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What factors should be considered when using red glue on circuit board?

What factors should be considered when using red glue on circuit board?

The SMD adhesive is commonly called red glue, which is mainly used in the double-sided circuit board mixing craft to temporarily fix the surface-assembled components on the PCB welding pad pattern, so that the subsequent wave soldering and other processes can be smoothly performed on the mounting surface. Before assembling the components, apply the adhesive on the set position of the PCB.

When using red glue, you must consider a variety of factors, especially the following three aspects:

1.Characteristics before curing

At present, most of the surface mounts use epoxy-based SMD adhesives. Commonly used SMD adhesives are available in color, usually in red and orange. The SMD adhesives are easily distinguishable. If used in excess, they are easily noticed and removed on the welding pads. The uncured SMD adhesive should have good initial adhesive strength. The initial adhesive strength refers to the strength of the SMD adhesive before curing, that is, the components are temporarily fixed, thereby reducing the flying or falling of the components when the components are mounted, and can withstand the vibration or bumps during the mounting and transmission process.

 

 red glue

red glue

2.Characteristics in curing

The characteristics in curing are related to the curing time and curing temperature required to achieve the desired bond strength. The shorter the time required to achieve the desired bond strength, the lower the temperature, indicating the better the SMD adhesive. The surface mount adhesive must have a fast curing speed at a lower temperature, and must have a certain bonding strength to fix the components after curing. If the bonding strength is too large, it is difficult to repair; on the contrary, if the bonding strength is too small, the components may fall into the soldering groove. The curing temperature of the SMD should be avoided too high to prevent PCB warpage and component damage. In order to ensure a sufficiently high productivity, the curing time is required to be short. Another characteristic of curing is that the shrinkage of the SMD adhesive during curing should be as small as possible, so that the bonded components are subjected to less stress and prevent excessive stress from damaging the components.

3.Characteristics after curing

Although the SMD adhesive loses its effect after wave soldering, it affects subsequent processes such as cleaning and rework. One of the important characteristics of the SMD adhesive after curing is the ability to be repaired. In order to ensure the ability to repair, the cured glass transition temperature should be low, generally should be 75 ~ 95 ° C. During the repair process, the temperature of the components generally exceeds 100 ° C, as long as the glass transition temperature of the cured adhesive is less than 100 ° C, and the amount of the adhesive is not excessive, the repair is not a problem.

May-10-2019

FASTPCBA