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Guide for Process Control Measures to Prevent Errors in SMT PCB Assembly

 Apr 23, 2021

SMT PCB Assembly

Surface Mount Technology (SMT) assembly is a popular mounting technique in PCB fabrication. This technique is highly adopted by PCB contract manufacturers due to the quality it offers in PCB fabrication. However, when it comes to the performance and reliability of the PCBs, it is essential to take a few preventive measures during fabrication. These are known as process control measures of SMT PCB assembly. These process control measures are defined for each step involved in the SMT PCB assembly process. In order to achieve quality, precision, and prevent errors the process control measures must be taken. This post guides through the process control measures for solder paste application, component mounting, and soldering to prevent errors in SMT PCB assembly.

Process Control Measures for SMT PCB Assembly

The process control measures to take during solder paste application, component mounting, and reflow soldering are listed below.

Solder Paste Application: The below-listed process control measures should be taken in order to avoid defects in solder paste printing or application.
  • The solder paste printing should be consistent and uniform across the board. Any deformation in the application of solder paste can jeopardize the distribution of electric current.
  • The PCB board pads should be prevented from oxidation.
  • Copper exposure or cross-marked copper traces should be prevented.
  • Bridging, turn-down edges, deviations, bows, and twists, etc should be prevented.
  • Printing should be done in an outward direction from the core to the edge of the board. The printing thickness should remain uniform.
  • Mounting holes on the stencil should match with fiducial marks on the board. This ensures appropriate solder paste application without interrupting the component mounting activity.
  • Solder paste mixing proportion should be 3:1 for old solder paste to new solder paste.
  • The solder paste application temperature should be maintained to 25° C.
  • Relative humidity during solder paste printing should be between 35% and 75%.

Solder paste application should be followed by automated optical inspection (AOI), flying probe test, etc. This detects the errors if any occurred and helps in making corrections on the spot.

Component/Chip Mounting: Since chip mounting is done using automated surface mount devices (SMDs), it is essential to take the following preventive measures to avoid errors.
  • The SMDs must be calibrated to the required chip mounting features. Any error during the calibration of SMDs may reflect the overall quality of PCB fabrication.
  • Since SMDs are automated using computer codes, the programming must be done meticulously, should be crosschecked, and edited for desired results.
  • Feeders and SMDs should be accurately mounted and interconnected so that reoccurrence of errors can be prevented.
  • Bug detection, debugging, troubleshooting and maintenance should be considered periodically. This helps in preventing device deflections, device errors occurred during the first cycle of chip mounting. Before each cycle of chip mounting, it is essential to debug the setup.
  • Interrelations between the devices’ components and the operational path of the SMDs must be analyzed.
  • Clarification of operational flow is essential to be known before performing a debugging process. This helps in the result-centric calibration of the system.
  • The redundancy of defects should be analyzed so that reoccurrence of errors can be understood. Once the time cycle, location, etc of the defect occurrence is known, the SMDs can be calibrated accordingly.
Reflow Soldering: Reflow soldering is a process of settling the applied solder pasted and fixate the mounted components. This process requires control over temperature and other parameters.
  • Analysis of temperature curve, thermal profile, etc is essential so that desired melting temperature can be set for reflow soldering.
  • Semilunar shapes of solder joints should be tested, as it ensures firm solder joints.
  • Check for any pseudo soldering, bridging, residues of solder paste or solder balls on the PCB surfaces.
  • Prevent vibrations and mechanical shocks during soldering.

Now that all the process control measures to prevent defects in SMT PCB assembly are discussed, you should choose the experts, as they know how to prevent all these errors during the PCB fabrication. You should source SMT PCB assembly services from experienced contract PCB manufacturers like Accelerated Assemblies. The company offers high-quality PCB production services that stand up to the industrial standards.