AOI: The Key to Controlling the Process, Part 2 -- In-process inspection is needed, but where should it be applied?

I am very fond of presenting a study in my seminars that has been attributed to ATT (Burlington, NC). An inspector was given 1,000 printed circuit board assemblies (PCBAs) with moderate component density and asked to inspect one side for soldering defects. The experienced inspector dutifully went about inspecting each board and marking the defects with little red arrows. As he went on to the next assembly, the one he had just completed inspecting was taken into the next room. Unbeknownst to him, the boards were serialized and for each board, the defects he detected were recorded for the board. The stickers were removed and the board was stealthily reinserted into the pile of boards to be inspected.

Rather than seeing 1,000 different PCBAs, he was actually inspecting the same 50 boards 20 times each. Much to the surprise of everyone, he agreed with himself regarding where a defect was (or wasn't) 44 percent of the time. In other words, more than half the time, he did not agree with himself as to whether a defect was present or not. Subjective, you say? Try this at home for yourself! Actually, these ATT inspectors were good-a number of my clients have done this test with their human inspectors and the agreement is usually in the area of about 35 percent. PCBA inspection is an area that is screaming for automation.

Post-Soldering Inspection

With regards to the assembly process, the key inspection points are post-solder paste print, post-component placement and post-soldering. However, assuming you are one of those very few, underprivileged process engineers who does not have an unlimited budget, where would you prioritize? Post-soldering inspection would seem to be the best investment. After all, a system placed at this point of the assembly process will detect any defects generated by the assembly process such as solder opens, solder shorts, missing parts, wrong parts, misoriented parts and skewed parts. Some of the defects can be caused by misprinting or by the pick-and-place machine. Hopefully, when detected, not only are the defects noted for repair, but also the root cause is rectified at the appropriate machine center. Another advantage of post-soldering inspection is that some of the defects detected include soldering defects caused by design deficiencies such as component shift due to pad imbalance, solder balls and tombstoning.

At APEX 2001, at least 25 different manufacturers of automated optical inspection (AOI) systems featured post-soldering inspection capability. However, inspecting a solder joint is not an easy task. Ideally, the joint is shiny, but the fact that it reflects light makes it difficult for the optics to see. Just how does one define or quantify a good solder joint? They all don't look exactly like the pictures in the IPC-610 specification, do they?

Granted, all systems learn by operator intervention on successive assemblies, and some of the better systems can measure the volume of the solder joint. Hang in there, the technology is getting better thanks to improvement in enabling technologies-better optics and faster microprocessors capable of carrying out more complex algorithms faster. Lots competition means that post-soldering inspection equipment will become more accurate, more versatile, quicker and will cost less.

Post-Print Inspection

While the spotlight, glamour and attention seems to be focused on post-soldering AOI, don't neglect post-print inspection. A prime motivation might be the fact that printing is such a variable and operator dependent operation. Even with the highest degree of automation, stencils and squeegees wear, solder paste reacts with the atmosphere and people make mistakes. A legendary industry study, attributed to Hewlett-Packard about 15 years ago, found that between 50 and 60 percent of assembly process-generated defects were due to solder paste or the deposition process. Try this yourself-take a look at your defect data and, even if your process is running at 3 sigma, you will find that at least half of the defects that are occurring are happening in the printing process. So, doesn't it make sense to catch those defects and correct the cause before you add value (components and time) to the assembly?

Post-print inspection is a very mature and capable technology. Solder presence and volume can be determined quickly and accurately. If ball grid arrays (BGAs), microBGAs, chip-scale packages (CSPs) or other area arrays are incorporated on the assembly, solder paste height monitoring is of paramount importance. Post-print inspection can be relatively inexpensive with benchtop systems for process qualification, starting around $20,000 and in-line systems available in the $80,000 range. These systems are far less expensive than post-soldering inspection. Of course, the task is easier, more straightforward and also lends itself to either full-or partial board in-line inspection or offline in an acceptable quality level (AQL) arrangement. AQL, with first article inspection and every nth board inspection, assures the process is correct. In-line, every article inspection assures the product is correct.

Post-Placement Inspection

Often overlooked is post-placement inspection. After all, pick-and-place is a very repeatable process, but, when people are involved in setup, the wrong feeder can be placed in the wrong location or the component manufacturer could incorrectly mark the tape reel. Due to a lack of standardization of pin 1 location and denotation on integrated circuits (ICs), the pick-and-place machine cannot assure the proper orientation of the component prior to placement. If the machine is fed incorrectly, the component will be placed incorrectly. While the optics on a number of machines could indeed note the orientation of the IC (by the direction of the writing), such checking would slow the system to a crawl. A post-placement inspection system can verify proper component at a very rapid rate and while the component is in wet solder paste. Finally, how often have you seen a line with a person situated between the last placement system and the entrance to the reflow oven tweaking parts

into the proper location?

Conclusion

Regardless of where you put an AOI system, one major benefit common to all systems on the market is that they collect and tabulate defects observed. Many of the systems can both enumerate and report these in real time in any form you might desire-including statistical process control (SPC) and Paretto. I cannot overstate how important defect data collection and review is to any process, so I'll save that for another column.

I previously alluded to the fact that, in many applications and many lines, AOI will replace in circuit test (ICT). The speed, accuracy, versatility (no tooling) and cost will drive this evolution very rapidly. Don't put your ICT on the blocks just yet, though, as some perfecting of the systems and methodology has yet to come. Isn't it interesting how forward thinking ICT manufacturers like Agilent and Teradyne have gotten involved in AOI in a big way? It would seem that the spray painting is on the wall!

Remember, we're all in this together.

Phil Zarrow is president and surface-mount process consultant for ITM Consulting, Durham, NH; [phone]; www.ITM-SMT.com

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