Characterizing Space-Flight Inductors and Transformer Failures

Ben Oni, Ph.D, NAFP Fellow, Marshall Space Flight Center/Tuskegee
University

Trent Griffin, Karen Cunningham, Bob Kapustka, Steve Luna, Marshall
Space Flight Center

## 

## Abstract 

There are many risks associated with developing space flight power
electronics hardware. Various design and manufacturing standards are
usually imposed on the hardware development processes with the objective
to reduce failure risks. In spite of these standards, some chip scale
package (CSP) assemblies and discrete components used in power
electronics hardware developments still fail in use and require
understanding of the failure mechanisms to establish improved confidence
in reliability. Failures of space flight inductors and transformers,
developed to MIL-STD 981, were studied. This paper characterizes these
failures in terms of type, cause, method of detection, and frequency of
occurrences. The paper also presents the relative importance of the
various MIL-STD 981 tests, as perceived by manufacturers of inductors
and transformers.

### Introduction

### There are many risks associated with designing power electronics space flight hardware. Failures in space flight hardware are immeasurably costly in terms of safety and replacement costs. The importance of reliability in space flight hardware cannot therefore be overemphasized. 

### 

### To ensure the highest standard of quality and reliability, many standards are usually imposed on components, design and manufacturing processes of space flight hardware with the goal to minimize failures in use. For custom electromagnetic devices for space applications, the MIL-STD 981 is the principal standard. 

### Since only a few space hardware units are designed, it is difficult to distinguish failures due to design flaws, overstress, or because of bad electronic piece parts. Each power electronic part used in these designs is derated per the project requirements to minimize component stress. In addition, many of these components are individually screened prior to installation in the flight unit. As an added insurance, the program office usually requires several additional analyses other than electrical stress to be performed including failure modes and effects, reliability, worst case circuit, safety, radiation, maintainability, mechanical stress, thermal, etc. Even after all of these analyses are performed, the protoflight unit is usually tested over as many operating and non-operating scenarios as possible. In spite of these standards, rigorous tests and associated high costs, some failures still occur. 

Objectively, it is desirable to see if there is a correlation between
the failure and the type of testing and/or analysis that is or is not
performed. If it could be determined that the performance of certain
tests and/or analyses on power electronics hardware greatly enhance
mission success and that the performance of other tests and/or analyses
do not, then one would be able to better advise the customer on which
test and/or analysis options would give them the best value per dollar
spent.

### Focus of study

For the goal to identify the failure mechanism of spaceflight hardware,
it was considered logical to focus on key discrete components rather
than on hardware assemblies treated as single units. One obvious reason
for this approach is that only few space hardware units are designed and
therefore limit the sample size available for evaluation. On the
contrary, certain components e.g. power inductors and transformers
constitute the basic building block of any power electronic hardware
design and therefore provide opportunity for reasonable sample size for
evaluation. Inductors and transformers were focused upon in this study,
partly because it is one of the business interest areas of the
study-originating department, and also because these are two components
that appear reasonably straightforward in design, robust in material and
mature in technology, and therefore arouse questions as to how much
value, testing to the MIL-STD 981 requirements, really adds to the
reliability of the components.

**Study Approach**

Failure data on space flight inductors and transformers were collected
and analyzed. The major sources contacted for data on inductor and
transformer included:

a.  Government Industry Data Exchange Program, (GIDEP)

b.  NASA Reliability Preferred Practices for Design and Test -- re:
    Lesson Learned Info

c.  Magnetics Manufacturers

d.  Electronic screening houses

For the magnetic manufacturers and Screening Houses, although the number
of surveys sent out was high, the number of respondents was however,
relatively low.

##### **Part I -- Analysis of Failure Data** 

The data collected was evaluated and classified as outlined below:

a.  Failure by source classification i.e. Manufacturer, Design,
    Installation

b.  Failure by fault type i.e. Open, Short, Fail Spec, others

    -   Frequency of occurrence

    -   Unit count

c.  Total failure by yearly incidence

d.  Failure by method of detection

# Failure Definitions adopted in study

From the failure reports investigated, the differentiating lines between
manufacturing, design, and installation failure classifications were
blurry. The definitions below were developed in this study to help
distinguish between the lines and to provide consistency in the
classification. Where the failure suggests involvement of two or more
classifications, the most likely classification was used.

1.  [Manufacturing Failures]{.underline} -- Failures resulting from:

    -   Improper manufacturing process

    -   Use of wrong material

    -   Poor Workmanship

    -   Incorrect assembly

    -   Improper Handling - packaging and transportation

2.  [Design Failures]{.underline} -- Failures resulting from:

    -   Fundamental design flaws

    -   Inappropriate/inadequate Material specification

    -   Improper test specification

    -   Improper/insufficient installation guidelines

3.  [Installation Failures]{.underline} -- Failure resulting from:

    -   Poor Installation workmanship

    -   Inappropriate connections

    -   Handling

    -   Non-conformance with installation specification

**Observations on failure results**

Based on the available GIDEP data used in this analysis, the following
observations are made:

1.  Magnetic components fail more in the Open mode than in the Short
    Circuit mode

2.  More failures result from manufacturing errors than design or
    installation errors

3.  Advances in manufacturing technology do not show decline in
    manufacturing related failures with years.

4.  More components fail the Thermal Cycle test than other tests

5.  About 20% of failures occur in use.

6.  Failure occurring in use is caused by a number of reasons; however,
    insulation breakdown and improper solder joints are more prevalent.

Failure results are graphically presented below.

