Solder paste printing is the most complex and unstable process in the SMT process. It is affected by multiple factors and changes dynamically. Even if the solder paste, printer, and printing parameters are unchanged, the printing quality may vary greatly. . Therefore, it is more and more common to set up inspection stations after printing to check the quality of solder paste printing in real time, especially for solder paste printing on circuit boards containing 0201 components and CSPs with a fine pitch of 0.5mm and below, and even some printing machines. Solder paste printing inspection systems such as AOI have been integrated.
3.2.1 Analysis of solder paste printing quality
As mentioned above, the solder paste printing process is very complicated. The “fishbone diagram” analysis of its printing quality is shown in Figure 3.2, which mainly includes solder paste, PCB, printer, squeegee, printing environment, operator, printing parameters, and templates. Etc., each major item is divided into a number of specific sub-items.
Printing parameters
1. The influence of solder paste quality
The viscosity, metal content, melting point, soldering temperature, and powder size of the solder paste are the main parameters that determine the performance of the solder paste. Generally speaking, different coating methods choose solder pastes with different viscosities. The viscosity for liquid dispensers is 100Pa·s~200Pa·s, 100Pa·s~300Pa·s for screen printing, and 200Pa·s~600Pa·s for stencil printing. Generally, a rotary viscometer is used to measure the viscosity of solder paste. In the process of viscosity measurement, attention must be paid to the temperature, rotation speed and measurement procedure during the measurement to make them consistent with the regulations. Generally speaking, as the temperature and rotation speed increase, the viscosity of the solder paste will decrease; in addition, the viscosity is quite sensitive to the operating procedures. The main factors affecting the viscosity of the solder paste are the metal content of the solder paste and the size of the solder ball.
The metal composition of solder paste generally includes Sn, Pb, Au, Ag, In, Bi, Ni, etc. At present, the main components of solder paste are Sn and Pb, and gradually transition to lead-free mainly Sn, Ag, and Cu. Among them, adding 2% Ag to the traditional Sn-Pb solder can effectively prevent corrosion and improve the creep resistance of the solder. Adding a small amount of In and Cd to the Sn solder can effectively prevent the co-chip-like microstructure, thereby effectively improving the shear fatigue life. With the strengthening of people’s environmental awareness, Pb will gradually decrease until it disappears in the solder, and the new solder will replace it. However, it is worth noting that although elements like Ag, Au, In, etc. can improve the welding quality to a certain extent, if the addition amount is not controlled properly, it will bring new welding defects. The weight percentage of the alloy is about 85%~90%, and the maximum metal content of the solder paste is 92.5%. Otherwise, the viscosity of the solder paste will hardly be affected when the viscosity of the flux changes. In addition, the viscosity of the entire solder paste will be very large. It is not conducive to the printing of solder paste. Too low metal content will relatively increase the flux content, and the increase of flux will inevitably lead to insufficient solder joints during soldering, resulting in less soldering and open soldering. And, as the flux increases, more voids will appear during soldering. The strength of the solder joint connection will be greatly reduced; at the same time, more bubbles will appear during soldering, which will easily cause solder balls and tin beads. In addition, the decrease in metal content will inevitably lead to a decrease in viscosity, which is prone to collapse and increase the occurrence of bridging.
Another important factor affecting the viscosity of solder paste is the shape and size of the solder powder. Ideally, the solder powder is spherical, and its powder diameter should not be larger than the opening size of the template and not more than 1/10 of the diameter of the nozzle of the dispenser. According to the size of the solder, the solder paste is divided into 6 types (TYPE1~TYPE6) according to the mesh size defined by the ASTM standard. The solder powder of the actual solder paste is not completely spherical, the particles are too small, satellite-shaped, flat, and there are The surface of the corner, the surface of the package, the irregular grain shape, etc. are all unsatisfactory, and they will more or less affect the final welding quality. The use of spherical solder powder mainly takes into account the oxidation of solder paste during production, transportation, and use. Compared with irregularly shaped solder powder, spherical powder effectively reduces the contact area between particles and the outside, which can effectively prevent Or reduce the oxidation of the solder paste, improve the wetting performance during soldering, and at the same time facilitate the rolling effect in the solder paste printing process to fill the template opening. As the size of the powder decreases, the viscosity of the solder paste increases significantly. This is because more particles contact the flux, thereby forming a greater bonding force between the powder and the solder, and correspondingly increasing the viscosity of the solder paste. For fine-pitch pin components, the use of small-size powder solder paste will greatly improve the printing performance, reduce the occurrence of insufficient solder and blockage of the template opening, thereby effectively improving the quality of soldering. In general, the metal content and powder size greatly affect the viscosity of the solder paste, and the appropriate solder paste should be selected according to the specific assembly requirements.
