|Wire sweep is deviation of the wire from a straight line to a curved or swept line, and can result in broken wires and bonds. |
X-Ray imaging offers a non-destructive, safe method to analyze the quality of manufacture for IC components of all types. X-ray imaging reveals the internal structural attributes of the package and complements visual microscopic inspection techniques that are used to evaluate external characteristics to detect defects in IC packaging.
The four main defect areas are Mold Voids, Wire Sweep, Die Bond Voids and Lid Seal Voids. X-Ray imaging is the only non-destructive method that allows manufacturers to quickly inspect the products for defects and make changes to the manufacturing process in a real-time environment. Destructive methods of inspection like micro-grinding do not provide instant feedback to the process engineer; x-ray inspection provides instant non-destructive feedback without the need for extended setup times. The typical x-ray inspection routine takes about three minutes from start to finish and requires no special fixturing.
Manual x-ray systems typically require the operator to adjust the x-y-z position, the mA-kV and magnification manually to allow for part position and density. X-ray systems with programmability allow manufacturers to inspect multiple components quickly and efficiently. The introduction of fully programmable x-ray systems has eliminated the need for operator intervention to compensate for part positioning and component density. The inspection routine setup is a point and click process and requires no software programming. The number of inspection routines is limited only by hard drive storage space.
IC Packaging. As conventional packing technology is extended to meet the new challenges, the production process must cope with an ever-decreasing tolerance for error. The need has never been greater for instant process monitoring in the IC manufacturing process. Using a real-time, non-destructive method like x-ray inspection to monitor the internal structures of the IC package during manufacture is critical.
Increasing Die Size. Die size is influenced by either the device complexity or device I/O. Complexity dictates die size as the area necessary to fabricate the number of transistors required by the device function. Device I/O dictates die area for interconnection bonding pads. Both influences drive the industry to larger die sizes and create a challenge for the die attach process to produce a voidless die bond.
Increasing I/O Counts. As device I/O increases, wire bonding as an interconnection process becomes less desirable for several reasons. First, wire bonding is a serial process and packaging throughput decreases with increasing I/O. Second, wire length generally increases with package pinout, raising interconnection impedance and compromising performance. Also increasing bond wire length leads to an increased susceptibility to wire sweep during the molding process. Wire sweep issues may result in catastrophic defects such as broken wires, shorted wires or latent defects caused by stress. Monitoring wire sweep on a regular basis allows for process feedback and control.
Reflow Assembly. A surface mount package is subjected to much more severe stress during board assembly than conventional through-hole designs using DIP packages and wave solder methods. Mold voids become a threat to component reliability because they can compromise the package's resistance to moisture penetration under assembly conditions.
Molded Packages. Molded packages are used in products that are characterized as high-volume commodities; they face competitive pressures for both price and quality. To succeed in commercial IC packaging, the manufacturer must have a price advantage without compromising the quality advantage.
Hermetic Packages. Hermetic packages are used when special component characteristics are required that exceed the capability of molded packages. Severe operational environments, quality specifications, and high heat dissipation are examples of conditions that would warrant the higher material and manufacturing costs of hermetic components. Control of the process helps maintain product quality and component reliability.
Quality inspections are designed as a process feedback mechanism to monitor and control the output at each process stage. Methods of inspection include x-ray and visual inspection using a microscope in the 40 to 60X magnification range. Inspection areas that use a microscope are at the wire bond stage, molding stage and final inspection. Areas of inspection using x-ray are at the die attach stage, molding stage, first and second electrical test stage and the final inspection. These quality control inspections are not intended to screen good components from bad ones; the inspections allow the process engineer to gauge the overall condition of the manufacturing process and to make real-time adjustments to the process. X-ray imaging plays a vital role in allowing the process engineer to implement adjustments to the manufacturing process in a real-time environment.
X-ray and the die attach process. The die provides a rigid support, electrical conductivity and heat dissipation properties that are crucial to the proper operation of all semiconductor devices. Several materials are used for semiconductor die attachment. A common concern for all die attach methods is the formation of voids and the impact voids have on reliability. Die attach voids impair component reliability in two ways; die stress fractures and temperature acceleration failure.
