ASTM B735-16(2022)

Standard Test Method for Porosity in Gold Coatings on Metal Substrates by Nitric Acid Vapor

SIGNIFICANCE AND USE
5.1 Gold coatings are often specified for the contacts of separable electrical connectors and other devices. Electrodeposits are the form of gold that is most used on contacts, although it is also employed as clad metal and as weldments on the contact surface. The intrinsic nobility of gold enables it to resist the formation of insulating oxide films that could interfere with reliable contact operation.  
5.2 In order that the nobility of gold be assured, porosity, cracks, and other defects in the coating that expose base-metal substrates and underplates must be minimal or absent, except in those cases where it is feasible to use the contacts in structures that shield the surface from the environment or where corrosion inhibiting surface treatments for the deposit are employed. The level of porosity in the coating that may be tolerable depends on the severity of the environment to the underplate or substrate, design factors for the contact device like the force with which it is mated, circuit parameters, and the reliability of contact operation that it is necessary to maintain. Also, when present, the location of pores on the surface is important. If the pores are few in number and are outside of the zone of contact of the mating surfaces, their presence can often be tolerated.  
5.3 Methods for determining pores on a contact surface are most suitable if they enable their precise location and numbers to be determined. Contact surfaces are often curved or irregular in shape, and testing methods should be suitable for them. In addition, the severity of porosity-determining tests may vary. This test method is regarded as severe.  
5.4 The relationship of porosity levels revealed by particular tests to contact behavior must be made by the user of these tests through practical experience or by judgement. Thus, absence of porosity in the coating may be a requirement for some applications, while a few pores on the critical surfaces may be acceptable for another. Such ac...
SCOPE
1.1 This test method covers equipment and procedures for using nitric acid vapor for determining porosity in gold coatings, greater than 0.6 μm (25 μin.) in thickness, particularly electrodeposits and clad metals used on electrical contacts.  
1.2 This test method is designed to show whether the porosity level is less or greater than some value that, by experience, is considered by the user to be acceptable for the intended application.  
1.3 A variety of other porosity testing methods are described in Guide B765 and in the literature.2,3 Other porosity test methods are Test Methods B741, B798, B799, and B809.  
1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Safety Data Sheet (SDS) for this product/material as provided by the manufacturer, to establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations prior to use. Specific precautions are given in Section 8 and 9.4.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Sep-2022
• ICS: 25.220.40 - Metallic coatings
• Technical Committee: B02 - Nonferrous Metals and Alloys
• Drafting Committee: B02.05 - Precious Metals and Electrical Contact Materials and Test Methods

Relations

• Refers: ASTM B765-03(2023) - Standard Guide for Selection of Porosity and Gross Defect Tests for Electrodeposits and Related Metallic Coatings
• Effective Date: 01-Nov-2023
• Refers: ASTM B542-13(2019) - Standard Terminology Relating to Electrical Contacts and Their Use
• Effective Date: 01-Nov-2019
• Refers: ASTM B809-95(2018) - Standard Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (“Flowers-of-Sulfur”)
• Effective Date: 01-Aug-2018
• Refers: ASTM B765-03(2018) - Standard Guide for Selection of Porosity and Gross Defect Tests for Electrodeposits and Related Metallic Coatings
• Effective Date: 01-Aug-2018
• Refers: ASTM B798-95(2014) - Standard Test Method for Porosity in Gold or Palladium Coatings on Metal Substrates by Gel-Bulk Electrography
• Effective Date: 01-Oct-2014
• Refers: ASTM B799-95(2014) - Standard Test Method for Porosity in Gold and Palladium Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
• Effective Date: 01-Oct-2014
• Refers: ASTM B765-03(2013) - Standard Guide for Selection of Porosity and Gross Defect Tests for Electrodeposits and Related Metallic Coatings
• Effective Date: 01-Dec-2013
• Refers: ASTM B809-95(2013) - Standard Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (“Flowers-of-Sulfur”)
• Effective Date: 01-Dec-2013
• Refers: ASTM B542-13 - Standard Terminology Relating to Electrical Contacts and Their Use
• Effective Date: 01-Feb-2013
• Refers: ASTM B374-06(2011) - Standard Terminology Relating to Electroplating
• Effective Date: 01-Apr-2011
• Refers: ASTM B799-95(2009) - Standard Test Method for Porosity in Gold and Palladium Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
• Effective Date: 01-Oct-2009
• Refers: ASTM B798-95(2009) - Standard Test Method for Porosity in Gold or Palladium Coatings on Metal Substrates by Gel-Bulk Electrography
• Effective Date: 01-Oct-2009
• Refers: ASTM B765-03(2008) - Standard Guide for Selection of Porosity and Gross Defect Tests for Electrodeposits and Related Metallic Coatings
• Effective Date: 01-Aug-2008
• Refers: ASTM B809-95(2008) - Standard Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor ("Flowers-of-Sulfur")
• Effective Date: 01-Aug-2008
• Refers: ASTM B542-07 - Standard Terminology Relating to Electrical Contacts and Their Use
• Effective Date: 01-May-2007

