Laser-based inspection aids material characterization
Vinod Mohan, Emerson & Cuming, Billerica, MA -- Test & Measurement World, 12/1/2004 2:00:00 AM
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When the Emerson & Cuming research team needed to characterize freshly dispensed (wet) electronic materials for R&D and process development, we were hampered by a lack of equipment that was able to acquire the necessary data. This presented a bottleneck that we solved by using a laser-based noncontact metrology system. Coupled with software tools, the metrology system enables us to acquire and analyze data about a wide range of materials and their characteristics, such as reflectivity and translucence.
Emerson & Cuming develops high-performance materials for the electronics industry as well as specialized materials used in other industries. Our facility includes an R&D site where new materials and processes are characterized, tested, and developed.
Since many of our products start out in liquid form, the manner in which they are applied to the product—as well as the shape and volume—is extremely important, as are the changes that the material may undergo as it transitions from a liquid to a solid state. Thus, in many applications, the physical profile of a dispensed product is extremely important. For example, the profile of a bead of adhesive at several stages—immediately after it is dispensed, prior to curing, during curing, and after curing—tells a great deal about a product's changes and performance during the critical curing process. Measurements taken at each stage relate directly to constraints within the application.
Inspecting the material involves more than simply measuring dot circumference; we need to precisely quantify the volume of material dispensed in a single dot. Typical measurement devices, such as contact gauges, can't measure the dimensions of a dot of liquid material, and we often measure materials immediately after dispense.
We needed a metrology tool that would let us make precise measurements of even wet materials in a noncontact manner. One major concern was whether the tool would be able to provide accurate readings across the range of products, which not only vary in color but also often have shiny or reflective surfaces. Some products, when applied to the substrate, offer a fairly low contrast with respect to the substrate. Therefore, we needed a noncontact-based system (laser or optical) that could accurately differentiate the material from the substrate.
We considered using an x-ray inspection system that was installed in our facility. When used with image-analysis software, the system can provide some quantitative figures, but it has limitations in terms of 3-D measurements. We need the 3-D measurements in order to characterize a dispensed pattern of wet material without touching it; especially when measuring a fairly large number of these small entities, typically in the 10-mil diameter range.
Noncontact
profilometer
We finally chose a CyberScan Vantage from Cyber Technologies. This laser-based noncontact inspection system combines a digital sensor with x- and y-translation stages for scanning any surface. It collects a series of high-resolution z-height measurements to create a 2-D profile (Figure 1). In addition, it performs a raster scan of a measurement location to produce a 3-D topographical map (Figure 2). Such noncontact 3-D inspections are required for microscale objects that are wet, fragile, pliable, or highly contoured.
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Fig. 1 High-resolution measurements produce a 2-D profile of a ball-grid arrray (BGA) semiconductor package. |
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Fig. 2 This raster scan from an inspection system shows a 3-D topographical map of a semiconductor wafer. |
The digital sensor automatically compensates for surface reflectivity variations and self-adjusts to obtain accurate measurement data from shiny, translucent, or multicolored surfaces. The software provides comprehensive profile measuring, such as height, area, length, roughness, slope, and radius. In 3-D mode, the volume, planar angles, 3-D height, and areas can be calculated.
With the AutoScan software feature, the system can be programmed to recognize and compare an array of dots, for example, with a preprogrammed "grid." The machine will compare the array with the grid and let the user know which, if any, objects do not conform or are out of spec.
Process benefits
The immediate benefit of implementing this system was that it let us assign quantitative numbers to products and processes. Measurements were precise, and the procedure was much faster than mechanical or contact measurement. Being able to characterize material while it is in a wet or uncured state saves a tremendous amount of time, because we can adjust or tweak the process quickly—in real time—without having to wait for the product to cure. Additionally, a material will change in dimension from a wet to a cured state, so the ability to accurately characterize a dot of wet, freshly dispensed, uncured material has tremendous importance.
Over time, we'll be able to develop methods of predicting how a material will slump after dispense, by measuring the same dot twice and comparing the data. With fillet characterization, we'll be able to determine what fillet shape, thickness, radius, and other dimensions a material will have when it is on the final product, or circuit board.
Time saved, measurement accuracy, and the ability to characterize wet materials and materials of all types, colors, and reflectivity—plus the comprehensive level of data now available—have had a significant positive impact on various process-development sequences. We expect that noncontact laser-based measurement technology, combined with appropriate software tools, has additional process improvement potential that still remains to be discovered.
| Acknowledgements | ||
| The author would like to acknowledge Matt Laffey and Mathew Eveline of Emerson &Cuming for their support in developing macros and routines to characterize material behavior in various challenging applications. | ||
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