|IC pins are partially hidden by through-hold components in conventional angled-view inspection. |
Automatic Optical Inspection (AOI) systems are an essential element in the PCB assembly process; they're needed as part of any quality assurance program. The user can choose from a virtual smorgasbord of system options characterized by many different performance parameters; so that a broad spectrum of quality demands can be satisfied. For optimal fault detection independent of the PCB layout and the assembly parameters, a high-end angled-view optical inspection is indispensable.
AOI systems are crucial for inspecting the solder joints of PLCC components and offers higher inspection quality for all kinds of IC components. The layout at the solder joint has a crucial influence on the reliability of optical inspection.
|Examples of lifted leads. |
EMS providers must deal with the difficulty of not being able to control the design of the pads and positioning of components; thus layout guidelines are needed. Unfortunately, these guidelines are not often followed. To overcome this obstacle, an angled view can provide image information for additional analysis. However, this additional viewing angle should not be considered a remedy, since it brings with it several boundary conditions which also influence the overall process of inspection. Even with angled-view inspection, the process requires more time because of the additional images to be captured and the considerably smaller field of view, plus the fact that the arrangement of the components on the PCB can place even further constraints on the view of the inspected solder joint.
|Large field of view of the angled-view "Chameleon" module at a 45° angle. |
Because of all of these potential problems, Goepel electronic GmbH has developed the rotating angled-view module — called "Chameleon" — which combines an intelligent verification function, plus layout-independent and reliable fault detection.
Detecting Critical Faults
Lifted leads are one of the most critical faults that can occur in the production of PCBs. They result from lifted pins or from insufficient wetting — as in pin oxidation. In the electrical test — ICT, boundary scan, functional test — these problem joints may show a perfect electrical contact to the pad on the PCB, but in actual operation, discontinuities can occur, resulting in a malfunctioning PCB.
Detecting lifted leads is a complex issue, since they can appear in many different ways. The lifted lead can result from:
- Poor solder quantity.
- Width of the pad.
- Length of the pad.
- Solder flow at the foot of the pin.
- Flow behavior on the pad.
- Height of the pin.
All of these criteria are factors in the case of the lifted lead, and can appear in a variety of ways. There may be lifted leads that can be detected with orthogonal inspection, however, this is greatly dependent on the PCB's layout and the amount of solder that is applied.
- The field of view (FOV) of the angled view is responsible for the inspection speed. Any complex PCB with several ICs must be inspected from an angled view, which in turn required a large FOV.
- The depth of focus (depth of field) of the camera's lens system is a limiting factor of the image quality of the angled view; depth of field is also a major controlling issue for the FOV on the PCB. This must take into consideration the height of the components above the board. Consequently, it influences the inspection quality and speed.
- The readily viewable front pins of an IC may present solder joints that appear to be good and solid, with well-formed meniscus on each joint, but this cannot be extrapolated to mean that all the joints under the array are good. An angled inspection of the IC pins enables an optical inspection of "hidden" pins — coming closer to an IPC-conform test.
- High resolution is needed, especially when inspecting fine pitch ICs up to 0.3mm.
- If some components in front of the tested pins are too high, there may be overlaps that cause visual obstructions in the angled field of view. PCB designers should try to avoid this predicament with design rules, but practical considerations may get in the way.
- Any movement or misalignment of the PCB during the inspection process can displace inspection positions in the camera image's angled-view. An appropriate compensation algorithm can help to solve this problem.
- Odd-angle components — other than the "standard" 90° — can cause other problems, requiring special programming, and often some kind of manual intervention by the test operator during the inspection cycle.
- An intelligent verification function is needed for such a system to provide a safe level of fault detection with few false calls.
Zeroing in on lifted leads, Goepel and its customers have cooperated to compile a catalog that shows virtually all kinds of lifted leads and how they appear in the angled-view optical inspection. Thus, the examples provided in the catalog are backed up by real-life lifted leads. The result is a pool of thousands of lifted leads in their many different guises. Taking this a step further, engineers have been able to develop a method of image capture by modifying viewing angles and illumination aimed at detecting virtually all types of lifted leads.
|Chameleon's angled-view coverage. |
To meet the needs of accurate image capture, an angled view module was developed that has an extraordinarily large field of view (42 x 42mm) with superior image quality and depth of focus, as well as a resolution of up to 10.5µm/pixel. Furthermore, an algorithm was developed that enables the automatic detection of lifted leads under all circumstances.
The angled-view inspection of IC pins make it possible to approach the ideal of meeting IPC specifications. This approach, using a "smart" optical system, allows the simultaneous inspection of two opposite pin rows of an IC. This provides a significant advantage in the inspection time; an SO-IC image capturing from only one direction is needed. Even for components with four pin rows, only two inspections are necessary with one 90° rotation of the viewing direction.
The final proof of the efficacy of this approach: in real-world production, test results demonstrated 100 percent fault detection with no increase in the rate of false calls.
|OptiCon family AOI systems that can have optional rotating angled-view module. |
Another accommodation in the software provides a solution to critical assemblies where there higher components block critical solder joints, along with components mounted at unusual angles. To handle these situations, the module can provide rotation in 1° steps through a full 360° of rotation. The results:
- For components in different angular positions (for example, 20°) no adjustment of the inspection position or the parameters is needed as the viewing direction only has to be adjusted to the respective angular position (20°), and the inspection can be carried out using a standard library sample in 0° position.
- If the viewing direction is obstructed by components, it is possible to turn the viewing direction a few degrees until the solder joints are completely visible. Thus, a true fault inspection is guaranteed.
Contact: Goepel electronics LLC, 9600 Great Hills Trail, Suite 150W, Austin, TX 78759 888-446-3735 or 512-782-2500 E-mail: info-AT-goepelusa.com Web: http://www.goepelusa.com or http://www.goepel.com