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Industry Experts voice differing opinions on Golden board vs. Parametric
December 31, 1969 |Estimated reading time: 4 minutes
The AOI Revolution:Where does it Really Matter?
Parametric measurement stems from the simplest of applications for automated optical inspection (AOI) in the printed circuit board assembly (PCBA) industry — component placement inspection. Restricted mainly to cell phone manufacture, industry analysts state that it accounts for about one-fifth of the total inspection market today. The vast majority of AOI investment is in solder joint inspection where more sophisticated and mature solutions exist and the IPC-610 standard applies.
Post-placement inspection is fully harnessed only when post-reflow inspection is not possible. In addition, post-placement component offsets self-correct during the solder reflow process. However, there is a considerable population of defect types that result from nonplacement-related effects due to pad, solder and component interactions in the reflow process environment under the high-temperature conditions and the resulting mechanical-related board stress effects. Opportunity for defects is significantly higher after reflow or wave soldering and this becomes more important with the implementation of 0201, finer pitch components and higher lead-free reflow oven temperatures.
Golden-board inspection methods offer ease-of-use in new product introduction (NPI) or high-mix environments in which fast programming and rapid product quality feedback are critical in the high-mix, low-volume environment. However, this must be supported by the more robust statistical analysis of solder joint quality for volume production with a copy exact system. Ease-of-transfer to high-volume production sites is vital.
The current available automated inspection technology has been in development for many years and golden-board inspection is a subset of AOI today. More importantly, it is the rapid advancement of vision algorithms and statistical modeling that has advanced the performance and value of AOI.
Paste inspection also is important to defect prevention. Paste coverage and height parameter control are important in identifying systematic defects from stencil printing and today's PCBA quality systems combine in-line paste and solder joint AOI.
Greater AOI defect coverage provides an effective low-cost repair solution prior to in-circuit test (ICT) or functional test. As a result, the industry has begun to traverse the next phase of the AOI implementation — test optimization. Test optimization will be achieved by exploiting the confidence that solder joint AOI provides and has opened the way for ICT probe reduction and the possible extinction of ICT altogether for some assemblies.
That's where I believe it really matters.
David Doyle, marketing manager, Electronics Assembly, may be contacted at Orbotech Inc., 44 Manning Rd., Billerica, MA 01821; (978) 901-5114; Fax: (978) 667-9969; E-mail: davidd@orbotech.com.
SMT Evolution = Parametric Revolution
Process capability (Cpk) is a static assessment at a point in time, but time waits for no Cpk figure. What matters is process performance in the morning vs. the afternoon, and on Monday vs. mid-week.
The fatal weakness of golden-board inspection is its exact-copy nature. Unlike the human inspector, an image correlation inspection must encounter the same precise image, or a fault is declared. A common complaint concerning golden-board systems is the need to continuously expand the reference image library as normal variations are encountered. The simple-but-inflexible nature of golden-board inspection is its downfall.
AOI is just another case of computer graphics, and graphical representations come in either vector-object or bitmap forms. Vector-object applications allow geometric object placement, and then provide the ability to move or stretch the objects later. Bitmap applications allow object placement, but once placed they lose geometric identifies and cannot be altered. Parametric inspection is analogous to the vector-object graphics, while golden board is bitmap-oriented. Under the parametric model, components on a circuit board are a set of geometric shapes related to one another by position and rotation. The ability of parametric inspection is not limited to outside edge detection, but to internal component feature detection and measurement. While golden-board inspection evaluates the entire component as a whole object, parametric inspection assesses internal features as well as outside component edges.
Color is crucial to parametric inspection. Parametric feature extraction requires color-contrast analysis. Most systems use grayscale systems evolved from pick-and-place part verification that use back lighted silhouette to ensure extremely high contrast. Unfortunately, circuit board component inspection requires a front lighted image where shadow contrast is not present. Only full-color vision provides the feature contrast required for parametric inspection.
Software cannot create information where it is not available, and no level of software sophistication can compensate for weak measurement. There has been a dramatic expansion in the availability of process control software applications, but this activity has been based on the hypothetical availability of key control parameters. The promise of these tools cannot be realized until real-time data becomes available. There is no other source of this data except AOI. AOI must cross over the line to true parametric measurement, and get on the path to full participation in the SMT evolution.
Robert Kane, VP of R&D, may be contacted at Vectron Inc., 10109 Carroll Canyon Rd., San Diego, CA 92131; (858) 621-2400; E-mail: bkane@vectroninc.com.