The hottest solder paste printing technology and t

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Solder paste printing technology and the impact of lead-free on it

1 Introduction

surface mount technology (SMT) has become the most common technology in today's electronic assembly technology, and solder paste printing is one of the key processes in the basic process of SMT, and its quality directly affects the quality and efficiency of SMT assembly. With the high density of electronic assembly and the miniaturization of components, fine pitch pins and lead-free share with you. Some strange phantom processes put forward higher requirements for solder paste printing. In order to ensure the product quality and formulate a reasonable printing process, it is necessary to discuss the solder paste printing technology and the setting of process parameters

2 solder paste printing technology

in SMT production, there are two kinds of technologies for solder paste deposition on PCB pads: one is the printing technology based on silk and metal stencil, which is a widely used solder paste coating method in industrial manufacturing and is suitable for mass production. Another kind of injection spot coating technology for coating solder paste for injection system is suitable for small batch production. This system is controlled by computer and can accurately deposit solder paste, which can well prevent solder paste waste

2.1 wire printing technology

wire printing mechanism is composed of PCB positioning system, scraper system and board. Board is the key component of silk printing machine, which is composed of frame, silk and mask graphics. Generally, the mask pattern is made on the wire in a proper way, and the wire is tightened on the frame

general frame materials include wood, aluminum alloy, stainless steel, etc. On the premise of meeting the strength requirements, light alloy should be selected as far as possible for convenient operation. According to the stretching method, there are two types of frames: fixed frame and self stretching frame. Fixing the frame is to fix the wire to the frame, which is usually fixed with adhesive; The self stretching frame stretches the silk directly onto the frame with the help of the stretching machine, so that the silk stretching position and the frame form a whole. With the help of "screw adjustment" or "stick frame" self stretching, it is suitable for occasions where cash inflow varieties and small batch printing benefit from business activities to a large extent

the commonly used etching mesh/Latex template for silk printing, and the commonly used material for silk is stainless steel or single fiber polymer resin. Usually, when used, a layer of photosensitive emulsion is coated on the silk to dry it into a photosensitive film. Then, the negative film is closely attached to the photosensitive film and exposed to ultraviolet light. The exposed part becomes a permanent coating, and the unexposed part is dissolved with developer. In this way, holes are formed in the adhesive part of the solder paste to be deposited, and wires are formed after drying. The diameter and opening size of the wire depend on the mesh number, which is usually 80. The diameter of the wire and the thickness of the emulsion mainly determine the thickness of the deposited solder paste. Generally speaking, the wire is only applicable to the solder joint height of 300 μ For occasions above M, the viscosity of solder paste is pa · s, and the scraper uses rubber or polyimide tree scraper with hardness of 70 ~ 90. The average size of alloy powder particles in solder paste should not be greater than 1/5 of the size of wire hole

wire printing is non-contact printing, which is prone to solder paste leakage defects (as shown in Figure 2). Generally, the operation speed of silk printing is slower than that of template printing, and the scraping gap is larger. At the same time, in order to facilitate printing, solder paste with lower viscosity should be used. In addition, when printing, adjust the wire to be parallel to the work frame and maintain a scraping gap of 0.5 mm

2.2 template printing technology

template missing printing is a direct printing technology, which uses metal templates to replace the silk in the silk printing machine. The so-called template is to carve a missing print on a piece of metal by chemical means or laser and other methods, unlike the silk opening to prevent the flow of solder paste, so the template can make 100% of celebrities make false advertisements and the solder paste that will be fined pass, while the silk can only make about 50% of the solder paste pass. According to the etching materials, templates can be divided into flexible metal templates and all metal templates

fine spacing solder paste printing usually adopts metal leakage printing, so the leakage plate opening controls the solder paste on the pad and the quality of leakage printing. The thickness of the template determines the thickness of the printing solder paste, and the size of the template hole determines the area and shape of the solder paste pattern. The thicker template is not conducive to the release of solder paste, and it is easy to bridge because the solder paste is too thick. Insufficient solder paste in the too thin formwork area affects the welding quality

generally speaking, the template is only applicable to the welding spot height of 100 ~ 300 μ For occasions within m, the viscosity of solder paste is 400 ~ 1200 PA · s (the spacing is 600 ~ 800 PA · s, and the fine spacing is 800 ~ 1200 PA · s). The average size of alloy powder particles should not be greater than 1/3 of the thickness and width of the template. In order to avoid the deformation and wear of the front end of the scraper, it is best to use a harder material, such as a metal scraper or a rubber or polyimide resin scraper with a hardness of 90

the aperture ratio of formwork refers to the ratio of width to thickness, i.e. w/T, which is generally 1.5:1. For CSP and other graphics, the area ratio (AR) must be used, as shown in Figure 3. Its value is generally greater than 0.66, and the solder paste release rate can reach more than 85%. The perfect solder paste deposition should have exactly the same shape as the template opening, that is, a=d, h=t or 2/3a. Where a is the diameter of the solder paste, D is the diameter of the opening, t is the thickness of the template, and H is the thickness of the solder paste. Figure 4 shows the relationship between the release rate of solder paste and the area ratio. In order to improve the release rate of solder paste, you can increase the aperture ratio by increasing the opening width or reducing the thickness, or you can choose the template technology with smooth opening wall

although template printing is more complex and expensive than silk printing, it has many advantages, such as insensitive to solder paste particle size, not easy to block, wide range of solder paste viscosity, uniform printing, clear graphics, relatively stable, long-term storage, etc., and it is very durable. Its service life is about 25 times that of silk, so it is suitable for mass production and assembly of products with high density and multi lead fine spacing

