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Publication Date: 03/1/2010
Archive >  March 2010 Issue >  Special Features: SMT and Production > 

AOI for Ultra-Tiny Components
Passive components have decreased in size keeping pace with the need for ultraminiaturization.

Smaller and lighter portable products means ultraminiaturization of component packaging found in today's manufacturing processes. This use of miniature components, such as the 01005, can generate significant challenges to the SMT manufacturing process and equipment providers. For automated optical inspection (AOI) manufacturers, the 01005 presents opportunities as well as challenges. On one hand, the utilization of the 01005 components promotes the widespread if not mandatory use of AOI, while it demands high precision and advanced inspection capability that is needed for assuring quality assemblies.

Human error, equipment limitations and design compromises can all lead to defects on the PCB. Bridging and tombstoning are among the manufacturing defects that are the most common for 01005 assemblies. Bridging can occur due to overly aggressive pad spacing (less than 150µm or 6 mils) or stencil misalignment. Stencil alignment becomes more critical as the pad geometry shrinks and for 01005 circuits it is absolutely critical to have perfect alignment. Tombstoning is the result of solder surface tension imbalance during the reflow process. Lead-free solder that requires a higher temperature to achieve wetting tends to increase the opportunity for surface tension imbalance, thus increasing the potential for tombstoning.

AOI Strategy
Traditionally, AOI can be deployed in three areas of the SMT process, post-paste print, pre-reflow and post-reflow. The rationale for inserting AOI early in the process is the strong belief in early defect detection and repair; the earlier in the process a defect is detected, the lower the repair cost. Fixing a solder paste defect is a lot easier and therefore less costly before reflow than after reflow. The problem with this line of thinking is that defects are produced throughout the assembly process, not just in the beginning. Component defects can only be detected after the components are placed, defects such as bridging and tombstoning can only be detected after reflow. If the AOI is only used upstream it will miss many of the early defects. Since the majority of the 01005 defects are bridging and tombstoning, post-reflow AOI is essential for assuring product quality.

Multiple AOI Locations
There are various characteristics of AOI machines that are essential for 01005 inspection.

Sensor and Optics. To accurately inspect a 01005 device it is important that the combined optical and digital magnification is sufficient to provide the necessary amount of resolution and information to the AOI software inspection algorithms. However, it is important to understand that more is not necessarily better. Higher magnification can lead to a smaller field of view (FOV), longer image acquisition times and more data to process.

With the ever-increasing cycle times of today's manufacturing process, it is important for an AOI system to be able to balance both magnification and image acquisition speed. One of the most popular AOI specifications to determine 01005 inspection capabilities is the "pixel size." The pixel size is determined by the properties of the imaging sensor and the optics of the AOI system. Let's take for example a CMOS sensor with an array size of 1280 x 1024 (1.3 Megapixels). When fitted with a lens at a focal distance that produces a field of view of 32mm x 25.6mm you would achieve a pixel size of 25µm.
FX Series AOI system from Nordson YESTECH.

If you consider that a 01005 chip resistor measures 200 x 400µm, then the projected image of the component would be 8 pixels x 16 pixels in size. This may not be enough information for the inspection algorithms to provide sufficient defect detection. However, by utilizing the same sensor with a lens that provides a field of view of 16mm x 12.8mm, the resulting pixel size of 12.5µm would display the component at 16 pixels x 32 pixels. This 4x increase in area may now be enough information for the inspection algorithms to accurately detect the defect conditions.

The increase in magnification comes with a reduction of FOV as well as an increase in inspection cycle-time. If high-speed inspection is a top priority, then sensor size should also be selected carefully. For example, if we were to select a sensor with an array size of 2448 x 2048 (5 Megapixels) then we would be able to achieve the same 12.5µm pixel size as in the above example, but at the same time, maintain an FOV of 30.6 x 25.6mm. Therefore the 5 megapixel sensor enables higher resolution while avoiding penalties to FOV and inspection cycle-time.

Lighting. In any machine vision application, lighting is one of the most important factors. To ensure the widest defect coverage for today's component and PCB configurations as well as those to come, it is crucial to have a dynamic light source. With the reduction in cost of light emitting diodes (LED) AOI equipment manufacturers are able to configure light arrays that are highly customizable within the inspection software. With multiple colors at various angles, the AOI programmer has the flexibility to enhance the contrast of the image to easily identify a multitude of defect conditions. Even within the 01005 component family there are many variations of color and surface properties. Combine this with the variations of paste composition, pad size and material, and PCB color and texture and the possibilities are endless. Given these conditions, programmable lighting is an invaluable tool for ensuring the widest range of defect detection.

Data Collection. One of the most important functions of an inline AOI system is data collection and retrieval. These data can be in the form of a text output, database, image collection, or combination of several formats. Collecting the data is a basic function of most AOI systems and can be as simple as enabling a checkbox in the software. However, retrieving the information can be a little more complex and depends on the configuration of the manufacturing line.

In a networked environment the AOI can simultaneously inspect a PCB while transmitting results from the previous assembly to a "downstream" review/rework station. The downstream station is then not only communicating with the AOI system but also storing inspection results and review operations into a SPC (Statistical Process Control) database. Operators have the ability to review the inspection results real-time or archive the data to be reviewed at a later time.
Magnified view of passive chips; ultra-tiny 01005 (far right) is so small it almost disappears from the photo.

Repair station, offline programming, and SPC software are common network tools often sold as options by AOI vendors. One new feature that has been recently introduced takes even greater advantage of the network environment of the manufacturing line. By placing multiple AOI systems in a single line, inspection results can be shared to actively affect the inspection and/or review at each stage. This new feature introduces a more dynamic inspection technique to ensure the highest possible defect detection. Since the 01005 assembly line lacks a feasible inline repair mechanism, the post reflow AOI needs to be able to utilize the inspection data from the upstream AOI (or AOIs) to enhance its defect detectability. In other words, they need to be able to collaborate to achieve the best possible inspection.

Insufficient Solder
For example, a defect such as insufficient solder is most effectively detected by the post-paste AOI. It is more difficult for the post-reflow AOI to detect this defect without generating some false calls. By combing the defect reports from the upstream AOIs, the post-reflow AOI can ensure the defect will not escape detection. In an ideal situation, there are up to three AOI machines on the line. The post-paste AOI should be tuned to detect stencil miss-alignment, and insufficient solder; the pre-reflow AOI should be tuned to detect component defects such as missing or improper polarity, the post reflow AOI should be tuned to detect bridging and tombstone defects. By dividing the tasks and combining results optimal detection can be achieved.

As the use of 01005 components steadily increases, AOI is challenged by the small size of the components and the inherent problems it causes, but the technology is ready and available to achieve accurate component location and reliable defect detection. The ideal scenario is to implement AOI at each of the fundamental steps of the SMT line: post-print, pre-reflow and post reflow. AOI systems must have a combination of advanced lighting, high performance optics, state-of-the-art inspection algorithms and reporting capabilities to provide the expected performance in a manufacturing process with 01005s.

Contact: Nordson YESTECH, 2762 Loker Avenue West, Carlsbad, CA 92010 760-918-8471 fax: 760-918-8472  

See at APEX Booth #1759.

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