What is SMT?(四) -- Process composition

author: Hellen

2022-09-09

Process composition

Printing (red glue / solder paste) --> detection (optional AOI automatic or visual inspection) --> placement (first paste small devices and then paste large devices: divided into high-speed placement and integrated circuit placement) --> detection (optional AOI optical / visual inspection) --> welding (using hot air reflow soldering for welding) --> detection (can be divided into AOI optical inspection appearance and functional test detection) --> maintenance ( Use tools: soldering table and hot air desoldering table, etc.)--> board separation (manual or board separator for cutting board)
Process flow is simplified to: printing ------- patch ------- welding ------- overhaul (each process can be added to the detection link to control the quality)
Solder Paste Printing
The role is to print the solder paste at an angle of 45 degrees with a squeegee onto the pads of the PCB, in preparation for the soldering of the components. The equipment used is the printing machine (solder paste printing machine), located in the forefront of SMT production line.
Part placement
Its role is to accurately install the surface assembly components to the PCB's fixed position. The equipment used is the mounter, located behind the printing machine in the SMT production line, and is generally a high-speed machine and a general-purpose machine used in accordance with production requirements.
Reflow soldering
Its role is to melt the solder paste, so that the surface assembly components and PCB firmly welded together. The equipment used is reflow oven, located in the SMT production line behind the bonder, for temperature requirements are quite strict, the need for real-time temperature measurement, the measured temperature in the form of a profile to reflect.
AOI Optical Inspection
Its role is to detect the solder quality of the soldered PCB. The equipment used is an automatic optical inspection machine (AOI), the location of which can be configured at the right place in the production line according to the needs of the inspection. Some before reflow soldering, some after reflow soldering.
Repair
Its role is to detect malfunctioning PCBs for rework. The tools used are soldering irons, rework workstations, etc. Configured after the AOI optical inspection.
Depanelization
Its role is to cut the multi-linked PCBA, so that it is separated into separate individuals, generally using V-cut and machine cutting method.
Basic knowledge
Solder paste is a paste made by mixing solder powder with paste solder with flux function, usually the solder powder accounts for about 90% and the rest is chemical composition.
We call the object that can change its form at will or divide it arbitrarily a fluid, and the science of studying the laws and characteristics of fluid deformation and flow behavior caused by external forces is called rheology. However, in engineering, the concept of viscosity is used to characterize the magnitude of fluid viscosity.
Rheological Behavior of Solder Paste
Solder paste is mixed with a certain amount of thixotropic agent and has pseudoplastic fluid properties. Solder paste in the printing, the role of the squeegee thrust, its viscosity decreases, when reaching the window of the template, the viscosity reaches its lowest, so it can smoothly through the window to the PCB pad, with the cessation of the external force, the solder paste viscosity and quickly back up, so there will be no collapse of the printed graphics and diffuse flow, to get a good printing effect.
Factors affecting the viscosity of the solder paste: solder powder content; solder powder particle size; temperature; shear rate.
1、Solder powder content: the increase of solder powder in the solder paste caused by the increase of viscosity.
2, solder powder particle size: solder powder particle size increases, viscosity decreases.
3, temperature: temperature increases, viscosity decreases. The best ambient temperature for printing is 23±3 degrees.

What is SMT?(五) -- Reflow soldering

What is SMT?(三) -- SMT Prospects