Roughly speaking, the components of solder paste can be divided into two major parts, namely flux and solder powder (FLUX&SOLDER POWDER).

(1) The main components and functions of flux:

A. Activator (ACTIVATION): This ingredient mainly plays the role of removing oxidized substances on the surface of PCB copper film pads and parts welding parts, and also has the effect of reducing the surface tension of tin and lead;

B. Thixotropic agent (THIXOTROPIC): This ingredient is mainly used to adjust the viscosity and printing performance of the solder paste, and plays a role in preventing tailing, adhesion and other phenomena during printing;

C. Resin (RESINS): This ingredient mainly plays a role in increasing the adhesion of the solder paste, and also protects and prevents the PCB from being oxidized again after soldering; this ingredient plays an important role in fixing parts;

D. Solvent (SOLVENT): This component is the solvent of the flux component. It plays a uniform role in the solder paste stirring process and has a certain impact on the life of the solder paste;

(2) Solder powder:

     Solder powder, also known as tin powder, is mainly composed of tin-lead alloy, with a general ratio of 63/37. When there are special requirements, there are also tin powders that add a certain amount of silver, bismuth and other metals to the tin-lead alloy. In summary, the relevant characteristics and quality requirements of tin powder are as follows:

A. The particle shape of tin powder has a great influence on the working performance of solder paste:

A-1. The important point is that the particle size distribution of tin powder is required to be uniform. Here we will talk about the issue of particle size distribution ratio of tin powder. Among domestic solder powder or solder paste manufacturers, distribution ratio is often used to measure the size of tin powder. Uniformity: Taking 25~45μm tin powder as an example, it is usually required that the proportion of particles around 35μm is about 60%, and the parts below and above 35μm each account for about 20%;

A-2. In addition, the shape of the tin powder particles is also required to be relatively regular; according to the relevant provisions in the "Electronic Industry Standard of the People's Republic of China "General Specifications for Tin-Lead Paste Solder" (SJ/T 11186-1998)": "The shape of the alloy powder should be It is spherical, but the maximum ratio of the major axis to the minor axis is allowed to be a nearly spherical powder of 1.5. If the user reaches an agreement with the manufacturer, it can also be alloy powder of other shapes." In actual work, it is usually required to be tin The ratio of the long and short axes of powder particles is generally less than 1.2.

A-3. If the above requirements of A-1 and A-2 cannot meet the above basic requirements, the printing, dispensing and welding effects of solder paste will most likely be affected during the use of solder paste.

B. The proportions of tin powder and flux in various solder pastes are also different. When selecting solder paste, you should choose based on the products produced, the production process, the precision of the welding components, and the requirements for the welding effect. different solder pastes;

B-1. According to the relevant provisions of the "Electronic Industry Standard of the People's Republic of China "General Specifications for Tin-Lead Paste Solder" (SJ/T 11186-1998)", "The percentage (mass) content of alloy powder in the solder paste should be 65% -96%, the deviation between the actual measured value of alloy powder percentage (mass) content and the predetermined value in the order form is not greater than ±1%"; usually in actual use, the tin powder content of the selected solder paste is about 90%, that is The ratio of tin powder to flux is roughly 90:10;

B-2. Ordinary printing processes mostly use solder paste with a tin powder content of 89-91.5%;

B-3. When using the needle injection process, it is better to use solder paste with a tin powder content of 84-87%;

B-4. Reflow soldering requires that the device pins are soldered firmly, the solder joints are full and smooth, and there is a 1/3 to 2/3 height solder climb in the height direction of the device (resistor device) end, and the metal alloy in the solder paste is Content has a certain impact on the thickness of the solder after reflow soldering (i.e., the fullness of the solder joints). In order to confirm the existence of this problem, relevant experts have conducted relevant experiments. The final experimental results are excerpted in the following table for reference:

 

   As can be seen from the above table, as the metal content decreases, the thickness of the solder after reflow decreases. In order to meet the requirements for the solder volume of the solder joints, a solder paste with a content of 85% to 92% is usually used.

C. The "low oxidation degree" of tin powder is also a very important quality requirement. This is also a problem that should be paid attention to during the production or storage of tin powder. If you do not pay attention to this problem, use tin powder with a higher oxidation degree to make The solder paste will seriously affect the quality of the welding during the welding process.

 

Solder paste classification and selection criteria

Under normal circumstances, first select the solder paste category, and then select based on alloy composition, particle size, viscosity and other indicators.

