pcb assembly

Determining Inspection Methods for PCB Assembly

Rush PCB suggests implementing inspection for Printed Circuit Boards (PCBs) at various stages during manufacturing and assembly processes to eliminate surface defects and ensuring they are of high quality and reliability. As a renowned PCB manufacturer and assembler, Rush PCB has professional inspectors capable of exposing leading defects prior to the PCB going for electrical tests.

Circuit boards consisting of surface mount devices (SMDs) assembled on them present a higher requirement for inspection, as solder joints for SMDs need to withstand greater stress than those with through hole components do. Adequate solder is usually not present for anchoring SMDs to the board, which leads to SMDs facing higher structural loads. That means, long-term reliability of boards with SMDs assembled on them depends to a large extent on the structural integrity of the solder joints of its components, thereby necessitating PCB assembly inspections.

Methods of inspection vary tremendously depending on the technology and defects the assemblers expect. Usually, manufacturers classify these into two broad categories:

  • Visual Inspection
  • Structural Process Tests

Visual Inspection

Manufacturers place visual inspection stages between specific processes during PCB assembly, and select the equipment according to the inspection necessary at that stage. Such visual inspection stages may be:

Assembly StageTools UsedDefects Checked
After Solder Paste PrintingNaked Eyes/Magnifying GlassPrinting defects and contamination
After Component PlacementNaked Eyes/Magnifying GlassMissing components, misplaced components, disoriented components
After ReflowNaked Eyes/Magnifying Glass, Light SourceSolder bridges, open joints, tombstoning, etc.


Structural Process Tests

By automating video capture in real time and digitizing the process of analysis, manufacturers can speed up the inspection process and improve the quality and reliability of the inspection process.

Depending on the volume of inspection and the speed required, Rush PCB suggests automated optical inspection (AOI) methods. These usually require multiple light sources, cameras, and computers with programs for detecting defects. Automated inspection methods are more expensive than manual visual inspection methods are, but much faster.

Complexity of modern PCBs with higher component density and specialized component structures such as BGA is leading manufacturers use different types of test systems with laser beams and X-rays, rather than visible light. Structural process tests usually do not need physical contact with the circuit board, and provide very high repeatability, while eliminating subjectivity from measuring defects.

Selecting the Method of Inspection to Use

OEMs usually are undecided on the proper method of inspection they want for their boards. To help in picking the inspection method most appropriate for our customers, Rush PCB has prepared the following guide. Prior to deciding whether to go with AOI or X-ray inspection, we suggest that our customers consider three elements:

  • Cost of Inspection
  • Defect Type
  • Inspection Speed

Designers need to consider the expenditure for inspection in relation to the cost of their boards. There is not much merit in conducting an expensive inspection procedure for a low-cost PCB.

The type of defects the designer is expecting in a PCB should define the choice of the inspection method. For instance, if the designer is concerned only about components missing or with wrong polarity, and simple solder joint defects, visual inspection would be enough to detect these defects in small volumes of PCBs.

On the other hand, if the volume of PCBs under production is large, detection of the above defects may require deployment of Automated Optical Inspection.

Additionally, if the PCB assembly consists of BGA and or fine pitch components, the designer may have to decide on X-ray inspection. Computerized X-ray inspection methods may be necessary if the production volume is very high and high inspection speed is the target.

Cost of inspection usually increases with requirements of speed and complexity of the method. For instance, while the cost of inspection is lowest for manual methods, it is higher for low speed inspection involving AOI methods, and the highest for automated X-ray methods for high speed inspection.

Inspection MethodDefect TypesSpeed of InspectionCost of Inspection
Manual OpticalPrinting defects and Contamination,

Missing components, Misplaced components, Disoriented components,

Solder bridges, Open joints, Tombstoning, etc.

Automated OpticalPrinting defects and Contamination,

Missing components, Misplaced components, Disoriented components,

Solder bridges, Open joints, Tombstoning, etc.

Automated X-raySemiconductor packaging, BGA and fine-pitch solder defects,

Voids in joints,

Internal defects in multi-layer PCBs



Defining the proper inspection method for the PCB assembly can result in enormous cost-savings for the OEMs. On one hand, this defines the expenditure incurred by the OEM for the inspection method, and on the other, it serves to bring down the rejection level and improve the quality and reliability of the PCB assembly. Rush PCB suggests following the above method for selecting the optimum way of inspecting PCB assemblies.