Today innovations in the automotive field are distinguished primarily by electronic applications. For example, such circuitry ensures proper operation of the airbags, ESP, engine management, and navigation aids. Some of these components are subject to high thermal stress. They must withstand contact with hot transmission oil without damage, as well as continuous operating temperatures of approx. 150°C. In the application range hybrid circuitry is preferable to the use of printed-circuit boards. Instead of printed-circuit boards, ceramic substrates are used on which the semiconductor chips have been attached directly without previous packaging and the connections between the ICs and circuit substrates are completed with the aid of wire bonding. At automotive-electronics supplier Continental Temic in Nuremberg, Germany, this technology is used in the area of powertrain and chassis components. Three Viscom AOI systems automatically check the formation of the ball and wedge, the wire route, and the bond position to ensure the quality of the electronic components.
Advantages of hybrid technology
An advantage of hybrid technology is the possibility of using semiconductor components without housings, thereby saving space. In such applications a fine gold or aluminum wire, processed with the aid of wire bonding, is used to complete the electrical connection. For thin wire bonding the company uses 32-μm gold wire processed using the ball-wedge method. For thick wire bonding, aluminum wire with a thickness of 350-μm is used and processed with the wedge-wedge method.
To ensure the quality of the components and connections it is necessary to check 600 wires per double panel. On the one hand, it is necessary to ensure that the components are positioned correctly and inspect the conductive glue. On the other, it is also necessary to check the bonded points and connections for correct position and damage. Accomplishing this with visual inspection would require hiring an immense number of employees, and then the error escape rate would be considerable and the danger of damaging the sensitive substrate by handing enormous.
AOI system inspect all significant criteria for wire bonding
For this reason the company decided in favor of automatic inspection using Viscom equipment. Walter Schneider, responsible for sales of Viscom inspection systems in Southern Germany, says, "At Viscom we developed a bond-inspection system capable of inspecting all significant characteristics practically and reliably." Currently three Viscom Model C6053BO bond inspection systems are in use in the hybrid production facilities at Continental Temic, and one Viscom S6054 AOI system is being used for inspecting solder joints in the printed-circuit-board sector.
Ralf Hollederer, of the process technology department at Continental Temic, adds: "The electrical test, which we also use, can only determine in a general manner if a connection exists or not. However, it is necessary for use to differentiate borderline cases. For example, the electrical test cannot determine when two wires come very close together. During subsequent potting, this can lead to short circuits."
Another problem, which can only be solved with AOI, is inspection of the position of a ball on a semiconductor pad. According to the specifications the ball must be positioned at least 75% on the semiconductor pad; however, this criterion cannot be measured electrically. Another advantage of automatic optical inspection is that the AOI system simultaneously checks the SMT components.
A peculiarity of the demanding bond inspection is the treatment of the pseudo-error/borderline case subject. "Due to process related variations we will never be able to achieve the extremely low PPM figures obtained with the SMT process," states Hollederer. "The input variables for AOI differ too highly due to the tolerances in the substrate structure, in the components and in the bonding process itself. When this is all added together the tolerances for the optical pattern for good parts is already very high." Here it is necessary to keep the processing time for indispensable manual evaluation of the errors indicated by AOI as low as possible with a suitable subsequent classification concept. Graphic indication of the error location in combination with illustration of the corresponding image material for the specific error location allows classification to be accomplished within just a few seconds.
Customers also appreciate automatic inspection, because this is the only way to reliably ensure quality. Moreover automatic optical inspection also offers the possibility of statistical evaluation. The number of errors detected can be recorded in relation to the component with the aid of a production database at Continental Temic. This ensures that parts can be traced back without problems. Finally the statistical data available with the use of AOI can also be used for process control and optimization to reduce the number of true errors occurring on the input side and therefore number of rejects.
In the future, the requirements for quality control will continue to increase. "The wire thicknesses will decrease even more," says Marcus Reichenberger, "and we cannot exclude the possibility that aluminum will also be used in the thin wire bonding sector. The number of wires will also increase and the components will be installed even more densely on the assemblies. There is simply no alternative to AOI."