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The “SMT dispensing” process, also known as the “SMT Red Glue” process in Asia, is a process where a glue is used to stick Surface Mount Device (SMD) parts to a Printed Circuit Board (PCB). The term “SMT Red Glue” is derived from the prevalent use of red adhesives in SMT processes, with a few instances of yellow adhesives. As seen in the illustration at the beginning of this article, beneath small chip components like resistors and capacitors, there is a red, glue-like substance in the center, commonly referred to as red glue.
Initially, the dispensing of glue was designed to stick SMD components to the PCB so that the PCB can pass through a wave soldering furnace, allowing the components can be wetted with tin and bonded to the solder pads on the PCB without falling into the hot wave soldering furnace.
The SMT red dispensing process was developed because many electronic components could not be immediately converted from Through Hole Device (THD) package to Surface Mount Device (SMD) package. Consider a scenario where half of the components on a PCB are THD, and the other half are SMD. How do you arrange these components so that they can all be automatically soldered onto the board? To solve this problem, the common approach involves designing all THD and SMD components on the same side of the circuit board. SMD components are then soldered using solder paste and reflowed through the oven. The remaining THD components, with all their soldering pins exposed on the opposite side of the circuit board, can be soldered in one go using a wave soldering furnace.
Later, ingenious engineers came up with a method to economize space on the circuit board. They sought ways to position components on the side that initially only featured pins for Through-Hole Devices (THD) but lacked any actual components. However, for the majority of THD components, this side couldn’t be in contact with the melting liquid soldering because of the numerous gaps in their bodies or their materials not being able to endure the high temperature of the soldering furnace.
In contrast to Through-Hole Devices, Surface Mount Device (SMD) components are specifically designed to withstand the elevated temperatures of the reflow oven. Although they can endure a brief immersion in the molting liquid soldering in the soldering pot, using only solder paste isn’t adequate to pass SMD components through the wave soldering furnace. This is because the temperature of the soldering furnace is necessarily higher than the melting point of the solder paste, causing SMD components to fall into the tin bath due to the melting of the solder paste.
Certainly, later on, engineers came up with the idea of using thermosetting adhesives (red glue) to bond SMD components to the circuit board. These adhesives solidify when heated, making them suitable for use with a reflow furnace. This innovation addressed the issue of components falling into the solder trough. The SMT red glue process was born, allowing for further reduction in the size of circuit boards. The image above illustrates SMD components with red glue underneath going through a wave soldering furnace, coexisting with the pins of THD components on the same side of the circuit board.
The reason for using thermosetting red glue is to prevent the already solidified adhesive from re-melting when passing through the wave soldering furnace. This way, the purpose of applying red glue would be defeated. So, if you ask, “Can red glue be reused like solder paste?” the answer is no.
Can all SMD components be processed through wave soldering with red glue?
Unfortunately, only some dual-row IC components with outward-extending leads/terminals or components with pads on both ends (such as SOIC, resistors, and capacitors) can undergo the wave soldering process. Workigbear has also seen QFP-type ICs with pins on all four sides going through wave soldering, but the pin spacing must be large enough to avoid soldering short.
Connectors or components with gaps are generally not suitable for the wave soldering process because the solder can contaminate their contact points or affect the component’s characteristics. Additionally, very small components (generally below size of 0402) may experience solder short due to their small spacing, making them unsuitable for wave soldering.
For more information on wave soldering, you can refer to the article “Can SMD Components Go Through Wave Soldering Process?”
How is red glue placed or dispensed onto the circuit board?
In the early days of the red glue process, SMT production lines would install dedicated dispensers to apply glue. During that period, there were occasional instances of a dual-process involving both red glue and solder paste (applying solder paste first, then dispensing red glue before component placement). This was done to reduce the shadow effect during wave soldering, which could lead to soldering defects.
Later on, as most electronic components gradually transitioned to SMD packaging, most of the glue dispensers were removed from SMT lines. Occasionally, when red glue was needed, it could be applied using a stencil printing method. However, with this approach, it’s not possible to simultaneously print solder paste and red glue. Nevertheless, where there’s a will, there’s a way. Some innovators have used thicker stencils (3D stencil), creating voids where solder paste is normally applied, and then printing red glue. While this method is more intricate, it allows for the coexistence of both solder paste and red glue, although controlling the amount of red glue can be a bit more challenging.
In addition to its role in securing components during wave soldering, red glue serves various other purposes on the circuit board
Because red glue provides strong adhesion, it is sometimes used to reinforce the attachment of components to the PCB. For example, some components are prone to detachment during drop tests, so engineers may use red glue to enhance the adhesion of these components to the board. I/O connectors that are occasionally complained about for being easily dislodged may also be reinforced using red glue. I’ve even seen engineers use red glue at the four corners of a BGA to reduce the incidence of solder cracking. Furthermore, since modern PCBs undergo double-sided SMT processes, heavier components placed on the first side during reflow may be at risk of falling off during the second reflow due to gravity. Applying red glue beforehand can mitigate the risk of component detachment.
However, most of these applications of the red glue process are considered “short-term solution.” In terms of production efficiency, adding an extra process increases the labor hours and, consequently, the production cost. Long-term solutions should ideally involve design strategies to eliminate the need for such red glue measures.
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