Most surface mount adhesives (SMA) used in PCB assembly are epoxy resins, but there are also acrylic resins for special purposes. After the introduction of high-speed glue dropping system and the electronics industry mastering how to deal with products with relatively short shelf life, epoxy resin has become the more mainstream glue technology in the world. Epoxy resin usually provides good adhesion to a variety of circuit boards and has very good electrical properties. The formula of epoxy resin patch adhesive with required characteristics provides users with many benefits, including: good dripping performance, continuous and consistent glue point contour and size, high wet strength and curing strength, rapid curing, flexibility and temperature shock resistance. Epoxy resin allows high speed of very small glue spots, provides good electrical characteristics of on-board curing, and does not drag or collapse during the heating curing cycle. (Because epoxy resin is heat sensitive, it must be stored in cold storage to ensure the maximum shelf life. ) Using visual inspection or automatic equipment, SMA must be compared with typical green or brown circuit boards. Because of the use of automatic visual control system to help the inspection process, red and yellow have become two basic glue colors. However, the ideal color depends on the visual contrast between cardboard and glue. Typically, the thermal curing of epoxy resin is carried out online in an infrared (IR) channel furnace. The lowest curing temperature is100 C. However, in fact, the curing temperature ranges from 1 10 to160 C..160 C. The curing process will be accelerated, but the glue point will be brittle. Adhesion strength is the key to the performance of adhesive, which is determined by many factors, such as the adhesion to components and PCB, the shape and size of glue points, and the degree of curing. The three most common reasons for insufficient adhesive strength are insufficient curing, insufficient adhesive content and poor adhesive force. The flow characteristics or rheological properties of the glue point profile glue affect the formation, shape and size of epoxy resin glue points. SMA allows the glue to drip quickly and controllably to form a clear glue spot (Figure 1). In order to ensure a good and stable glue point profile, the glue is cleverly designed to be soluble (that is, it becomes thinner when stirring and thicker when standing). In this process, SMA is subjected to shear force in the process of glue dripping, and its viscosity decreases and it is easy to flow. When the glue touches the PCB surface, it will quickly recombine and restore its original viscosity. The gel point distribution is also affected by the recovery rate of shaking solubility, viscosity at zero shear rate and other factors. The actual glue dot shape can be "pointed"/conical or hemispherical. However, the outline of the glue dot is defined by non-viscous parameters, such as the volume of the glue dot, the diameter of the glue dropping needle and the height from the plate. That is to say, for a given glue grade, it is possible to produce a very high narrow glue point or a low wide glue point by adjusting their parameters. After mounting, the dripped glue points have two requirements: the diameter must be smaller than the gap between pads, and there is enough height to connect the gap between PCB surface and component body without disturbing the mounting head. The gap of glue is determined by the height of the pad above the solder mask of PCB and the thickness difference between the terminal metal and the component body. This gap may vary, ranging from 0.05 mm for flat components to 0.3 mm for SOP (Small Package) and QFP. The glue point with high drop ensures the coverage area of good glue on components with high height from the ground. The high glue point also allows glue to be squeezed out between components with low floor height without worrying about contaminating the pads. Usually, for the same level of glue, two sets of glue dropping parameters are used together: one set is to generate high and large glue spots for components with higher height from the ground; The other provides glue spots with medium height and glue amount for MELF and MELF. The size of the glue point is also controlled by the ratio of the inner diameter of the needle mouth to the height from the ground. Generally, the ratio of the width to the height of the glue point is1.5:1~ 5:1(h/w = 0.2 ~ 0.6), which depends on the parameters of the glue dropping system and the grade of the glue. By adjusting the machine settings, these ratios can be optimized for any component. Avoid the possibility that the water in the cavity glue point may boil during the curing process, resulting in a cavity, weakening the glue point, and opening a path for solder to penetrate under the component, which may cause a short circuit due to tin bridge. There is almost no water in the glue in the syringe, but if the glue is in an uncured state and exposed to indoor conditions, especially in a humid environment, the glue may absorb water. For example, using the needle transfer method to drop glue, because the glue is open and the exposed area is large, moisture is a problem. This problem may also occur when using a syringe to drip glue, if the dripping and curing time is long, or if the indoor conditions are very humid. In view of this, most surface patch adhesives are prepared from raw materials with low hygroscopicity to minimize their influence. Slow curing at low temperature and long heating time can help moisture escape before curing and solve the problem of cavity formation. Similarly, moisture can be eliminated by storing the components in a low-temperature dry place or pretreating the materials in a drying oven at a suitable temperature. Avoiding the process pause before curing and using special adhesive with low hygroscopicity are helpful to reduce the cavity problem. Dispensing SMA can be applied to PCB by syringe dispensing, needle transfer or template printing. Less than 65,438+00% of all applications use the needle transfer method. It uses an array of needles to immerse in a plastic tray. Then the suspended droplets are transferred to the plate as a whole. These systems require glue to have low viscosity and good hygroscopicity resistance because it is exposed to indoor environment. The key factors to control the transfer of glue to the needle include the diameter and style of the needle, the temperature of the glue, the immersion depth of the needle and the period length of glue dripping (including the delay time before and during the contact between the needle and PCB). The tank temperature should be between 25℃ and 30℃, and the viscosity of glue and the number and form of glue spots should be controlled. Template printing is widely used in solder paste and dispensing. Although less than 2% of shape memory alloys are printed by stencil at present, interest in this method has increased, and new equipment is overcoming some early limitations. Correct template parameters are the key to get good results. For example, contact printing (zero off-board height) may require a delay time in order to form a good glue spot. In addition, the non-contact printing of polymer template (about 1mm gap) requires the best scraper speed and pressure. The thickness of the metal template is generally 0. 