Process Inspection and SMA Boards Inspection

Today let’s talk about the process inspection and SMA Board inspection when we assemble a printed circuit board.

Process inspection, also known as process testing, is a crucial aspect of SMT (Surface Mount Technology) assembly. It encompasses inspection of various stages including solder paste printing, component placement, reflow soldering, and cleaning processes. Process inspection serves as a key method for controlling product quality throughout production with the following objectives:

1. Solder Paste Printing Process Inspection:

Purpose: To ensure the quality of solder paste printing, which is vital for SMT assembly.
Inspection Criteria: Follow internal company standards or reference industry standards such as IPC or SJ/T10670-1995. Criteria may vary depending on the specific process, such as whether post-soldering cleaning is required.
Inspection Methods: Visual inspection using magnifying glasses or microscopes, as well as solder paste inspection machines (SPI). Different types of components may require different inspection techniques, such as 2D SPI for larger components and 3D SPI for smaller, high-density components.

Solder Paste Printing Defect Illustration

2. Component Placement Process Inspection:
Purpose: To inspect components before soldering to prevent errors in component type, polarity, or placement, which could lead to costly rework or affect component reliability.
Inspection Criteria: Follow internal company standards or reference industry standards such as IPC or SJ/T10670-1995.
Inspection Methods: Inspection methods vary based on equipment and assembly density, including visual inspection, magnification, microscopy, or automated optical inspection (AOI).

3. Reflow Soldering Process Inspection:
Purpose: 100% inspection after soldering, with particular attention to double-sided reflow soldering processes.
Inspection Criteria: Follow internal company standards or reference industry standards such as IPC or SJ/T10670-1995.
Inspection Methods: Depending on available equipment, visual inspection or magnification using magnifiers or microscopes may be utilized.

 

Common Solder Joint Defect Examples

 

4. Cleaning Process Inspection:
Purpose: To meet cleanliness requirements, especially for products with specific demands such as military, medical, or precision instruments.
Inspection Criteria: Use instruments such as Omega meters to measure cleanliness indicators like ionic contamination and insulation resistance for specialized products, or visual inspection for general products.

 

Surface Mount Assembly (SMA) boards require 100% inspection. For high-density boards, inspection should be conducted using a 2-5x magnifying glass or a 3-20x stereo microscope.
Boards with electrostatic-sensitive components must be inspected by personnel wearing antistatic wristbands on antistatic workbenches. During inspection, the surface mount assembly boards should be handled with care. They should be put in antistatic containers or racks, clearly labeled once the inspection is completed.

1. Visual Inspection Quality Requirements:
Components should be undamaged and clearly marked.
Through-hole inserted components should be properly aligned, without exceeding allowable tolerances for twisting or tilting.
PCB and component surfaces should be clean, and free from excess solder balls, and other contaminants.
Component installation positions, models, nominal values, and characteristic markings should match assembly drawings.
Solder joints should be well wetted, with joints being complete, continuous, and smooth. Solder volume should be appropriate, and joint positions should be within specified ranges, without defects such as solder bridging, open joints, solder balls, or inadequate soldering.
Solder joints may have holes, but their diameter should not exceed 1/5 of the solder joint size, and no more than two holes are allowed per joint.

2. Surface Mount Component Solder Joint Quality Standards (Generally following IPC-A-610 standard):
Definition of Good Solder Joint: A joint that maintains electrical performance and mechanical strength within the specified usage environment, conditions, and lifespan.

Appearance Criteria for Good Solder Joints:
Good wetting of solder joints;
Appropriate solder volume, avoiding excess or insufficient solder;
Solder joint surfaces are intact, continuous, and smooth, without voids or holes;
Component leads or pins are properly positioned on the solder pads;
Surface-mounted components are undamaged after soldering, with no detached terminal electrodes.

Internal Criteria for Good Solder Joints:
Good solder joints must form proper Intermetallic Compounds (IMCs) at the interface;
No cracks or fractures within the solder joint;
Adhering to these inspection and quality standards ensures the reliability and performance of surface mount assemblies.

By conducting thorough process inspections and SMA board inspections, it is possible to control and prevent the propagation of defects between stages, identify root causes of defects, minimize rework after soldering, enhance SMT assembly quality and productivity, and ensure the reliability of the final product.

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