ASTM B735-16(2022)

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Substrates By Paper Electrography (Withdrawn 2005)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 service@astm.org. 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-
reliability of contact operation that it is necessary to maintain. sectional area. This is to ensure that movement of air and vapor
Also, when present, the location of pores on the surface is within the vessel will not be restricted during the test.
B735 − 16 (2022)
6.3 Stereomicroscope, having a 10× magnification, shall be 9.2 The ambient temperature and the temperature of the
used for pore counting. In addition a movable source of specimens and solution are 23 °C 6 3 °C at the beginning of
illumination capable of giving oblique lighting on the specimen the test and maintained throughout the test period.
surface is also useful.
9.3 The relative humidity in the immediate vicinity of the
test chamber shall be no greater than 60 %, although 55 % or
7. Reagent
below is preferred. If the relative humidity is greater than
7.1 Nitric Acid, Reagent Grade Concentrated 70 % 6 2 % 60 %, do not run this test.
HNO , sp gr 1.415 to 1.420.
9.4 Add fresh HNO to the bottom of the clean and dry test
chamber, and immediately close the cover. After 30 min 6
8. Safety Hazards
5 min, load the samples, using suitable fixtures, and replace the
8.1 Carry out this test method in a chemical fume hood, cover. The ambient relative humidity shall be no greater than
since the gases that are released, mainly when the reaction 60 % during both the addition of the HNO and the insertion of
vessel is opened at the end of each test, are very corrosive. the samples. (Warning—Do not grease the rim of the
desiccator nor its cover. If desired, press a minimum of three
8.2 Use caution, however, to ensure that drafts that are often
strips of pressure sensitive polytetrafluoroethylene tape (adhe-
found in fume hoods do not cause significant cooling of the
sive side down) at equal intervals around the desiccator rim.)
chamber walls, that could lead to a rise in the relative humidity
and acceleration of the test (see 9.3). It is often convenient to 9.5 Unless otherwise specified, the exposure time to nitric
acid vapor shall be 60 min 6 5 min. An exposure time of
enclose the reaction vessel in a box with a loose-fitting cover.
75 min 6 5 min is also commonly used for gold thicknesses in
8.3 Observe normal precautions in handling corrosive acids.
the 2 μm to 2.5 μm (75 μin. to 100 μin.) range. A table of
In particular, wear goggles completely enclosing the eyes when
convenient exposure times is given in the appendix.
handling nitric acid, and make eye wash facilities readily
available.
NOTE 3—Variations in exposure time with thickness are often recom-
mended because pores in thicker coatings are deeper and their average
sizes are smaller than those in thinner coatings. The nitric acid medium
9. Procedure
would therefore take longer to penetrate an average pore in thicker
9.1 Handle specimens as little as possible, and only with coatings compared to thinner ones. On the other hand, when exposure
times are too long, the corrosion products will overlap and impair pore
tweezers, microscope-lens tissue, or clean, soft cotton gloves.
delineation. A detailed discussion of these effects is given in Footnote 3.
Prior to the test, inspect the samples under 10× magnification
for evidence of particulate matter. If present, such particles 9.6 Remove the samples at the end of the test and dry in an
oven at 125 °C 6 5 °C for 30 min 6 5 min. Then remove from
shall be removed by blowing them off with clean, oil-free air.
If this is not successful discard the sample. Then, clean the the oven, and place directly into a desiccator containing active
desiccant, and allow to cool to room temperature.
samples with solvents or solutions that do not contain chlori-
nated hydrocarbons,
...