Stress Fractures. One purpose of die attachment is to balance the mechanical stress on the die that will occur from the physical forces on the package or thermal forces caused by different expansion coefficients that act during temperature transitions. Localized stress on the die can make a small crack propagate quickly, causing opens in the active circuit layers and resulting in component failure.
Fractures often start as microcracks along the die edge that are a result of the dicing operation. Voiding that is present at the die corners and edges create a localized tension force and allow this type of fracture propagation.
|Subtle grayscale differences show die voids, are measured by using image processing software. |
Hot Spots. Temperature-accelerated failure mode results in hot spots where a higher transistor junction temperature is generated above voids that impede proper thermal dissipation. Higher operating temperatures that are caused by die attach voiding lead to significant reductions in component reliability. Power transistors and hybrids are very susceptible to this type of failure.
X-Ray Inspection of Die Attach. Inspection of die attach voiding can be accomplished non-destructively and instantaneously by using a micro-focus high-magnification, high-resolution x-ray system. Flat panel digital x-ray detectors offer greater sensitivity and gray scale when inspecting low silver epoxy compounds where subtle grayscale differences representing voids are present. Die attach voiding is measured by using image processing software that measures the percentage of void area relative to the percentage of die area. The data results can be saved and compared to previously collected data to help the process engineer determine the conditions of the manufacturing process.
Molding Process. The molding process constantly changes with the introduction of new encapsulation materials, package styles and reliability concerns. Much smaller tolerances for error force manufactures to focus on this critical process step.
Latent Defects in Molding. Two types of defects are produced at this stage of manufacture and are directly related to latent device failure: wire sweep and mold voids. These defects are internal to the package, and x-ray imaging technology will enable the process engineer to quickly monitor and adjust the process. Wire sweep occurs during the molding process when the encapsulation material viscosity is too low or the flow rate is too high, causing the forces to displace the bond wires. Wire sweep is measured in the perpendicular x-ray image as a deviation of the path of the wire from a straight line to a curved or swept line. Extreme wire sweep will result in broken wires and bonds. Moderate wire sweep will result in shorts and crossed wires. Critically weakened wires will pass electrical test and the failure will only become evident in final assembly.
|Die void grayscale differences in higher-magnification image. |
Mold voids are another indicator of process problems. The use of an x-ray system to reveal mold characteristics such as void size, shape, location and the die location inside the package provides valuable information to the process engineer. Mold voids pose a serious threat to component reliability. The major problem would be die corrosion and the package's ability to resist moisture ingress. Mold voids can cause delamination of the epoxy from the lead frame that will compromise the IC package's ability to resist moisture penetration. The thermal shock from the reflow process causes moisture trapped in the mold void to vaporize, which results in high internal pressures. Mold void defects that lead to package failure after circuit board assembly will enable moisture ingress to begin during the assembly cleaning stage and result in early failure when placed in service. Mold voids can be inspected using the same x-ray imaging techniques used for inspecting die attach voids.
Lid Seal Process. Hermetic component packages are produced in many forms to meet specific conditions that are more demanding than molded package types. Extreme mechanical, thermal and environmental specifications require specialized hermetic packaging methods using exotic materials and difficult manufacturing processes.
Latent Defects In The Lid Seal. The lid seal can be produced in a variety of ways: glass to ceramic, glass to metal, epoxy to metal and metal to metal. In all cases, lid seal defects pose a threat to component reliability.
Hermetic components rely on the lid seal to maintain an inert atmosphere in the cavity space around the die. If the area becomes compromised, corrosion will begin on the bond wires, bond wire interfaces and unpassivated die surfaces, leading to rapid and catastrophic failure.
X-Ray Inspection of Lid Seals. Epoxy seals are detectable during x-ray imaging due to the powdered silver that has been added to enable it to conduct electricity. Glass seals are typically larger and contain lead as an additive to improve reflow and adhesion properties. Metal seals are denser and produce excellent void details. Seal voids are typically small and require a high magnification high-resolution x-ray system.
Contact: Scienscope International, 5751 Schaefer Ave., Chino CA 91710 909-590-7273 fax: 909-494-5313 Web: http://www.scienscope.com