Comparison of AOI, ICT and AXI and When to Use Them during PCB SMT Assembly

Comparison of AOI, ICT and AXI and When to Use Them during PCB SMT Assembly


As a type of classical inspection technology in recent years, AOI (Automated Optical Inspection) has developed at such a high speed that AOI equipment has been widely applied for SMT (Surface Mount Technology) PCB (Printed Circuit Board) assembly. AOI works to capture images through a rotating camera that will automatically scan PCB and then comparison will be made between tested solder joints and eligible parameters in database. Image processing will lead defects on surface mount PCBs to be exposed and those defects will be displayed or indicated through monitor or automatic marker so that rework staff will be able to work on them.

AOI is mainly responsible for the following inspection goals.

a. End Quality, namely the inspection implemented for products' status before they leave manufacturing line. This inspection goal should go first when manufacturing issues are clear, SMT PCB assembly covers a high mix, and quantity and speed should be critically considered. Under such condition, AOI equipment is usually placed at the end of assembly line where it is capable of generating a large amount of process control information on a large scale.

b. Process Tracking, namely the process during which AOI equipment is leveraged to monitor surface mount assembly process typically in terms of detailed defect classification and component mounting displacement information. Manufacturers should first take advantage of this goal when products' reliability matters, high volume low mix assembly is in need and components supply falls into a stable situation. As soon as this goal is determined, AOI equipment should be placed on several places along surface mount assembly line so that specific manufacturing situation can be monitored on line and necessary basis will be provided to manufacturing technique adjustment.

Although AOI equipment can be placed at multiple positions along manufacturing line and its placement on each position will lead to different defects inspection, AOI equipment should be placed at a position where most defects can be recognized and corrected. Three inspection positions can be considered:

a. After Solder Paste Printing. If solder paste printing process totally conforms to requirement, defect count inspected during ICT (in-circuit test) will dramatically go down. Typical solder paste printing defects include:

• Insufficient solder paste on pad

• Excessive solder paste on pad

• Mismatching between solder paste and pad

• Solder bridging between pads

In the process of ICT, probability of defects mentioned above is relatively proportional to severity of issues. Slightly insufficient solder paste will seldom lead to defects while absence of solder paste, however, will almost lead to defects during ICT. Insufficient solder paste may be one of the reasons leading to missing components or open circuits. Nevertheless, missing components are caused by other reasons, which is the premise to determine where to place AOI equipment. Moreover, those reasons have to be contained into inspection schedule. Specific placement inspection directly supports process tracking and characterization. During this stage, quantitative process control data includes printing displacement and solder paste volume and qualitative information on printing solder paste will be generated as well.

b. Before Reflow Soldering. When AOI equipment is placed before reflow soldering, automated optical inspection is implemented after solder paste printing and before reflow soldering, which is a typical inspection position for AOI because this position is able to make most defects exposed deriving from solder paste printing and component mounting. The quantitative process control information generated at this position provides alignment information about IC mounter and fine pitch component mounter that can be used to modify component mounting or calibrate surface mounter. Generally speaking, this position inspection enables to meet the target of process tracking.

c. After Reflow Soldering. AOI equipment is placed after reflow soldering, that is, the final phase of surface mount assembly. This position is the most prevalent selection of AOI since full-scale assembly issues can be captured when AOI equipment is placed after reflow soldering. Automated optical inspection after reflow provides high security since it recognizes issues caused by solder paste printing, component mounting and reflow soldering.


ICT equipment is the most fundamental device applied in electrical test. The traditional ICT device takes advantage of specialized bed nail that contact with components that have been soldered solidly on PCB board and certain voltage and current are used to carry out final test so that component defects can be known including missing, displacement, misplacement, parameter deviation, solder joint bridging, opens and shorts etc. Bed nail works appropriately for simple PCBAs and mass volume production due to its high speed and low cost. However, as PCB assembly density gradually goes up, fine pitch SMT assembly and new product introduction goes through an increasingly short period and PCB boards go to diversity, bed nail test have to be confronted with some indispensible issues that fail to be overcome. It even fails to carry out test on some high-density SMT PCB assembly.

Another prevalent ICT method for SMT PCB assembly is flying probe test which depends on a large amount of flying probes to test electrical performance of circuits. Nevertheless, it is widely accepted to be applied in PCB fabrication test. As it is used in surface mount assembly test, it is able to do more.


Compared with AOI, AXI is a newly-developed inspection method. As assembled PCB boards enter AXI equipment along rail, the X rays will be absorbed by the detector below after being transmitted by transmitting tube and going through PCB. Since solder joints contain so much lead that can absorb X rays, well-formed solder joints will display black dots on the image while X rays penetrating some materials such as glass fiber, copper or silicon. Therefore, X ray inspection makes solder joint so direct and clear that solder joint defects can be automatically inspected through image analysis algorithms.

Thanks to the development of modern technology, X ray inspection has developed from 2D to 3D. As an inspection method through X ray transmission, the former generates clear images of component solder joints that are placed on single-sided board while performing badly on double-sided board. The latter, however, takes advantage of layering technology so that it is capable of inspecting solder joints on double-sided board. Moreover, 3D X-ray inspection is also able to inspect defects of those unseen solder joints such as BGA solder joints and PTH solder joints. Furthermore, solder in PTH holes can be inspected to ensure its sufficiency so that solder joint quality will be dramatically improved.