Fast Moiré Interferometry
08/01/2002
COVER STORY
Inspecting leadless packages
BY RONNY THERIAULT
Semiconductor manufacturers are pressured these days to decrease the size of components while increasing their functionality. Driven by a new generation of electronic devices, ranging from portable hand-held devices to large boards used in telecommunications routers, new integrated circuit (IC) packages have been developed based on quad flat no-lead (QFN) and micro lead frame (MLF) designs to maximize silicon real estate and boost performance.
These devices do away with large gull wing leads in favor of micro pads (planar exposed frame contacts) that can be densely packed into areas of a few dozen square millimeters. The reduction in component size, coupled with the accompanying functional requirements (greater thermal dissipation in QFN designs, reduced power consumption and greater logic ability), has resulted in smaller, feature-rich and expensive-to-manufacture packages. Consequently, as the value of these micro frame components increases, so does the need for greater component inspection. Unfortunately, much is unknown regarding the defects associated with the various steps of manufacturing QFN packages.
Inspecting Leadless Packages
New "leadless" packages are pushing the limits of automated optical inspection (AOI). Experts estimate that a standard 0201 form factor component requires inspection systems with a minimum resolution of 20 µm in three dimensions. In some cases, that figure can be even smaller, depending on the type of defect (such as incorrect laser mark vs. plastic cracks, flashing and burrs).
Traditional approaches, including laser scanning, multi-camera stereovision and interferometric techniques, can achieve 2-D and sometimes 3-D inspection of defects on thin shrink small outline packages (TSSOP) and ball grid array (BGA) packages, but they fail to deliver a complete 3-D inspection capability for high-speed lines that produce QFNs, TSSOPs, BGAs, and advanced packages.
 Figure 1. Although manufacturing processes vary from fab to fab, each step of the QFN manufacture process requires careful inspection to maintain high yields. |
A technology called fast Moiré interferometry (FMI) was first deployed in 1999 for difficult inspection challenges with BGAs and TSSOPs. This technology simplifies the inspection hardware by using only one camera, one moving grid filter, and one light source to collect X-, Y- and Z-axis volumetric pixel acquisition data for each pixel position with resolution down to 3 µm. Systems using FMI technology have been shown to be useful for identifying micro defects associated with QFN, MLF and other types of micro lead packages. The inspection speeds can be up to 20,000 units per hour (UPH), with low false failure rates, using turret feeders.
This article will detail the QFN manufacturing process with special emphasis on defects associated with packaging steps and how an inspection system can identify these defects at high speeds.
The No-lead Packaging Process
As I/O counts increase and shrink with QFN designs, and because no-lead architectures require even tighter planarity and pitch requirements, automated inspection processes are used at more steps of the manufacturing process. As shown in Figure 1, a QFN package typically is inspected at many points in the fab process: before and after lithography; after die and wire bonding; and after molding, singulation and packaging. The complexity and cost of these components, along with dropping margins, require that testing be done - not only for customer satisfaction - but also to correct process-related defects as soon as they happen. During the molding stage, bubbles, contaminants and variations in flow pressure can lead to voids in the plastic casing, grooves between contact pads, and other surface irregularities that affect both the aesthetic quality of the IC and its ability to make contact with the PCB. Deflashing, cutting the lead frame sections into strips, and saw or punch-out singulation can lead to chip-outs and cracks in the plastic molding.
Other defects associated with these steps include "bridging" leads (such as when the saw deposits metal fragments between contact pads that could lead to a short), "smeared" leads damaged during the saw or punch-out process, and overexposed lead frames. Mechanical steps in the process also can add to coplanarity mismatch among contact pads, or excessive pitch variations of the lead frame or package. Laser marking can experience mechanical failure of the write beam or control mechanism, or put on the wrong identifying data. Finally, major manufacturers of QFN components have reported a new defect associated with tape remnants stuck to the exposed lead frame that are not removed during chip wash. After reviewing the variety of defects, a final inspection of the chip package becomes an obvious requirement. However, this inspection cannot bottleneck the manufacturing process.
Inspection Choices
Choosing the right tool for a job requires more than an answer to the simple question, "Can it work?" Today's competitive market requires that a tool be capable of identifying 100 percent of the defects with low false reject rates - and at speeds that keep the cost of inspection per component to a minimum.
