, and if it is really necessary to use, it can be mixed with the freshly opened solder paste in a ratio of 1:3 and then used.

 4. When the solder paste is placed on the steel mesh for more than 30 minutes and has not been used, it should be re-stirred before it can be used again. If the interval is long, put the solder paste back into the storage jar, close the cap tightly and put it in the refrigerator.

 5. According to the area of the printed board and the number of solder joints, determine the amount of solder paste added to the missing plate for the first time, generally add 200 - 300 grams for the first time, and then add it appropriately after printing for a period of time.

6. The solder paste should be placed within 24 hours after printing, and the solder paste on the PCB should be cleaned off and reprinted after the time is exceeded. When cleaning, you should use alcohol or cleaning solution to carefully clean to prevent tin powder from remaining on the PCB, resulting in poor soldering, and the PCB needs to be air-dried before use after cleaning, otherwise PCB bubbles may occur.

 7. The best temperature of solder paste printing is 25 °C±3 °C, and the humidity is 30 - 60%RH. In the SMT production process, the performance and quality of the solder paste are directly related to the quality of the final product. Its composition, characteristics, classification, rheological behavior, as well as various test items and performance requirements and other factors need to be strictly controlled. From the perspective of composition, the ratio and composition of alloy solder powder and flux have a great impact on the performance of solder paste. Different alloy solder powder shapes, particle sizes, and flux activity and content all play important roles and restrict each other. In terms of classification, the classification according to the melting point and flux activity is different, which is helpful to choose under different production needs. In terms of rheological behavior, the thixotropic agent in the solder paste makes it have pseudoplastic fluid properties, and good printing results can be obtained through the change of viscosity during the printing process, while various factors such as solder powder content, particle size, ambient temperature and printing rate will affect its viscosity.

In terms of testing items, many testing items cover the appearance, composition, performance and other aspects of solder paste, only all indicators meet the production standards, in order to ensure the quality of solder paste, and then ensure the production quality of SMT. At the same time, a good solder paste should have a variety of characteristics such as long storage life, working life, maintaining the original shape, good wetting performance, etc. When selecting solder paste, it is also necessary to consider various aspects such as storage stability, printability, adhesion, bonding state after soldering, and flux composition characteristics. In addition, the precautions for the use and storage of solder paste can not be ignored, from the temperature and humidity of preservation, the order of use, stirring, the use time after opening, the amount of printing, the placement time after printing to the temperature and humidity during printing, etc., each link is strictly required, in order to maximize the role of solder paste in SMT production, reduce product defects caused by solder paste problems, and ensure the smooth progress of SMT production process and high quality products.