(1) Classification method:

A. Ordinary rosin cleaning type [divided into RA (ROSIN ACTIVATED) and RMA (ROSIN MILDLYACTIVATED)]: This type of solder paste shows better "tinning speed" during the soldering process and can ensure good "soldering effect"; After the welding work is completed, there is relatively much rosin residue on the PCB surface, which can be cleaned with appropriate cleaning agents. After cleaning, the board surface is smooth and has no residue, ensuring that the cleaned board surface has good insulation resistance and can pass various electrical performance technical tests. ;

B. No-clean solder paste [NC (NO CLEAN)]: After the soldering of this type of solder paste is completed, the PCB board surface is smoother and has less residue. It can pass various electrical performance technical tests and does not need to be cleaned again, while ensuring the welding quality. At the same time, the production process is shortened and the production progress is accelerated;

C. Water-soluble solder paste [WMA (WATER SOLUBLE PASTES)]: Due to technical reasons, the solder paste produced in the early days generally had too much residue on the PCB board surface and the electrical performance was not ideal, which seriously affected the product quality; CFC cleaning agents were mostly used at that time. Because CFC is not good for environmental protection, it has been banned in many countries. In order to meet the needs of the market, water-soluble solder paste has been produced. After the soldering work is completed, the residue of this solder paste can be cleaned with water, which not only reduces the customer's production cost and meets environmental protection requirements.

(2) Selection criteria:

1. Alloy composition: Under normal circumstances, the Sn63/Pb37 solder alloy composition can meet the welding requirements; for welding of devices with silver (Ag) or palladium (Pd) coating, the alloy composition is generally selected as Sn62/Pb36/ Ag2 solder paste; for PCB welding with components that are not resistant to thermal shock, choose Bi-containing solder powder.

2. Viscosity of solder paste (VISCOSITY):

In the SMT workflow, there is a process of moving, placing or transporting the PCB from printing (or dispensing) the solder paste and attaching the components to sending it to the reflow heating process; during this process, in order to ensure that the PCB has been The printed (or dotted) solder paste will not deform, and the components that have been attached to the PCB solder paste will not shift. Therefore, the solder paste is required to have good viscosity and retention time before the PCB enters the reflow soldering heating.

A. The viscosity index (i.e. viscosity) of solder paste is often expressed in the unit "Pa·S"; among them, 200-600Pa·S solder paste is more suitable for use in pin-type dispensing systems or production processes with a higher degree of automation. Equipment; the printing process requires a relatively high viscosity of solder paste, so the viscosity of the solder paste used in the printing process is generally around 600-1200 Pa·S, and is suitable for manual or mechanical printing;

B. High-viscosity solder paste has the characteristics of good solder joint staking effect and is more suitable for fine-pitch printing; while low-viscosity solder paste has the characteristics of faster falling during printing, no need to wash tools, and time saving;

C. Another characteristic of the viscosity of solder paste is that its viscosity will change as the solder paste is stirred, and its viscosity will decrease during stirring; when stirring is stopped and the viscosity is left to stand for a while, its viscosity will return to its original state; this This point plays an extremely important role in how to choose solder pastes of different viscosities.

In addition, the viscosity of solder paste has a great relationship with temperature. Under normal circumstances, its viscosity will gradually decrease as the temperature increases.

3. Mesh number (MESH):

Domestic solder paste manufacturers often use the "granularity" of tin powder to classify different solder pastes, while many foreign manufacturers or imported solder pastes use the concept of "mesh number (MESH)" to classify different solder pastes. The basic concept of mesh number (MESH) refers to the number of mesh holes per square inch of the screen; in the actual tin powder production process, several layers of screens with different meshes are mostly used to collect tin powder, because the size of each layer of screen mesh The mesh sizes are different, so the particle size of the tin powder passing through each layer of mesh is also different. The particle size of the finally collected tin powder particles is also a regional value;

A. From the above concept, the larger the mesh number index of the solder paste, the smaller the particle diameter of the tin powder in the solder paste; and when the mesh number is smaller, it means the larger the particles of the tin powder in the solder paste; refer to the following Table comparison:

B. If the manufacturer of solder paste selects solder paste according to the mesh number index of the solder paste, it should be determined based on the spacing between the smallest solder joints on the PCB: if there is a larger spacing, you can choose a smaller mesh number. Solder paste, on the contrary, when the spacing between solder joints is small, solder paste with a larger mesh size should be selected; generally, the particle diameter should be within about 1/5 of the template opening.

