ASTM B799-95(2000)
(Test Method)Standard Test Method for Porosity in Gold and Palladium Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
Standard Test Method for Porosity in Gold and Palladium Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
SCOPE
1.1 This test method covers equipment and methods for determining the porosity of gold and palladium 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 which 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 the literature. Other porosity test methods are B735, B741, B798, and B809. An ASTM Guide to the selection of porosity tests for electrodeposits and related metallic coatings is available as Guide B765.
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. For specific hazards, see Section 5.
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Designation:B 799–95 (Reapproved 2000)
Standard Test Method for
Porosity in Gold and Palladium Coatings by Sulfurous Acid/
Sulfur-Dioxide Vapor
This standard is issued under the fixed designation B 799; 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 765 Guide for Selection of Porosity Tests for Electrode-
posits and Related Metallic Coatings
1.1 This test method covers equipment and methods for
B 798 Test Method for Porosity in Gold or Palladium
determining the porosity of gold and palladium coatings,
Coatings on Metal Substrates by Gel-Bulk Electrography
particularly electrodeposits and clad metals used on electrical
B 809 Test Method for Porosity in Metallic Coatings By
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 which 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 the literature. Other porosity test methods are B 735,
3.2 Definitions of Terms Specific to This Standard:
B 741, B 798, and B 809. An ASTM Guide to the selection of
3.2.1 corrosion products—those reaction products emanat-
porosity tests for electrodeposits and related metallic coatings
ingfromtheporesthatprotrudefrom,orareotherwiseattached
is available as Guide B 765.
to, the coating surface after a vapor test exposure.
1.4 The values stated in SI units are to be regarded as
3.2.2 measurement area (or 88significant surface’’)—the
standard. The values given in parentheses are for information
surface that is examined for the presence of porosity. The
only.
significant surfaces or measurement areas of the part to be
1.5 This standard does not purport to address all of the
tested shall be indicated on the drawing of the part or by
safety concerns, if any, associated with its use. It is the
provision of suitably marked samples.
responsibility of the user of this standard to establish appro-
3.2.3 Discussion—For specification purposes, the signifi-
priate safety and health practices and determine the applica-
cant surfaces or measurement areas are often defined as those
bility of regulatory limitations prior to use. For specific
portions of the surface that are essential to the serviceability or
hazards, see Section 6.
function of the part, such as its contact properties, or which can
2. Referenced Documents be the source of corrosion products or tarnish films that
interfere with the function of the part.
2.1 ASTM Standards:
3.2.4 metallic coatings—include platings, claddings, or
B 374 Terminology Relating to Electroplating
other metallic layers applied to the substrate. The coatings can
B 735 Test Method for Porosity in Gold Coatings on Metal
comprise a single metallic layer or a combination of metallic
Substrates by Nitric Acid Vapor
layers.
B 741 Test Method for Porosity In Gold Coatings on Metal
3.2.5 Porosity—the presence of any discontinuity, crack, or
Substrates by Paper Electrography
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 that 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 799 – 88. Last previous edition B 799 – 93.
For example see: Nobel, F. J., Ostrow, B. D., and Thompson, D. W.,“ Porosity
4.1 The test method employs concentrated sulfurous acid
Testing of Gold Deposits,” Plating, Vol 52, 1965, p. 1001.
(H SO ),whichemitssulfurdioxide(SO )gasaccordingtothe
2 3 2
S. J. Krumbien, Porosity Testing of Contact Platings, Proceedings, Connectors
equilibrium reaction:
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.
B 799
H SO 5 SO 1 H O (1)
5.4 Methods for determining pores on a contact surface are
2 3 2 2
most suitable if they enable their precise location and numbers
The procedure is similar to one first proposed by Lee and
tobedetermined.Contactsurfacesareoftencurvedorirregular
Ternowski.
in shape, and testing methods should be suitable for them. In
4.2 Exposure periods may vary, depending upon the degree
addition, the severity of porosity-determining tests may vary
of porosity to be revealed. Reaction of the gas with a
fromprocedurescapableofdetectingallporositytoprocedures
corrodable base metal at pore sites produces reaction products
that detect only highly porous conditions.
that appear as discrete spots on the gold or palladium surface.
