In PCB drilling process, the following possible quality problems need to be controlled: porosity, leakage, displacement, wrong drilling, incomplete drilling, hole loss, partial waste, flaring, and plug hole. At present, the management and control methods of various manufacturers are mainly to standardize the drilling operation process before drilling and strengthen the inspection methods after drilling. In actual production, as pre drilling means can only reduce the probability of errors, and cannot be completely eliminated, post drilling inspection must be relied on to ensure product quality.  

In the post drilling inspection, many domestic manufacturers are still using the method of plug gauge combined with manual visual film (film) package inspection and comparison: focus on the inspection of large holes and small holes through plug gauge, focus on the inspection of holes, leaks, displacement, unpenetration, and unpenetration through film, and other hole damage, flaring, and hole plugs are completed through manual visual inspection. When using film for inspection, drill a red film sample plate for each product when drilling, fix it with the product plate through pins during inspection, and conduct visual inspection manually under the light box. In theory, this method can detect various defects, but in practice, the effect discount is very large.  

The main problems are as follows:

First, the inspection requirements for small aperture cannot be guaranteed: production practice shows that for PCBs with the minimum aperture ≥ 0.5mm, high inspection results can be achieved manually on the premise of ensuring a certain production efficiency. This is determined by the minimum recognizable visual angle of the human eye, working distance and attention duration. With the reduction of the aperture size, the inspection ability of the human eye will decline rapidly for the product plates below 0.5mm. For the product plates ≤ 0.25mm, it is difficult to ensure the quality of manual sampling inspection.  

Second, the efficiency of manual inspection is limited: the efficiency of manual inspection is directly related to the number of holes and the minimum hole diameter. The actual production experience shows that when the hole size is more than 10000 and the minimum hole size is less than 0.5mm, the efficiency will be significantly reduced, and manual work is only suitable for spot check. For high-density boards, the quality of drilling can no longer be guaranteed by manpower.  

Third, the stability of quality cannot be guaranteed: people will be affected by experience, emotion, fatigue, sense of responsibility and other factors, and it is difficult to ensure the stability of quality. Some manufacturers shall not adopt the method of multiple manual and repeated inspections, but still cannot ensure the stability of quality.  

In order to solve the above problems, many large PCB manufacturers have used hole inspection AOI equipment to replace manual work in a large range. Especially for Japanese funded enterprises and Taiwan funded enterprises, years of practice has proved the effectiveness of this new method, which is worthy of attention and reference by many domestic PCB manufacturers.  

The hole inspection AOI equipment is an automatic optical inspection equipment. According to the image morphology of various defects in the drilling, it is divided into: porous, few holes, large holes, small holes, debris, hole deviation, and hole morphology. It can be divided into two types: one is hole inspection machine, and the other is hole position measurement and inspection machine (Hole AOI). In fact, there is also an X-Ray inspection machine, which is mainly used for embedded blind holes and multi-layer board alignment analysis. It is inconsistent with the target of manual film inspection, and is not within the scope of this analysis.  

According to the equipment matching experience of PCB manufacturers, it is recommended to use multiple sets of hole inspection machines for the full inspection of the first plate and bottom plate, focusing on the inspection of multiple holes, fewer holes, large holes, small holes and debris; A hole position measuring and checking machine is used for spot check, focusing on hole deviation. The characteristics of the two types of equipment are as follows:

Hole inspection machine: it has the advantages of low price and fast inspection efficiency. One piece of 600mm × The 600mm PCB has an average of 6-7 seconds, which can realize the inspection of multiple holes, fewer holes, larger holes, smaller holes, and debris. The disadvantage is that the inspection ability for hole position is not high, and only serious defects can be detected. According to the actual production experience of the manufacturer, an average of 15 drilling rigs are equipped with 1 hole inspection machine.  

Hole position measuring and checking machine: the advantage is that it can check all items. The disadvantage is that the price is high (about 3~4 times that of the hole testing machine), and the inspection efficiency is low. It takes several minutes or even longer to inspect one piece. It is generally recommended to configure one set for random inspection of products to supplement the deficiencies of hole position inspection by the hole inspection machine.  

The inspection principle of hole inspection AOI equipment: the optical system is used to collect PCB borehole imaging and compare it with the design file (drilling tape file or Gerber file). If the two are consistent, the borehole is correct; otherwise, it indicates that there is a problem in the borehole, and then the defect type is analyzed and classified according to the image morphology. The hole inspection equipment is compared with the drilling design document, and the manual visual inspection is compared with the film. The inspection principle can avoid problems caused by film drilling errors, and the reliability is higher.