Benchmark of MOST components
After establishing the first hardware for MOST25 Technology, the release recommendations were introduced. This method was used to investigate and validate new products according to the requirements for automotive applications. In order to have a common basis for this work an application recommendation was written  were released. Nevertheless, this approach also realizes some of the principles of robustness validation. Within the MOST community this approach is well proven, and mandatory not only for the German car manufacturers but for others as well.
After having some experience with different products also coming from diverse manufacturers, the concept of benchmarking was introduced. The goal of this method is to compare products with functionality with the best performance. The definition of best performance is based on the requirements for the automotive application. The challenge of this approach is that all relevant aspects are taken into account. In Figure 1 the relationship between these different limits is shown.
2. Principle of recommendation process
The recommendation process can be divided into several steps:
- Valid specification – based on the real operating conditions and common description of requirements
- Assessment of project – review of project flow, technical realization, planned verification steps, quality strategy, and assessment of the manufacturing facility if new technologies are used
- Review of measured data – the measurements are mainly done by the manufacturer Judgment – based on the results of the developed investigation tool
This paper describes the method to review the data and make an objective judgment of the tested devices. For this goal the test sequences are separated into different sections:
- Characterization respectively electrical distribution:Measurement of all important parameters over the whole range of conditions. These are normally the operating temperatures, supply voltages, and (for the receiver) the specified optical input power range
- Life time tests:Examples for this section are high temperature operating life (HTOL), temperature humidity bias (THB), and temperature cycling (TC)
- Characterization after life test: Same procedure as before, performed with only a few devices that were stressed by the complete required duration of life test
- Short time tests: Examples for this section are mechanical shock (MS), solder ability (SD), and physical dimension (PD)
In each section for each single test row some statistical values are calculated to describe the behavior of each particular parameter. An example for this calculation is given in Figure 2. The criteria in this calculation are the number of failures, capability index, the distance to the specification limits, and the distribution of the measurements.
For a complete description it is also necessary to consider the behavior over the total set of different conditions. This is true especially for the characterization and the life time test sequences. For this purpose the applicable automotive specification describes a method for calculating an index, called a design index, similar to the capability. Figure 3 shows this approach for a test sequence that has several readouts from start to 1000 hours. The statistical characteristics of the different distributions are the relevant criterion for this judgment. For this design index a minimum value of 1.0 is required. This is quite similar to the capability index where a minimum of 1.66 is required.
Figure 3: Principle of using the design index for judgment, defined in automotive specification. For full resolution, click here.
3. Extension of the method to benchmark approach
The calculated values of the investigation are set in relation to the specification limits. Together with the automotive requirements, perfect performance is defined and set to 100%. With this definition it is possible to describe the real performance of the product as a percentage.
For better understanding of the benchmark the values from each single calculation were merged together to come to one single value for the judgment. This procedure is similar to the guideline for auditing of quality systems at automotive suppliers . In this approach a classification from A to C was defined to characterize the performance of the product.
- Classification A >90% Status: green
- Classification AB >80%; <90% Status: green; improvement possible
- Classification B >70%; <80% Status: yellow; improvement highly recommended
- Classification BC >60%; <70% Status: yellow; improvement required
- Classification C <60% Status: red
The total counts of measurements are mentioned as additional information to increase the confidence of the judgment.
4. Results and examples
In the investigation process the information is transferred to the customer first in concrete values. In addition to this, the information is given in a diagram to make the behavior more clear. For example, to see the outliers of one single test row, see Figure 4.
An example of the behavior of the design index for one parameter regarding all conditions during characterization is given in Figure 5 . Most of the conditions fulfill the requirement. Only at two supply voltage conditions at low temperature are the limits exceeded.
Figure 5. Behavior of design index for one test sequence, 2 supply voltages and 3 temperatures (low: -40°C, room: +25°C, high; +95°C). For full resolution, click here.
After the required duration of each life test sequence it is necessary to take a few devices and measure the parameter for the characterization again. The criterion in this case is the comparison to the original characterization measurement. It is clear that these are not the same devices but these devices should have a similar distribution. Therefore the general behavior should be similar and within the distribution of the original characterization. An example for demonstrating the behavior is given in Figure 6.
The example of the total result is showing acceptable performance – see Figure 7.
Some of the tests are very good. These tests are marked with “A”. Others should perform better. This is mainly true for the characterization which is marked with “BC”. The reason for this low value is that some of the measured parameters are outside the limits of the specification. A solution for this could be to increase the specification range. If this is not possible because the limits are fixed by the MOST specification, a small design change could increase this performance.
 Failure Mechanism based Stress Test Qualification for Integrated Circuits; Automotive Electronics Council (AEC-Q-100); 14-May 2007; Rev G
 Guideline for Characterization of Integrated Circuits; Automotive Electronics Council (AEC-Q-003); 31-July 2001; Rev initial
About the author:
Viktor Tiederle is President, of RELNETyX AG, 70771 Leinfelden-Echterdingen, Germany, Phone: +49-711-6939780, email: email@example.com.
Tiederle works as Senior Consultant and President. He has long experience in the area of quality, reliability, and validation of electronic products. He has worked with MOST Technology since 2001, mainly dealing with the MOST Full Physical Layer for single components like FOT and pigtail.
Article published in Elektronik automotive, Special Edition MOST, April 2011, www.elektroniknet.de
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