ISO 9621:2024

Space systems - Methods to decide thermal vacuum test cycles of recurring production according to precipitation efficiency and reliability

This document provides technical methods to calculate the precipitation efficiency and liability of a flight model by measuring the screening effectiveness of thermal cycles. This document is applicable to the recurring production unit and other hardware assembly levels, as either an option to reduce or a method to tailor the baseline number of cycles for thermal vacuum and thermal cycle acceptance tests.

Systèmes spatiaux — Méthodes pour déterminer les cycles d'essais sous vide thermique de la production récurrente en fonction de l'efficacité et de la fiabilité des précipitations

General Information

• Status: Published
• Publication Date: 12-May-2024
• ICS: 49.140 - Space systems and operations
• Technical Committee: ISO/TC 20/SC 14 - Space systems and operations
• Drafting Committee: ISO/TC 20/SC 14 - Space systems and operations
• Current Stage: 6060 - International Standard published
• Start Date: 13-May-2024
• Due Date: 08-Nov-2024
• Completion Date: 13-May-2024

Overview

ISO 9621:2024 - Space systems - Methods to decide thermal vacuum test cycles of recurring production according to precipitation efficiency and reliability - defines technical methods to determine the number of thermal vacuum test (TVT) cycles for recurring production flight hardware. The standard provides quantitative approaches to calculate precipitation efficiency (PE) and reliability from failure data, enabling test-cycle tailoring or reduction for mass-produced spacecraft hardware, constellation units, and repeated manufacturing blocks.

Key topics

• Purpose: Measure screening effectiveness of thermal cycles to screen out latent defects (infant mortality) and justify the number of TVT or thermal cycle (TCT) acceptance test cycles.
• Two calculation methods:
  • Method 1 - Precipitation efficiency method: Estimation of PE (probability of screening latent defects) from observed failures and test parameters.
  • Method 2 - Reliability method: Estimation of item reliability to support tailoring of TVT cycle counts.
• Failure data collection and analysis: Required inputs and best practices include:
  • Cycle number when each failure occurred and test parameters (minimum/maximum temperatures; minimum temp suggested at −55 °C for failure data).
  • Function-test results during/after TVT to ensure failures are detected.
  • Treatment of failures from sequential or combined environments (e.g., vibration before/after TVT).
  • Handling data across different production lots and similarity justification when aggregating data.
  • Inclusion of thermal cycle test (TCT) fatigue data where relevant.
• Data handling: Sorted failure-cycle lists and associated temperature ranges are used to derive PE and reliability estimates.
• Supporting material: Annex A explains theoretical estimation of PE and lower bounds; Annex B contains a worked example.

Applications

ISO 9621:2024 is intended for:

• Spacecraft test engineers and environmental test laboratories planning thermal vacuum and thermal cycle acceptance tests.
• Reliability and quality engineers in satellite manufacturers, especially for recurring production and constellation programs.
• Program managers and procurement teams who need a defensible basis to tailor or reduce baseline TVT cycles while maintaining screening effectiveness. Practical uses include reducing cost and test time for mass-produced flight hardware, tailoring test requirements based on accumulated failure data, and improving production-process feedback loops.

Related standards

• ISO 15864 - Space systems - General test methods for spacecraft, subsystems and units (normative reference for TVT/TCT requirements).
• ISO 10795 - Referenced for terminology and definitions used in ISO 9621.

ISO 9621:2024 provides a data-driven framework to optimize TVT cycles, balancing test cost and defect screening effectiveness for recurring space-system production.

Standards Content (Sample)

International
Standard
ISO 9621
First edition
Space systems — Methods to decide
2024-05
thermal vacuum test cycles of
recurring production according
to precipitation efficiency and
reliability
Systèmes spatiaux — Méthodes pour déterminer les cycles
d'essais sous vide thermique de la production récurrente en
fonction de l'efficacité et de la fiabilité des précipitations
Reference number
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and abbreviated terms . 1
3.1 Terms and definitions .1
3.2 Abbreviated terms .2
4 Methods of TVT cycles determination . 3
4.1 General .3
4.2 Failure data collection and analysis .3
4.3 Method 1 – Precipitation efficiency method .4
4.3.1 General .4
4.3.2 Precipitation efficiency estimation .5
4.4 Method 2 – Reliability method .6
4.4.1 General .6
4.4.2 Reliability method .6
4.5 Empirical equivalent fatigue exponent by failure data .7
Annex A (informative) Theory of estimating PE and its lower band limit . 9
Annex B (informative) Worked example .11
Bibliography .16

