RELIABILITY CONSIDERATIONS OF FLIP CHIP COMPONENTS FOR AUTOMOTIVE ELECTRONIC APPLICATIONS

Electronic devices for automotive electronic applications have to be operated under extreme environmental conditions and therefore are required to have higher reliability compared with general electronic equipment. Recently automotive voltage regulators, ignition systems, etc. have been changing from mechanical constructions to electronic ones using thick film technology. This paper presents results that shows that our flip chip IC technology can satisfy the high reliability requirements of automobile electronics.


INTRODUCTION
Thick film integrated circuits using hybrid circuits are being applied to automotive electronics and the volume of such business is increasing every year. Such a market requires high reliability in the component and in the circuit, particularly from the point of view of maintaining safety. Furthermore the environmental conditions that automobile electronics find themselves in are continually changing, and are often extreme, and it is therefore, necessary to establish methods of operation that take into account the various extreme conditions under which the circuits will operate.
Up to the present time hybrid circuits involving integrated circuits have been applied to many electronic equipments. However, in the case of automotive applications completely different design standards must be introduced for the hybrid integrated circuits. Table I   To realise the latter the structure of the solder bumps needs to be complex. Figure 1 shows the type of structure used in our work. A side view of a component attached by the solder reflow bonding process is shown in Photo 1. This was obtained using a scanning electron microscope.
To obtain easy strong and reliable bonding the bump has to be solder covered. A thick copper layer has been used to prevent the chip from touching the solder covered conductor at the time of bonding, in order to improve the reliability of the bonding process. Chromium is used to obtain good adhesion between the copper and the aluminium or the sputtered silicon dioxide which is often used to protect the integrated circuit chip surface from being scratched after manufacture.  Reliability of bonding strength under various environmental tests has been examined. Environmental temperature conditions for automotive electronics depends on simulating conditions due to the weather and due to heat conduction and radiation from the engine. Of special importance is the maximum temperature experienced by a circuit placed near the distributor or the alternator housing in the engine compartment. Such a circuit can reach temperatures ranging from 90C to 130C as shown in Figure 5. Also the minimum temperatures the circuits will be required to operate under must be considered and starting temperatures as low -30C to -40C can be experienced, (see Figure 6)a. From these considerations it was decided that the temperature conditions required for storage of the bonded devices should be fixed at + 130C and -40C. Twenty chips were randomly selected for bonding strength measurements after having been stored under these extreme conditions for varying periods of time. The results are shown in Figures 7 and 8. This data shows that the bonding strength was negligibly affected by either the high or low temperature storage test.
The rate of change of temperature inside the engine compartment also needs to be considered. This can change rapidly particularly for a car started in a cold climatic condition as the temperature in the engine compartment can reach 100C in only a few minutes. 2 To test the effect of such rapid changes in temperature on flip chip bonded integrated circuits, chips attached to substrates were subject to the test conditions shown in Figure 9. 500 Cycles of this temperature variation were performed. In order to protect chips from mechanical shock in the production process bonded chips are generally coated with rubber which itself causes thermal stress to be applied to the bonding. Given this, the tested samples used for the temperature cycling were originally coated with silicone/rubber for testing. In order to protect integrated circuit chips from the atmosphere the bonded chips are usually housed in hermetic sealed packages or plastic encapsulation. Hermetically sealed chips are easily protected from humidity but in the case of plastic encapsulation it is necessary for the chips to have a low susceptibility to any humidity conditions. For this reason humidity tests were made with uncoated chips at 60C and 95% humidity and this condition is the same as that recommended by JASO or SAE. Figure 11 shows the results of such a test.

ELECTRICAL RELIABILITY
The design of automotive electronics must fully consider the electrical environmental met in use, such as battery voltage changes and various kinds of surge voltages. In order to guarantee reliability under such conditions, electrical endurance tests must be applied to flip chip hybrid circuits used for voltage regulators. In this section the results of such reliability tests will be reported.
The surge voltages experienced can be due to a variety of causes such as the thermal change in circuit loads, use of the ignition system and finally the switching off of inductive loads. The ignition surge voltage is the worst condition experienced and voltages obtained from such a cause can reach values up to 300 volts. With these considerations and given an equivalent circuit of the flip chip hybrid circuit used for an automotive voltage regulator as shown in Figure  12b, a surge current wave form as shown in Figure 12a was used as a test.
Surge currents were allowed to flow between the first and fourth terminal as in Figure 12b at an ambient circuit temperature of 125C. The voltage drop between the first and fourth terminal was measured at 1 mA and 30 mA as shown in Figure 13. After surge voltages were applied for 2 108 cycles, the voltage change observed was less than +_0.1% from the original value.  In terms of the electrical reliability of automotive electronics the effect of high temperature operation is considered the most important factor. In order to evaluate this factor even more strictly, the flip chip integrated circuits under test were subject to high temperature tests with a voltage bias applied at the same time. Such a test gave a more accelerated effect than ordinary high temperature testing, particularly with regard to the importance of evaluating the surface passivation technology used. For such a test a maximum applied voltage of about 10 volts is normally used, but we decided to use a bias voltage of 20 volts between the collector and base of our transistors.
Results of such tests using an applied temperature, T of 125C, are shown in Figure 14. No change in leakage current was detected after such tests and it can be concluded that the passivation of the devices was satisfactory for automotive electronic applications.

CONCLUSIONS
It has been shown that flip chip integrated circuits and passive devices attached to thick film circuits can withstand various extreme environmental conditions required for the operation of such circuits in automotive environments. Hybrid circuit voltage regulators constructed using flip chip technology have been tested in the field, and have achieved a failure rate less than 0.001% It is expected that in the near future more and more automobile electronic circuits will be involved in each automobile and the fact that it has been shown that our flip chip technology together with hybrid circuits can achieve the environmental reliability required indicates that the spread of such applications is likely to be rapid.