3 template printing process

taking template printing as an example, the solder paste printing process is shown in Figure 5, which is divided into two parts: pressing the solder paste into the opening of the printing template and moving the solder paste to the substrate pad. The setting and adjustment of printing process parameters plays a very important role in printing quality. The following details each part

3.1 press the solder paste into the opening part of the printing template

Figure 6 shows the process of pressing the solder paste into the opening part of the printing template. At this time, the rotation of the solder paste plays a great role. When moving the solder paste through the scraper, there is friction between the solder paste and the printing template surface, which is opposite to the moving direction of the solder paste. Under the action of this friction, the solder paste will rotate, that is, roll. Once rolling occurs, the solder paste will often collide with the front of the scraper, change the direction, and generate pressure in the front of the scraper. This pressure is the force that presses the solder paste into the printing template. At the same time, through rolling, it also plays a powerful role in the direction of lifting the scraper. Therefore, in order to achieve correct printing, the force of pressing the solder paste into the opening of the printing template and the force of lifting the scraper must be properly controlled

(1) the best setting of scraper speed and scraper angle

scraper speed and scraper angle are the two basic factors to control the pressing force (as shown in Figure 7). The best setting of the scraper speed is to set the solder paste so that it will not slide on the printing template, but roll and move. The scraper speed can vary in the range of 10 ~ 150 mm/s, generally between 25 ~ 50 mm/s, the OFP with spacing less than 0.5 mm is 20 ~ 30 mm/s, and the ultra-fine spacing is 10 ~ 20mm/s, generally 12.7 mm/s. It is worth noting that when the rubber scraper performs large spacing printing or large pressure printing, the deformation will form scraping pits, resulting in insufficient printing volume. Therefore, the printing speed of the rubber scraper is higher than that of the metal scraper, which is generally close to twice that of the metal scraper

if the speed of the scraper is fast, the speed at which the solder paste collides with the front of the scraper is relatively fast, and the generated force is large. Considering that the time for the scraper to pass through the opening part at this time, that is, the time for pressing the solder paste, is relatively short, the final result is that the pressure applied to the whole opening part during printing remains unchanged, that is, the number of solder paste pressed into the opening part does not change. Generally, the printing speed is low and the filling property is good, so the drag behind the scraper as shown in Figure 8 will not occur

the pressure distribution at the front of the scraper obtained by the computer is shown in Figure 9. The pressure produced by the front of the scraper is distributed in a very narrow range of only 2 ~ 3 mm from the front of the scraper, so it can be said that the impact of the scraper angle and its change on the pressure produced is only on the angle of the front of this part

the angle of the scraper is generally controlled at 45 ° ~ 75 °. Let the transfer coefficient = transfer depth/limit pressure to express the relationship between the angle of the scraper and the limit printing pressure. The test results show that the best setting of the scraper is in the range of 60 ℃ ~ 70 ℃, and the best printing effect and transferability can be obtained by matching the appropriate printing pressure and speed. The angle of the scraper is too small, with excellent rolling and filling performance, it can be widely used in many fields, such as new energy vehicles, aerospace, etc., but it is prone to leakage. It is generally used for through-hole reflow welding and fine spacing template printing to increase the amount of solder paste coating

(2) the best setting of printing pressure

the force applied to the scraper is called printing pressure. If this force is too large, the front of the scraper will deform and affect the angle of the scraper, which plays an important role in the pressing force. Figure 1o shows the phenomenon that the front of the scraper is lifted during the rolling of the solder paste. If the front of the scraper is lifted during the rolling of solder paste, there will be a gap between the front of the scraper and the printing template, and solder paste will remain on the printing template. The printing pressure should generally be the same as the pressure generated by rolling, which can generally be set within the range of 5 ~ 100 MPa. If it is too large, collapse and leakage defects will occur, usually 10 ~ 30 MPa. In addition, since the force generated by rolling changes with the amount of solder paste supplied, the operator needs to adjust the optimal value appropriately

3.2 process of moving solder paste to the substrate pad

in order to move solder paste from the opening part of the printing template to the pad of the printed circuit board, it is necessary to fix the printing template and press the printed circuit board down in the vertical direction. At this time, the force generated has the adhesion between the substrate pad and the solder paste, and the friction between the wall of the printing template and the solder paste when the solder paste filled in the opening part moves. When the friction force is greater than the adhesion force between the substrate pad and the solder paste, the solder paste does not move in the opening part and cannot be printed. On the contrary, when the friction is small, the solder paste moves smoothly. It can be seen that the design of the opening part of the printing template and the setting of the demoulding speed are very important in this process

(1) the best design of the template

the best design requirements of the template are obtained from the printing mechanism: ① the opening area of the template is larger than the inner wall area of the opening, so that the adhesion of the welding joint surface is greater than the friction force; ② The wall surface of the opening part should be as smooth as possible, and at the same time, considering the detachability, it should be designed into an "eight" shape (in the case of laser template making, if it is processed from the substrate side, it will naturally become this shape)

(2) optimal setting of demoulding speed

when the substrate drops, the printing template becomes larger and flexes due to the adhesion of solder paste. If the deflection of the printing template becomes larger, the template will return to its original position due to the elastic force of the deflection. The result is that at a certain position, the template is quickly reset due to its elasticity, and the surrounding of the solder paste is lifted, forming an extremely raised printing shape at both ends. The lifting height is proportional to the deflection of the template. In serious cases, the solder paste will be scraped off, leaving the solder paste in the opening. Figure 11 shows the relationship between demoulding speed and printing quality. Generally, the demoulding speed is set to 0.3 ~ 3 mm/s. During printing, the gap between the template and PCB is less than 0.5 mm, and the demoulding distance is generally 3 mm. See Table 1 for the specific disengagement speed, while the actual demoulding speed

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