Chip adhesive SMA and glue dropping process for th

Patch adhesive (SMA) and glue dropping process for PCB assembly

most surface patch adhesives (SMA) used in PCB assembly are epoxy resins, although polypropylene (acrylics) is also used for special purposes. After the introduction of high-speed glue dropping system and the mastery of how to deal with products with relatively short shelf life by the electronic industry, epoxy resin has become a more mainstream glue technology in the world. Epoxy resin generally provides good adhesion to a wide range of circuit boards and has very good electrical properties

desired characteristics

the formulation of epoxy resin patch adhesive provides users with many benefits, including: good dripping adhesive performance, continuous and consistent glue dot contour and size, high wet strength and curing strength, rapid curing, flexibility and temperature shock resistance. Epoxy resin allows the high speed of very small glue points, provides good electrical characteristics of curing on the board, and does not drag lines and collapse during the heating and curing cycle. (since epoxy resin is heat sensitive, it must be stored under cold storage conditions to ensure the maximum shelf life.)

when using visual inspection or automatic equipment, SMA must be compared with typical green or brown circuit boards. Due to the use of automatic visual control system to help the inspection process, red and yellow have become two basic colors of glue. However, the ideal color depends on the visual comparison between board and glue

typically, the heating curing of epoxy resin takes place in-line, in an infrared (IR) channel furnace. The minimum temperature for starting curing is 100 ℃, but in fact, the curing temperature range is 110~160 ℃. The temperature above 160 ℃ will speed up the curing process, but it is easy to cause weak glue spots

bonding strength is the key to the performance of adhesive, which depends on many factors, such as the adhesion to components and PCB, the shape and size of adhesive dots, and the curing level. The three most common reasons for insufficient bonding strength are insufficient curing, insufficient amount of adhesive and poor adhesion

glue dot profile

the flow characteristics or rheology of the glue affect the formation of epoxy resin glue dots and its shape and size. SMA allows rapid and controlled glue dripping to form a shaped glue dot. In order to ensure a good and stable glue dot profile, the glue is cleverly designed to be shake soluble (that is, it becomes thinner when stirred and thicker when stationary). In this process, the viscosity of SMA decreases when subjected to shear force during glue dropping, allowing easy flow. When the adhesive hits the PCB surface, it quickly restructures and restores its original viscosity

the profile of glue spots is also affected by the recovery rate of shake solubility, viscosity at zero shear rate and other factors. The actual glue dot shape may be "pointed"/conical or hemispherical. However, the glue dot profile is defined by non viscous parameters such as glue dot volume, glue dropping needle diameter and off board height. That is, for a given glue grade, by adjusting their parameters, it is possible to produce either very high narrow glue spots or low wide glue spots

there are two requirements for the dripping glue spots after the patch: their diameter must be less than the space between the pads, and they must be high enough to connect the gap between the PCB surface and the component body without interfering with the patch head. The glue gap is determined by the height of the pad above the PCB solder mask and the thickness difference between the end metal and the component body. The gap may be different. The small gap may be smaller than 0.05mm of flat sheet components, and the large gap may be larger than 0.3mm of SOP (small outline package) and QFP

the high drop of glue ensures the coverage area of good glue on the components with high ground clearance. High glue spots also allow glue to be extruded between components with low ground clearance without worrying about polluting the pad. Usually, for the same grade of glue, there are two sets of glue dropping parameters used together: one is to produce high and large amount of glue spots for components with high ground clearance; The other is to provide glue dots with medium height and glue amount for flat sheet components and metal electrode face (MELF) components

the size of the glue dot is also controlled by the ratio of the inner diameter of the selected needle nozzle to the height above the ground. Generally, the ratio of glue dot width to height ranges from 1.5:1 to 5:1 (h/w=0.2 to 0.6), depending on the parameters of the glue dropping system and the grade of glue. These ratios can be adjusted by adjusting the machine settings; Tang chuanrudder, general manager of Tongling Guochuan Electronic Material Technology Co., Ltd., said he component optimization

avoid voids

the moisture in the glue point may boil during curing, causing voids, weakening the glue contact, and opening a path for solder to penetrate under the components, which may cause a short circuit due to the tin bridge. In the syringe, there is little moisture in the glue, but the glue may absorb moisture when it is left in the non curing state and exposed to indoor conditions, especially in a humid environment. For example, when using needle transfer method to drop glue, moisture is a problem, because the glue is open and the exposed area is large. This problem may also occur when using a syringe to drip glue. If the dripping and curing stay for a long time, or the indoor conditions are very humid. In view of these, most surface patch adhesives are prepared with raw materials with low moisture absorption to minimize their impact

low temperature slow curing and long heating time can help the moisture run out before curing. It can be detected by tensile test, which can solve the problem of cavity formation. Similarly, moisture can be eliminated by storing components in a low-temperature dry place or by pre-treatment of materials in a drying oven at an appropriate temperature. Avoiding the process pause before curing and using a special adhesive with low moisture absorption can help reduce the problem of voids