15~2.00mm, which should be slightly larger than (+0.05mm) the gap between the component and the PCB. At present, more than 90% of SMT glue is dripped by syringes (Figure 2), and syringes can be further divided into two categories: pressure time system and volume control system. Pressure time syringe drip is the most common method, and the rest of this section will talk about this technology. The syringe can reach the dropping speed of 50,000 points per hour and can be adjusted to meet the ever-changing production requirements. Fault analysis of dripping defects There are several unresolved dripping problems that may lead to the final process defects. These include pull wires, discontinuous glue spots, glue-free spots and satellite glue spots. Bonding cables will lead to pad contamination and poor welding points. When the nozzle is retracted, the glue must break quickly and clearly (Figure 3). Even those glues specially prepared for high-speed dripping may be broken if the parameters are incorrect. For example, when the amount of glue is equal to the diameter of the drip hose, and the required height from the ground is too small, the danger of pulling the wire is extremely high, resulting in a very high and thin glue point. Although the combination of smaller needle nozzle diameter and ground height can solve this problem, the pulling wire may still be caused by other parameters unrelated to the glue itself, such as electrostatic discharge to the circuit board, incorrect Z-stroke adjustment height, flexibility of the circuit board or insufficient supporting force of the circuit board. Without the bonding point, the component will not be installed correctly. If the air pressure of the production line is not enough to drip the glue (that is, the pressure of the injection machine is not enough to cause the glue to drip discontinuously), the glue dripping point may not appear. Typologically, the size of discontinuous glue spots will affect the overall bonding strength between circuit board and components. There are several reasons for this phenomenon: the needle mouth leaves the ground and the pillar falls on the pad. This problem can be solved by changing the needle mouth at different off-ground support positions. The time allocated for glue recycling is not enough. Increasing the delay can solve the recovery problem. If the pressure time is not enough to complete the glue dropping cycle (or decreases with the horizontal line of the glue surface), increasing the ratio of pressure to cycle time, usually expressed as a percentage of the maximum value, will correct the problem of discontinuous glue dot size. Because of the irregularity of satellite points, it may cause pad pollution or insufficient bonding strength. When the needle mouth is too high from the ground, reducing the height can eliminate the satellite point. If the amount of glue is too large, reducing the pressure or using a needle with a larger inner diameter can solve the problem. Factors affecting the drying property of glue. Good dripping depends not only on the quality of glue. For the method of glue dripping by pressure time injection machine, many factors related to the machine affect the fluidity of glue dripping and the formation of glue spots. The inner diameter of the needle mouth is the key to the formation of glue spots, which must be much smaller than the diameter of glue spots on the board. In principle, the ratio should be 2: 1. 0.7~0.9mm glue point requires 0.4mm inner diameter; 0.5 ~ 0.6 mm glue point needs 0.3 mm inner diameter. Equipment manufacturers usually provide technical specifications and operating instructions to produce the required glue spot size and shape. The distance between PCB and needle mouth, or the height of plug, controls the height of glue point (Figure 4). Must be suitable for the amount of glue dripping and needle mouth ID. For a given amount of glue, the ratio of glue point height to width will increase with the height of the plug. Usually, the maximum stop height is half of the inner diameter of the needle mouth; Beyond this point, there will be discontinuous glue dripping and glue pulling. Today's high-speed equipment uses a glue dripping cycle, and the pressure can start regularly before the needle nozzle is in place. The exit speed, exit height of the needle nozzle and the time delay between glue dripping and needle nozzle exit will all affect the shape of the glue point and the pull wire. Finally, the temperature will affect the viscosity and the shape of the glue point. Modern glue droppers mostly rely on the temperature control device on the needle mouth or in the chamber to keep the temperature of glue higher than room temperature. However, the outline of the glue point may be damaged. If the temperature of PCB is higher than that of the previous process, keeping the needle mouth and plug bent or worn may have a crucial impact on glue dripping. Too much glue around the needle mouth may affect the smooth and continuous formation of glue points. In extreme cases, the glue may bridge over the stop pin, thus interrupting the dripping of the glue. The general solution is to keep the periphery of the needle mouth as clean as possible. The cleanliness of the inner surface of the needle nozzle is another common source of glue dripping problems. Glue may accumulate on the ID, thus restricting the flow. If left in a warm environment or an incompatible solvent for a long time, the glue may also partially solidify in the needle mouth. Changing the grade of glue may cause side pollution and needle mouth blockage. Before cleaning with solvent, a drill should be used to remove the blockage caused by solidified or semi-solidified glue. ) The nozzle of the glue dropping needle should be checked regularly, but it should be cleaned only when the glue dropping problem becomes obvious. When an empty rubber nozzle is installed in the syringe, cleaning will increase the problems encountered. Soaking the hidden needle tip in a solvent is a common but inefficient cleaning method. When soaking the needle tip, use a compatible solvent, but don't just rely on soaking to remove all uncured materials. The high-pressure injection of compatible solvent can blow the glue out of the inner hole of the needle nozzle. Then dry compressed air is blown into the inner hole to dry the needle mouth. Another cleaning method includes ultrasonic or static soaking. The uncured glue should be removed mechanically by using a passivation tool and a drill needle or a piano wire with a diameter suitable for the inner hole of the needle mouth. Soak the parts to be cleaned in clean solvent. Soak in ultrasound and set&; 40 degrees; Drive at maximum power for three minutes. For static immersion, stir the immersed parts until the solvent is contaminated by the adhesive. Wash parts in clean solvent to ensure cleanliness. The needle nozzle with small inner hole is treated by high-pressure spray, and the parts are dried by blowing dry compressed air through the inner hole.