AOI (Automated Optical Inspection)

Saki Corporation designs and manufactures both 2D and 3D automated optical inspection (AOI) systems for printed circuit board (PCB) production. Automated Optical Inspection is a method of using optics to capture images of a PCB to see if components are missing, if they are in the correct position, to identify defects, and to ensure the quality of the manufacturing process. It can inspect all size components such as 01005, 0201, and 0402s and packages like BGAs, CSPs, LGAs, PoPs, and QFNs.

The introduction of AOI enabled real-time inspection. With the advent of high-speed, high-volume production lines, an incorrect machine setting, placing the wrong part on a PCB, or having a problem with alignment could result in a large number of manufacturing defects and subsequent rework in a short amount of time. The original AOI machines were capable of 2-dimensional measurements, such as checking board features and component features to determine X and Y coordinates and measurements. 3D systems have expanded on 2D, adding the height dimension to the equation, thereby providing X, Y, and Z coordinates and measurements.

NOTE: Some AOI systems don’t actually “measure” the height of the components. Saki’s system is a true inspection and measurement system.

AOI detects errors early in the manufacturing process and assures process quality before the board is moved to the next manufacturing step. AOI helps yield improvement through feedback to the line and provides historical data and production statistics. Making sure that quality is controlled throughout the process saves time and money incurred from material waste, repair and rework, added manufacturing labor, time, and expenses, not to mention all the costs of device failure.

There are 3 critical requirements for AOI machines:

  • To detect any errors in the production line and immediately feed that information back upstream so as not to repeat the error.
  • To accommodate high-speed capabilities to align with takt time so corrective measures can be taken in a timely manner.
  • To be quick and easy to program and operate so inspection can be accomplished in real time and with reliable inspection results.

Where in the assembly process is AOI used?

Saki's AOI systems are perfectly suited for both pre-reflow and post-reflow applications due to Saki's powerful inspection and measurement algorithms and lighting and imaging systems.

AOI Pre-Reflow

AOI pre-reflow is used to check component placement status and location. Pre-reflow inspection typically shows a lower level of defects, generally being limited to defects such as missing or extraneous components, or misplaced components that would result in bridging during the reflow process. Pre-reflow inspection, therefore, is generally used as an additional process for SMT applications utilizing exorbitantly expensive components, to avoid defects that might be exceptionally difficult to repair post-reflow, or for inspection of boards prior to the last pick-and-place machine operation, which would be dedicated to placing EMI/RFI shields that would obstruct the ability of an AOI to inspect components beneath the shields.

AOI Post-Reflow

AOI post-reflow is the most common use of AOI in an SMT line and is pivotal for quality assurance to prevent defects from passing to the next stage of production. All defects must be detected by AOI inspection at this critical step. Post-reflow AOI inspection allows detection of defects that may be attributed to causes prior to reflow, as discussed in the pre-reflow scenario, but post-reflow AOI provides defect detection for all defects throughout the PCB assembly process, including defects whose causal factors might be attributed to the thermal dynamics to which a PCB and its components are subject during the reflow process.

Inspection Capabilities: 2D and 3D

Inspection Categories 2D 3D
Bent Lead
Copper Exposure
Damaged Components
Defective/Damaged Components
Dry Joints/Absence of Solder
Excess Solder
Fillet Defect
Foreign Material
Gold Finger
Height Defects  
Inclined Components  
Incorrect Component
Insufficient Solder
Lifted Chip  
Lifted Lead  
Missing Components
Package Height  
Paste Registration
Polarity (Chip & IC)
Solder Ball
Through-Hole Pins (PTH)


Guidelines for choosing 2D vs. 3D technology

There are technological factors that make either a 2D or a 3D AOI system a better choice for a given application. However, there are other factors to consider, such as cost and the intended use of the PCB assembly. In some cases, the application, cost factors, and acceptable process parameters may make a 2D machine the better choice, but in other cases, those same factors may deem a 3D AOI system as the better choice. Ideally, an AOI system which utilizes both 2D and 3D technology provides the most optimal inspection capability, since some board, component, and solder characteristics are best detected by 2D technology, while 3D technology is best for others. The proper blend of 2D and 3D technology provides the greatest range of recognition and inspection capability, with the most accurate inspection and measurement results, and the lowest false call and escape rates.