 

Things to note when using solder paste

(1) Storage requirements for solder paste:

The solder paste should be stored in a sealed form in a refrigerator with constant temperature and humidity at a storage temperature of 0°C to 10°C. If the temperature is too high, the alloy powder in the solder paste will not react chemically with the flux and the viscosity and activity will be reduced. Reduction affects its performance; if the temperature is too low, the resin in the flux will crystallize, causing the solder paste to deteriorate. During the storage process, the more important point is to pay attention to the issue of maintaining a "constant temperature". If the solder paste continuously undergoes different temperature changes from various environments in a short period of time, the solder paste will also be damaged. The performance of the flux changes, thereby affecting the welding quality of the solder paste.

(2) Requirements before use:

When the solder paste is taken out of the freezer (or refrigerator), it should be in a sealed state, and it should be opened after it returns to room temperature, about 2-3 hours; if it is opened just after being taken out of the freezer, the existing temperature difference will cause the solder paste to The paste will condensate and condense into moisture, which will cause solder beads to be produced during reflow soldering; however, it is not possible to heat the solder paste back to room temperature. Rapid temperature rise will deteriorate the performance of the flux in the solder paste, thus affecting the performance of the solder paste. Welding effect. This is also a problem that manufacturers of solder paste should pay attention to during use.

(3) Precautions when using:

1. Squeegee pressure: Ensure that the printed solder joints have clear edges, smooth surfaces, and appropriate thickness;

2. Squeegee speed: Ensure that the solder paste rolls rather than slides relative to the scraper. Under normal circumstances, 10-20mm/s is appropriate;

3. Printing method: contact printing is suitable;

In addition, the solder paste must be fully stirred during use, and then added to the printing stencil according to the printing set amount. If the dot injection process is used, the dot amount must also be adjusted.

In the case of long-term printing, the volatilization of the solvent in the solder paste will affect the demoulding performance of the solder paste during printing. Therefore, the container for storing the solder paste cannot be reused (can only be used once). After printing, the stencil The remaining solder paste should be stored in other clean containers. The next time it is used, check whether there are any agglomerations or solidification in the remaining solder paste. If it is too dry, add the solder paste diluent provided by the supplier. Dilute before use.

When operating, operators should be careful to avoid direct contact between solder paste and skin. In addition, the printed substrate should be soldered on the same day.

(4) Working environment requirements:

The best conditions for a solder paste workplace are: temperature 20-25°C, relative humidity 50-70%, clean, dust-free, and anti-static.

 

Adjustment of reflow curve

   The purpose of reflow soldering: to correctly and reliably weld surface-mounted electronic components (SMD) and PCB together;

   Process principle: When the temperature of the solder, component and PCB reaches above the melting point of the solder, the solder melts, filling the gap between the component and the PCB, and then as it cools, the solder solidifies to form a welded joint;

Process flow:

1. The first heating zone (preheating zone)

The purpose of heating is to raise the temperature of solder paste, PCB and components from room temperature to a predetermined preheating temperature; the preheating temperature is a temperature lower than the melting point of the solder.

An important parameter of the heating section is the "heating rate". Generally, its value should be 1-2.0 degrees/S; due to the different heat absorption rates of PCB and components, the heating rate of each component will also be different, resulting in PCB There is a gradient in the temperature distribution on the plate. Because the temperature at all points in this section is below the melting point of the solder, the existence of a "temperature gradient" is not a big deal. At the end of the first heating zone, the temperature is about 100-110 degrees; the time is about 30-90 seconds, preferably about 60 seconds.

2. Insulation zone (also called dry penetration zone)

The purpose of "heat preservation" is to allow the flux in the solder paste to have sufficient time to clean the solder joints and remove the oxide film on the solder joints, and at the same time, to allow the PCB and components to have sufficient time to reach temperature equilibrium and eliminate the "temperature gradient"; this The stage time should be set at 60-120 seconds; at the end of the heat preservation section, the temperature is 140-150 degrees.

3. The second heating zone

The temperature rises from about 150 degrees to 183 degrees. This temperature zone is the activation period of the activator. The area where the temperature of the PCB board is uniform is generally 30-45 seconds. The time should not be long, otherwise it will affect the welding effect.

4. Welding area

   The solder melts in the welding area and achieves good soldering between the PCB and the component pins. When the temperature in the welding area begins to rise rapidly, the components will still absorb heat at different rates, causing a temperature difference again, so the temperature must be controlled to eliminate this temperature difference. Generally speaking, the maximum temperature in this section should be 30-40 degrees higher than the melting point of the solder (183 degrees), and the time should be about 30-60 seconds. However, the time above 225 degrees should be controlled within 10 seconds, and the time above 215 degrees should be controlled within 10 seconds. It should be controlled within 20 seconds; if the temperature in this section is too high, components will be damaged, and if the temperature is too low, some solder joints will be wetted and welded poorly. In order to avoid and overcome the above defects, forced hot air reflow soldering is currently more effective.

