Since the advent of the surface mount technology in the 1960s, after 30 years of continuous interruption, the placement equipment has developed in the direction of speed, high precision, and flexible production; surface mount devices have become highly integrated and ultra-compact. The typical process flow for package development and surface mount is: Solder paste is printed onto the placement device and reflow soldered to the test. In the surface mount process control process, excellent solder paste printing quality, we must strictly control solder paste selection, template production and printing process parameters and process management.
First, the choice of solder paste
Solder Paste is a mixture required by the reflow process, including metal powdered solder, flux, organic material as a carrier, suspended media of various ingredients, and other additives. The basic principles of solder paste selection are: 1 grain size and shape of alloy powder; 2 content of metal powder; 3 type of flux; 4 stability of solder paste.
1. The size of the solder particles is generally -200 mesh/+325 mesh, that is, at least 99% by weight of the powder particles can pass through the 200 (hole/in2) mesh, and less than 20% by weight of the powder particles can pass through the 325 mesh. The size of particles other than this size is not more than 10%. When solder paste is printed on a device with a 0.5mm pitch, the solder particle size should be smaller than conventional, and a solder paste with a particle size of -300 mesh + 500 mesh can be used.
2. The shape of the solder powder depends on the oxide content of the powder and also determines the printability of the solder paste. The spherical solder powder has the smallest total surface area at a given volume, reduces the area of ​​surface oxidation that may occur, and has good consistency, creating conditions for the solder paste to have excellent printing properties. The solder paste viscosity tester can detect the shape of the solder powder from the display.
3. The amount of metal in the solder paste determines the size of the weld. As the percentage of metal increases, the weld size increases. However, at a given viscosity, as the metal content increases, the tendency of the solder to bridge increases. When the thickness of the solder paste is constant, the metal content affects the reflow thickness. Metal content % Thickness (in) Wet solder paste Solder after reflow 90 0.009 0.0045 85 0.009 0.0035 80 0.009 0.0025 75 0.009 0.0020 Solder pins are required to be firmly bonded after reflow. The solder is plump, smooth, and 1/3 to 2/3 of the height of the solder in the direction of the height of the tip of the device. From the above table, it can be seen that as the content of metal decreases, the thickness of solder after reflow is reduced. In order to meet the solder paste amount requirements for solder joints, solder paste of 85%--92% metal content is generally used. Manufacturers generally control the metal content at 89% or 90%, the better the effect.
4. The flux is one of the main components of the solder paste carrier. The existing solder paste fluxes are of three types: R type (rosin flux), RMA type (moderately activated rosin) and RA type (fully activated rosin). Commonly used fluxes containing RMA type consist of rosin and a salt solution called an activator. This flux, activated by salt, is suitable for eliminating minor oxide films and other contamination and increasing wetting between the molten solder and the pad, element tip or hole. It can also be used as a temporary adhesive before reflow, so that the surface mount device is always fixed in place during the placement and transmission without deviation. Some brand placement machines, especially high-speed placement machines, This characteristic is particularly important at the time when the mounting speed is high and both the mounting head and the printed board are moving at a high speed.
5. After the solder paste is printed and reflowed, it must be transported, patched, etc. The solder paste has a dwell time in the air. In addition, the temperature of the working environment sometimes fluctuates, and during the trial production of new product projects, due to problems such as device packaging of the chip mounter, some devices require manual placement, so the solder paste stays in the air for a relatively long time. Longer, this requires better solder paste stability, slower flux loss, and longer operating life.
6. Solder paste is a thixotropic fluid. When a constant shear stress or tensile stress is applied, the viscosity of the solder paste decreases with time. This means that the structure gradually disintegrates and the viscosity of the solder paste acts on it. The shear stress increases and decreases, while its viscosity is also sensitive to temperature, and the viscosity decreases with increasing temperature. The paste used for the stencil printer has a viscosity of 300-700Pa.s. The effect of increasing the shear stress and raising the temperature on the paste viscosity with the same multi-component content and flux carrier. The solder paste manufacturer will configure the solder paste in a fixed recipe. After that, the viscosity of the solder paste was measured. According to the sensitive characteristics of the solder paste, in addition to testing the solder paste before use, on the one hand to measure the stability of the solder paste quality, while ensuring that the degree of paste used in the production and the factory configuration paste viscosity is basically the same, play the best Use effect. At this point, it should be noted that the test equipment and the viscosity unit are the same as those used by the solder paste manufacturer. If they are inconsistent, they must be converted to arrive at the correct conclusion.
