What is the red glue on the SMT PCB?

Red glue is a polyolefin compound and belongs to SMT materials. Unlike solder paste, it solidifies after being heated. When the temperature reaches 150℃, the red glue begins to change from a paste to a solid. This article will lead you to understand what the red glue on the pcb board is, what the red glue on the pcb does, the role of the red glue in pcb patch processing, and the standard process of SMT red glue.

What is the red glue on the pcb board?

In the mixed process of SMT and DIP, in order to avoid the situation of double furnace passing through the single-sided reflow soldering once and the wave soldering once, the red glue is applied to the center point of the chip components and devices on the wave soldering surface of the PCB, so that the tin can be applied once during the wave soldering, eliminating the solder paste printing process.

The correct name of the SMT "red glue" process should be the SMT "glue dispensing" process. Because most of the glue is red, it is commonly known as "red glue". In fact, there is also yellow glue, which is the same as the "solder mask" on the surface of the circuit board that we often call "green paint".

Generally, you can find a ball of red glue in the middle of the bottom of small parts such as resistors and capacitors. This is red glue. The red glue process was developed because there were many electronic parts that could not be immediately transferred from the original plug-in (DIP) package to the surface mount (SMD) package.

There are half DIP parts on a circuit board and the other half are SMD parts. How to place these parts so that they can all be automatically soldered to the board? The general practice is to design all DIP and SMD parts on the same side of the circuit board. The SMD parts are printed with solder paste and soldered in the soldering furnace. As all the solder feet of the remaining DIP parts are exposed on the other side of the circuit board, all the DIP solder feet can be soldered at once using the wave soldering furnace process. Therefore, two soldering processes are required at the beginning to solder all the components.

In order to save the layout space of the PCB and put more components in, SMT components need to be placed on the bottom side. At this time, the components need to be glued to the circuit board, and then the circuit board can pass through the wave soldering furnace, so that the components can be tinned and connected to the pads on the circuit board without falling into the hot wave soldering furnace.

If you want to complete the soldering in one go in order to reduce the process flow, you can use through-hole reflow soldering; however, many plug-in components selected cannot withstand the high temperature environment of reflow soldering, so through-hole reflow soldering cannot be used. Only some large companies with large-scale products may consider through-hole reflow soldering, and they can purchase some high-priced plug-in components that can withstand high temperatures.

The general SMD parts have been designed to withstand the reflow soldering temperature, but the reflow soldering temperature is higher than the wave soldering temperature, so the SMD devices will not have any problems even if they are immersed in the wave soldering tin furnace for a short period of time. However, there is no way to make the SMD pass the wave soldering furnace with printed solder paste, because the temperature of the tin furnace must be higher than the melting point of the solder paste, so the SMD parts will fall into the tin furnace tank due to the melting of the solder paste.

Therefore, the SMD devices need to be fixed first, so red glue is used.

What is the role of red glue on PCB

1. Red glue generally plays a fixing and auxiliary role. Solder is the real welding role.

2. Prevent components from falling off in wave soldering (wave soldering process). When using wave soldering, in order to prevent components from falling off when the printed board passes through the solder tank, it is used to fix the components on the printed board.

3. Prevent components on the other side from falling off during reflow soldering (double-sided reflow soldering process). In the double-sided reflow process, SMT adhesive should be used to prevent large devices on the soldered side from falling off due to the melting of solder.
4. Prevent displacement and vertical position of components (reflow process, pre-coating process). Used in reflow process and pre-coating process to prevent displacement and vertical position during mounting.
5. Marking (wave soldering, reflow, pre-coating). In addition, when the printed circuit board and components are changed in batches, SMT adhesive is used for marking.

SMT adhesive is also called SMT red glue. It is usually red (also yellow or white). The paste is evenly distributed with adhesives such as hardeners, pigments, solvents, etc. It is mainly used to fix SMT components on the printed circuit board. It is generally distributed by dispensing or steel screen printing. After the components are attached, they are placed in an oven or reflow oven for heating and hardening.

The patch glue for patch processing solidifies after being heated. The solidification temperature of patch processing is generally 150 degrees, and it will not melt even if heated again. In other words, the thermal hardening process of patch processing is irreversible. The patch processing effect will vary depending on the thermal curing conditions, the connected objects, the equipment used, and the operating environment. When using it, the patch glue should be selected according to the printed circuit board assembly process.

The patch processing red glue is a chemical compound, the main components of which are polymer materials, patch processing fillers, curing agents, and other additives. The patch processing red glue has viscosity fluidity, temperature characteristics, wetting characteristics, etc. According to this characteristic of the patch processing red glue, the purpose of using red glue in production is to make the parts firmly adhere to the PCB surface to prevent them from falling.

The patch processing red glue is a pure consumable material and is not a necessary process product. Now with the continuous improvement of surface mount design and technology, patch processing through-hole reflow soldering and double-sided reflow soldering have been realized, and the mounting process using patch processing patch glue is showing a trend of becoming less and less.

SMT red glue standard process

The standard process of SMT red glue production is: screen printing → (glue dispensing) → mounting → (curing) → reflow soldering → cleaning → detection → rework → completion.

1. Screen printing: Its function is to print solder paste (solder paste) or red glue (chip glue) on the pads of the PCB circuit board to prepare for the welding of components. The equipment used is a screen printer (screen printer), which is located at the forefront of the SMT process production line.

2. Glue dispensing: It is to put red glue on the fixed position of the PCB. Its main function is to fix the components on the PCB board. The equipment used is a glue dispensing machine, which is located at the forefront of the SMT production line or behind the detection equipment.

3. Mounting: Its function is to accurately install surface assembly components on the fixed position of the PCB. The equipment used is a chip mounter, which is located behind the screen printer in the SMT production line.

4. Curing: Its function is to melt the red glue (chip glue) so that the surface mounted components and PCB boards are firmly bonded together. The equipment used is a curing furnace, which is located behind the SMT production line.

5. Reflow soldering: Its function is to melt the solder paste (solder paste) so that the surface mounted components and PCB boards are firmly bonded together. The equipment used is a reflow soldering furnace, which is located behind the SMT production line.

6. Cleaning: Its function is to remove the soldering residues such as flux that are harmful to the human body on the assembled PCB board. The equipment used is a cleaning machine, which can be in a non-fixed position, online or offline.

7. Inspection: Its function is to inspect the welding quality and assembly quality of the assembled PCB board. The equipment used includes magnifying glasses, microscopes, online testers (ICT), flying probe testers, automatic optical inspection (AOI), X-RAY inspection systems, functional testers, etc. The position can be configured in a suitable place on the production line according to the needs of the inspection.

8. Rework: Its function is to rework the PCB board that has been detected to be faulty. The tools used are mainly hot air guns, soldering irons, rework workstations, etc. They are configured at any position in the production line.

9. After the red glue is applied and the components are mounted, they can be sent to the curing furnace for curing. Curing is a key process in the red glue-wave soldering process. In many cases, due to poor or incomplete curing of the red glue (especially when the components on the PCB are unevenly distributed, it is most common), components will fall off during transportation and welding. Therefore, the curing work should be done carefully. The glue is cured thoroughly.

Conclusion

SMT red glue fixes components in PCB processing, though its use declines with tech advances, remaining technologically significant.