Wafer processing advancements and wafer testing now provide us with high-quality functional die. However, the high I/O node counts and tight line spacing common in most of today’s ICs present a new set of problems. It is becoming more difficult to provide interconnection reliability. Open and short circuits within the IC package as well as opens between the IC and the PCB are becoming an increasing concern.
Today, many assembly operations are performed automatically, calling for tight control of all dimensional parameters. Chances for positional misalignment or unacceptable volumetric depositions exist at the die, the package interconnection points, and the IC package-to-PCB interface. To ensure the dimensional integrity to achieve high-quality interconnections, a variety of inspection processes is being used at every level.
Wire-Bonding and Bump-on-Die-Pad Inspection
The circuits on a die are accessed through conductive pads, produced by a combination of etching and electrodeposition. Connections between the pads and the external IC package terminals—leads, pins, or ball grid arrays (BGA)—may be made in a variety of ways.
Wire bonding is a predominant method. Most automated wire-bonding machines accommodate gold or aluminum wires.
With gold wire, the process used usually is ball bonding. The bonding tool feeds a short piece of wire toward the pad, and the tip of the wire is melted to form a ball which is compressed and attached to the pad. The other end of the wire is formed in a crescent shape and attached to a lead frame.1
The wire-bonding operation is so fast that human operators find it difficult to perform a 100% inspection. Fortunately, automated optical inspection (AOI) systems now keep pace with the attachment process and are precise enough to deal with today’s small IC geometries.
Wire-bonding inspection tasks performed by AOI systems include exact sizing of the bond, the location of the wire and ball. AOI systems also check ball size, crescent size and shape, crescent position, and wire height. Typical rejection limits may be set automatically. Providing timely pad size and location measurement data feedback aids wafer-process control. Wire-related inspection data is used to adjust wire-bonding machine parameter settings.2
An alternative connection method growing in popularity deposits bumps on the pads. The bumps are mated with an intermediate interconnection medium, such as a polymer film carrying a pad-matching pattern (as in tape automated bonding), or are directly interfaced with conductors extending to the exterior of the package.
To ensure proper mating, the size and position of these minute bumps are critical. As a result, AOI equipment now is specifically developed to perform these
measurements. Some wafer-bump inspection systems, for example, use a combination of 3-D and 2-D gray-scale data to assess the diameter, height, position, and volume of the bumps or balls deposited on the pads, their coplanarity, and the regression plane.
Lead and Ball Terminal Inspection
Today, most ICs are surface-mount devices (SMDs) with fine-pitch leads or high-density ball-grid I/O connections. For leaded devices, lead dimensions, lead spacing, and their shape are now critical since less solder is used for connecting them to PCB pads to avoid creating shorts between adjacent terminations.
To assure proper lead integrity, more AOI systems have been installed at semiconductor and electronic assembly plants. Inspection tasks determine relative lead height using the lead frame as a reference, positional tolerances referred to the package outline, and skew measurements.
Single or multiple light sources combined with single or multiple cameras or laser scanner and sensor arrangements obtain the required 2-D and 3-D data. To minimize process steps, AOI systems performing lead inspection often are used in conjunction with lead-forming equipment or device handlers.
A different set of challenges is being posed by BGA packages which, instead of using leads, provide connections through an array of solder balls located on the underside of the device. While offering space savings and improved electrical characteristics over other SMD configurations, their connection integrity is difficult to verify after they are soldered to the PCB.
More importantly, if the interconnections between the BGA package and the PCB are faulty (shorts between solder balls or opens), package removal and rework are difficult or impossible. Consequently, it is essential that semiconductor manufacturers ensure that all the solder spheres or balls on each BGA device are accurately placed and properly formed.
Solder balls are attached to the device package by placing them onto the internal interconnection pads using gravity feed and a pattern-matching tooling plate or through vacuum transfer. During a subsequent reflow operation, the balls are soldered to the device. AOI systems inspect the packages and ball arrays at the ball-attach phase and after the reflow phase.
