AOI before reflow and AOI after reflow

AOI (Auto Optical Inspection) is standard on PCBA processing plants and can help increase product passed yield. The AOI is divided into the AOI before reflow and AOI after reflow. Then, when should we use "AOI before reflow" and "AOI after reflow "?

In the early days, the application of AOI mostly used on the quality and appearance inspection after the soldering of the reflow oven on the circuit board to replace the original visual inspection or supplement the artificial fatigue leakage, called it “AOI after reflow”. With the rapid development of electronic products and the increasing requirements of EMI/EMC, RF products use a large number of shields, with the rise of the concept of prevention is better than treatment, so there is the emergence of "AOI before reflow".

The purpose of the "AOI after reflow" detection is to immediately react to undesirable phenomena to the SMT process and improve product passed yield. Another purpose is to ensure that PCBA (assembled circuit board) has no quality problems in subsequent processes, otherwise once the PCB is assembled into a complete machine and find the problem, it is necessary to disassemble the PCBA to repair it, which wastes time and may result in scrapping. Therefore, the "AOI after reflow" test is usually the last step of the SMT assembly to ensure quality. Some PCBAs will also have test procedures such as ICT and FVT, in order to improve the test coverage of PCBA to 100%.

The focus of AOI detection is on the appearance, by optical image comparison principle, it is basically possible to detect whether there are missing parts, parts deflection, etc. on the circuit board, and it is also possible to conditionally detect whether there is a wrong parts, polarity reverse, parts foot warping, foot deformation, tin bridge (can not detect tin wire), less tin, cold welding, empty welding (can not detect false welding) and other issues.

It is precisely because the "AOI after reflow" is the final inspection of the solder after the reflow, it is already an established fact when it is found to be defective. At this time, it is necessary use the soldering iron to repair or scrap it. If the possible defects can be grabbed and corrected before reflowing, the subsequent possible solder defects or parts problems can be solved before the soldering of the circuit board, and the ratio of repair after the reflow can be greatly reduced.

Therefore, the rise of "AOI before reflow" actually has two major points:

First, find out the quality of the SMT before the reflow

"AOI before reflow" can pre-check the SMT before the reflow oven whether have missing parts, parts deflection, polarity reverse, and wrong parts, etc. problems, and correct it without use the soldering iron.

Second, check the quality of parts under the shielding case or large parts

Because countries have requirements for EMI of electronic products, and RF products are popular, a large number of electronic products have begun to use designs like shielding case to isolate electronic interference. Some electronic products may even require SMT processes directly solder the shielding case to the circuit board, or some designs place parts underneath large parts to strive for utilization of the circuit board. This type of design make the "AOI after reflow" cannot inspect the soldering quality of the parts under the shielding case or large parts.

So, another major purpose of the "AOI before reflow" is to solve such parts that cannot be inspecting by the "AOI after reflow".