Reliability Models for Hot Carrier Injection, Wirebond and Fatigue Failures

Reliability Models for Failure Mechanisms

Hot Carrier Effects

Hot carrier effects is a phenomenon involving the injection of highly energetic carriers into the gate oxide layer and the silicon substrate, resulting in volume charge build-up that can shift transistor threshold voltages. This mechanism is accelerated by low temperatures.

AF = tf_use/ tf_test

AF = e^{([Ea/k]
[1/Tuse-1/Ttest] + C [Vtest-Vuse])}

where:

V = voltage accelerating the carriers

Ea = -0.2 eV to -0.06 eV

C = constant

Bond/Solderability Failures

Bond/solderability failures related to intermetallic growths, e.g., ball lifting due to Kirkendall voids, Cu-Sn intermetallic growths towards the leadfinish surface, etc. are modeled as follows.

tf = Ae^(Ea/kT)

AF = tf_use/ tf_test

AF = e^{(Ea/k)
(1/Tuse-1/Ttest)}

where:

A = constant

Ea = 1 eV for Au-Al bonds

Ea = 0.5-0.75 eV for Sn-based leadfinish

TC-induced Package Cracking

The occurrence of fracture anywhere in the package after it has undergone several temperature cycles has also been modeled. Since the zero-stress condition of the package is at a high temperature (around 175 deg C) , the low temperature (cooling) cycle has the main effect on this mechanism.

AF = (DTaccel/DTuse)^m

where:

DTaccel = Tmin(accel) - Tneutral

DTuse = Tmin(use) - Tneutral

Tneutral = zero stress temperature (approx. 175 deg C)

m = 20 (fracture property-dependent)

Fatigue Failures

Fatigue failures are failures due to application of cyclical stresses.

AF = (DTaccel/DTuse)ⁿ

Nf = C(DT)^-n

where:

Nf = cycles to failure

DT = temperature difference

n = temperature difference factor

Package Failures; Die Failures

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