What is the SMT adhesive on the PCB board?

SMT adhesive is a polyene compound, unlike solder paste, it solidifies when heated, and its freezing point temperature is 150°C, at this time, the adhesive starts to change from a paste to a solid, it belongs to SMT material.

What is the SMT adhesive on the PCB board?

In the mixing process of SMT and DIP, in order to avoid twice high temperature for reflow soldering and wave soldering, the adhesive on the center point of the chip component when wave soldering, can once soldering, omit the solder paste printing process.

The SMT adhesive commonly is red, in fact, there are other colors, such as white, yellow, green etc.

We can find that there are a bunch of red gel-like objects in the middle of the small components (such as resistors and capacitors) bottom side, this is SMT adhesive. The SMT adhesive process was originally developed because there were still many electronic components that could not be transferred from the original DIP to the surface mount solder (SMD) package.

If there is half of the DIP components on one board and the other half are SMD components, how do you place these components so that they can be automatically soldered to the circuit board? The general approach is to design all DIP and SMD components on the same side of the circuit board, SMD components are soldered use solder paste by reflow soldering, and the remaining DIP parts because the foots of the components are exposed on the another side of the board, so can be soldered once in wave soldering.

In order to save the layout space of the PCB, we hope to put in more components. Therefore, SMT components are also required on the bottom side of the PCB. At this time, in order to stick the components on the circuit board, and then let the board pass the wave soldering, the components can be soldered and bonded to the pads on the circuit board without falling to the wave soldering machine.

In order to reduce the process, we hope to complete the soldering at one time. Through-hole reflow soldering can be used, but many of the DIP components we have chosen cannot withstand the high temperature environment of reflow soldering. Therefore, through-hole reflow soldering cannot be used. So, through-hole reflow soldering is only possible for mass production from large companies, because it can purchase some high-priced DIP components that can withstand high temperatures.

Since the general SMD components have been designed to withstand the reflow soldering temperature, the reflow soldering temperature is higher than the wave soldering temperature, so the SMD components will not be problematic even if they are immersed in the a wave soldering for a short period of time, but solder paste is no way to make SMD components pass wave soldering, because the temperature of the tin furnace must be higher than the melting temperature of the solder paste, so that the SMD components will fall into the tin furnace tank because the solder paste melts.

So we need to fix the SMD components first, so we used SMT adhesive.