What is the red glue on the SMT PCB?

Red glue is a heat-solidifying polyolefin adhesive used in SMT to fix components during wave soldering. It prevents detachment in mixed DIP/SMD PCB assembly processes. Usage declines with advanced soldering techniques.
What are the differences between FPC, PCB and SMT?

Essential guide to SMT workshop operations: PCB/FPC materials, environmental controls (temp/humidity), solder paste handling, process flow, ESD protection, quality systems & standards for efficient electronic assembly.