![](media/image1.wmf){width="3.0097222222222224in"
height="2.390277777777778in"}![](media/image2.wmf){width="3.009027777777778in"
height="2.28125in"}

![](media/image3.wmf){width="3.1347222222222224in"
height="2.1118055555555557in"}![](media/image4.wmf){width="3.384027777777778in"
height="2.0493055555555557in"}

##### **Part II - Relative priority ranking of Mil-Std 981 Group B tests for Power transformers and power inductors by Magnetics Manufacturers**

Magnetic manufacturers routinely perform a number of tests to validate
their products according to customer specifications. Some of these
tests, as stated in the Mil-Std 981 handbook, table IV -- Group B tests
for Power transformers and power inductors include:

1.  Thermal Shock

2.  Burn-in

3.  Seal (when applicable)

4.  Dielectric withstanding voltage

5.  Induced voltage

6.  Insulation resistance

7.  Electrical Characteristics

8.  Radiographic inspection

9.  Visual and dimensional examination (external)

Magnetics manufacturers were surveyed to evaluate how they viewed, based
on their experiences, the relative importance of Mil-Std 981 tests.

Some of the manufacturers surveyed performed tests to the Mil. 981
standard, others did not. After analyzing the response from both groups,
the similarities of the response profile gave the confidence that both
data sets could be lumped without significant errors in the conclusions.

Similar survey sent to Electronic screening houses did not yield any
feedback.

### Observations on Magnetics Manufacturers' Survey Results 

Based on the survey responses from magnetics manufacturers, the
following observations are made regarding the relative importance of
Mil-Std 981 tests:

1.  Tests performed depend on customer applications

2.  Irrespective of application, majority of magnetics manufacturers
    consider "Dielectric" and "Electrical" tests as highest priority
    tests.

3.  Seal and Radiographic tests were generally considered least
    critical. It is noted however that in some applications, these tests
    may be absolutely necessary.

4.  Estimated average cost to conduct each test varied so widely and
    inconsistently that the data set could not be considered reliable to
    assess cost of tests.

The survey results are graphically presented and reflect the relative
importance of the indicated tests based on the propensity of the test to
generate failures. A ranking of "5" indicates most critical in ranking,
and suggests that the particular test will most likely expose a
potential failure in the component, while on the other hand, a ranking
of "1" indicates that the test is least critical in ranking and least
likely to expose potential failure in the component. The % value shown
on the graph indicates the percentage of the manufacturers who gave the
indicated ranking to the test in question.

![](media/image6.wmf){width="3.0381944444444446in"
height="2.4305555555555554in"}![](media/image7.wmf){width="3.1354166666666665in"
height="2.4305555555555554in"}

![](media/image8.wmf){width="3.3847222222222224in"
height="2.7694444444444444in"}![](media/image9.wmf){width="3.1354166666666665in"
height="2.7125in"}

![](media/image10.wmf){width="3.2604166666666665in"
height="2.678472222222222in"}![](media/image11.wmf){width="3.384027777777778in"
height="2.5625in"}

![](media/image12.wmf){width="3.0368055555555555in"
height="2.6875in"}![](media/image13.wmf){width="3.2618055555555556in"
height="2.384027777777778in"}

![](media/image14.wmf){width="2.7888888888888888in"
height="1.8527777777777779in"}

# Conclusions 

Failures in space flight power Inductors and Transformers have been
analyzed and classified according to failure source, fault type,
frequency of occurrence, count by yearly incidence and method of
detection. Additionally, magnetics manufacturers were surveyed regarding
their experiences as to which Mil-Std 981 tests had potential to
generate the most failures.

The results lead to the observations that:

1.  Failure encounters in magnetic devices, particularly inductors and
    transformers, are not very common.

2.  Data collected show that

    -   Magnetic components fail more in the Open mode than in the Short
        Circuit mode

    -   More failures result from manufacturing errors than design or
        installation errors

    -   Advances in manufacturing technology do not show decline in
        failures with years due to manufacturing errors.

    -   More components fail the Thermal Cycle test than other tests

    -   Insulation breakdown and improper solder joints account for
        majority of the failures occurring in use

3.  Survey responses from magnetics manufacturers indicate that:

    -   Magnetics manufacturers generally consider "Dielectric" and
        "Electrical" tests as highest priority tests while Seal and
        Radiographic tests were generally considered least critical.

The ever-rising costs in space flight hardware developments and the need
to maintain high reliability necessitate maintaining active database of
failure incidences to support analysis and understanding of the failure
mechanisms. In this regard, this paper strongly suggests reactivating
the GIDEP program and streamlining a uniform method of failure
reporting.

Reference:

1.  Denson, William K., Reliability Assessment of Critical Electronic
    Components, RL-TR-92197 IIT Research Institute, Final Technical
    Report, Rome Laboratory, Air Force Command, Griffiss Air Force Base,
    NY 13441-5700

2.  Devaney, John, R., Hill, Gerald L., Seippel, Robert G., Failure
    Analysis Mechanisms, Techniques, & Phot Atlas. Failure Recognition &
    Training Services, Inc, East 204 Nora Spkane, WA 00207

3.  Ghaffarian, Reza, Quality Assurance and Reliability: A system
    Approach, NASA EEE Links, Vol. 4 No. 4, Dec 1998

4.  Ghaffarian, Reza, Variables Affecting CSP Reliability, NASA EEE
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5.  Ghaffarian, Reza, Solderability Test and Correlation to SMT solder
    Joint Quality, NASA EEE Links, Vol. 2 No. 1, March 1996.

6.  GIDEP

7.  Inductors and Transformers Manufactures

8.  Screening Houses

9.  Automotive Electronics Council - Component Technical Committee
    "Stress Test Qualification for Passive Components" document
    AEC-Q200-REV B, March 15, 2000.