PCBA Tech - Understand the bad reasons and process of SMT

Poor SMT patch processing is due to missing parts. There are many reasons for missing parts in SMT patch proofing processing. In addition, the process of SMT patching is screen printing, dispensing, placement, curing and PCB reflow soldering, etc.

Poor SMT patch processing is due to missing parts. There are many reasons for missing parts in SMT patch proofing processing. In addition, the process of SMT patching is screen printing, dispensing, placement, curing and reflow soldering, etc. Next, I will introduce you Here is the detailed content.

1. Reasons for poor processing of SMT patches

1. Missing parts There are many reasons for missing parts in SMT patch proofing processing, such as: vacuum pump carbon film is not good enough to cause missing parts, component thickness difference is too large, SMT patch machine parts parameter setting error, placement height setting Improperly waiting.

2. Offset SMT package material after the patch glue is cured, the component shift phenomenon occurs, and in severe cases, even the component pins of the SMT patch proofing are not on the pad. The reason may be that the positioning reference point of PCBA processing is not clear, or the positioning reference point on the PCBA board is not aligned with the reference point of the steel mesh. This phenomenon can also be caused by the failure of the optical positioning system of the printer in the SMT small-batch chip processing plant, or the solder paste of the PCB electronic processing plant that does not match the opening of the stencil and the design file of the circuit board.

3. The short circuit of the PCBA may be caused by undesirable reactions such as bridging, or the distance between the stencil and the PCBA board may be too large, which may cause the solder paste printing to be too thick and short, or the component placement height may be set too low to make the solder paste Extrusion causes short circuit, solder paste collapse, stencil opening is too large or thickness is too large, etc.

4. The tombstone phenomenon in SMT patch processing may be caused by clogging of the steel mesh, clogging of the nozzle, deviation of the feed port, excessive spacing between the pads, and poor temperature settings. SMT small-batch patch processing plants can only provide high-quality SMT labor and materials services if they do their best in SMT processing and treat the products required by each customer with the most enthusiastic attitude.

Second, the process of SMT patch

1. Silk screen: Its function is to leak solder paste or patch glue onto the PCB pads to prepare for the soldering of components. The equipment used is a screen printing machine (screen printing machine), located at the forefront of the SMT production line.

2. Dispensing: It is to drip glue onto the fixed position of the PCB board, and its main function is to fix the components on the PCB board. The equipment used is a glue dispenser, located at the forefront of the SMT production line or behind the testing equipment.

3. Mounting: Its function is to accurately mount the surface mount components to the fixed position of the PCB. The equipment used is a placement machine, located behind the screen printing machine in the SMT production line.

4. Curing: Its function is to melt the patch glue, so that the surface assembly components and the PCB board are firmly bonded together. The equipment used is a curing oven, located behind the placement machine in the SMT production line.

5. PCB reflow soldering: Its function is to melt the PCB solder paste, so that the surface PCB assembly components and the PCB board are firmly bonded together. The equipment used is a reflow oven, located behind the placement machine in the SMT production line.

6. Cleaning: Its function is to remove the solder residues such as flux that are harmful to the human body on the assembled PCB board. The equipment used is a washing machine, and the location may not be fixed, it may be online or offline.