Capabilities vary, but programming time is the general differentiator.

Inspection is not the same as electrical testing. Rather, it is performed via optical means during PCB assembly and directed at ferreting out defects. Integrated with assembly equipment, automatic optical inspection (AOI) and automatic optical test (AOT) are at the epicenter of these test and inspection methods.

A competent T&I technical team with diverse experience is critical for creating methods that can universally solve problems for all types of PCBs. Reason: Creating and implementing an AOI program is not an exact science. Most often, it is not perfect, but can be regarded as a "work in progress" for a long period before it is ideal for covering all critical features and expanding inspection coverage area. Consequently, technical personnel designing a T&I methodology should have extensive flying probe, in-circuit and functional test experience, as well as thorough knowledge of how AOI captures and analyzes. These skills and experience are critical to fully comprehend and handle the nuances of AOI programming. Engineers and technicians will likely encounter false defects and faults during AOI of a PCB. To detect these anomalies, they must have a trained and experienced eye.

Inexperience can often lead to time-consuming mistakes. For example, tighter-than-necessary AOI tolerances can produce many false defects. When this occurs, Quality Assurance spends time reviewing and separating those false defects from true ones.

AOI Refresher

AOI is a system for inspecting electronic circuits comprised of components and subassemblies. It inspects and identifies defects during assembly by deploying an optical device to capture images, which are then compared with those of a board known to have no defects ("golden board").

AOI first captures a "master" or known good image of the PCB (Figure 1). Next, it analyzes the image, divides it into separate but smaller segments, captures many images of these segmented images and then analyzes the captured image to determine, detect and capture the faults on the boards. The third AOI component is defect designation, which defines and describes defect types the master image has found when it is compared to the board under inspection.

AOT, on the other hand, is relatively new and is designed to find faults on the PCB. AOI and AOT are different, but operate in tandem. AOT is a combination of AOI and electrical test. It combines the fault detection logic found in conventional probe-type electrical test systems (Figure 2) with inspection systems using optic-scanning devices. The fault detection logic is based on the principles of circuit connectivity and isolation. This means circuits are connected at proper points on the PCB and disjointed where they are supposed to be disjointed.

Conventional electrical testers probe the panel and analyze connectivity. AOT not only does that, but also optically captures images. It performs image analysis like AOI and then does defect designation. As an electrical test, AOT generates the necessary signals to measure input versus output and then scans the targeted object to accurately perform a fault report on a PCB under investigation (Table 1). These steps verify a fault as being a true fault. AOT takes further steps to check every IC and other testable component on the PCB to ensure it is of the correct value and placed at proper orientation, and that these components are within tolerances specified at the component selection stages of hardware design. AOT performs additional checks to discover other problems such as cosmetic defects, opens and shorts. It is more comprehensive than AOI and is less likely to generate false defects, a problem AOI normally encounters. AOT is applicable to all circuit types, be they analog, digital or mixed in nature. Moreover, it is free of limitations associated with electrical test and AOI.

Optical inspection can be used at different assembly stages to build products that require little to no rework or quality control. As a process, AOI detects such defects as component presence and absence, orientation and positioning, and misplaced components. For example, AOI would flag a capacitor if incorrectly placed where a resistor should be, or detects components of the wrong polarity.

It also detects visible and hidden solder joints, especially solder fillets (Figure 3) using a camera that can view fillets at 45° angles (versus a top-down view of the components). (A fillet refers to the amount of solder paste applied to solder components on to the PCB surface. It is the amount of solder from the tip of the component toward its heel; that distance is called a fillet.) Fillets are checked to determine whether the amount of solder is sufficient for proper adhesion of components.

AOI inspects exposed and sealed assemblies in plastic or metal packages. For example, an IC may not be operating properly. A very powerful AOI unit can check wire bonds, which may or may not be properly connected underneath the device packaging. For that matter, it is capable of detecting all problems or errors associated with small components. AOI checks defects resulting from upside down or skewed/misaligned components, as well as detecting tombstoning.

AOI's database includes libraries to compare right components against the ones used on a given PCB. For example, a capacitor from one supplier may be coded with one color while a similar capacitor from another vendor is a different color, but both are identical in terms of function. AOI distinguishes the colors and determines if the capacitor is the correct one to use, even if it is from a different manufacturer. Moreover, a good AOI system is capable of reading the different color codes to verify their values.

2-D vs. 3-D

AOI systems come in 2-D and 3-D versions with a vast cost difference between the two. A 3-D AOI can look at the height of the component as well as the length and width to capture, identify and detect errors. 2-D units use high-speed cameras and special lenses to ensure accurate measurement for post-placement AOI. Here, three tiers of image analysis run simultaneously with the inspection process. This permits the AOI system to operate at the same speed as the high-speed placement equipment.

2-D inspection checks on component lead positioning, spacing, length, lead tip and missing leads. Leads are those portions of a component making contact with the PCB surface. This level of inspection ensures leads are properly positioned and spaced apart in relation to other leads, and ensures they are neither too short nor too long. If a lead is too short, the flux may not wet sufficiently and the component will not properly solder to the board. If the lead is too long, it may lead to bridging. 2-D also checks for "mouse bites" on the PCB, which are the holes drilled through a panel of boards used to separate them at the end of the process. This is part of drilling process that takes place during board fabrication.

Solder reflow, if not profiled properly, can introduce an additional set of defects. For that reason, 3-D AOI is important. These include components that are missing, tombstoned, billboarded, misaligned, offset or unnecessary, plus an array of solder-related defects including solder bridges, insufficient or excess solder, and solder balls. Other defects include lifted component leads, gold-finger contamination, incorrect jumper position and switch settings, through-hole pins and improperly inserted connectors.

To cover this spectrum of possible defects, 3-D AOI uses flexible and highly specialized technology. Some AOI systems deploy as many as five to six cameras with one directly inspecting the PCB and the others checking multiple PCB locations. With this in-depth coverage, most of the assembly defects are exposed by 3-D AOI systems

Not all AOI systems are created equal. Programming time is the general differentiator, and foretells the amount of resources required to structure a program. Some AOI systems can incur from one to two days for programming time for a job, while superior systems require only three to four hours for the same job.

Other considerations include GUI user-friendliness, ease-of-use menus, amount of expertise needed, offline programming, specialized or easy programming languages, data translation across platforms, library depth, ease of minimizing false rejects, and others.

 

Zulki Khan is founder and president of Nexlogic Technologies Inc. (nexlogic.com); zk@nexlogic.com.