How to understand SMT red glue process

How to understand SMT red glue process

In fact, SMT red glue process is called because most of the glue is red, so it is commonly known as red glue. In fact, there is also yellow glue, which is the same as we often call the solder mask on the circuit board surface green paint.

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    What is SMT red glue process

    It is usually found that there is a mass of red glue at the bottom and center of the small parts such as resistors and capacitors. This is red glue, which is originally designed for the purpose of sticking SMD parts to the circuit board,so the process is also called SMT red glue process. And then the circuit board can pass through wave soldering furnace, so that the parts can be stained with tin and bonded to the welding pad on the circuit board. Without falling into the hot wave soldering furnace.

    The SMT red glue process was developed because there were many electronic parts that could not be immediately converted from their original HTD (through hole device) package to SMD surface weld package. Imagine a circuit board with half HTD parts and half SMD parts.

    How to arrange SMD parts so that they can all be welded to the board automatically​

    The common practice is to design all the THD and SMD parts on the same side of the circuit board. The SMD parts are printed with solder paste and go back to the welding furnace for welding, while the remaining THD parts can be welded at once using the wave soldering furnace process because all the welding feet are exposed on the other side of the circuit board.

    Later, the engineer came up with a way to save space on the circuit board, that is, to put parts on the side where there were only THD parts feet but. However, most THD parts could not be placed on the side where the board passed the tin furnace because there were too many gaps in the body, or the part material could not withstand the high temperature of the tin furnace.

    Ordinary SMDs have been designed to withstand reflow temperatures, even if immersed in a wave soldering furnace for a short period of time will not be a problem, but only printing solder paste will not be able to get SMDs through the wave soldering furnace, because the temperature of the tin furnace must be higher than the melting point of the solder paste, so the SMD parts will melt into the tin furnace.

    Later, of course, some engineers came up with the idea of using a thermosetting adhesive to stick SMD parts on the circuit board. This adhesive will solidify after heating and can be heated in reflow furnace. In this way, the problem of parts falling out of tin slots can be solved. The picture above shows that the red glue wave welding furnace under the SMD and the pin of the THD coexist on the same side of the circuit board.

    Why the thermosetting type of red glue is used is to avoid the solidified glue

    Why the thermosetting type of red glue is used is to avoid the solidified glue

    The reason why the thermosetting type of red glue is used is to avoid the solidified glue in the wave welding furnace when the remelting, so as to lose the purpose of a little red glue, and red glue can not be reused like amount of solder paste.

    Whether all SMD parts can go through the wave soldering by adding SMT red glue process.
    Only some double row feet to extend the IC parts or welding pads at both ends of the parts (such as resistance, capacitance) can go through the wave welding process, have also seen QFP this kind of four sides have feet of the IC walk the wave welding, but the foot spacing must be large enough, otherwise there will be a short circuit problem.

    General connectors or parts with gaps can not walk the wave welding process, because the tin liquid will contaminate its contact points or affect the characteristics of the parts, and too small parts (generally below 0402) because the spacing is too small easy to produce tin short circuit and can not walk the wave soldering furnace problem.

    How does red glue be put on PCB

    In the early days when the SMT red glue process was prosperous, special dispenser would be installed in the SMT production line to point red glue. During that period, SMT red glue process were used together with solder paste occasionally aiming to reduce the shadow effect caused by soldering empty.

    soldering-empty

    Later, after most of the electronic parts were gradually changed to SMD packaging, most of the dispensing machines were removed from the SMT line. Occasionally, when SMT red glue process is needed, it can also be achieved by the way of plate printing, but in this way, there is no way to print solder paste and red glue at the same time. However, some people use thicker steel plates (3D steel plates), dig out bomb shelters in the original place of printing solder paste, and then apply SMT red glue process. But problem is that the amount of red glue is not easy to control.

    What else can the red glue be used for

    Because of the good fixing effect of red glue, it is sometimes used to strengthen the ability of parts to fix on the circuit board. For example, some parts are easy to fall in the fall test, so some engineers want to use red glue to increase the strength of parts sticking on the circuit board.

    Some external connectors are sometimes complained by customers that they are easy to be pushed off, and some engineers want to use SMT red glue process to reinforce; More have seen engineers use red glue points in the four corners of BGA to reduce the defective rate of tin crack.

     double-sided SMT process

    Thirdly, because the circuit board is already double-sided SMT process, some heavy parts if placed on the first side of the furnace, in the second welding furnace will be easy to fall because of the gravity, in advance to point red glue can avoid the risk of parts falling.

    However, most of these processes using red glue can only be classified as short-term countermeasures, in terms of production efficiency, to increase a process will increase a certain number of hours, that is, to increase the cost, long-term countermeasures are still to use the design method to remove the red glue measures.

    FAQ

    In fact, SMT red glue process is called because most of the glue is red, so it is commonly known as red glue. In fact, there is also yellow glue, which is the same as we often call the solder mask on the circuit board surface green paint.

    It is usually found that there is a mass of red glue at the bottom and center of the small parts such as resistors and capacitors. This is red glue, which is originally designed for the purpose of sticking SMD parts to the circuit board,so the process is also called SMT red glue process. And then the circuit board can pass through wave soldering furnace, so that the parts can be stained with tin and bonded to the welding pad on the circuit board. Without falling into the hot wave soldering furnace.

    The SMT red glue process was developed because there were many electronic parts that could not be immediately converted from their original HTD (through hole device) package to SMD surface weld package. Imagine a circuit board with half HTD parts and half SMD parts.

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