Overview

ASTM B735-16(2022) is the Standard Test Method for Porosity in Gold Coatings on Metal Substrates by Nitric Acid Vapor. Developed by ASTM International, this standard outlines the procedures and equipment for evaluating porosity in gold coatings, especially those used on electrical contacts and connectors. By applying nitric acid vapor to coated surfaces, the method reveals pores and defects that expose underlying base metals, providing key information on the integrity and durability of the gold coating.

Porosity in gold coatings can significantly impact electrical reliability and component longevity, particularly in demanding environments. Therefore, this test method is highly valued in industries where performance and reliability are critical.

Key Topics

• Porosity Detection: The standard specifies the use of nitric acid vapor to detect discontinuities such as pores, cracks, or other defects in gold coatings thicker than 0.6 μm (25 μin.). Reaction products formed at defect sites visibly indicate the location and number of pores.
• Applicability: Optimized for electrodeposited and clad gold coatings on metal substrates, particularly copper, nickel, and their alloys. The method accommodates complex geometries common in electrical contacts.
• Test Severity: This nitric acid vapor test is considered severe, suitable for applications that require a stringent assessment of porosity. It is not suitable for coatings thinner than 0.6 μm or metals less noble than gold.
• Interpretation: Allows users to determine whether the porosity is within acceptable limits based on experience and application requirements. It provides qualitative and quantitative data on pore distribution and severity.
• Health, Safety, and Environmental Considerations: The standard mandates proper handling and disposal procedures due to the use of corrosive acids, highlighting the importance of adherence to safety data sheets and appropriate lab practices.

Applications

ASTM B735-16(2022) is integral in several sectors including electronics, aerospace, communications, and critical electrical systems. The primary applications include:

• Quality Assurance: Ensures that gold coatings on connectors and contacts meet performance and reliability criteria by detecting porosity that might lead to corrosion or electrical failure.
• Product Development and Evaluation: Assists engineers and manufacturers in selecting appropriate gold plating processes or evaluating alternate coating solutions based on quantitative porosity data.
• Failure Analysis: Used for root cause investigations in cases of contact failure, enabling the identification of coating defects that compromise performance.
• Conformance Testing: Supports compliance with industry and customer requirements for gold-plated components, serving as an accepted standard method for porosity assessment during incoming inspections or lot release.

Related Standards

Several ASTM standards relate closely to ASTM B735-16(2022) and offer complementary test methods or terminology for porosity testing in metallic coatings:

• ASTM B374 - Terminology Relating to Electroplating
• ASTM B542 - Terminology Relating to Electrical Contacts and Their Use
• ASTM B741 - Test Method for Porosity in Gold Coatings on Metal Substrates by Paper Electrography (Withdrawn)
• ASTM B765 - Guide for Selection of Porosity and Gross Defect Tests for Electrodeposits and Related Metallic Coatings
• ASTM B798 - Test Method for Porosity in Gold or Palladium Coatings by Gel-Bulk Electrography
• ASTM B799 - Test Method for Porosity in Gold and Palladium Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
• ASTM B809 - Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (“Flowers-of-Sulfur”)

These related documents can provide additional context or alternative methods suited to specific materials or application requirements.