5. Cooling area

   Purpose: to solidify the solder, form a welded joint, and eliminate the internal stress of the solder joint as much as possible; the cooling rate should be less than 4 degrees/second, and it can be cooled to 75 degrees.

   In short, the principle for establishing the reflow temperature curve is that the temperature rise rate before the welding zone should be as small as possible. After entering the second half of the welding zone, the temperature rise rate should increase rapidly, and the maximum temperature time of the welding zone should be short, so that PCB and SMD Due to thermal shock, it takes a long time to adjust the temperature curve before production. At the same time, reflow equipment with several temperature zones should be selected based on product characteristics and batch size.

 

In the process of using solder paste, from solder paste printing, SMD mounting to reflow soldering, we often encounter various problems. These problems often plague users of solder paste. How to analyze and solve them? These problems have also become a topic for our solder paste manufacturers; therefore, it is necessary for solder paste manufacturers to continuously strengthen the professional quality and professional level of their marketing personnel. After the products are delivered to users, they should assist users to properly and timely Handling these issues properly can also reflect the supplier's service strength. Here, I will only briefly introduce several common problems and analysis of their causes, which are also problems that I often encountered when serving customers in my previous work. They are only for the reference of readers and users:

(1) When double-sided patch welding, components fall off

Double-sided soldering is becoming more and more common in the SMT surface mount process. Generally, usersThe first side will be printed, components mounted and welded first, and then the other side will be processed. In this process, the problem of components falling off is not very common; some customers, in order to save processes and costs, save Instead of welding the first side first, welded both sides at the same time. As a result, components falling off during welding became a new problem. This phenomenon is due to the insufficient vertical fixing force of the solder on the components after the solder paste melts. The main reasons are:

1. The components are too heavy;

2. The soldering feet of the components have poor solderability;

3. The solder paste has poor wettability and solderability;

We always put the solution to the first reason last and start improving the second and third reasons first. If the second and third reasons are improved and the phenomenon still exists, we will advise the customer to When welding these fallen components, they should be fixed with red glue first, and then reflowed and wave soldered. The problem can basically be solved.

(2) Tin beads are produced on the PCB board after soldering:

   This is a common problem in the SMT welding process, especially when users are using a new supplier's products in the early stages, or when the production process is unstable, such problems are more likely to occur. With the cooperation of the customer, and through our After a lot of experiments, we finally analyzed that the reasons for the generation of tin beads may be as follows:

1. The PCB board is not sufficiently preheated during reflow soldering;

2. The reflow temperature curve setting is unreasonable, and there is a large gap between the board temperature before entering the welding area and the temperature in the welding area;

3. The solder paste failed to fully return to room temperature when taken out of the cold storage;

4. The solder paste is exposed to the air for too long after opening;

5. Tin powder is splashed on the PCB board during patching;

6. During the printing or transportation process, oil stains or moisture stick to the PCB board;

7. The flux itself in the solder paste is not properly formulated and contains non-volatile solvents or liquid additives or activators;

The first and second reasons above can also explain why newly replaced solder paste is prone to such problems. The main reason is that the current temperature curve does not match the solder paste used, which requires customers to change suppliers. When doing so, be sure to ask the solder paste supplier for the temperature curve that the solder paste can adapt to;

The third, fourth and sixth reasons may be caused by improper operation by the user; the fifth reason may be caused by improper storage of the solder paste or failure of the solder paste after the shelf life has expired. The solder paste has no viscosity or has too low viscosity. Tin powder was splashed during patch placement; the seventh reason was caused by the solder paste supplier's own production technology.

(3) There are many residues on the board surface after welding:

The presence of more residues on the PCB board surface after welding is also a problem often reported by customers. The presence of more residues on the board surface not only affects the smoothness of the board surface, but also has a certain impact on the electrical properties of the PCB itself; causing more The main reasons for excessive residues are as follows:

1. When promoting solder paste, we do not understand the customer’s board conditions and customer requirements, or make selection errors due to other reasons; for example: the customer requires no-cleaning and residue-free solder paste, but the solder paste manufacturer provides rosin. Resin-based solder paste has caused customers to report that there is more residue after soldering. Solder paste manufacturers should pay attention to this aspect when promoting their products.

2. The rosin resin content in the solder paste is too high or the quality is poor; this should be a technical problem of the solder paste manufacturer.

(4) Problems such as tailing, adhesion, and blurred images may occur during printing:

This reason is often encountered in the printing process. After summary, we found that the main reasons are as follows:

1. The viscosity of the solder paste itself is low and is not suitable for the printing process. This problem may be caused by the wrong selection of solder paste, or it may be that the solder paste has expired, etc. This problem can be solved by coordinating with the supplier.