Squeegee type
The wear, pressure, and hardness of the squeegee determine the print quality and should be carefully monitored. For acceptable print quality, the squeegee edges should be sharp and straight. Low squeegee pressure causes omissions and rough edges, while high squeegee pressure or a very soft squeegee will cause smeared printing and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop solder paste from wide openings, causing solder fillets to be insufficient. There are two types of scraper that are common: rubber or polyurethane scrapers and metal scrapers. When a rubber squeegee is used, a squeegee having a hardness of 70-90 durometer is used. When excessive pressure is used, solder paste that penetrates into the bottom of the stencil may cause bridging, requiring frequent bottom wipes. In order to prevent bottom penetration, pad openings must provide a gasketing effect when printing. This depends on the roughness of the template opening wall.
Metal scrapers are also commonly used. With the use of more closely spaced components, the amount of metal scrapers is increasing. They are made of stainless steel or brass and have a flat blade shape with a printing angle of 30 to 45°. Some scrapers are coated with a lubricating material. Because of the lower pressure, they do not scoop solder paste out of the openings, but also because they are metallic, they are not as easily worn as the rubber squeegee and therefore do not require sharpness. They are much more expensive than rubber squeegees and can cause stencil wear.
Different types of squeegee are used in printed circuit assembly (PCA) using standard components and clinch elements. The amount of solder paste required is very different for each type of component. Fine pitch components require much less solder than standard surface mount components. Pad area and thickness control the amount of solder paste.
Some engineers use double-thickness stencils to apply the proper amount of solder paste to dense-footed components and standard surface mount pads. Other engineers use a different approach - they use more economical metal scrapers that do not need to be sharp. The use of a metal spatula can more easily prevent changes in the amount of solder paste deposited, but this method requires a modified template aperture design to prevent excessive solder paste deposition on the fine pitch pad. This method has become more popular in the industry, but the use of double-thickness printed rubber scrapers has not yet disappeared.
First, the choice of solder paste
Solder Paste is a mixture required by the reflow process, including metal powdered solder, flux, organic material as a carrier, suspended media of various ingredients, and other additives. The basic principles of solder paste selection are: 1 grain size and shape of alloy powder; 2 content of metal powder; 3 type of flux; 4 stability of solder paste.
1. The size of the solder particles is generally -200 mesh/+325 mesh, that is, at least 99% by weight of the powder particles can pass through the 200 (hole/in2) mesh, and less than 20% by weight of the powder particles can pass through the 325 mesh. The size of particles other than this size is not more than 10%. When solder paste is printed on a device with a 0.5mm pitch, the solder particle size should be smaller than conventional, and a solder paste with a particle size of -300 mesh + 500 mesh can be used.
2. The shape of the solder powder depends on the oxide content of the powder and also determines the printability of the solder paste. The spherical solder powder has the smallest total surface area at a given volume, reduces the area of ​​surface oxidation that may occur, and has good consistency, creating conditions for the solder paste to have excellent printing properties. The solder paste viscosity tester can detect the shape of the solder powder from the display.
3. The amount of metal in the solder paste determines the size of the weld. As the percentage of metal increases, the weld size increases. However, at a given viscosity, as the metal content increases, the tendency of the solder to bridge increases. When the thickness of the solder paste is constant, the metal content affects the reflow thickness. Metal content % Thickness (in) Wet solder paste Solder after reflow 90 0.009 0.0045 85 0.009 0.0035 80 0.009 0.0025 75 0.009 0.0020 Solder pins are required to be firmly bonded after reflow. The solder is plump, smooth, and 1/3 to 2/3 of the height of the solder in the direction of the height of the tip of the device. From the above table, it can be seen that as the content of metal decreases, the thickness of solder after reflow is reduced. In order to meet the solder paste amount requirements for solder joints, solder paste of 85%--92% metal content is generally used. Manufacturers generally control the metal content at 89% or 90%, the better the effect.
