With the continuous improvement in the precision of electronic products, component sizes have been shrinking.
PCB substrates and finished products have also become more diverse in type.
As a result, various electronic auxiliary materials have been widely adopted.
Solder paste and SMT red adhesive are indispensable materials in SMT production.
This article elaborates on the differences between these two materials and relevant selection principles.
Introduction to Red Adhesive
Red adhesive is a polyolefin compound with properties completely different from solder paste.
It undergoes a curing reaction when heated, with a curing temperature of 150°C; once reaching this temperature, the paste-like red adhesive will turn solid directly.
Red adhesive has physical properties including viscosity, fluidity, temperature responsiveness and wettability.
Its core purpose in production is to firmly bond electronic components onto PCB surfaces and prevent component falling off in subsequent processes.
Introduction to Solder Paste
Solder paste is a new type of welding material developed alongside SMT technology.
It is a paste mixture formulated by solder powder, flux, surfactant, thixotropic agent and other raw materials.
It is mainly used to solder surface-mount resistors, capacitors, ICs, and other SMT components onto PCBs, with the core function of conducting electricity and forming reliable solder joints.
Differences Between Solder Paste and Red Adhesive
For PCBA products with single-sided or double-sided SMT mounting that do not require wave soldering, the solder paste process is used.
When the SMT side and the through-hole soldering side are on the same PCB surface, red adhesive is used for component mounting.
The specific differences between the red adhesive reflow process and the solder paste reflow process are listed below:
1. Red adhesive only provides mechanical fixation and is non-conductive; solder paste realizes both component fixation and electrical conduction.
2. Components fixed by red adhesive must go through wave soldering to complete electrical soldering.
3. The wave soldering temperature matched for red adhesive is lower than that for solder paste.
4. Red adhesive is generally used as an auxiliary fixing material, while solder paste serves as the core soldering material to realize electrical connection during welding.
Function of Hybrid Red Adhesive + Solder Paste Dual Process
The hybrid process is designed to avoid two heating cycles (one reflow soldering plus one wave soldering) on a single side.
For components located on the wave soldering side of the PCB, dots of red adhesive are applied to hold components in place.
Solder can be applied in a single wave soldering pass, eliminating the solder paste printing step on this side.
Red adhesive and solder paste have clear respective functions: red adhesive plays an auxiliary fixing role without conductivity, while solder paste undertakes the actual soldering and conductive function.
Molten solder during wave soldering forms electrical connection on component pins.
Selection Criteria for Red Adhesive and Solder Paste
1. Products without traditional through-hole components (single-sided, double-sided, and multi-layer boards) normally adopt the solder paste process.
2. Single-sided boards fitted with traditional through-hole components use the red adhesive process on the soldering side.
3. For double-sided boards with through-hole components on one side, solder paste is used on one side for SMT mounting, while red adhesive is used on the opposite side for soldering.
The red adhesive process requires an extra wave soldering step to finish soldering between components and PCBs, resulting in longer procedures, higher labor and manufacturing costs.
By contrast, the solder paste process completes soldering only through reflow soldering.
It features shorter workflows. It also has lower labor and production costs.
In addition, it results in shorter product lead times and stronger market competitiveness.
Conclusion
In PCBA manufacturing, solder paste is primarily used to achieve electrical connections through reflow soldering.
Red adhesive serves as a non-conductive auxiliary material.
It provides temporary mechanical fixation during wave soldering processes.
The choice between these two materials depends on PCB structure, component layout, and process requirements.
In general, solder paste-based processes offer simpler workflows, lower cost, and higher efficiency, while red adhesive is applied in more complex assemblies requiring additional wave soldering steps.
A proper understanding of their differences enables more efficient process planning and improved production quality in SMT manufacturing.
