ASTM B735-95(2000)
(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
SCOPE
1.1 This test method covers equipment and procedures for using nitric acid vapor for determining porosity in gold coatings, 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 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 establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautions are given in Section 8 and Note 2.
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Designation:B735–95(Reapproved2000)
Standard Test Method for
Porosity in Gold Coatings on Metal Substrates by Nitric
Acid Vapor
This standard is issued under the fixed designation B 735; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope B 798 Test Method for Porosity in Gold or Palladium
Coatings on Metal Substrates by Gel-Bulk Electrography
1.1 This test method covers equipment and procedures for
B 799 Test Method for Porosity in Gold and Palladium
using nitric acid vapor for determining porosity in gold
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
coatings, particularly electrodeposits and clad metals used on
B 809 Test Method for Porosity in Metallic Coatings by
electrical contacts.
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
1.2 This test method is designed to show whether the
porosity level is less or greater than some value that, by
3. Terminology
experience, is considered by the user to be acceptable for the
3.1 Definitions: Many terms used in this test method are
intended application.
defined in Terminology B 542 and terms relating to metallic
1.3 Avarietyofotherporositytestingmethodsaredescribed
,
2 3 coatings are defined in Terminology B 374.
in Guide B 765 and in the literature. Other porosity test
3.2 Definitions of Terms Specific to This Standard:
methods are B 741, B 798, B 799, and B 809.
3.2.1 corrosion products—those reaction products emanat-
1.4 The values stated in SI units are to be regarded as
ingfromtheporesthatprotrudefrom,orareotherwiseattached
standard. The values given in parentheses are for information
to, the coating surface after a vapor test exposure.
only.
3.2.2 measurement area (or “significant surface”)—the
1.5 This standard does not purport to address all of the
surface that is examined for the presence of porosity. The
safety concerns, if any, associated with its use. It is the
significant surfaces or measurement areas of the part to be
responsibility of the user of this standard to establish appro-
tested shall be indicated on the drawing of the part or by
priate safety and health practices and determine the applica-
provision of suitably marked samples.
bility of regulatory limitations prior to use. Specific precau-
3.2.3 Discussion—For specification purposes, the signifi-
tions are given in Section 8 and Note 3.
cant surfaces or measurement areas are often defined as those
2. Referenced Documents portions of the surface that are essential to the serviceability or
function of the part, such as its contact properties, or which can
2.1 ASTM Standards:
be the source of corrosion products or tarnish films that
B 374 Terminology Relating to Electroplating
interfere with the function of the part.
B 542 Terminology Relating to Electrical Contacts and
3.2.4 metallic coatings—include platings, claddings, or
Their Use
other metallic layers applied to the substrate. The coating can
B 741 Test Method for Porosity In Gold Coatings on Metal
comprise a single metallic layer or a combination of metallic
Substrates By Paper Electrography
layers.
B 765 Guide to the Selection of Porosity Tests for Elec-
3.2.5 Porosity—the presence of any discontinuity, crack, or
trodeposits and Related Metallic Coatings
hole in the coating that exposes a different underlying metal.
3.2.6 Underplate—a metallic coating layer between the
This test method is under the jurisdiction of ASTM Committee B02 on substrate and the topmost layer or layers. The thickness of an
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
underplate is usually greater than 0.8 µm (30 µin.).
B02.11 on Electrical Contact Test Methods.
Current edition approved April 15, 1995. Published June 1995. Originally
4. Summary of Test Method
published as B 735 – 84. Last previous edition B 735 – 92.
For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity
4.1 This test method employs nitric acid (HNO ) vapor at
Testing of Gold Deposits,” Plating, Vol 52, 1965, p. 1001.
low relative humidity. Reaction of the gas mixture with a
S. J. Krumbein, Porosity Testing of Contact Platings, Proceedings, Connectors
corrodible base metal at pore sites produces reaction products
and Interconnection Technology Symposium, Oct. 1987, p. 47.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 02.04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B735–95 (2000)
that appear as discrete spots on the gold surface. Individual that could participate in substrate corrosion reactions. The test
spots are counted with the aid of a loupe or low-power stereo is rapid, simple, and inexpensive. In addition, it can be used on
microscope. contactshavingcomplexgeometrysuchaspin-socketcontacts.