Inspection operations at these two phases include verifying the presence or absence of solder balls, measuring the spacing between them, and determining their size and roundness. Ball placement accuracy usually is 0.001″ @ 3 sigma.3
In addition, the coplanarity of the ball seating plane must be assessed. If the substrate carrying the die and providing the base for the pads or the package itself is not completely flat, then the top of the solder balls will not lie in an ideal seating plane. If the contact pressure applied during final test is too high or uneven, the ball shapes may become distorted, affecting the coplanarity of the device.4
Some AOI systems inspect leaded as well as BGA devices using different algorithms to process the acquired raw data. Others deal exclusively with one style.
In addition to performing mechanical inspection tasks, several AOI systems assess the quality and legibility of the identifying markings on the device packages. For some applications, dedicated mark inspection and character verification systems may be preferred.5
Assessing IC-to-PCB Interconnections
Reflowing solder paste is the primary SMD-to-PCB interconnection method used today. While providing the advantages of low cost and compatibility with most SMD package styles, the process requires tight control of the amount and location of the paste. This is especially true for high-density devices where a slight misplacement of paste can result in solder bridges or an insufficient amount of paste can cause an open or marginal connection.
“Proper solder-paste deposition is the foundation of a good surface-mount process,” emphasized David Clark, director of the SVS Product Group at View Engineering. “If the paste application is not correct, there is a good chance that joint defects will occur. However, by inspecting the solder paste, the deposition process can be kept under control and defects can be avoided.”
Good process control is most important when BGA packages are involved, since visual defect identification and solder touch-up are not practical because the solder joints are under the device. “Initially, process engineers concerned about interconnection quality insisted on X-ray inspection of BGA joints, but they find that the key to a good BGA assembly process lies in controlling the solder-paste application,” commented Dominic Haigh, business unit manager, Control Automation Group, Teradyne, Assembly Test Division. “Today, users watching the solder paste now report 1- to 3-ppm defect rates for BGA assembly.”
The same AOI system used for solder-paste inspection may carry out post-reflow joint-quality evaluations. It also can perform other inspection tasks, such as determination of component presence and their orientation. AOI is limited only because the item to be assessed must be visible; otherwise, X-ray techniques must be employed.
References
Hanneman, R. J., et al, Physical Architecture of VLSI Systems, John Wiley & Sons, Inc., 1994.
Schmitt, L. A., “The Automated Inspection of Wire Bonds: Vision Techniques and Their Performance in Production Installations,” Semicon West Proceedings, 1994.
Caracappa, E., “Advantages of Machine Vision in BGA Manufacturing,” RVSI/Acuity Corp., 1997.
Smets, C., et al, “Using Vision for Lead Inspection and Coplanarity Measurements,” Semicon West Proceedings, 1994.
Snyder, C. J., “Machine Inspection and Optical Character Verification in the Semiconductor Industry,” International Robots and Vision Automation Conference, May 1997.
IC Inspection Products
System Measures BGA
Sphere Placement and Quality
The PowerBGA™ is a 2-D vision inspection system for ball grid array (BGA) devices. You can customize field-of-view device layout parameters, BGA pattern descriptors, and tolerances for proper monitoring. A setting dialog box allows you to configure global settings such as reporting, I/O, graphics, and hardware. Ball-placement and pitch-measurement accuracies at 3 sigma are 0.001″ and 0.002″, respectively; and the speed is 1,200 spheres/s. The ball diameter range extends from 0.012″ to 0.040″. Acuity Imaging, (603) 577-5845.
Solder-Paste Inspection System
Interfaces With Repair Station
The InterScan™ 5507 A/B AOI Systems perform solder-paste inspection. They analyze stencil alignment, find clogged stencil patterns, carry out 3-D analysis of selected pads to measure paste deposit thickness, and determine the percentage coverage of solder paste over pads. Inspection also is performed for solder-paste bridging. A component-template-based teaching methodology or CAD-based data entry may be employed. The field of view is 0.4″ for the 5507A and 1″ for the 5507B. The corresponding speeds are 2 to 3 in.2/s and 3 to 4 in.2/s. Teradyne, Control Automation Division, (510) 932-6900.
