By Terrence Thompson, Senior Editor
Despite some impressive system-on-chip (SoC) designs, packages and modules may require more than one IC. Some may handle several monolithic semiconductors and one or more discrete components. How do you know what's good? How do you know what's going to fail in the field? One thing is almost certain: Growing device and package complexity have provided a substantial boost to developers of automated inspection gear. Today, the substrate may be a package, a board, other chips or a wafer stack of ICs, MEMS and/or opto devices. Accor-dingly, product cost, required usable lifetime and liability concerns are among the factors that determine how much inspection and test-if any-are needed. Since familiar ICs and discrete electronic components are being assembled in the same package with MEMS/MOEMS and/or photonic (opto) devices, getting the correct component in the correct place-properly oriented-is becoming a greater challenge. Fortunately, automated optical inspection systems provide an ideal way to validate and verify product quality. Extremely Precise Alignment Needed Keep in mind that MEMS and photonic components require extremely precise alignment, which is typically not required with ICs. Discrete active and passive components must be in the correct location, properly aligned and interconnected, to function properly. Mixed-technology component inspection techniques are becoming invaluable. PWBs, many with microvias and constructed of a variety of materials, add to the inspection challenge. Options and More Options When reviewing defect inspection technologies, there are several choices, including automated optical inspection (AOI), acoustic imaging scanning (AIS), infrared (IR), laser imaging (LI) and moiré interferometry (MI), or combinations, in a given system. (Another option, automated x-ray inspection (AXI) was covered in our October issue.1, 2) Perhaps the first question a buyer must consider is how the machine will be used: inline, stand-alone or both? The next consideration should be the vision system or imaging technology. Vision and imaging systems can be 2D, 3D, 2D/3D, black and white, gray scale, pseudo color or true color. Each technology offers some advantages such as cost, speed or accuracy-and typically a few tradeoffs. It is also essential that you be familiar with the terms manufacturers employ to spec their equipment. For example, camera resolution for the area inspected is a specification that describes pixels per unit area.
Avoiding Performance Loss "Mechanical stability of substrate and package with temperature excursions continues to be a 'hot' point for customers," notes Patrick Hassell of AkroMetrix, Atlanta. "We see an increasing trend for characterization across operating environments and for reliability analysis (i.e., temperature cycling)," he adds. "We are told that optical devices can suffer performance losses with even the most minor of deformations in the z-axis direction." These losses occur even when using stable substrates such as ceramics and metals. Hassell points out that the AkroMetrix machine is able to account for CTE mismatches between device, substrate, adhesives and housing material. The system also considers warpage across temperatures ranging from -55°C to 140°C at the same time. Acoustics Reveal MEMS Flaws Steve Martell of Sonoscan, Inc., Elk Grove, Ill., says, "We have been evaluating MEMS devices for years. In fact, the first C-SAM type system sold specifically to QC a MEMS device was shipped in 1995. Since then, we have seen a slow but steady growth of interest in integrated MEMS and MOEMS devices and the need to evaluate them." He adds that there has been a growing interest in SOI-bonded wafer quality assurance for microelectronics and MEMS/ MOEMS usage. Dr. John Dralla of Electroglas' Inspection Products Division, San Jose, adds, "To help our customers avoid costly damage to expensive die and probe cards, and yet get the critical wafer sort electrical information they need, we are developing AOI methods to spot electrical probe marks on top of electroplated reflowed solder and gold flip-chip bumps."
Electroglas provides a range of inspection, wafer probing, and software technologies and is in a position to observe emerging trends on the sort floor, Dralla says. "Our customers are being challenged to find the proper non-damaging probe card and prober setup on semiconductor die using wafer-level packaging technologies," Dralla adds. Traditional AOI approaches, Dralla reports, rely on probe marks on the under-bump bonding pad to set up an electrical probing system that may cause damage to the bonding layer prior to bumping." What will they do when the pad isn't damaged?" The ultimate goal for every quality assurance program is not to inspect quality into the product, but to assure quality, says Dralla. Obviously, the only reason to employ AOI is so that vendors can guarantee the production of quality products. Cliff Bockard of ERSA Inc., Plymouth, Wis., contends that the use of X-ray and in-circuit test alone does not meet the requirements where BGAs are densely populating a substrate or board. X-ray and other tests must be supplemented by optical inspection of the BGA solder connections. These tests should be specifically designed to meet the demands of highly dense, low stand-off height components such as Tessera's µBGA, CSP and flip-chip packages, says Bockard. David Clark of GSI Lumonics notes, "3D inspection of solder paste and components on PWBs is a very important operation, calling for precise, accurate measurements of critical SMT processes." This inspection, adds Clark, is an important step in process control where the goal is not detecting defects but preventing them in the first place. 2D/3D Now "Real 2D/3D inspection is becoming essential for wafer-level CSP packaging, since you must detect not only coplanarity, height and diameter of bumps, but surface defects such as cracks, chip-outs and contamination on the die too," says Ronny Theriault of SolVision, Ottawa, Canada. He adds this is where a moiréeacute; interferometry-based machine excels. Scans for Tiny Features "We use a high-resolution, PC-based flatbed scanner platform combined with color image recognition and vectorization software to provide off-line and inline inspection," says Bill Loving of ScanCAD International, Inc., (Morrison, Colo.). It has worked on PWB fabrication and assembly for the last 12 years with great success, he adds. "Printed or dispensed materials of all kinds can be inspected, including balls, bumps, dielectrics, conductors and resistors," notes Loving. Since stencil/screen inspection is needed for bumping, the systems can make certain that stencils/screens match original Gerber/CAD data after fabrication or cleaning, eliminating errors before they enter the production line. Inspection Strategies AOI is very widely used and continues to evolve. One recent trend has been improved vision technology resolutions with true color (16 million colors) detection capabilities. In the past, grayscale with 256 shades has been more common. Should your vision system employ analog or digital imaging? The answer will impact the degree of image analysis possible.
Some AOI systems are available with resolutions as high as 16 million pixels/ image. Some systems also employ parametric feature-based techniques rather than the familiar known-good "golden board" approach. Selecting an AOI system is becoming more challenging with the growing number of vision technologies that range from simple black and white through pseudo and true color, all impacted by the lighting schemes employed. Good, but Not Perfect Each inspection technology has numerous merits yet none will catch everything. In-circuit testing is increasingly handicapped by limited access to test points, so more expensive functional testing is required to improve final product screening. Infrared inspection systems can compare known-good powered device, package and board thermal signatures against in-process products. This can reveal abnormal thermal signatures and defects that other systems miss. Many systems now provide statistical data and/or closed-loop feedback to make real-time process changes to boost yields. Assemblies must have the correct parts, be they ICs, MEMS, opto or discretes, in precisely the right location and oriented to function properly. The technology choices must be made based on throughput, required yields and the impact on users if the assembly fails in use. Conclusions Automated optical inspection systems have been widely used at the board level for years for spotting defects and using those defect evaluations to help improve yields. The Inspection System Suppliers listing that follows will help make the selection process easier. References 1. "Shrinking IC Package Sizes Are Driving the Quest for More Powerful X-Ray Inspection," Ron Iscoff, Chip Scale Review, pp. 53-55, October 2002. 2. "X-Ray Inspection System Suppliers," Chip Scale Review, pp. 57-60, October 2002.
|
Copyright © 2002