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Feature Articles

Machine Vision in the Assembled Printed Circuit Board Market - Part 2

by Nello Zuech, Contributing Editor - AIA

 

This is the second part of a two-part article on machine vision in the assembled printed circuit board market. Part 1 covered a brief review of the history of machine vision-based AOI systems in applications in the electronic industry, as well a discussion of why today there is more urgency and more compelling reasons to adopt the technology. It also discusses where on the production line such systems can be deployed and where it actually makes the most sense. Some of the application issues, concerns, and requirements are also described. Part 1 concludes with the exhortation that it's the DATA that has the value but only if it will be used to make the line adjustments flagged to optimize yield.

What follows - Part 2 - depicts an example of a return-on-investment analysis that suggests in a typical assembly shop the ROI is in months and not in years. Tables are also developed that depict defect type by specific line location and vendors by product offerings. A questionnaire is developed designed to guide a prospective buyer in evaluating product offerings vis-à-vis his specific requirements. Some tips are also provided to assure a mutually rewarding experience between a vendor and buyer of an AOI system.

Return on Investment
Table 1 depicts one attempt to develop a justification for an AOI system. In this example the application involves post reflow. Many assumptions have been made: production is 20,000 boards/month; labor rates with benefits around $20.25; percent solder joint rejects 0.2%; etc. The analysis also takes into account warranty costs, indirect labor costs associated with inspection functions, inspector training costs, value of inventory in rework, etc. The net result is that under this scenario the cost of an AOI system can be recovered in less than 5 months. Note the costs of the AOI system include annual service costs and the labor costs to operate the system.

Applications
Table 2 depicts the defect types associated with the respective applications. An analysis should be conducted to determine the frequency the respective defects are actually experienced within your own board assembly operations. This should lead to conclusions about which defects a system must be able to detect reliably as distinguished from those that it would be nice if the system detects as well. In other words, a list of 'needs' versus 'wishes' will result from this exercise. Following the table is a checklist that reviews the questions that a prospective vendor should review. In ultimately determining the most qualified vendor, the answers to these questions should match your operation's specific requirements.

Questions to Ask Prospective Vendors of AOI Systems
As you examine the products from different vendors you will find most make the same claims. It is clearly important to get them to put their claims in writing. The following questions are meant to provide the framework for a systematic analysis of the competitive landscape. The answers given should be consistent with the application requirements anticipated. This list is not meant to be complete. Because of different quality management philosophies within the board assembly industry, the set of questions used should be consistent with your own espoused quality strategy. 