4. The flux is one of the main components of the solder paste carrier. The existing solder paste fluxes are of three types: R type (rosin flux), RMA type (moderately activated rosin) and RA type (fully activated rosin). Commonly used fluxes containing RMA type consist of rosin and a salt solution called an activator. This flux, activated by salt, is suitable for eliminating minor oxide films and other contamination and increasing wetting between the molten solder and the pad, element tip or hole. It can also be used as a temporary adhesive before reflow, so that the surface mount device is always fixed in place during the placement and transmission without deviation. Some brand placement machines, especially high-speed placement machines, This characteristic is particularly important at the time when the mounting speed is high and both the mounting head and the printed board are moving at a high speed.
5. After the solder paste is printed and reflowed, it must be transported, patched, etc. The solder paste has a dwell time in the air. In addition, the temperature of the working environment sometimes fluctuates, and during the trial production of new product projects, due to problems such as device packaging of the chip mounter, some devices require manual placement, so the solder paste stays in the air for a relatively long time. Longer, this requires better solder paste stability, slower flux loss, and longer operating life.
6. Solder paste is a thixotropic fluid. When a constant shear stress or tensile stress is applied, the viscosity of the solder paste decreases with time. This means that the structure gradually disintegrates and the viscosity of the solder paste acts on it. The shear stress increases and decreases, while its viscosity is also sensitive to temperature, and the viscosity decreases with increasing temperature. The paste used for the stencil printer has a viscosity of 300-700Pa.s. The effect of increasing the shear stress and raising the temperature on the paste viscosity with the same multi-component content and flux carrier. The solder paste manufacturer will configure the solder paste in a fixed recipe. After that, the viscosity of the solder paste was measured. According to the sensitive characteristics of the solder paste, in addition to testing the solder paste before use, on the one hand to measure the stability of the solder paste quality, while ensuring that the degree of paste used in the production and the factory configuration paste viscosity is basically the same, play the best Use effect. At this point, it should be noted that the test equipment and the viscosity unit are the same as those used by the solder paste manufacturer. If they are inconsistent, they must be converted to arrive at the correct conclusion.
Squeegee type
The wear, pressure, and hardness of the squeegee determine the print quality and should be carefully monitored. For acceptable print quality, the squeegee edges should be sharp and straight. Low squeegee pressure causes omissions and rough edges, while high squeegee pressure or a very soft squeegee will cause smeared printing and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop solder paste from wide openings, causing solder fillets to be insufficient. There are two types of scraper that are common: rubber or polyurethane scrapers and metal scrapers. When a rubber squeegee is used, a squeegee having a hardness of 70-90 durometer is used. When excessive pressure is used, solder paste that penetrates into the bottom of the stencil may cause bridging, requiring frequent bottom wipes. In order to prevent bottom penetration, pad openings must provide a gasketing effect when printing. This depends on the roughness of the template opening wall.
Metal scrapers are also commonly used. With the use of more closely spaced components, the amount of metal scrapers is increasing. They are made of stainless steel or brass and have a flat blade shape with a printing angle of 30 to 45°. Some scrapers are coated with a lubricating material. Because of the lower pressure, they do not scoop solder paste out of the openings, but also because they are metallic, they are not as easily worn as the rubber squeegee and therefore do not require sharpness. They are much more expensive than rubber squeegees and can cause stencil wear.
Different types of squeegee are used in printed circuit assembly (PCA) using standard components and clinch elements. The amount of solder paste required is very different for each type of component. Fine pitch components require much less solder than standard surface mount components. Pad area and thickness control the amount of solder paste.
Some engineers use double-thickness stencils to apply the proper amount of solder paste to dense-footed components and standard surface mount pads. Other engineers use a different approach - they use more economical metal scrapers that do not need to be sharp. The use of a metal spatula can more easily prevent changes in the amount of solder paste deposited, but this method requires a modified template aperture design to prevent excessive solder paste deposition on the fine pitch pad. This method has become more popular in the industry, but the use of double-thickness printed rubber scrapers has not yet disappeared.
Hospital Bed Series,Nursing Bed,Pediatric Bed,Hospital Folding Bed
Hebei Boxin Rehabilitation Equipment Co., Ltd , https://www.cnsmartcare.com