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Keywords: Porosity testing, gold coatings, nitric acid vapor test, electrical contacts, ASTM B735, plating reliability, metallic substrate analysis, connector porosity, quality control in electroplating

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B735 − 16 (Reapproved 2022)
Standard Test Method for
Porosity in Gold Coatings on Metal Substrates by Nitric
Acid Vapor
This standard is issued under the fixed designation B735; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers equipment and procedures for
B374 Terminology Relating to Electroplating
using nitric acid vapor for determining porosity in gold
B542 Terminology Relating to Electrical Contacts and Their
coatings, greater than 0.6 μm (25 μin.) in thickness, particularly
Use
electrodeposits and clad metals used on electrical contacts.
B741 Test Method for Porosity In Gold Coatings On Metal
1.2 This test method is designed to show whether the 5
Substrates By Paper Electrography (Withdrawn 2005)
porosity level is less or greater than some value that, by
B765 Guide for Selection of Porosity and Gross Defect Tests
experience, is considered by the user to be acceptable for the
for Electrodeposits and Related Metallic Coatings
intended application.
B798 Test Method for Porosity in Gold or Palladium Coat-
ings on Metal Substrates by Gel-Bulk Electrography
1.3 A variety of other porosity testing methods are described
2,3
B799 Test Method for Porosity in Gold and Palladium
in Guide B765 and in the literature. Other porosity test
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
methods are Test Methods B741, B798, B799, and B809.
B809 Test Method for Porosity in Metallic Coatings by
1.4 The values stated in SI units are to be regarded as
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
standard. The values given in parentheses are for information
only.
3. Terminology
1.5 This standard does not purport to address all of the
3.1 Definitions—Many terms used in this test method are
safety concerns, if any, associated with its use. It is the
defined in Terminology B542 and terms relating to metallic
responsibility of the user of this standard to become familiar
coatings are defined in Terminology B374.
with all hazards including those identified in the appropriate
3.2 Definitions of Terms Specific to This Standard:
Safety Data Sheet (SDS) for this product/material as provided
3.2.1 corrosion products, n—those reaction products ema-
by the manufacturer, to establish appropriate safety, health,
nating from the pores that protrude from, or are otherwise
and environmental practices, and determine the applicability
attached to, the coating surface after a vapor test exposure.
of regulatory limitations prior to use. Specific precautions are
3.2.2 measurement area (or “significant surface”), n—the
given in Section 8 and 9.4.
surface that is examined for the presence of porosity. The
1.6 This international standard was developed in accor-
significant surfaces or measurement areas of the part to be
dance with internationally recognized principles on standard-
tested shall be indicated on the drawing of the part or by
ization established in the Decision on Principles for the
provision of suitably marked samples.
Development of International Standards, Guides and Recom-
3.2.2.1 Discussion—For specification purposes, the signifi-
mendations issued by the World Trade Organization Technical
cant surfaces or measurement areas are often defined as those
Barriers to Trade (TBT) Committee.
portions of the surface that are essential to the serviceability or
function of the part, such as its contact properties, or which can
be the source of corrosion products or tarnish films that
This test method is under the jurisdiction of ASTM Committee B02 on
interfere with the function of the part.
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
B02.05 on Precious Metals and Electrical Contact Materials and Test Methods.
Current edition approved Oct. 1, 2022. Published November 2022. Originally
approved in 1984. Last previous edition approved in 2016 as B735 – 16. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/B0735-16R22. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity Standards volume information, refer to the standard’s Document Summary page on
Testing of Gold Deposits,” Plating, Vol 52, 1965, p. 1001. the ASTM website.
3 5
Krumbein, S. J., Porosity Testing of Contact Platings, Proceedings, Connectors The last approved version of this historical standard is referenced on
and Interconnection Technology Symposium, Oct. 1987, p. 47. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B735 − 16 (2022)
3.2.3 metallic coatings, n—include platings, claddings, or important. If the pores are few in number and are outside of the
other metallic layers applied to the substrate. The coating can zone of contact of the mating surfaces, their presence can often
comprise a single metallic layer or a combination of metallic be tolerated.
layers.
5.3 Methods for determining pores on a contact surface are
3.2.4 porosity, n—the presence of any discontinuity, crack, most suitable if they enable their precise location and numbers
or hole in the coating that exposes a different underlying metal. to be determined. Contact surfaces are often curved or irregular
