Issue



Automatic Optical Inspection of IC Connections


09/01/2007







Automatic Optical Inspection of IC Connections

Die and wire bonding are among the most important IC connection technologies in electronic production. Meeting quality requirements for these processing steps requires 100% monitoring of the finished products. To accomplish this reliably, objectivly, and economically, automatic optical in-line inspection systems (AOIs) are being used more frequently.

Die bonding is the placement and formation of an insoluble connection between a bare die and substrate. Wire bonding defines the method of forming an insoluble electrical connection between a semiconductor chip and its respective substrate by wires. This technology is also used to bond substrate connections one beneath the other, and for contacting adjacent wiring levels to one another.

Application Areas for Bond Technology

Application areas for these technologies reach from the back-end areas of integrated semiconductor component production, through the direct application of bare chips to electronic assemblies, and - in the case of the wire bond - the connection of separate electronic modules to one another. The main difference between application fields lies in the differing substrates used. While in the first case thin lead frames are primarily used, the second deals with ceramic substrates (hybrid or thick-film technology), customary PCBs for chip-on-board (CoB) technology, or massive metallic base plates (power electronics).


Figure 1. Bare chip with gold thin wire bond
Click here to enlarge image

Although die bonding is similar in most cases, wire bond technologies vary in materials, wire strengths, and connection methods. The wire circumferences used in automatic bonding can reach from 17 µm through 25 µm/32 µm/60 µm (Figure 1), up to 300 µm/500 µm (Figure 2). For thin wire bonds, various gold and aluminum alloys are used; for thick wire bonds, aluminum is the main choice. Further, the imprint at the contact points varies according to the contact method used. These include ball-wedge bonding that uses a capillary tool; wedge-wedge bonding, which uses a bond-wedge tool; and special bonding, such as security double bonds or wedge-on-ball. Thermal compression bonds, which establish connection using temperature and pressure, have been widely replaced by ultrasonic technology. In case of ball-wedge bonding with thin gold wires, thermosonic (TS) bonding comes into play. Here, the micro-weld between wire and connection pad is accomplished by the combination of temperature and mechanical ultrasonic energy. In wedge-wedge bonds with aluminum wires, increased temperatures can be eliminated, and the process is referred to as ultrasonic (US) bonding.

Bond Quality Through AOI

These technologies are frequently found in critical and safety-relevant applications, which places a great deal of emphasis on quality and reliability. Chief among them is the automobile industry. Typical products include engine controls, ABS, or airbag systems.


Figure 2. Wedge-shaped aluminum thick wire bond foot.
Click here to enlarge image

A combination of inspection approaches is relied upon to guarantee required quality. As a rule, assemblies undergo a 100% electrical inspection, or in-circuit test (ICT). With this test, however, no conclusions about the load capacity or mechanical stability of the bond connections can be made. For this, pull tests are performed, which gauge the tear-off force of individual bonds on a random sample basis. In a pull test, the selected component is destroyed. Further, there are a range of defect criteria that cannot be assessed with electrical tests, such as too-narrow minimum distances between wires; bonds made to contaminated surfaces, which prevents sufficiently robust connections; or missing wires, which frequently occurs when multiple bonds between connections with the same electric potential are necessary. Therefore, it is critical to perform additional optical inspection of all bond connections to positively identify the multitude of defects that can occur during the production process.

Problems arise when optical inspection is only done manually. With typical cycle times of 1,000 bonds and more per minute on one line, many people would be needed. Human inspection is subject to fatigue due to repeated evaluation of the same image material, and is not adequately reliable. These problems can be solved with AOI. Such systems can recognize all relevant defect features with 100% inspection coverage, and indicate them positively and in reproducible form. The inspection coverage of an AOI system includes die bonds, in which presence of the die, displacement and rotation, damage and contamination of the bond surfaces, and ensuring conductive adhesive or epoxy discharge along the die edge are all evaluated. Wire bonds are examined for presence and correct assignment of the wires; loop characteristics; indications of impermissible form deviations such as curling or wire sway; compliance with a minimum distance between adjacent wires (Figure 3); over-bonding, which involves incorrect placement of multiple wires for the lack of correct wire tear-off (Figure 4); position of the bond ends on the connection pads; wire tear-off; and correct formation of the bond end, whether ball or wedge.

The inspection scope for wire bonds depends heavily on the type of bond involved. For a ball bond, the criteria can be ball formation, radius, and wire exit point. For simple wedges, stitch width and symmetry criteria are checked. With a thick-wire wedge bond, tail length, configuration of the bond foot, formation and integrity of the bond impress, and presence of bond ears (material exuded along the sides by the high pressure) are evaluated.

The base inspection program - position and type of the individual inspection sites - is automatically generated from CAD data. The specific choice of individual inspection criteria and their sensitivity setting can be parameterized and flexibly adapted according to need. Depending on available cycle time, additional inspection content can be added as needed, such as the correct placement of SMT components, or damage to the substrate such as craters and edge tears.


Figure 3. Analysis to ensure minimum wire spacing.
Click here to enlarge image

In addition to the higher inspection certainty and reliability - as well as economy - AOI offers still another decisive benefit. Through the classification of indicated defects according to their causes by skilled persons, defect classes can be differentiated. Through the continual accumulation of feature values such as displacement, rotation angle, radius values, etc., valuable process data can be automatically gathered and steered back into production control. In this way, through the definition of suitable intervention limits, faulty parameters or unstable drifting processes can be corrected before critical defects occur.


Figure 4. Overbonding.
Click here to enlarge image

Technical Base
The base for this accurate inspection is a high-precision axis system combined with a solid inspection cell and a sophisticated high-resolution camera module. The axis system consists of a high-precision linear axis for accurate positioning of the sensor head. The camera module* contains integrated illumination unit to descern features required to investigate inspection criteria with sharp clarity. Inspection depth is determined by the sensor head resolution. For bond applications, this is markedly higher than AOI systems designed for standard PCB assemblies. Resolution for standardized AOI systems is, for a simple component inspection, 20 µm/pixel. For demanding applications, a resolution of 10 µm/pixel is used. At 8.4 µm/pixel, the lowest resolution for bond applications is higher than that of a high-resolution standard camera. For the inspection of thin wires, a bond HR resolution is used. At 5.4 µm/pixel, it is able to inspect wires thinner than 25 µm. Inspecting wires with diameters below 25 µm can be achieved with the high-resolution VHR camera module, which is equipped for application-specific resolution of 2 to 5 µm per pixel. This VHR sensor technology accomplishes reliable inspection of bond wires with extremely small diameters. The VHR camera module is able to record images of the inspection object with different illumination settings and various diffusions and to find defects reliably. In the process, the inspection scope includes the course of the wire, distance between adjacent wires, and the positions and geometries of balls and wedges.

Conclusion

More than ever, AOI is essential to guarantee zero-defect quality of completed products and to ensure sufficiently robust connections against thermal and mechanical loads. In some cases, with 100% optical inspection, electrical tests or the continual destruction of inspection pieces by random sample testing can be eliminated completely.

*Developed and manufactured by Viscom


CHRISTIAN FABER, leader of strategic development, can be contacted at Viscom AG, Carl-Buderus-Str. 9-15, 30455 Hannover, Germany; +49/511-94996-632; E-mail:christian.faber@viscom.de.