Microprocessor failures

Fig. 14.4 FIB cross section of a failed chip showing the two last layers of copper metallization and C4 bump left). Migration of Sn through cracks in the BLM layer and its reaction with cooper metaUization led to shorting and microprocessor failure. Cracks in the Ti layer (after selective etching of solder and solderable layer) is clearly visible (right)...

Some Case Studies of Software and Microprocessor Failures... 

Example 4. A particular microprocessor (MPU) is assigned for a fuel-injection system. The failure rate must be estimated, and 100 MPUs are tested. The test is terrninated when the fifth failure occurs. Failed items are not replaced. This type of testing, where n is the number placed on test and ris the number of failures specified, is termed a Type II censored life test. 

The use of computers and microprocessors (also known as programmable electronic systems [PES]) in process control continues to grow. They have brought about many improvements but have also been responsible for some failures. If we can learn from these failures, we may be able to prevent them from happening again. A number of them are therefore described below. Although PES is the most precise descnption of the equipment used, I refer to it as a computer, as this is the term usually used by the nonexpert. 

Most microprocessor-based analyzers permit direct comparison to two machine-trains or components. The form of direct comparison, called cross-machine comparison, can be used to identify some types of failure modes. 

However, the developments of microprocessor or computer-based instrumentation that can be used to monitor the operating condition of plant equipment, machinery and systems have provided the means to manage the maintenance operation. They have provided the means to reduce or eliminate unnecessary repairs, prevent catastrophic machine failures, and reduce the negative impact of the maintenance operation on the profitability of manufacturing and production plants. 

Since a comprehensive program will include trending and projected time-to-failure, multiple readings are required on all machinery to provide sufficient data for the microprocessor to develop trend statistics. Normally during this phase, measurements are acquired every two weeks. 

The microprocessor-based systems should be programmed, challenged, and validated to eliminate the exposure of one product to another through control failure. Valves and actuation devices used to divert product flows should be programmed properly and validated for their adequate functionality. 

Microprocessor control generally results in less operator attention required, higher levels of reliability, and ease of changing groups of set points. Other advantages are automatically programmed startup sequences, over temperature alarm, thermocouple loss alarm, heater failure alarms, and closer temperature control accuracy (Chapter 3). 

Production—Upon failure of microprocessor control, the process must be operated manually. 

Protection against loss of data due to power failure. The microprocessor and digital clock are provided with reserve power supplies, which ensures that, if the normal mains supply should fail, the timing continues uninterrupted and there is no loss of data from the RAM. After restoration of the normal supply following a power failure, the time, date and stored data are recorded by the output equipment and normal automatic operation is resumed. Recharging of the reserve power-pack also takes place when the operation from the mains supply is restored. No reserve supply is required by the PROM, whose memory contents are unaffected by disturbances to the mains. 

What happens if the microprocessor fails, especially with a pump running which could empty the day tank and probably damage the pump Design a computer failure mode that signals an alarm to the operator and shuts off the pumps. 

What happens if there is a power failure Because a microprocessor has a read only memory, nothing will happen to the system on power failure. But be sure to design the system software and hardware so that it will come back onstream following power failure with no interruptions when service is restored. 

A software component designed and coded either manually or with the help of tools may be subject to a wide variety of faults. The root cause of these faults is to be found in the specification, in the design or in the implementation. A software fault can be seen as a deviation in the content and/or in the order of instructions or data stored in memory causing the microprocessor not to behave as expected under some event or sequences of events. Trying to consider all possible faults that could affect even a simple software component is not practicable. Nevertheless, it is possible to consider the consequences of such faults, as they will lead to a few numbers of software failure modes... 

PSA systems are moderately reliable. The numerous valves associated with the process can cause unexpected shutdowns. The new PSAs are designed with alternate modes of operation, in which 100% of design capacity can be achieved while bypassing any failed valve or instrument, with only a slight loss of recovery. Failures are automatically detected and bypassed by the microprocessor-based control system. However, stronger and periodic control cycles are required. 

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