glue dropping method

sma can be applied to PCB by syringe glue dropping method, needle transfer method or template printing method. The use of needle transfer method is less than 10% of all applications. It uses needle array to dip in the tray of glue. Then the suspended rubber drops are transferred to the board as a whole. These systems require a low viscosity adhesive and good resistance to moisture absorption because it is exposed to the indoor environment. The key factors to control the glue dripping of needle transfer include the diameter and style of the needle, the temperature of the glue, the depth of the needle immersion and the length of the glue dripping cycle (including the delay time before and during the needle contact PCB). The tank temperature should be between 25~30 ℃, which controls the viscosity of the glue and the number and form of glue spots

template printing is widely used in solder paste, and can also be used to distribute adhesives. Although currently less than 2% of SMA is printed by template, interest in this method has increased, and new equipment is overcoming some of the earlier limitations. Correct template parameters are the key to achieve good results. For example, contact printing (zero off board height) may require a delay period to allow good glue dot formation. In addition, the non-contact printing of polymer template (about 1mm gap) requires the best scraper speed and pressure. The thickness of metal formwork is generally 0.15~2.00mm, which should be slightly greater than the gap between (+0.05mm) components and PCB

more than 90% of SMT glue is currently dripping through syringes (Figure 2), which can be further divided into two categories: pressure time system and volume control system. Pressure time syringe dripping is the most common method, and the rest of this section will cover this technology. The syringe can reach the glue dropping speed of 50000 points per hour, and can be adjusted to meet the changing production requirements

failure analysis of glue dripping defects

there are several unsolved glue dripping problems that may lead to the final process defects. These include cable, discontinuity of glue point size, no glue point and satellite glue point. The stay wire of glue can cause pad pollution and poor welding points. When the glue dropping nozzle retracts, the glue must be broken quickly and clearly (Figure 3). Even those specially prepared for high-speed glue dropping may have pull wires. If the capacity retention rate is stable at 80%, the parameter is incorrect. For example, when the amount of glue is equivalent to the diameter of the glue dropping nozzle and the required height off the ground is too small, the danger of pulling wire is very high, resulting in a very high and thin glue point. Although the combination of small needle nozzle diameter and ground clearance height can solve this problem, the stay wire may still be caused by other parameters unrelated to the glue itself, such as electrostatic discharge on the plate, incorrect z-stroke adjustment height and the flexibility of the plate or insufficient support of the plate

in case of no glue spots, the components will not be installed correctly. If the air pressure of the production line is not enough for glue dripping (that is, the pressure of the syringe is not enough to cause discontinuous glue dripping), no glue points may occur. Typically, the size of discontinuous glue dots affects the overall bonding strength between the plate and the element. The following are the reasons for this phenomenon:

· the ground support of the needle mouth falls on the pad. This problem can be solved by changing the needle nozzle with different ground support positions

· insufficient time is allocated for glue recovery. Increase the delay to 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 satellite points appear irregularly, they may cause pad contamination or insufficient bonding strength. When the needle tip is too high from the ground, reducing the height can eliminate the satellite points. If the glue amount is too large, reducing the pressure or using a needle with a larger ID (inner diameter) will solve the problem

factors affecting the drivability

good drivability does not depend solely on the quality of the glue. For the pressure time syringe dispensing method, many machine related factors affect the dripping ability and the formation of glue spots. The inner diameter of the needle tip is key to the formation of the glue point, which must be much smaller than the diameter of the glue point on the plate. As a principle, the ratio should be 2:1. 0.4mm ID is required for 0.7~0.9mm glue point; 0.3mm ID is required for 0.5~0.6mm glue point. Equipment manufacturers usually provide technical specifications and operation instructions to produce the desired size and shape of glue dots

pcb needle nozzle spacing, or the height of the stopper, control the height of the glue point (Figure 4). It must be suitable for the amount of glue dripping and needle tip ID. For a given amount of glue, the ratio of glue dot height to width will increase with the height of the stopper. Generally, the maximum stopper height is half of the needle nozzle ID; Beyond this point, discontinuous glue dripping and pulling will occur

the use pressure of current high-speed equipment can start the glue dripping cycle at a fixed time before the needle nozzle is in place. The withdrawal speed and height of the needle nozzle and the time delay between glue dropping and needle nozzle withdrawal all affect the shape of the glue point and the pull line

finally, temperature will affect viscosity and dot shape. Most modern glue dropping machines rely on the temperature control device on the needle mouth or the chamber to keep the glue temperature higher than room temperature. However, the outline of the glue point may be damaged. If the PCB temperature is increased from the previous process,

Maintenance

the bending or wear of the glue dropping needle nozzle and stopper may have a critical impact on the glue dropping. Too much glue around the needle tip may affect the formation of smooth and continuous glue spots of YAG laser with working wavelength of 1064 nm. In extreme cases, the glue may bridge on the stopper pin and interrupt the glue dripping. The universal solution is to keep the periphery of the needle mouth as clean as possible

the cleanliness of the inner surface of the needle mouth is another common source of glue dripping problems. The accumulation of glue may occur on the ID, limiting the flow. The glue may also partially solidify in the needle mouth if it is left in a warmer environment