1. Introduction
Generally, PCB manufacturing involves a full inspection after two steps : first, the circuit is completed (inner and outer layers), and second , the finished product is inspected. This chapter focuses on the inspection after the circuit is completed .
2. Inspection Method
2.1 Electrical Measurement
2.2Visual inspection
A magnifying glass with a circular lamp is used to inspect the quality of the circuit and the accuracy of the alignment . If the outer layer also needs to be inspected for the quality of the holes and plating , a 10x eyepiece is usually provided for further confirmation . This is a very traditional operating mode , so the manpower requirement is quite large . However, the high-density design of the current boards is almost impossible to inspect with the naked eye , so AOI, which is described below, will be used extensively .
2.3AOI – Automated Optical Inspection
As line density gradually increases and specifications become more stringent, visual inspection with magnifying lenses is no longer sufficient to filter out all defects , hence the application of AOI (Automated Optical Inspection).
2.3.1 Scope of Application
A. Board type
- Signal layer, power layer, post-drilling (i.e., both inner and outer layers are acceptable).
- Film, dry film, copper layer. (Working film , after dry film development , after circuit completion)
B. Currently, most AOI applications are concentrated on the inspection of inner layer circuits after completion . However, a larger process that can replace manual labor is the surface finishing of boards after the green paint has been applied . Especially for BGA boards , the board size is small , the lines are fine , and the quantity is large , so the demand for manpower is staggering . However, the application of AOI in this field still requires technological breakthroughs .
2.3.2 Principle
The industry generally uses two types of automated optical inspection: CCD and Laser. The former primarily uses halogen lamps to transmit light, targeting the un-blackened copper surface of the board, utilizing its reflective effect to identify open circuits, short circuits, or dents. It is applied to the inner layers before blackening or the outer layers before line coating. The latter, Laser AOI, is mainly used for the substrate portion of the board.
The interpretation is based on the difference in intensity of fluorescence produced after reflection from the substrate (copper surface). Early LaserAOI did not produce very strong fluorescence from "dual-function" lasers, and often required the addition of a small amount of "fluorescent agent" to enhance its effect and reduce false alarms. When the substrate was thinner than 6 mil, the laser light often penetrated the board and reached the copper lines on the other side, causing misinterpretation.
The "four-functional" substrate itself has a pale yellow tint, which already enhances fluorescence. Laser automated optical inspection technology is relatively mature and has become the main source of AOI lamps in recent years .
Modern AOI (Automated Optical Inspection) technology utilizes laser fluorescence, reflected light from smooth metal surfaces, and laser light signals through holes to significantly improve circuit detection capabilities.
2.3.3 Detection Items
The capabilities of each brand can be found in their datasheets . Typical detection items are listed below.
A. Disadvantages of the signal layer circuitry
B. Power supply and ground plane ,
C. Hole, SMT
AOI (Automated Inspection) is a highly advanced inspection device that replaces manual labor . It utilizes technologies such as lasers , optics , and intelligent judgment software to theoretically perform its operations . Here, we should pay attention to whether its future development can completely replace all visual inspections at each stage of PCB manufacturing .
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