# Radiation Qualification by Means of the System-Level Testing: Opportunities and Limitations
## 5 Discussion  
Regardless of the different limitations of the system-level testing that were presented in chapter 4, this approach might be still useful in product development. Particularly, let us further discuss the example of SiP modules from 3D PLUS: the company has a database of tests at the device level, so they have knowledge of the device sensitivity to radiation and lot-to-lot variations. They are then able to identify risky devices and build systems with expected initial radiation performance reasonably good. In this context, system-level radiation tests might be used to verify the performance of 3D PLUS products. Moreover, the electronic circuits are created by the company designers, who are then able to apply their own proprietary mitigations technics. It is essential to take benefit of the test at the system level because they are able to identify the efficiency of the mitigation techniques and to identify what could be improved. As the SiP is a white box for the company designers, they can increase the number of observability points, to have better insight into system details, particularly during the prototype-level (board-level) testing. Furthermore, at all design phases, it is possible to identify failure modes and error signatures for the system and to evaluate general TID performance. This might be an interesting added value in the design process. Qualification limitations presented in chapter 4, might be at least partially addressed in the SiP product development flow:  
* to define the worst case test conditions, testing in different configurations is needed, but the knowledge about the system design may facilitate defining the configurations to verify;
* defining the number of SUTs required to have good enough statistical results is important; testing the SiP models at early development phases (e.g., without full mechanical, thermal qualification) might significantly reduce the cost, if it turns out that a high number of SUTs is needed to test;
* in terms of beam limitations, which are particularly strong for SEE tests, testing the SiP at 2D board level (instead of the 3D module) would usually increase the value of performed tests, because the beam penetration might be lower, therefore the available LET range of particles increases;
* in terms of the system observability, limitations are again stronger for SEE tests, but here the solution could be the use of the supplementary instrumentation in digital SUTs [31] to increase the observability (for the cost of additional development). Nevertheless, the complete system qualification, i.e., the one that would overcome all the qualification limitations discussed in chapter 4, is difficult. Furthermore, the cost of performing such complete qualification might be too high to make it a useful tool for the product or mission development. In some cases, simplification of the qualification is possible thanks to the knowledge about the system design or thanks to the knowledge about the target environment. However, for most of the cases, complete system qualification by system-level test remains difficult, and it is justified to consider the development of the limited qualification approach, that might be easier to apply, give valuable information, while bringing some level of risk that needs to be accepted. Limited qualification scenarios and risk acceptance should be considered in the context of the system reliability and availability. High reliability space missions require maximum reliability (the system is not destroyed and is working within the specifications) and availability close to 100% (availability defined as a percentage of time that the system can be used during the mission, this percentage might be decreased by, e.g., power cycling of the system due to SEFI (single event functional interrupt)). High reliability missions will never accept qualification scenarios that introduce some risk in the reliability and will rarely accept any risk in the availability, even if the effort of the qualification will be lower.