A lack of standardized medical grade
To date, there are no standards that electronic components must meet to be used in medical devices. Many « medical grade » can be found in some component manufacturers, their meanings, however, change from one to another.exhilaration at their freedom ran through them all.
Example of a conversation with a manufacturer:
- “Hello Sir, I saw that you offered your component in medical grade, what is it ?
- It’s the exact same as the others, but with additional tests.
- Oh, what are the extra tests you do ?
- Well, the ones you want…”
If we read a component datasheet, few standards are indicated. And yet, any serious manufacturer refers to standards to qualify its processes and / or its products. Some of these standards are generic (J-STD-020 to define MSL levels for example), others are related to the type of component (CECC 30300 for electrolytic capacitors), and finally others are related to the application (AEC-Q200 for passive components in the automotive industry).
Obviously, not all components meet all standards, and it’s not even always easy to know which standards are met. Almost all of these standards require component testing.
However, these can’t be made on 100% of the pieces, which is good, otherwise their cost would be unaffordable.
If we take the example of the AEC-Q200 standard, which concerns passive component in automobile, it defines a whole range of tests to be carried out on the components, including electrical measurements before and after stress in temperature, humidity, vibrations … and the drift that would be expected. This is only a process qualification, which means that if the component manufacturer does not change their manufacturing process, they do not need to requalify the components.
To be able to market a medical device, it is necessary to provide certification authorities with data relating to its reliability and the impact of a possible failure on the patient.
Generally speaking, you can never be sure that a component won’t fail. Its impact on the electronics and the operation of a device is generally considered when it comes to the system architecture, by applying the principles of redundancy / dissimilarity.
Not only are the electronics duplicated (redundancy) so that a random failure is not enough to corrupt the operation of the equipment, but the duplications are not similar (dissimilarity): they are designed by different teams, with different components, different means so that a single fault cannot simultaneously corrupt all the redundancies.
Although this principle is widely used in systems requiring high reliability (aviation, nuclear, etc.), the strong integration of medical equipment, and more particularly when it is implantable, does not allow the number of redundancies to be multiplied sufficiently to ensure system reliability. The reliability of the components themselves then becomes paramount in determining the reliability of the device.
The need for exhaustive information on the reliability of components becomes important. It follows that commercial range components should not be used. However, we must compensate for the lack of « medical » grade on the components in another way, in order to understand this notion of reliability. The best would be to start from the needs of the device itself and the descriptions of the different stresses that it must be able to undergo and to deduce from it the resulting stresses for the components.
First difficulty: the stress seen from the component is not the same as that of the device: its temperature can be higher in operation, it can benefit from a housing protecting it from humidity, the vibrations can be attenuated or accentuated by function of the mechanical assembly…
Second difficulty: component reliability data is only available from the most mature manufacturers. For the others, it will be necessary to ask them again and again….
Third difficulty: component reliability data can be found on tests carried out during the qualification of their manufacturing process and it is not uncommon to recover data from 10 or 15 years ago. It will then be necessary to perform tests on the received batches to ensure that any deviation from the manufacturing process of the components does not have a significant impact on their reliability.
In exceptional cases, it may even be necessary to separate parts to only keep the components which are closest to normal characteristics, the heart of the grinding wheel.
Reliability methodology for electronic systems AEC-Q100 (AEC-Q101, AEC-Q200): Failure mechanism-based stress test qualification for integrated circuits (for discrete semiconductors, for passive components)
Medical device directive 93-42-EEC EN 60601-1: General requirements for basic safety and essential performance of medical devices