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28 PCB007 MAGAZINE I APRIL 2018 duced by temperature cycling and materials have been optimized for this failure mode. However, humidity and temperature are criti- cal as well, although many applications in the car demonstrate self-heating during operation- al mode. Before switching on the system, cars may be parked for days or weeks in a humid environment and humidity finds its way into the electronics via plastic or barometric pres- sure compensations elements. The effects of humidity, both on the surface and within the structure of the PCBs, are ar- eas of critical concern, and the possible fail- ure modes have been studied in great detail. Therefore, this article highlights failures due to temperature, humidity and bias voltage (THB). Figure 9 shows conductive dendritic growth during dewing (water condensation) on a PCB, something which must be avoided. Even high humidity without dewing can pro- duce electrical shorts if the materials are not selected carefully. Surface isolation resistance (SIR) can drop and may result in failures in the electronics. Our approach is to understand very well the climate conditions in the protected (metal or plastic housing) electronics by simulation and experimental tests. On the other hand, we qualify the materials used (like PCB, devices, flux, thermal interface materials or conformal coating) and design ele- ments according to the IPC-9202 SIR method at different temperature and humidity conditions. With this approach, which is shown in Figures 10 and 11, we are on the safe side regarding the selected design elements and materials, ensur- ing a proper function during product life cycle. Inside the PCB, humidity is also critical and lead to a different class of failure modes re- sulting from electrochemical migration (ECM). Conductive anodic filaments (CAF) or hollow fibres are already well-known in the industry. any disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights. Dendrites on surface after direct dewing Dendrite growth Figure 9: Failure mode on a PCB. (Source: Bosch) Figure 10: Realistic prediction of local humidity in ECUs by validated simulation models. (Source: Bosch) Automotive Electronics | AE/EAI1 | 12/10/2017 © Robert Bosch GmbH 2017. All rights reserved, also regarding any disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights. 12 Local humidity load on PCB: Figure10