November_EDFA_Digital

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 18

PACKAGING AND PCB LAYOUT As mentioned earlier, a mixed superimposition of stress factors applies when the car leaves the factory. Qualification tests—even with the latest AEC-Q200 revi- sions—cannot consider all superimposed stress scenarios. Thus, engineers without sufficient field experience in automotive electronics should be very careful when designing in ultra-modern packages. Fromfailure analysis experience, these critical points should be considered: • Avoid packages with only a small tolerance of thermomechanical relaxation and low robustness against vibrations. Micro leadframe (MLF)/quad flat no-lead (QFN)-packages should not be used in auto- motive applications due to their tendency to shear interconnects under vibration and/or thermal cycling stress. • Extremely small mold compound overlap to pins may result in stitch bond cracking; this often happens when the chip size is only a little smaller than the fully encapsulated device (Fig. 6). While stress to the pin and mold from excessively hot soldering, pin forming, or PCB bending and stamping from the leadframe causes a small, micron-like pin delami- nation from the potting, the bond wire remains absolutely fixed. If the pin then starts moving, the

Fig. 3 Contactless electrostatic voltage measurement probe, mounted close to the sensor housing.

repair, the failures never reoccurred. At amileage of more than 10,000 km, enough dirt anddust in themotor cabinet adhered to the rubber tubes that they became electrically dissipative—thus replacing the missing GND connection. Another effect needs to be considered in comparable setups—the electric field that typically develops between the sensor and the ground. If a charge separation (e.g., sputtering of oil or fuel drops) occurs within such a field (even a weak one), rather high voltage can be achieved. The voltage level depends on the distance reachedby sep- arated charge partners (e.g., drops or particles) after they suffered separation during the spraying process. It is the same as if the distance of the plates of a charged capaci- tor is increased (Fig. 4)—because the charge remains the same but capacitance decreases, voltage must increase. Therefore, ESD-sensitive sensors experiencing a high- speed flowof gas or liquid need to be externally protected against charging (Fig. 5). Chip-internal ESD protection structures usually cannot cope with operation induced, continuous charge/ESD stress.

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Fig. 5 Protectionof a sensor toavoidelectrostatic charging: A = metal case; B = rubber tube; C = metal cap, grounded by a resistor (optional); D = sensor housing (grounded); E = sensor (grounded via resistance or [optional] by a capacitor); and F = signal line (shielded).

Fig. 4 If the capacitance of a charged capacitor is reduced, voltage increases. The same applies if charge separation happens within spraying/sputtering of powders, liquids, and sometimes even within high- speed gas flows.

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