Individual spots are counted with the aid of a loupe or
5.5 The present test method is capable of detecting virtually
low-power stereo microscope.
all porosity or other defects that could participate in corrosion
4.3 This test method is suitable for coatings containing
reactions with the substrate or underplate. The test is rapid,
95 % or more of gold or palladium on substrates of copper,
simple, and inexpensive. In addition, it can be used on contacts
nickel, and their alloys which are commonly used in electrical
having complex geometry such as pin-socket contacts (al-
contacts.
though with deep recesses it is preferred that the contact
4.4 This porosity test involves corrosion reactions in which
structures be opened to permit reaction of the sulfur dioxide
the products delineate defect sites in coatings. Since the
with the interior significant surfaces).
chemistry and properties of these products may not resemble
5.6 Therelationshipofporositylevelsrevealedbyparticular
those found in natural or service environments this test is not
teststocontactbehaviormustbemadebytheuserofthesetests
recommended for prediction of the electrical performance of
through practical experience or by judgment. Thus, absence of
contacts unless correlation is first established with service
porosity in the coating may be a requirement for some
experience.
applications, while a few pores in the contact zone may be
acceptable for others.
5. Significance and Use
5.7 This test is considered destructive in that it reveals the
5.1 Gold coatings are often specified for the contacts of
presence of porosity by contaminating the surface with corro-
separable electrical connectors and other devices. Electrode-
sion products and by undercutting the coating at pore sites or
posits are the form of gold that is most used on contacts,
at the boundaries of the unplated areas. Any parts exposed to
although it is also employed as inlay or clad metal and as
this test shall not be placed in service.
weldmentsonthecontactsurface.Theintrinsicnobilityofgold
5.8 This test is intended to be used for quantitative descrip-
enables it to resist the formation of insulating oxide films that
tions of porosity (such as number of pores per unit area or per
could interfere with reliable contact operation.
5.2 Palladium coatings are sometimes specified as alterna- contact) only on coatings that have a pore density sufficiently
low that the corrosion sites are well separated and can be
tives to gold on electrical contacts and similar electrical
component surfaces, both as electrodeposits and as inlay or readily resolved. As a general guideline this can be achieved
clad metal. This test method is particularly suitable for deter- for pore densities up to about 100/cm . Above this value the
mining porosity in palladium coatings, since the reactive tests are useful for the qualitative detection and comparisons of
atmosphere that is used does not attack the palladium if the porosity.
specified test conditions are followed. In contrast, palladium
5.9 For these purposes, the measurement area,or significant
coatings are attacked by nitric acid (HNO ) and other strong
3 surface, shall be defined as those portions of the surface that
oxidizing agents, so thatTest Method B 735 cannot be used for
are essential to the serviceability or function of the part, such
determining the porosity in such coatings.
as its contact properties, or which can be the source of
5.3 In order for these coatings to function as intended,
corrosion products or tarnish films that interfere with the
porosity, cracks, and other defects in the coating that expose
function of the part. The significant surfaces shall be indicated
base-metal substrates and underplates must be minimal or
on the drawings of the parts, or by the provision of suitably
absent, except in those cases where it is feasible to use the
marked samples.
contacts in structures that shield the surface from the environ-
ment or where corrosion inhibiting surface treatments for the
6. Safety Hazards
deposit are employed. The level of porosity in the coating that
6.1 Carry out these test procedures in a clean, working fume
may be tolerable depends on the severity of the environment to
hood. The SO gas that is emitted is toxic, corrosive, and
the underplate or substrate, design factors for the contact
irritating.
device like the force with which it is mated, circuit parameters,
and the reliability of contact operation that it is necessary to
6.2 Use caution, however, in actually performing the tests
maintain. Also, when present, the location of pores on the
that the drafts often found in hoods do not cau
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