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
The thermal vacuum test is one of the most important and expensive environment tests of space systems.
The thermal vacuum test is required in general to demonstrate the ability of the test item to meet the design,
function and performance requirements under the combination of vacuum conditions and temperature
extremes experienced during spaceflight, and to screen out initial failure, known as infant mortality, such
as workmanship error, integration error and latent material defect (ISO 15864). The number of thermal
cycles, to be referred hence forth as simply cycles, is one of the test conditions in thermal vacuum test, is an
essential parameter used to determine the screening effectiveness of initial failure. The number of cycles
should be determined, generally based on technical aspects and with careful consideration of various factors
such as test item complexity, heritage, and maturity of design and manufacturing, as well as reliability
required of the test article. However as experienced in mass production industry, it is a natural expectation
that as the design and manufacturing process of an item matures through continuous improvement of the
manufacturing process, workmanship defects and initial failure will be reduced.
This document provides two technical methods specified to calculate the precipitation efficiency and
reliability by the failure data to measure the quantity of screening effectiveness used to determine number
of cycles of thermal vacuum test. These methods can be applied to reduce the number of cycles performed
during a thermal vacuum test specified for recurring production of flight hardware, such as higher volume
unit production runs and hardware produced for large constellation programs. This document supplements
ISO 15864 used as either an option to reduce or a tailoring method to the baseline of thermal vacuum and
thermal cycle acceptance tests specific for the recurring production hardware.

v
International Standard ISO 9621:2024(en)
Space systems — Methods to decide thermal vacuum test
cycles of recurring production according to precipitation
efficiency and reliability
1 Scope
This document provides technical methods to calculate the precipitation efficiency and liability of a flight
model by measuring the screening effectiveness of thermal cycles. This document is applicable to the
recurring production unit and other hardware assembly levels, as either an option to reduce or a method to
tailor the baseline number of cycles for thermal vacuum and thermal cycle acceptance tests.
2 Normative references
The following documents are referred to in the text in such a way that some or all content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 15864, Space systems — General test methods for spacecraft, subsystems and units
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 15864 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1.1
failure
termination of the ability of an item (3.1.5) to perform a required function
[SOURCE: ISO 10795:2019, 3.98]
3.1.2
flight model
model dedicated to being launched and operated in orbit and subjected to acceptance testing
3.1.3
hardware
items (3.1.5) of identifiable equipment including piece parts, components, assemblies, subsystems and systems
[SOURCE: ISO 10795:2019, 3.119, modified — The abbreviated term "H/W" has been removed.]
3.1.4
initial failure
probability of failure (3.1.1) or defect under an environmental test