  1. Post solder paste:
    1. Is your system an on-line or offline system? 
    2. Does it perform 100% inspection or sample inspection? In either case, please provide some measure of board density versus throughput? E.g. For board with 4000 components on an 8' board, system can handle 2 sq. in/second or whatever.
    3. Is your system 2D or 3D?
    4. If 2D, what does the system do and what are the specs? Accuracy, repeatability of measurements, etc.? Do you have a recommended calibration procedure to demonstrate accuracy of the system and, if so, what is it?
    5. What is the finest pad pitch that can be handled?
    6. If 3D what does the system do and what are the specs? Accuracy, repeatability of measurements, etc.? Do you have a recommended calibration procedure to demonstrate accuracy of the system and, if so, what is it?
    7. If 3D, how is the height of the specific solder paste pad measured - based on a local reference plane? A global reference plane? How does the system handle board warpage issues?
    8. If 2D is system based on area camera or line scan camera or laser scanner?
    9. If 3D can you describe the fundamental underlying principles for capturing 3D data?
    10. Does your system have difficulty handling a range of solder paste secularity?
    11. How is the system trained to handle a new board design? CAD compatibility? Gerber file compatibility? Train-by-showing? Other? Combination?
    12. How long does it take to train on a new board?
    13. What is the changeover time where boards have been previously trained?
    14. What is the throughput at what specific pixel size?
    15. What is your false reject rate? Escape rate? How have these been demonstrated?
    16. Is there an action that takes place if there are 'x' number of consecutive rejects at the same location? Or 'Y' over the entire board?
    17. Is the system design based on your own proprietary hardware or commercially available products such as frame grabbers or vision processors or is it a host-based processing system?
    18. Do you offer an upgrade patch for future generation products?
    19. How many cameras does your system have and why?
    20. Can you comment on the strength of your lighting arrangement and relevance to the application?
    21. Can you comment on the strength of your optics and relevance to the application?
    22. Does your system have the ability to adapt field-of-view/resolution as a function of the board design for a specific board design?
    23. Does your system have Internet trouble-shooting compatibility?
  2. Pre-reflow:
    1. Is your system an on-line or offline system? 
    2. Does it perform 100% inspection or sample inspection? In either case, please provide some measure of board density versus throughput? E.g. For board with 2000 components, system can handle 2 sq. in/second or whatever.
    3. Is your system 2D or 3D?
    4. Is your system color-based? What advantages does color offer?
    5. What is the finest pitch component that can be handled?
    6. What does the system do:
      1. Component presence
      2. Component missing
      3. Correct component
      4. Polarity
      5. Orientation
      6. Misplaced/offset
      7. Skewed
    7. If it makes measurements, what are the specs? Accuracy, repeatability of measurements, etc.? Do you have a recommended calibration procedure to demonstrate accuracy of the system and, if so, what is it?
    8. If 2D is system based on area camera or line scan camera or laser scanner?
    9. If 3D can you describe the fundamental underlying principles for capturing 3D data?
    10. Does your system have difficulty handling a range of solder paste and board secularity?
    11. How is the system trained to handle a new board design? CAD compatibility? Gerber file compatibility? Train-by-showing? Other? Combination? Do you include part libraries? Do you have the capability for offline programming to avoid system downtime during training on new board design?
    12. How long does it take to train on a new board?
    13. What is the changeover time where boards have been previously trained?
    14. What is the maximum height of a component that the system can handle?
    15. What is the throughput at what specific pixel size?
    16. What is your false reject rate? Escape rate? How have these been demonstrated?
    17. Does board warpage affect results or what amount of board warpage can the system handle without degrading results?
    18. Is there an action that takes place if there are 'x' number of consecutive rejects at the same location? Or 'Y' over the entire board?
    19. Is the system design based on your own proprietary hardware or commercially available products such as frame grabbers or vision processors or is it a host-based processing system?
    20. Do you offer an upgrade patch for future generation products?
    21. How many cameras does your system have and why?
    22. Can you comment on the strength of your lighting arrangement and relevance to the application?
    23. Can you comment on the strength of your optics and relevance to the application?
    24. Does your system have the ability to adapt field-of-view/resolution as a function of the board design for a specific board design?
    25. Can you comment on your fundamental inspection approach and indicate strengths: template matching (discuss basis of template), normalized gray scale correlation, edge segmented-based pattern recognition, vector-based modeling, neural net, other, etc.? Does approach have capacity for continuous learning to adapt to ongoing board/component appearance variables?
    26. Can you comment on your system's suitability for: low mix/high volume operations, medium mix/medium volume and high mix/low volume?
    27. Does the system interface to a rework station? Do you offer a rework station? 
    28. Does your system have Internet trouble-shooting compatibility?
    29. What is the price range of your systems?
    30. What options, if any, are offered for your system?
  3. Post reflow and post wave solder, use the same questions as 2 with the following modified version of (f):
    1. What does the system do:
      1. Component presence
      2. Component missing
      3. Correct component
      4. Polarity
      5. Orientation
      6. Misplaced/offset
      7. Skewed
      8. Tombstones
      9. Solder presence
      10. Insufficient solder
      11. Solder bridges both between leads and between components
      12. Solder wick
      13. Cold solder joint
      14. dewetting
      15. Solder voids
      16. Bent leads
      17. Lifted leads/chips
      18. Solder balls
    2. Can your systems handle lead-thru-hole components and, if so, pre-wave or post-wave or both? If there are different answers to the above performance related questions for the versions of the systems that you offer for pre or post wave applications, please comment accordingly.

Vendors
The table depicts some of the vendors known to be selling systems into the North American electronic market. It is understood that there are over 25 companies aggressively pursuing these AOI applications in the worldwide market. An attempt has been made to identify the specific applications the respective companies suggest their products address. This data was largely obtained from information on their website. An attempt was made to verify this information from the vendors themselves but very few cooperated.

What is Required to Succeed
Bob Ries of cyberoptics offers the following counsel. The vendors must 'start with setting the right expectations about what AOI will and will not do. Then the user and vendor must deploy enough resources (engineering and production) to make it work. These systems do not today (and will never) run themselves. Programming and day to day maintenance take ongoing resources.'

Chuck Gamble at Leica Microsystems amplifies on the vendor's responsibility by suggesting 'The vendor has SIX AREAS where he has to provide a proper level for the AOI implementation to be successful. 1. Repeatable data, both 2D and 3D, 2. Speed, the system must be able to keep up with pulse rates, 3. Programming and ease of use. The system must be able to be programmed in less than 30 minutes and must be capable of being run and programmed by line operators. 4. Reliable and Robust machine performance in a 24 X 7 environment, 5. Support Services, such as training, spares, documentation, global support network, rapid response, etc. and 6. Price - All of this at a reasonable price, …'

 

 

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