STEP 9 Test and Inspection: Maximizing Production Efficiency through Automated Inspection - Surface Mount Technology
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STEP 9 Test and Inspection: Maximizing Production Efficiency through Automated Inspection


Lean manufacturers are relying on automated inspection equipment (SPI/AOI/AXI) to streamline the manufacturing process and provide real-time root cause analysis of manufacturing defects. The objective is to increase profitability through improved production yields and reduced costly rework. Brian D’Amico, MIRTEC Corp., reviews the types of SPI and AOI technology, and how to choose and implement inspection machines.

In these tough economic times, electronics manufacturers must maximize their production efficiency by implementing lean manufacturing initiatives and optimizing production processes. With this in mind,

Figure 1.
manufacturers are relying on automated inspection equipment (SPI/AOI/AXI) to streamline the manufacturing process and provide real-time root cause analysis of manufacturing defects. The objective is to increase profitability through improved production yields and reduced costly rework.

 There are two generally adopted forms of automated inspection that provide fast return on investment (ROI) when deployed in the electronics manufacturing process: automated solder paste inspection (SPI) and automated optical inspection (AOI). Figure 1 shows an SMT assembly line that uses SPI and AOI technology after each significant stage of the assembly process.

Automated Solder Paste Inspection

The majority of manufacturing defects are attributed to the solder paste printing process; some claim as many as 80% of overall defects. By implementing SPI post-print, manufacturers are able to monitor and maintain the screen printing operation in real time. Defects such as insufficient or excessive solder paste volume, clogged stencil apertures, mis-registration, solder bridging, shape deformity etc., are detected by SPI at the beginning of the assembly process with minimal cost to rectify. Rework costs significantly increase with each progressive stage of the assembly process. A defect detected post-print can be resolved by simply cleaning the PCB, removing the solder paste and the associated defect; a defect detected post-reflow requires costly rework and repair with a risk of damage to the PCB and/or components.

Selecting an SPI system. All SPI systems use one of two basic technologies for post-print solder profilometry: laser scan technology or Moiré phase shift image processing. Laser scan is the fastest and least expensive means of inspecting for solder profilometry; however, these attributes come with a tradeoff. Laser systems are more susceptible to reflective noise and variations in PCB surface and density. There is also an inherent lack of accuracy due to the shadow effect, whereby one side of the solder deposition cannot be illuminated and characterized by the laser system.

Figure 2.
Moiré phase shift image processing is the most accurate means of inspecting for post-print solder profilometry. Using Moire phase shift image processing, a structured light projects a pattern such as a grid, or series of stripes onto the solder paste deposition. This pattern is then incrementally shifted across the deposition and a series of images are captured by a downward high-resolution CCD camera. The most advanced systems have a shadow-free design in which both sides of the solder deposition are characterized simultaneously (Figure 2).

The main benefits of Moiré technology include superior solder paste height resolution capability and the ability to withstand external factors such as PCB variations and 

Figure 3.
substrate warpage. The result is a highly accurate 3D image from which the system can extract precise volume and shape characteristics (Figure 3).

Automated Optical Inspection (AOI)

The evolution toward advanced miniature packaging continues to increase PCB density and complexity. As the manufacturing process becomes more complex, there is a higher probability of defects on finished PCB assemblies (PCBAs). Manufacturers are able to minimize defects by implementing AOI in the inspection process. But exactly where should AOI occur in the production line? Should the machine be inserted after the placement machine (post-placement), or after the reflow oven (post-reflow)? The answer to this question is a resounding yes. Most manufacturers, however, are forced to select either one location or the other due to budgetary constraints.

AOI is used post-placement to inspect assembled PCBs for missing components, wrong components, and proper component orientation. Since this process is performed prior to reflow, defects are relatively simple to repair and less costly than if these defects were discovered post-reflow.

Figure 4.
Another benefit of post-placement inspection is that component placement data can be gathered for trend analysis. Figure 4 shows a scatter plot of data collected by an AOI system for post-placement trend analysis. This data is useful for early detection of placement machine calibration issues, which, if unresolved, increase process defects. Post-reflow trend analysis is somewhat ambiguous due to the fact that component placements will be modified by the reflow process. Without post-placement inspection, the need for placement machine calibration may go unnoticed until a significant rise in the number of defective boards occurs.

Post-reflow AOI is perhaps the most widely accepted methodology among electronics manufacturers. An AOI system placed at this point in the assembly process will detect any defects generated throughout the entire process including missing components, wrong components, proper component orientation, insufficient solder, excessive solder, solder opens, solder bridges etc. Some defects may be attributed to the printing process, others may be caused by the placement system, and yet others may result from the reflow process. The purpose of post-reflow AOI is to provide defect data necessary for PCB rework and repair, as well as a means of collecting this data across multiple assemblies for root cause analysis and continuous process improvement.

Selecting an AOI system. Not only do manufacturers need to determine the most suitable location for AOI in the assembly line, they are also faced with a selection of AOI system from a field of no less than 25 suppliers. This may seem to be a daunting task, but domestic manufacturing requirements and current market conditions help narrow the field to a manageable handful of equipment makers. To address low-to-medium volume domestic manufacturing requirements, the AOI machine must be quick and easy to program. This will allow manufacturers to change over multiple products in a given production day. Furthermore, in today’s lean manufacturing 

Figure 5.
environment, engineering resources are at a premium, so AOI programming and operation must require minimal technical expertise. Lastly, to contend with the complexity of today’s PCB assemblies, the AOI system must provide technically advanced optics, lighting, and measurement systems necessary for extremely accurate and repeatable inspection results. Figure 5 shows how laser inspection technology can be used in conjunction with optical inspection to yield superior inspection results.

Conclusion

Current market conditions are such that electronics manufacturers are competing to capture as much business as possible, facing ever-decreasing profit margins. These market conditions likely will not change in the foreseeable future. Electronics manufacturers are relying upon automated solder paste inspection and automated optical inspection to maximize efficiency and acquire quantitative information that helps streamline the manufacturing process and reduce manufacturing defects. By increasing first pass production yields (FPY), manufacturers are able to decrease costs, save time, and reduce the need for non-value-added rework and repair. EMS providers and electronics OEMs are becoming more selective when purchasing equipment, looking to add value to their business and keep up with industry changes.

Brian D’Amico, president, MIRTEC Corp., may be contacted at (203) 881-5559; [email protected]; http://www.mirtecusa.com.



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