AOI systems incorporate multiple capabilities
Automated optical inspections systems no longer specialize on just one area of printed-circuit board inspection.
Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 8/13/2008 2:40:00 PM
The use of automated optical inspection (AOI) technology for inspecting solder paste, pre-reflow solder, and post-reflow solder on printed-circuit boards (PCBs) is not new. But traditionally, a different AOI system has been required for each of these steps.
Now, some vendors are combining many of these inspection capabilities in the same machine, such as the Agilent Technologies Medalist sj5000 AOI system, which the company introduced at the APEX show in April. Stacy Johnson, Agilent’s Americas marketing development manager, recently discussed the trend in AOI systems and explained that software is the key to the sj5000's functionality.
Q: Why has Agilent combined the use of AOI for inspecting 2-D solder paste, pre-reflow solder, and post-reflow solder in one machine?
Stacy Johnson
Americas Marketing Development Manager
Agilent Technologies
A: We have been combining these capabilities in our systems for nearly a decade. However, combining more types of test and inspection in a single system is actually a larger, overall trend within test and measurement these days. A more flexible system with multiple capabilities can be adapted to multiple products. This built-in flexibility boosts the value that users can get from each machine.
When it comes to AOI, most manufacturers know they need it, but the question is at which stage: solder paste, pre-reflow, post-reflow, or all three? And should they use AOI on just certain product lines, or on all of them? A machine that does all of these can be used at many stages, or moved around as needs change, therefore helping to increase its ROI.
Q: What are the challenges involved in combining these functions and how does the sj5000 overcome those challenges?
A: We’re definitely not the only ones combining these steps, but our approach is different. In general, others are using existing hardware by adding cameras and mirrors, and then moving the stages. We use the hardware to take an image and then encode that image in software.
This approach has a single hardware imaging chain, and the software does the image analysis and necessary post-processing. You turn on a certain algorithm and that determines whether you’re doing 2-D paste inspection, or post- or pre-solder reflow inspection. So, we aren’t combining multiple, older technologies. Instead, we are trying to stay at the cutting edge in terms of the types of cameras and imaging that are available, and then we are wrapping software around them.
Q: How else does the use of software contribute to greater user value?
A: Another major trend in AOI test systems is the push to make them easier to use and more scalable. Software gives you both greater flexibility and greater scalability, so you can shut functions on or off as needed. You can use the menu of algorithms to optimize test coverage of the board under test based on what is actually present, without sacrificing inspection speed.
For instance, if a specific device type is not on the board, then you won’t need a certain algorithm to perform a particular function based on that device type. That means you can shut off that algorithm. Alternately, if a board has a lot of technologies on it, you can optimize which algorithms are on or off to balance coverage vs. inspection speed.
Software also makes upgrading a system easier and faster. As advanced packaging continues to be used and new technologies and problems show up on the board, how do you cope with them? If your inspection technology is focused on hardware, you have to change the hardware or add more of it. But if it’s focused on software, you can develop a new algorithm and add it to the existing hardware, such as we have done with the lead ringer solder-joint algorithm for lifted leads. The startup time is faster using this method since the user already knows how to work your interface.
Q: What are the technical, functional, and cost reasons for either combining 2-D and 3-D solder-paste inspection in one machine or for keeping them separate?
The Medalist sj5000 AOI system.
A: The sj5000 includes 2-D inspection only. It has been shown that volumetric information, which only comes from 3-D inspection, can be a good predictor of long-term solder joint quality. Combining the 2-D and 3-D paste-inspection technologies on one machine is technically possible. In our case, we use laser triangulation for our 3-D paste inspection.
For our series 2 and 3 product lines, we chose to go with a unified platform approach that enables both 2-D and 3-D inspection methods. Users of those machines can quickly swap out the 2-D lighting heads and swap in the 3-D optics to make a component inspection system capable of 3-D paste analysis. They perform either a 3-D solder paste inspection on 100% of the board before components are placed, or a 2-D presence/absence type of test during their existing AOI in either pre- or post-reflow.
This is a more scalable approach and lets the users decide where to place their AOI. If they are building high-reliability and complex parts, they must do 3-D paste inspection, but in other scenarios, the 2-D paste inspection may be good enough. We plan to extend this unified platform capability to the sj5000 in the future.
Q: What else is driving the use of AOI in PCB inspection?
A: As part complexity is increasing, so is the cost of those parts. Not finding defects until late in the process can be costly. Using AOI pushes the knowledge of defects further up in the production process. In 3-D paste and pre-reflow inspection, defect information becomes available before parts are committed to the board. This can mean a significant cost savings. The main trend here is defect prevention rather than defect detection.
Q: How much faster are line rates today?
A: In the past, an inspection rate of 1 to 3 square inches per second was considered good. Now, that’s shifted to upwards of 5 to 6 square inches per second. But you have to balance speed with an inspection system’s capabilities and make tradeoffs. For example, if you shut off some functions, either software or hardware, you might be able to get to 7 square inches per second.
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