ASTM B735-06(2011)
(Test Method)Standard Test Method for Porosity in Gold Coatings on Metal Substrates by Nitric Acid Vapor
Standard Test Method for Porosity in Gold Coatings on Metal Substrates by Nitric Acid Vapor
SIGNIFICANCE AND USE
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.
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.
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.
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 acceptance (or pass-f...
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. , 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 Material Safety Data Sheet (MSDS) for this product/material as provided by the manufacturer, to establish appropriate safety and health practices, and determine the applicability of regulatory limitations prior to use. Specific precautions are given in Section 8 and 9.4.
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Standards Content (Sample)
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Designation: B735 − 06(Reapproved 2011)
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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope B542Terminology Relating to Electrical Contacts andTheir
Use
1.1 This test method covers equipment and procedures for
B741Test Method for Porosity In Gold Coatings On Metal
using nitric acid vapor for determining porosity in gold
Substrates By Paper Electrography (Withdrawn 2005)
coatings,greaterthan0.6µm(25µin.)inthickness,particularly
B765GuideforSelectionofPorosityandGrossDefectTests
electrodeposits and clad metals used on electrical contacts.
for Electrodeposits and Related Metallic Coatings
1.2 This test method is designed to show whether the
B798Test Method for Porosity in Gold or Palladium Coat-
porosity level is less or greater than some value that, by
ings on Metal Substrates by Gel-Bulk Electrography
experience, is considered by the user to be acceptable for the
B799Test Method for Porosity in Gold and Palladium
intended application.
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
1.3 Avarietyofotherporositytestingmethodsaredescribed B809Test Method for Porosity in Metallic Coatings by
2,3
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
in Guide B765 and in the literature. Other porosity test
methods are Test Methods B741, B798, B799, and B809.
3. Terminology
1.4 The values stated in SI units are to be regarded as
3.1 Definitions—Many terms used in this test method are
standard. The values given in parentheses are for information
defined in Terminology B542 and terms relating to metallic
only.
coatings are defined in Terminology B374.
1.5 This standard does not purport to address all of the
3.2 Definitions of Terms Specific to This Standard:
safety concerns, if any, associated with its use. It is the
3.2.1 corrosion products, n—those reaction products ema-
responsibility of the user of this standard to become familiar
nating from the pores that protrude from, or are otherwise
with all hazards including those identified in the appropriate
attached to, the coating surface after a vapor test exposure.
Material Safety Data Sheet (MSDS) for this product/material
as provided by the manufacturer, to establish appropriate
3.2.2 measurement area (or “significant surface”), n—the
safety and health practices, and determine the applicability of
surface that is examined for the presence of porosity. The
regulatory limitations prior to use. Specific precautions are
significant surfaces or measurement areas of the part to be
given in Section 8 and 9.4.
tested shall be indicated on the drawing of the part or by
provision of suitably marked samples.
2. Referenced Documents
3.2.2.1 Discussion—For specification purposes, the signifi-
cant surfaces or measurement areas are often defined as those
2.1 ASTM Standards:
B374Terminology Relating to Electroplating portions of the surface that are essential to the serviceability or
functionofthepart,suchasitscontactproperties,orwhichcan
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
3.2.3 metallic coatings, n—include platings, claddings, or
B02.11 on Electrical Contact Test Methods.
Current edition approved Oct. 1, 2011. Published October 2011. Originally other metallic layers applied to the substrate. The coating can
approved in 1984. Last previous edition approved in 2006 as B735–06. DOI:
comprise a single metallic layer or a combination of metallic
10.1520/B0735-06R11.
layers.
For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity
Testing of Gold Deposits,” Plating, Vol 52, 1965, p. 1001.
3.2.4 porosity, n—the presence of any discontinuity, crack,
Krumbein,S.J.,PorosityTestingofContactPlatings, Proceedings,Connectors
orholeinthecoatingthatexposesadifferentunderlyingmetal.
and Interconnection Technology Symposium, Oct. 1987, p. 47.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B735 − 06 (2011)
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 Therelationshipofporositylevelsrevealedbyparticular
teststocontactbehaviormustbemadebytheuserofthesetests
4. Summary of Test Method
throughpracticalexperienceorbyjudgement.Thus,absenceof
porosity in the coating may be a requirement for some
4.1 This test method employs nitric acid (HNO ) vapor at
applications, while a few pores on the critical surfaces may be
low relative humidity. Reaction of the gas mixture with a
acceptable for another. Such acceptance (or pass-fail) criteria
corrodible base metal at pore sites produces reaction products
should be part of the product specification for the particular
that appear as discrete spots on the gold surface. Individual
product or part requiring the porosity test.
spots are counted with the aid of a loupe or low-power stereo
microscope.
