Aerospace and Electronic Systems Magazine April 2017 - 21

Wu et al.
mechanism. When two dissimilar metals form an intimate contact,
atoms in the metals diffuse across the interface. The difference of
the diffusion rates between the two metals results in the depletion
of atoms in one of the metals. Interdiffusion is a time-dependent
mechanism. High temperature accelerates the interdiffusion process. An example of interdiffusion failure in electronic packages
is the purple plague of aluminum wire bonds on gold bond pads.
The components in a BLDC drive, such as transistors and ICs,
may fail when they are exposed to thermal or mechanical shock
from the environment. This is an overstress failure. The major
components of a BLDC fan are chosen from the analysis of the
hardware architecture in Figure 2: bearings, blades, BLDC drive,
and stator. An overview of the potential failure modes, causes,
and mechanisms of BLDC fans is listed in Table 1. The selection
of an optimum fault tolerant fan driven by an electric motor with
electronics or mechanical reliability considerations is crucial to the
EMA system.
Usually the reliability and useful life of a fan can be separated
to electronic failures and mechanical wear-out failures. Electronic
failures are usually random failures that are caused by defective
electronic components. With proper screening of defective electronic components, the likelihood of failure due to random failures
is low for most modern electronic components. However, if a circuitry design is faulty or components are misused in a design, the
failure rate of misused electronic components could be high and
could cause premature failure. Compared to mechanical wear-out
failures such as bearing wear-out (> 5 years, if running continuously), electronic failures tend to occur in the earlier stage of operation, most likely in the first three to five years. The best way
to avoid electronic failures is to do a thorough design review to
make sure the circuitry design is correct and the life of electronic
components could be as long as the life of mechanical components.
But the effort spent in improving electronics reliability alone may

still not be enough to significantly improve the overall reliability
of a fan.
Most fan product specific sheets state their products have an
overall life-time of ">50,000 hours under normal use". The fault
probability is the reciprocal of the expected life-span in hours of
fan use. For example, a fault probability of 2 × 10−5 means the
expected life-span is 50,000 hours of continuous use (here, "continuous use" means 24 hours/day and 7 days/week). Ametek Propimax-II loss of output is O (2 × 10−5), i.e., 50,000 hours' life span.
This has to be improved significantly to meet the fault probability
requirement for cooling EMAs in aircraft. Considering all the factors provided, the calculated mean-time-to-failure was found to be
50,000 hours, or approximately six years if the fan products are
used continuously. If a nonoperation factor of 2 is considered, the
fault probability is about 1/100,000 = 1 × 10−5.
The life span of 50,000 hours is from many fan manufacturers'
product specific sheets, though there are some variations. If the
fault probability of a single fan is improved to 1 × 10−5, a dual-fan
system could have 1 × 10−10 which means a cooling system life of 1
× 1010 hours. It should be noted that the reliability requirement for
the fan cooling system has not been established for aircraft EMA
cooling applications as the requirement would depend on a specific
application. However, for discussion purposes in the next section,
the fault probability desired is assumed to be 1 × 10−10. This was
suggested from Defense Advanced Research Projects Agency solicitation (SB132-005) [4].

FOUR APPROACHES FOR RELIABILITY IMPROVEMENT
DUAL-BEARING SYSTEM
Fault-tree analysis is a crucial part of any trade study for an aircraft component. For ball bearings: lubricant deterioration, fatigue,

Table 1.

Potential Failure Modes, Causes, and Mechanisms of BLDC Fans
Failure Sites
Ball bearing

Blade

Failure Modes

Failure Causes

Failure Mechanisms

Jam

Thermal overload

Lubricant deterioration

Ball cracking

Cyclic loading

Fatigue

Wrinkles

Moisture

Corrosion

Surface crack

Mechanical overload

Yielding

Surface fouling

Dust

Adhesion

Crack

Cyclic loading

Fatigue

Open circuit

High temperature

Inter-diffusion

Short circuit

Voltage bias, moisture

Dendritic growth

Crack on solder
interconnect

Thermal cycling, cyclic
load

Fatigue

Cracked and peeling
winding wire film

Thermal overload,
mechanical overload

Thermal aging of
insulating materials

BLDC motor
BLDC drive

Stator

APRIL 2017

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine April 2017