Several solutions exist for chip package inspection, but many of these systems cannot detect defects less than 10 µm in diameter. Laser triangulation, for example, offers high-resolution inspection. A laser scans across the IC and a position-sensitive detector measures the reflected light, which carries information related to the Z-position for each point of the object being tested. Although lasers have dropped in cost and complexity in recent years, speckle or optical noise related to the nature of coherent light continues to be a problem for most low-cost laser sources. Also, a laser with a 30 µm beam diameter cannot resolve items smaller than 30 µm without complex processing.
Perhaps the greatest challenge using a laser triangulation system is its dependence on multiple mechanical systems. Laser triangulation requires high-precision scanning mechanisms (usually spinning mirrors) and precise stage controls to deliver full 3-D data. A failure among any one system can halt the inspection system. While fabs contain some of the most complex machinery in the manufacturing industry, any addition to the complexity can be a drawback. Simplicity reduces operational costs and reduces production downtime related to maintenance.
Scanning systems take longer to inspect a single part than systems that process a wide field of view. Scanning systems also require that the part be placed carefully in relation to fiducial marks before scanning, adding to system cost through a vision-guided pick-and-place support system.
Stereovision systems, composed of two charge-coupled device (CCD) cameras linked to dedicated image processing hardware, provide larger fields of view but also add complexity because of the need for multiple imagers. These systems take several pictures of a single part simultaneously. Based on small variations between the same pixel location in different images, image-processing boards determine the X, Y and Z position of all points within the field of view. High-resolution inspection using stereovision requires either interpolation of image data smaller than pixel sizes or high-numeric aperture optics for each of the cameras, adding system costs, vibration issues and complexity. Calibration of multiple camera heads on a single field of view also increases integration time and complexity, while increasing potential operational and maintenance operations.
Moiré Interferometry
Moiré interferometry is an accepted method for reducing calibration requirements while boosting resolution of automated inspection systems. For years, interferometry has been used in many industries, including electronics, as a high-precision inspection technique. In the past, however, systems that offered the highest resolution tended to be the slowest, typically using a pair of scanning laser beams - one for reference, one for measurement - and collecting precise Z-coordinate data based on the phase difference between the two wave fronts.
Finding a simpler system with the same resolution led to system designs that replaced the laser with a noncoherent light source coupled to a projection grating. This fringe skeletization technique projects a series of lines on the component surface. Distortions in the projected lines indicated height along the Z-axis relative to a reference plane, but these systems lacked the resolution required by the electronics industry. Further enhancements led to Moiré interferometry, where a second grating was introduced in front of the camera.
Passing light through two gratings produces a Moiré interferometric pattern similar to the effect one sees while looking through two chainlink fences. By observing the distortions of the Moiré pattern, inspection systems can deduce the Z-coordinate relative to a reference plane for each pixel with higher resolution than skeletization and without processing overhead. However, the algorithms and mechanical systems needed to extract both 2- and 3-D data using a Moiré interferometric approach typically have been too slow for high-speed inspection of semiconductors.
FMI
The next step in IC package inspection was combining the best of fringe skeletization and Moiré interferometry to create FMI. A modular system was designed for integration into existing QFN or similar package circuit test equipment, or as a stand-alone on-line or off-line inspection system. It simplifies the mechanical systems while optimizing load sharing of the algorithm among multiple microprocessors.
This system uses one digital CCD camera positioned directly over the part or tray to accommodate the widest possible field of view and limit false rejects because of occlusion of the field of view by variations or structures on the package surface. Simple white light is delivered to the object under test at a 30° angle off the vertical axis through fiber optics. A servo-controlled grating is positioned in front of the light aperture. The grating is moved and an image is acquired four times per inspection cycle in less than 150 ms. The four images are superimposed one atop the other using a workstation for fast volumetric pixel acquisition (Figure 2) and a complete X-, Y- and Z-topography for the entire field of view. AP
Conclusion
A new approach to package inspection - FMI - is suitable for the new no-lead micro packages, such as QFN. The speed (20,000 UPH) and resolution (4 to 5 µm) address the throughput and geometric requirements of these package types.
*MLF is a trademark of Amkor.
**FMI is a trademark of SolVision.
Ronny Theriault, worldwide sales and marketing manager, can be contacted at SolVision, 2111 Boulevard Fernand-Lafontaine, Suite 130, Longueuil, Quebec, Canada J4W 2V6; (450) 679-9542; Fax: (450) 679-9477; E-mail: [email protected].