ASTM B920-16(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: B920 − 16 (Reapproved 2022)
Standard Practice for
Porosity in Gold and Palladium Alloy Coatings on Metal
Substrates by Vapors of Sodium Hypochlorite Solution
This standard is issued under the fixed designation B920; 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 practice covers equipment and methods for
B374 Terminology Relating to Electroplating
revealing the porosity of gold and palladium coatings, particu-
B542 Terminology Relating to Electrical Contacts and Their
larly electrodeposits and clad metals used on electrical con-
Use
tacts.
B735 Test Method for Porosity in Gold Coatings on Metal
1.2 This test practice is suitable for coatings containing gold
Substrates by Nitric Acid Vapor
or 75 % by mass of palladium on substrates of copper, nickel,
B741 Test Method for Porosity In Gold Coatings On Metal
and their alloys, which are commonly used in electrical
Substrates By Paper Electrography (Withdrawn 2005)
contacts. B765 Guide for Selection of Porosity and Gross Defect Tests
for Electrodeposits and Related Metallic Coatings
1.3 A variety of full porosity testing methods is described in
B798 Test Method for Porosity in Gold or Palladium Coat-
2,3
the literature. These porosity Test Methods are B735, B741,
ings on Metal Substrates by Gel-Bulk Electrography
B798, B799, and B809. An ASTM Guide to the selection of
B799 Test Method for Porosity in Gold and Palladium
porosity tests for electrodeposits and related metallic coatings
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
is available as Guide B765.
B809 Test Method for Porosity in Metallic Coatings by
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
1.4 The values stated in SI units are to be regarded as
standard. The values given in parentheses are for information
3. Terminology
only.
3.1 Definitions—Many terms used in this practice are de-
1.5 This standard does not purport to address all of the
fined in Terminology B542 and terms relating to metallic
safety concerns, if any, associated with its use. It is the
coatings are defined in Terminology B374.
responsibility of the user of this standard to establish appro-
3.2 Definitions of Terms Specific to This Standard:
priate safety, health, and environmental practices and deter-
3.2.1 corrosion products, n—those reaction products ema-
mine the applicability of regulatory limitations prior to use.
nating from the pores that protrude from, or are otherwise
For specific hazards, see Section 6.
attached to, the coating surface after a vapor test exposure.
1.6 This international standard was developed in accor-
3.2.2 metallic coatings, n—include platings, claddings, or
dance with internationally recognized principles on standard-
other metallic layers applied to the substrate. The coatings can
ization established in the Decision on Principles for the
comprise a single metallic layer or a combination of metallic
Development of International Standards, Guides and Recom-
layers.
mendations issued by the World Trade Organization Technical
3.2.3 porosity, n—the presence of any discontinuity, crack,
Barriers to Trade (TBT) Committee.
or hole in the coating that exposes a different underlying metal.
3.2.4 underplate, n—a metallic coating layer between the
substrate and the topmost layer or layers. The thickness of an
This practice is under the jurisdiction of ASTM Committee B02 on Nonferrous
underplate is usually greater than 0.8 μm (30 μin.).
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 2001. Last previous edition approved in 2016 as B920 – 16. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/B0920-16R22. 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
Krumbien, 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
B920 − 16 (2022)
4. Summary of Practice outside of the zone of contact of the mating surfaces, their
presence can often be tolerated.
4.1 The test practice employs a solution of sodium
5.3 Methods for determining pores on a contact surface are
hypochlorite, a material readily available as household bleach.
The test is recommended primarily as a qualitative means for most suitable if they enable their precise location and numbers
to be determined. Contact surfaces are often curved or irregular
assessing the plating quality in electrical connectors and is
desirable because it uses readily available reagents and equip- in shape, and testing methods should be suitable for them. In
ment and is extremely inexpensive, simple, and fast. In the test, addition, the severity of porosity-determining tests may vary
from procedures capable of detecting all porosity to procedures
the coated parts to be evaluated are suspended above a solution
of sodium hypochorite in a vessel that is closed but not sealed. that detect only highly porous conditions.