2. Printing parameters
The printing process of solder paste is the decisive factor of quality. The printing process involves various printing process parameters and settings, such as squeegee speed, pressure, angle, hardness and material, and demolding speed. The speed of the squeegee determines the time for the solder paste to fill the opening of the template. A long time indicates that the solder paste will have ample time to fill the opening. The general speed is 10mm/s-25mm/s. The rolling of the solder ball must be maintained during the printing process. Too fast is not conducive to the rolling of the solder balls and is easy to cause omissions, while too slow will cause solder paste to overflow. Theoretically, the squeegee speed should be proportional to the pad spacing and inversely proportional to the thickness of the template and the viscosity of the solder paste.
Too small a squeegee pressure can not make the solder paste completely fill the opening and may also cause too much printed solder paste. Too large will cause the end of the squeegee to dig into the stencil opening and make the printed volume of solder paste too thin. Too much squeegee pressure will also cause the solder paste to be printed. Squeeze to the reverse side of the template and produce poor printing. In addition, for PCBs with weak rigidity, in order to prevent the PCB from warping under pressure, the method of supporting the ejector pin (needle) is often used to improve the printing quality. The angle of the squeegee should be paid attention to, one is the angle made when the squeegee itself is manufactured, and the other is the pushing angle during the printing process (the angle between the squeegee and the PCB when the squeegee is in contact with the PCB). The angle of the squeegee generally includes 45\55\60°, etc. The pushing angle is related to the angle of the squeegee, the pressure, the hardness of the squeegee, and the printing speed. In comparison, the pushing angle is generally larger, which can prevent the solder paste from sticking to the squeegee and cause the defect of insufficient solder paste printing.
The hardness and material of the squeegee is a very important factor. The squeegee is generally divided into a rubber squeegee and a metal squeegee. For rubber squeegees with relatively low hardness, when the squeegee is in contact with the template during the solder paste printing process, due to the squeegee The presence of pressure at the opening of the template causes the end of the scraper to deform and enter the opening to form a “digging” phenomenon. The deformation of the metal squeegee is very small and has almost no effect on the printed solder paste. Since the metal scraper is harder than rubber, it is not easy to deform during the printing process. It can effectively reduce the amount of scraper entering the opening of the template, thereby greatly reducing the occurrence of “digging”. For rubber scrapers, it is easy to wear the scraper. , Solder paste residue is easy to appear on the template. The hardness of the rubber squeegee is generally (55~95) Hs, and the commonly used squeegee has a hardness of 90Hso. The higher hardness (greater than 9OHs) squeegee is mainly used in the welding of fine-pitch components. Paste printing. The alloy squeegee using elastic alloy as the base and hard metal as the blade combines the advantages of both a metal squeegee and a rubber squeegee. It has the hardness of a metal squeegee and the flexibility of a rubber squeegee, and can print solder paste with a flat surface.
The demolding speed is a factor that affects the shape of the printed solder paste, and the demolding speed is generally O.hnm/s~0.2mm/s. Theoretically speaking, the demolding speed should not be exactly the same throughout the demolding process. It should be a slow-fast-slow process (as shown in Figure 3.3). The demolding speed should be at the beginning and the end of the demolding process. Both should be slower. At the beginning, because the template is in close contact with the PCB, if the demolding is too fast, the solder paste may collapse due to excessive atmospheric pressure. At the same time, slow demolding can effectively reduce the rebound of the template and the PCB; the template is about to separate Accelerating the demolding of solder paste can effectively maintain the integrity of the printed solder paste shape and effectively reduce the occurrence of solder paste sticking and tailing. At present, advanced printing machines can set the demolding speed to be accelerated, and the effect is very good. In addition, the demolding speed is related to the lead spacing. The smaller the spacing, the lower the demolding speed.
(A) Fast demolding position; (b) Slow speed to prevent collapse; (c) Slow speed to prevent rebound; (d) Slow speed to prevent tailing.
The thickness of the solder paste is also controlled by the thickness of the template. In addition, the thickness of the solder paste is also related to the lead spacing. The greater the spacing, the greater the thickness of the print. For example, for pins with a pitch of 0.3mm, the thickness of the template is generally 0.1mm, while the thickness of the solder paste is 0.09mm~0.1mm. For device pins with a pitch of 0.5mm and above, the thickness of the template is It is 0.12mm, 0.15mm, and the printing thickness is 0.11mm~0.15mm. The upper surface of the printing template should have a certain degree of roughness to ensure that the solder balls roll rather than move during the printing process. The trapezoidal shape of the opening can reduce the defect rate to a certain extent. The inner wall of the opening needs good surface quality, and if necessary, an electro-polishing process is adopted. According to the IPC7525 standard, in order to ensure good model release, the key control parameter is the area ratio and width to thickness ratio of the template opening. For traditional tin-lead solder paste, the template opening size requires the width to thickness ratio to be greater than 1.5, and the area ratio to be greater than 0.66° due to lead-free The solder paste filling and demolding ability is relatively weak, and the requirements for the width to thickness ratio and area ratio of the template opening are relatively high. It is recommended that the template opening width to thickness ratio is greater than 1.6 and the area ratio is greater than 0.71.