Patch adhesive is a pure consumption of non-essential process products, now with the continuous improvement of PCA design and technology, through hole reflow, double-side reflow welding has been realized, the use of patch adhesive PCA mounting process is becoming less and less.
SMT adhesive, also known as SMT adhesive, SMT red adhesive, is usually a red (also yellow or white) paste evenly distributed with hardener, pigment, solvent and other adhesives, mainly used to fix components on the printed board, generally distributed by dispensing or steel screen printing methods. After affixing the components, place them in the oven or reflow furnace for heating and hardening. The difference between it and the solder paste is that it is cured after heat, its freezing point temperature is 150 ° C, and it will not dissolve after reheating, that is to say, the heat hardening process of the patch is irreversible. The use effect of SMT adhesive will vary due to the thermal curing conditions, the connected object, the equipment used, and the operating environment. The adhesive should be selected according to the printed circuit board assembly (PCBA, PCA) process.
Features, applications and prospects of SMT adhesive:
SMT red glue is a kind of polymer compound, the main components are the base material (that is, the main high molecular material), filler, curing agent, other additives and so on. SMT red glue has viscosity fluidity, temperature characteristics, wetting characteristics and so on. According to this characteristic of red glue, in the production, the purpose of using red glue is to make the parts firmly stick to the surface of the PCB to prevent it from falling. Therefore, the patch adhesive is a pure consumption of non-essential process products, and now with the continuous improvement of PCA design and process, through hole reflow and double-sided reflow welding have been realized, and the PCA mounting process using the patch adhesive is showing a trend of less and less.
SMT adhesive is classified according to the mode of use:
Scraping type: The sizing is carried out through the printing and scraping mode of steel mesh. This method is the most widely used and can be used directly on the solder paste press. The steel mesh holes should be determined according to the type of parts, the performance of the substrate, the thickness and the size and shape of the holes. Its advantages are high speed, high efficiency and low cost.
Dispensing type: The glue is applied on the printed circuit board by dispensing equipment. Special dispensing equipment is required, and the cost is high. Dispensing equipment is the use of compressed air, the red glue through the special dispensing head to the substrate, the size of the glue point, how much, by the time, pressure tube diameter and other parameters to control, dispensing machine has a flexible function. For different parts, we can use different dispensing heads, set parameters to change, you can also change the shape and quantity of the glue point, in order to achieve the effect, the advantages are convenient, flexible and stable. The disadvantage is easy to have wire drawing and bubbles. We can adjust the operating parameters, speed, time, air pressure, and temperature to minimize these shortcomings.
SMT patch adhesive typical curing conditions:
100℃ for 5 minutes
120 ° C for 150 seconds
150℃ for 60 seconds
1, the higher the curing temperature and the longer the curing time, the stronger the bonding strength.
2, because the temperature of the patch adhesive will change with the size of the substrate parts and the mounting position, we recommend to find the most suitable hardening conditions.
The thrust strength requirement of the 0603 capacitor is 1.0KG, the resistance is 1.5KG, the thrust strength of the 0805 capacitor is 1.5KG, the resistance is 2.0KG, which can not reach the above thrust, indicating that the strength is not enough.
Generally caused by the following reasons:
1, the amount of glue is not enough.
2, the colloid is not 100% cured.
3, PCB board or components are contaminated.
4, the colloid itself is brittle, no strength.
Thixotropic instability
A 30ml syringe glue needs to be hit tens of thousands of times by air pressure to be used up, so the patch glue itself is required to have excellent thixotropy, otherwise it will cause instability of the glue point, too little glue, which will lead to insufficient strength, causing the components to fall off during wave soldering, on the contrary, the amount of glue is too much, especially for small components, easy to stick to the pad, preventing electrical connections.
Insufficient glue or leak point
Reasons and Countermeasures:
1, the printing board is not cleaned regularly, should be cleaned with ethanol every 8 hours.
2, the colloid has impurities.
3, the opening of the mesh board is unreasonable too small or the dispensing pressure is too small, the design of insufficient glue.
4, there are bubbles in the colloid.
5. If the dispensing head is blocked, the dispensing nozzle should be cleaned immediately.
6, the preheating temperature of the dispensing head is not enough, the temperature of the dispensing head should be set at 38℃.
The causes of over-wave soldering are very complex:
1. The adhesive force of the patch is not enough.
2. It has been impacted before wave soldering.
3. There is more residue on some components.
4, the colloid is not resistant to high temperature impact
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