in shape, and testing methods should be suitable for them. In
3.2.5 underplate, n—a metallic coating layer between the
addition, the severity of porosity-determining tests may vary.
substrate and the topmost layer or layers. The thickness of an
This test method is regarded as severe.
underplate is usually greater than 0.8 μm (30 μin.).
5.4 The relationship of porosity levels revealed by particular
tests to contact behavior must be made by the user of these tests
4. Summary of Test Method
through practical experience or by judgement. Thus, absence of
4.1 This test method employs nitric acid (HNO ) vapor at
porosity in the coating may be a requirement for some
low relative humidity. Reaction of the gas mixture with a
applications, while a few pores on the critical surfaces may be
corrodible base metal at pore sites produces reaction products
acceptable for another. Such acceptance (or pass-fail) criteria
that appear as discrete spots on the gold surface. Individual
should be part of the product specification for the particular
spots are counted with the aid of a loupe or low-power stereo
product or part requiring the porosity test.
microscope.
5.5 This test method is highly sensitive and is capable of
4.2 This test method is suitable for inlays or claddings
detecting virtually all porosity or other defects in gold coatings
containing 75 % or more of gold or for electroplatings con-
that could participate in substrate corrosion reactions. The test
taining 95 % or more of gold on substrates of copper, nickel,
is rapid, simple, and inexpensive. In addition, it can be used on
and their alloys, that are commonly used in electrical contacts.
contacts having complex geometry such as pin-socket contacts.
However, it is preferred that deeply recessed sockets be opened
4.3 The nitric acid vapor test is too severe to be used for
to expose their critical surfaces prior to testing.
gold coatings less than 0.6 μm (25 μin.) in thickness. It is also
not suitable for coatings that are less noble than gold or
5.6 This test method is considered destructive in that it
platinum, such as palladium and its alloys, or gold-flashed
reveals the presence of porosity by contaminating the surface
palladium and its alloys. Gold-flashed is defined as a plated
with corrosion products and by undercutting the coating at pore
thickness of gold between 3 μin and 5 μin.
sites or at the boundaries of unplated areas. Any parts exposed
to these tests shall not be placed in service.
4.4 This porosity test involves corrosion reactions in which
the products delineate defect sites in coatings. Since the
5.7 This test method is intended to be used for quantitative
chemistry and properties of these products may not resemble
descriptions of porosity (such as number of pores per unit area
those found in natural or service environments, these tests are
or per contact) only on coatings that have a pore density
not recommended for prediction of the electrical performance
sufficiently low that the corrosion sites are well separated and
of contacts unless correlation is first established with service
can be readily resolved. As a general guideline this can be
experience.
achieved for pore densities up to about 100/cm or per 100
contacts. Above this value the tests are useful for the qualita-
5. Significance and Use
tive detection and comparisons of porosity.
5.1 Gold coatings are often specified for the contacts of
6. Apparatus
separable electrical connectors and other devices. Electrode-
posits are the form of gold that is most used on contacts, 6.1 Test Chamber, may be any convenient size glass vessel
although it is also employed as clad metal and as weldments on
capable of being sealed with a glass lid, such as a glass
the contact surface. The intrinsic nobility of gold enables it to desiccator of 9 L to 12 L capacity. The ratio of the air space in
resist the formation of insulating oxide films that could
the chamber (in cubic centimetres) to the nitric acid surface
interfere with reliable contact operation. area (in square centimetres) shall not be greater than 25:1.
6.2 Specimen Holders or Supports—Supports or hangers
5.2 In order that the nobility of gold be assured, porosity,
cracks, and other defects in the coating that expose base-metal shall be made from glass, polytetrafluoroethylene or other inert
materials. It is essential that the holders be so designed, and the
substrates and underplates must be minimal or absent, except
in those cases where it is feasible to use the contacts in specimens so arranged, that the circulation of the vapor is not
impeded. Specimens shall be at least 75 mm (3 in.) from the
structures that shield the surface from the environment or
where corrosion inhibiting surface treatments for the deposit liquid surface and at least 25 mm (1 in.) from the vessel walls.
Also, the measurement areas of the specimens shall be at least
are employed. The level of porosity in the coating that may be
tolerable depends on the severity of the environment to the 12 mm (0.5 in.) from each other.
underplate or substrate, design factors for the contact device 6.2.1 Do not use a porcelain plate or any other structure that
like the force with which it is mated, circuit parameters, and the would cover more than 30 % of the liquid surface cross-
reli
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