5.5 This test method is highly sensitive and is capable of
detectingvirtuallyallporosityorotherdefectsingoldcoatings
4.2 This test method is suitable for inlays or claddings
that could participate in substrate corrosion reactions. The test
containing 75% or more of gold or for electroplatings con-
israpid,simple,andinexpensive.Inaddition,itcanbeusedon
taining 95% or more of gold on substrates of copper, nickel,
contactshavingcomplexgeometrysuchaspin-socketcontacts.
and their alloys, that are commonly used in electrical contacts.
However,itispreferredthatdeeplyrecessedsocketsbeopened
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
5.6 This test method is considered destructive in that it
not suitable for coatings that are less noble than gold or
reveals the presence of porosity by contaminating the surface
platinum, such as palladium and its alloys, or gold-flashed
withcorrosionproductsandbyundercuttingthecoatingatpore
palladium and its alloys. Gold-flashed is defined as a plated
sites or at the boundaries of unplated areas.Any parts exposed
thickness of gold between 3 and 5 µin.
to these tests shall not be placed in service.
4.4 This porosity test involves corrosion reactions in which
5.7 This test method is intended to be used for quantitative
the products delineate defect sites in coatings. Since the
descriptions of porosity (such as number of pores per unit area
chemistry and properties of these products may not resemble
or per contact) only on coatings that have a pore density
those found in natural or service environments, these tests are
sufficiently low that the corrosion sites are well separated and
not recommended for prediction of the electrical performance
can be readily resolved. As a general guideline this can be
of contacts unless correlation is first established with service
achieved for pore densities up to about 100/cm or per 100
experience.
contacts. Above this value the tests are useful for the qualita-
tive detection and comparisons of porosity.
5. Significance and Use
6. Apparatus
5.1 Gold coatings are often specified for the contacts of
separable electrical connectors and other devices. Electrode-
6.1 Test Chamber, may be any convenient size glass vessel
posits are the form of gold that is most used on contacts,
capable of being sealed with a glass lid, such as a glass
althoughitisalsoemployedascladmetalandasweldmentson
desiccatorof9to12Lcapacity.Theratiooftheairspaceinthe
the contact surface. The intrinsic nobility of gold enables it to
chamber (in cubic centimetres) to the nitric acid surface area
resist the formation of insulating oxide films that could
(in square centimetres) shall not be greater than 25:1.
interfere with reliable contact operation.
6.2 Specimen Holders or Supports—Supports or hangers
5.2 In order that the nobility of gold be assured, porosity,
shallbemadefromglass,polytetrafluoroethyleneorotherinert
cracks, and other defects in the coating that expose base-metal
materials.Itisessentialthattheholdersbesodesigned,andthe
substrates and underplates must be minimal or absent, except
specimens so arranged, that the circulation of the vapor is not
in those cases where it is feasible to use the contacts in
impeded. Specimens shall be at least 75 mm (3 in.) from the
structures that shield the surface from the environment or
liquid surface and at least 25 mm (1 in.) from the vessel walls.
where corrosion inhibiting surface treatments for the deposit
Also, the measurement areas of the specimens shall be at least
are employed. The level of porosity in the coating that may be
12 mm (0.5 in.) from each other.
tolerable depends on the severity of the environment to the
6.2.1 Donotuseaporcelainplateoranyotherstructurethat
underplate or substrate, design factors for the contact device
would cover more than 30% of the liquid surface cross-
liketheforcewithwhichitismated,circuitparameters,andthe
sectionalarea.Thisistoensurethatmovementofairandvapor
reliability of contact operation that it is necessary to maintain.
within the vessel will not be restricted during the test.
Also, when present, the location of pores on the surface is
6.3 Stereomicroscope, having a 10× magnification, shall be
important.Iftheporesarefewinnumberandareoutsideofthe
used for pore counting. In addition a movable source of
zoneofcontactofthematingsurfaces,theirpresencecanoften
illuminationcapableofgivingobliquelightingonthespecimen
be tolerated.
surface is also useful.
5.3 Methods for determining pores on a contact surface are
7. Reagent
most suitable if th
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