2. It is caused by improper machine settings or improper operating methods by the operator during printing. Improper settings such as the speed and pressure of the scraper are likely to affect the printing effect. In addition, the proficiency of the operator (including speed, pressure, repeated printing, etc. during printing) also has a great impact on the printing effect.

3. The gap between the stencil and the substrate is too large;

4. The solder paste has poor overflow properties;

5. The solder paste is not fully stirred before use, resulting in uneven mixing of the solder paste;

6. When using screen printing, the latex mask coating on the screen is uneven;

7. The metal component in the solder paste is low, that is, the proportion of flux components is high;

(5) The solder joints are not full of tin:

The main reasons why the solder joints are not full of tin are as follows:

1. The flux in the solder paste is not active enough and cannot completely remove the oxidized substances on the PCB pad or SMD soldering position;

2. The wetting properties of the flux in the solder paste are not good;

3. The PCB pad or SMD soldering position has severe oxidation;

4. During reflow soldering, the preheating time is too long or the preheating temperature is too high, causing the flux activity in the solder paste to fail;

5. If some solder joints are not full of tin, it may be that the solder paste was not fully stirred and the flux and tin powder were not fully integrated before use;

6. The temperature of the reflow soldering area is too low;

7. The amount of solder paste in the solder joints is not enough;

(6) The solder joints are not bright:

In the SMT welding process, generally customers have requirements for the brightness of solder joints. Although this is also a problem that exists in daily work, it is often just a subjective awareness of the customer, or it can only be achieved through comparison. We can conclude whether the solder joint is bright or not, because there is no standard for the brightness of the solder joint. Generally speaking, the reasons for the non-brightness of the solder joint are as follows:

1. If you compare the products soldered with silver-free solder paste and the products soldered with silver-containing solder paste, there will definitely be some differences. This requires customers to explain their solder joint requirements to the supplier when choosing solder paste;

2. The tin powder in the solder paste is oxidized;

3. The flux itself in the solder paste has additives that cause matting effects;

4. There are rosin or resin residues on the surface of the solder joints after welding. This is a phenomenon we often see in actual work, especially when using rosin-type solder paste. Makes the solder joints slightly brighter, but the presence of residues often affects this effect, especially in larger solder joints or IC feet. If it can be cleaned after soldering, I believe the gloss of the solder joints should be improved;

5. The preheating temperature is low during reflow soldering, and non-volatile residues remain on the solder joint surface;

(7) Component displacement:

"Component displacement" is a foreshadowing of other problems during the welding process. If this problem is not detected before entering reflow soldering, more problems will occur. The main reasons for component displacement are as follows:

1. The viscosity of the solder paste is not enough, causing parts to shift due to vibration during transportation;

2. The solder paste has expired and the flux has deteriorated;

3. The air pressure of the suction nozzle is not adjusted properly during placement, the pressure is not enough, or there is a mechanical problem with the placement machine, causing the components to be placed in the wrong position;

4. During the transportation process after printing and patching, vibration or incorrect transportation occurs;

5. The flux content in the solder paste is too high, and the flow of flux during the reflow soldering process causes components to shift;

(8) Erection of components after welding:

Compared with other welding methods, "post-weld component monument" is a unique phenomenon in the SMT welding process, and this problem is often encountered. After analysis, we believe that the main reasons for this problem are as follows:

1. The reflow temperature zone line setting is unreasonable, and the drying and penetration work before entering the soldering area is not done well, so that there is still a "temperature gradient" on the PCB, causing the solder paste melting time on each solder joint to be inconsistent in the soldering area, thus As a result, the stresses borne by both ends of the component are different, resulting in the phenomenon of "erected monuments";

2. The preheating temperature is too low during reflow soldering;

3. The solder paste is not fully stirred before use, and the flux in the solder paste is unevenly distributed;

4. Some components are misaligned before entering the reflow soldering area;

5. This phenomenon may also occur when the solderability of SMD components is poor.

 

Summarize:

 

With the development of science and technology, SMT technology is becoming more and more popular, but with it comes the emergence of various problems. If these problems cannot be prevented and solved well, it will affect the future development of SMT; this also requires that In every aspect of cooperation with the SMT process, learn as much as possible about the characteristics of SMT and solutions to various problems. People related to the SMT process, including solder paste manufacturers, SMD manufacturers, SMT process users, SMT Equipment suppliers and others should master as much professional knowledge as possible. Only through continued and orderly cooperation among various industries can the long-term improvement and development of the SMT process be possible.