Double-Speed 3-D Laser Scanner
Speeds Solder-Paste Inspection
The new double-speed 3-D laser scanner doubles the data acquisition rate of the Model 8100 SMT Inspection System. Available in two resolutions, the scanner measures solder-paste volume down to a 12-mil (0.3 mm) pitch. It is an option on new Model 8100 Systems or available as a field upgrade. The Model 8100 performs high-speed, in-line data acquisition and image-processing and provides data for SMT prototyping, process characterization, and real-time process control. SVS Products Group, View Engineering Division of General Scanning, (313) 975-7664.
Mini Mirror Helps
Inspect BGAs
The Ball Grid Array Mirror helps inspect the outer and inner rows of solder balls on a mounted BGA device. The mirror is hand held or self standing and measures 0.754″ L × 0.190″ W. A diamond coating helps protect the mirror from scratching. Metron Optics, (619) 755-4477.
Wafer Bump Inspection System
Offers 3-D/2-D Gray-Scale Data
The Model 890 Automated Inspection System for wafer, die, and package substrate bump inspection features a 3-D scanning laser. It acquires, analyzes, and displays 3-D height information and 2-D gray-scale data at a rate of several millions of pixels/s. The software allows you to monitor process measurement histograms, trend plotting of individual balls, and percent yield, pass, fail, and rework. Measurements include ball height, missing ball, position, volume, coplanarity, regression plane, and ball diameter. Semiconductor Products Group, View Engineering Division of General Scanning, (805) 522-8439.
Solder-Paste Inspection System
Is 3× Faster Than Prior Models
The Sentry™ 2000 In-Line Solder-Paste Inspection System uses dual 200-MHz Pentium® Pro Processors running Windows NT. Typical inspection time for a 13 x 13, 50-mil-pitch BGA site is less than 8.5 s. Sentry measures solder-paste height, area, and volume at multiple user-selected sites per board. Measurement data can be saved to a diskette, hard drive, Zip™ drive, or network drive for SPC analysis. CyberOptics, (800) 746-6315.
Vision Processor Inspects
Ball Grid Arrays
The BGA Inspection Package is a PCI-bus plug-in vision processor with accompanying software to inspect BGAs. It inspects BGAs for missing, misplaced, or improperly formed solder balls at a rate up to 4,000/s. A Windows™-based software package helps you describe the proper ball pattern. The system also performs calibration to convert training and measurement parameters from pixels to physical units and to correct for camera skew, optical distortion, and scaling. Cognex, (508) 650-3000.
Solder-Paste Inspection System
Features 2-D, 3-D Capabilities
The 1820/L Inspection System checks solder paste as well as components on PCBs. An integrated laser adds a 3-D inspection capability to a fourth generation 2-D system. The 1820/L features a statistical process control package and user-definable procedures for a variety of inspection needs, including 100% checking of all components in 3-D. Machine Vision Products, (800) 260-4MVP.
In-Tray Lead Scanner Provides
Dual Inspection Sites
The LS-3900DB In-Tray Lead Scanning System has enhanced sensor electronics and more streamlined software than previous versions in the LS-3000 Series. It provides dual inspection sites to eliminate tray-to-tray handling time. Throughput of quad flat packs is up to 2,800 units/h. Options include automatic Z adjustment, solder-flake detection, and mark inspection. RVSI, (516) 273-9700.
Inspection Machine
Analyzes Solder Paste
The Panasert IPJ-V uses color line-scanner technology for area-calculated solder-paste and surface-mount inspection with 30-µm resolution. The automated inspection machine performs in-line quantitative analysis of solder-paste printing and component-mounting conditions. It detects blurred, smeared, bridged, misaligned, or missing solder paste as well as missing or misplaced components. Panasonic Factory Automation, (708) 288-4400.
Copyright 1997 Nelson Publishing Inc.
September 1997
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