3.1.5
item
node of a product breakdown structure
Note 1 to entry: An item can be any functional unit, subsystem, or system in ISO 15864 that can be individually
considered.
Note 2 to entry: An item can be considered either as a “product” or a “component” on a “product breakdown structure”
of more than two levels of decomposition. Items are designated “products” when described as being decomposed and
designated “components” when described as decompositions.
[SOURCE: ISO 10795:2019, 3.134, modified — The domain "" has been removed; in note 1 to
entry, a reference has been made to ISO 15864.]
3.1.6
latent defect
defect caused by workmanship error, integration error or latent material, which is not detectable in a stress-
free environment, but is either screened under environmental test conditions or flight environment
3.1.7
precipitation efficiency
PE
probability of screening out latent defects (3.1.6) in a specific environmental test
3.1.8
reliability
ability of an item (3.1.5) to perform as required under given conditions for a given time interval
Note 1 to entry: In this document, reliability is equivalent to the probability that the hardware (3.1.3) is failure-free.
[SOURCE: ISO 10795:2019, 3.198, modified — "a required function" has been replaced by "as required"; the
original two notes to entry have been replaced by a new one.]
3.1.9
tailoring
process by which individual requirements of specifications, standards, and related documents are evaluated
and made applicable to a specific project by selection and, in some exceptional cases, modification of existing
or addition of new requirements
[SOURCE: ISO 10795:2019, 3.237]
3.1.10
test temperature range
difference between the maximum and minimum temperatures in a thermal vacuum test
Note 1 to entry: The thermal vacuum test is specified in ISO 15864:2021, 7.18.
3.2 Abbreviated terms
AT acceptance test
MPE maximum predicted environment
PE precipitation efficiency
TCT thermal cycle test
TESS thermal environmental stress screening
TTC telemetry, tracking and command
TVT thermal vacuum test
4 Methods of TVT cycles determination
4.1 General
Thermal cycle and thermal vacuum tests (TVT) are required for system and certain types of subsystem/
unit acceptance test (AT), as specified in ISO 15864:2021, Table 1 and Table 3. The number of cycles is
determined in general by consideration of the overall development and test history of the hardware under
test, for example, the complexity and heritage of design and tests of lower-level assemblies before integrated
assemblies, TVT conditions such as temperature range and duration. Initial screening for latent defects of
flight hardware, also called infant mortality, which is caused by manufacturing, material and workmanship
defects, shall be demonstrated by the acceptance test. The determination of TVT cycles depends on the
essential parameter used to measure the screening effectiveness for initial latent defects by the precipitation
efficiency and reliability of the test. Although in ISO 15864 there is no statement for the value of number of
cycles, the expected number of cycles for thermal environmental stress screening (TESS) is determined by
[1],[2]
the required precipitation efficiency (PE) and temperature range in TVT .
The number of TVT cycles for the relevant kinds of hardware (system, subsystem and unit) of recurring
production can be tailored to reduce the number of cycles specified in baseline requirement by prior
experience and studies of the failure database. This document provides the technical methods to support
tailoring thermal vacuum test cycles of the relevant hardware. Methods used to calculate the precipitation
efficiency and reliability are used to measure the quantity of the TESS effectiveness, by the priorly revealed
failure database collected from the objective hardware. The failure database used in these methods shall
support the justification of number of cycles determination.
The term of recurring production stated herein shall be replaced by spacecraft, subsystem or unit defined in
ISO 15864 in case of tailoring the baseline of thermal vacuum and thermal cycle acceptance tests. Recurring
production can be hardware used for constellation programs, or more generally, the hardware made with
a constant block of design and manufacture, such as mass production hardware, replicating manufactured
hardware.
The failure database may be extended to the hardware that has similarity in design, manufacture and part
grade integrated in the production, but the technical rationale of supporting the similarity claim shall be
justified. An example to support similarity can be when the hardware is designed and manufactured by
relative standards, reliability and quality control by same management standards, high class parts (e.g.
[3]
class I) are used in the hardware .
4.2 Failure data collection and analysis
The failure distribution function is used for estimating TESS effectiveness by the fatigue induced stress.
Failure data collected shall be revealed that failures are relevant to initial defects due to foreign substance
contamination in manufacturing and workmanship of the relevant hardware. Failure data collection at
acceptance TVT shall provide the following basic information.
a) The cycle numbers and test parameters, including minimum and maximum temperatures, in the TVT
when the failure occurred. The minimum temperature of failure data collected should be 55 °C.
b) The results from function tests performed. A function test shall be performed during and after exposure
to the TVT environment to ensure the perceptiveness of potential failures. In cases that failures are
discovered after exposing to TVT environment and the cycles when the failure occurred are difficult to
be identified, an increased number of cycles should be assigned. This assignment of increased cycles is
to obtain a conservative estimation.
c) Failure data from previous and/or follow-on environmental tests. If the hardware is exposed to a
number of environments sequential, for example, a vibration environment could be exposed on the
hardware before or after TVT, the failure data collected shall be carefully analysed. Failures escaping
from a previous exposure environment or into a follow-on exposed environment shall be carefully
analysed to confirm if there are TVT environment related failures. If the failure related environment is
indistinct, this kind of failure data shall not be included.

d) Failure data from different lot tests. If the failure data are collected from different lots or replicating
manufactured hardware, the failure data should be carefully analysed to determine if there are different
number of failure cycles between lots or replicating hardware. Test parameters to influence the TESS,
such as temperature range, temperature transition rate, hardware operation, failure modes, should be
carefully examined.
e) Failure data from thermal cycle tests (TCT). Thermo-mechanical fatigue failures in TCT may be included
in the failure database.
Typical failure data and initial defects of electronic units are shown in Table B.1.
The collected failure data shall be sorted in ascending order by the cycle number x when the failure is
discovered and their corresponding test temperature range ΔT . If there are k failures, the failure data shall
be sorted to,
xx= ,,xx, (1)
()
12 k
ΔΔTT=(),,ΔΔTT, (2)
12 k
where
th
x
is the cycle number when the failure is discovered; i is the index of i failure;
i
th
ΔT
is the TVT temperature range corresponding to the i failure.
i
If there are different TVT temperature ranges, temperature effect shall be considered by normalizing
[4]
equivalent fatigue with the low-cycle fatigue equivalence. The equivalent temperature normalized cycle
′
number x shall be decided by Formula (3).
bb bb
xx′′= ′′,,xx,/′ = xTΔΔTx,/ΔΔTT′′,,xTΔΔ/ T (3)
() () () ()
12 k ()11 22 kk
where
b
is the low-cycle fatigue equivalent exponent value;
ΔT′
is the reference test temperature range in the TVT for equivalent temperature normalization; the
minimum reference temperature for normalization shall be 55 °C.
Fatigue equivalent exponent b shall be determined by the material and failure mode of the test hardware.
[5]
b = 2 is recommended for solder joint, which is widely used for electric unit. Empirically equivalent value
comprising multiple failure modes, and materials should be calculated following the method in 4.4.
NOTE Examples of electric unit failure data related to manuf
...