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Printed Circuit Board Assembly Process – Some Recommendations

 Mar 31, 2015

The process of printed circuit board assembly involves several phases, and a large number of steps. It is important to follow these steps and certain guidelines to ensure assembly yield and board-level reliability.

Mounting Recommendations

Considering the following mounting recommendations helps you create rigid and high-quality circuit boards with unfailing performance.

  • Stenciling Solder Paste – Solder paste is applied to the PCB using solder stencil. It must be noted that the stencil thickness (preferably 100-125 µm thickness) must match to the needs of all the components on the PCB. It is recommended to use laser-cut or electroformed stencils for applying solder paste onto a board. To ensure excellent print quality, these stencils must be wiped between print passes.
  • Solder Paste – Solder paste meant for printed circuit board assembly consists of solder alloy and a flux system. The selected solder paste should be suitable for printing solder stencil aperture dimensions. Since solder paste is sensitive to storage, time, temperature, and humidity, it is recommended to select a paste after analyzing your PCB application requirements. It is preferable to go through the handling recommendations of the respective solder paste manufacturer.
  • Pick and Place Components – It is recommended to use advanced automatic component placement machines to measure and place PCB components at their programmed positions. These machines use location and orientation information for accurately placing the components onto the board. Picking and placing PCB components manually is not recommended, as it may lead to misplacing the components.
  • Soldering – After picking and placing the components, the next step is passing the board through a soldering machine. A highly recommended soldering technique is forced-convection reflow. Ensure to execute this process with an established reflow temperature profile. Consider consulting the manufacturer for an appropriate reflow temperature profile before starting the soldering operation.
  • PCB Cleaning – The soldering process may leave some flux residues around the solder joints. The removal/drying of even a small quantity of residue requires utmost care. It is recommended to select ultrasonic, spray, or vapor cleaning techniques based on the type of flux system used during the printed circuit board assembly. Consider consulting the solder-paste manufacturer to determine the appropriate cleaning method.
  • Inspecting the Final Assembly – Cleaning must be followed by rigorous quality inspection. Some of the recommended systems for inspection include:
    • Automatic X-ray Inspection (AXI) Systems
    • Side-Looking Optical Microscopy

These inspection systems reliably detect soldering defects such as poor soldering and misalignment, bridging, voiding, and missing parts. In case of any defects detected, it must be reworked using recommended procedures.

Adhering to the aforementioned steps and recommendations allow for an easy printed circuit board assembly process with improved functionality. The PCBs that are manufactured according to standard recommended procedures helps you produce a board with zero defects.

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