Paper towels extend from the solution in the bottom part of the
5.4 The present test practice is capable of detecting virtually
way up the sides of vessel providing a wicking action for the
all porosity or other defects that could participate in corrosion
solution. After exposure in this manner for 30 min to 45 min,
reactions with the substrate or underplate. In addition, it can be
the parts are removed from the vessel, dried with hot air and
used on contacts having complex geometry such as pin-socket
examined for the presence of corrosion products that indicate
contacts (although with deep recesses it is preferred that the
porosity.
contact structures be opened to permit reaction of the vapors
with the interior significant surfaces).
4.2 For more quantitative characterization of porosity it is
better to use one of the previously mentioned porosity test
5.5 The relationship of porosity levels revealed by particular
standards. This practice is oftentimes used as an early predictor
tests to contact behavior must be made by the user of these tests
of the likelihood of failure in a full mixed flowing gas (MFG)
through practical experience or by other forms of testing. Thus,
test used as an accelerated environmental test.
absence of porosity in the coating may be a requirement for
some applications, while a few pores in the contact zone may
4.3 Exposure periods may vary, depending upon the degree
be acceptable for others. The acceptable number, sizes and
of porosity to be revealed. Reaction of the gas with a
locations of the pore corrosion products shall be as specified on
corrodable base metal at pore sites produces reaction products
the appropriate drawing or specification.
that appear as discrete spots on the gold or palladium surface.
Individual spots may be counted with the aid of a loupe or
5.6 This test is considered destructive in that it reveals the
low-power stereomicroscope.
presence of porosity by contaminating the surface with corro-
sion products and by undercutting the coating at pore sites or
4.4 This porosity test involves corrosion reactions in which
at the boundaries of the unplated areas. Any parts exposed to
the products delineate defect sites in coatings. Since the
this test shall not be placed in service.
chemistry and properties of these products may not resemble
those found in natural or service environments this test is not
5.7 The test is simple and inexpensive. The cost associated
recommended for prediction of the electrical performance of
with the test is very low, using standard basic equipment found
contacts unless correlation is first established with service
in an industrial laboratory. There are minimal waste disposal
experience.
issues associated with the procedure. The test is very popular
because of its very quick means of assessing the likelihood of
5. Significance and Use
plating quality problems, prior to the performance of acceler-
ated environmental testing on the 1 to 2 week scale at much
5.1 Palladium and gold coatings are often specified for the
greater expense.
contacts of separable electrical connectors and other devices.
Electrodeposits are the form of gold that is most used on
6. Safety Hazards
contacts, although it is also employed as inlay or clad metal
and as weldments on the contact surface. The intrinsic nobility
6.1 Carry out this test procedure in a clean, working fume
of gold and palladium alloys enables it to resist the formation
hood. The vapor emitted is toxic, corrosive, and irritating.
of insulating oxide films that could interfere with reliable
6.2 Because the test is conducted in a reaction vessel using
contact operation.
a loose-fitting cover, it is desirable to insure that the drafts
5.2 In order for these coatings to function as intended,
often found in hoods are not so high as to adversely effect the
porosity, cracks, and other defects in the coating that expose
reproducibility of the test within the reaction vessel.
base-metal substrates and underplates must be minimal or
6.3 Observe good laboratory practices when handling the
absent, except in those cases where it is feasible to use the
sodium hypochlorite (household bleach) solution. In particular,
contacts in structures that shield the surface from the environ-
wear eye protection completely enclosing the eyes, and make
ment or where corrosion inhibiting surface treatments for the
eye wash facilities readily available.
deposit are employed. The level of porosity in the coating that
may be tolerable depends on the severity of the environment to
7. Apparatus
the underplate or substrate, design factors for the contact
device like the force with which it is mated, circuit parameters, 7.1 Test Glassware, a vessel of sufficient size such that the
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