4.2 This test method is suitable for inlays or claddings However,itispreferredthatdeeplyrecessedsocketsbeopened
containing 75 % or more of gold or for electroplatings con- to expose their critical surfaces prior to testing.
taining 95 % or more of gold on substrates of copper, nickel, 5.6 This test method is considered destructive in that it
and their alloys, that are commonly used in electrical contacts. reveals the presence of porosity by contaminating the surface
4.3 The nitric acid vapor test is too severe to be used for withcorrosionproductsandbyundercuttingthecoatingatpore
gold coatings less than 0.6 µm (25 µin.) in thickness. It is also sites or at the boundaries of unplated areas.Any parts exposed
not suitable for coatings that are less noble than gold or to these tests shall not be placed in service.
platinum, such as palladium and its alloys, or gold-flashed 5.7 This test method is intended to be used for quantitative
palladium and its alloys. descriptions of porosity (such as number of pores per unit area
4.4 This porosity test involves corrosion reactions in which or per contact) only on coatings that have a pore density
the products delineate defect sites in coatings. Since the sufficiently low that the corrosion sites are well separated and
chemistry and properties of these products may not resemble can be readily resolved. As a general guideline this can be
those found in natural or service environments, these tests are achieved for pore densities up to about 100/cm or per 100
not recommended for prediction of the electrical performance contacts. Above this value the tests are useful for the qualita-
of contacts unless correlation is first established with service tive detection and comparisons of porosity.
experience.
6. Apparatus
5. Significance and Use
6.1 Test Chamber, may be any convenient size glass vessel
5.1 Gold coatings are often specified for the contacts of
capable of being sealed with a glass lid, such as a glass
separable electrical connectors and other devices. Electrode-
desiccatorof9to12Lcapacity.Theratiooftheairspaceinthe
posits are the form of gold that is most used on contacts,
chamber (in cubic centimetres) to the nitric acid surface area
although it is also employed as clad metal and as weldments on
(in square centimetres) shall not be greater than 25:1.
the contact surface. The intrinsic nobility of gold enables it to
6.2 Specimen Holders or Supports—Supports or hangers
resist the formation of insulating oxide films that could
shall be made from glass, polytetrafluoroethylene or other inert
interfere with reliable contact operation.
materials. It is essential that the holders be so designed, and the
5.2 In order that the nobility of gold be assured, porosity,
specimens so arranged, that the circulation of the vapor is not
cracks, and other defects in the coating that expose base-metal
impeded. Specimens shall be at least 75 mm (3 in.) from the
substrates and underplates must be minimal or absent, except
liquid surface and at least 25 mm (1 in.) from the vessel walls.
in those cases where it is feasible to use the contacts in
Also, the measurement areas of the specimens shall be at least
structures that shield the surface from the environment or
12 mm (0.5 in.) from each other.
where corrosion inhibiting surface treatments for the deposit
6.2.1 Do not use a porcelain plate or any other structure that
are employed. The level of porosity in the coating that may be
would cover more than 30 % of the liquid surface cross-
tolerable depends on the severity of the environment to the
sectionalarea.Thisistoensurethatmovementofairandvapor
underplate or substrate, design factors for the contact device
within the vessel will not be restricted during the test.
liketheforcewithwhichitismated,circuitparameters,andthe
6.3 Stereomicroscope, having a 103 magnification, shall be
reliability of contact operation that it is necessary to maintain.
used for pore counting. In addition a movable source of
Also, when present, the location of pores on the surface is
illuminationcapableofgivingobliquelightingonthespecimen
important. If the pores are few in number and are outside of the
surface is also useful.
zone of contact of the mating surfaces, their presence can often
7. Reagent
be tolerated.
5.3 Methods for determining pores on a contact surface are
7.1 Nitric Acid, Reagent Grade Concentrated 70 6 1%
most suitable if they enable their precise location and numbers HNO , sp gr 1.415 to 1.420.
tobedetermined.Contactsurfacesareoftencurvedorirregular
8. Safety Hazards
in shape, and testing methods should be suitable for them. In
addition, the severity of porosity-determining tests may vary. 8.1 Carry out this test method in a chemical fume hood,
This test method is regarded as severe. since the gases that are released, mainly when the reaction
5.4 Therelationshipofporositylevelsrevealedbyparticular vessel is opened at the end of each test, are very corrosive.
teststocontactbehaviormustbemadebytheuserofthesetests 8.2 Use caution, however, to ensure that drafts that are often
throughpracticalexperienceorbyjudgement.Thus,absenceof found in fume hoods do not cause significant cooling of the
porosity in the coating may be a require
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