ASTM B798-95(2020)
(Test Method)Standard Test Method for Porosity in Gold or Palladium Coatings on Metal Substrates by Gel-Bulk Electrography
Standard Test Method for Porosity in Gold or Palladium Coatings on Metal Substrates by Gel-Bulk Electrography
SIGNIFICANCE AND USE
5.1 Noble metal coatings, particularly gold or palladium, are often specified for the contacts of separable electrical connectors and other devices. Electrodeposits are the form of gold or palladium which is most used on contacts, although gold and palladium are also employed as clad metal and as weldments on the contact surface. The intrinsic nobility of gold and to a certain extent palladium enables them to resist the formation of insulating films that could interfere with reliable contact operation.
5.2 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 or are outside of the zone of contact of the mating surfaces, their presence can often be tolerated.
5.3 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 from procedures capable of detecting all porosity to procedures that detect only gross defects. The test method in this document is generally regarded as severe.
5.4 The relationship of porosity levels revealed by particular tests to contact behavior must be made by the user of the...
SCOPE
1.1 This test method covers equipment and techniques for determining porosity in noble metal coatings, particularly electrodeposits and clad metals used on electrical contacts.
1.2 The 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 Other porosity testing methods are outlined in Guide B765. Detailed critical reviews of porosity testing are also available.2 Other porosity test methods are B735, B741, B799, and B809.
1.4 The values stated in SI units are to be regarded as standard. The values 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, health, and environmental practices, and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see Sections 7 and 8.
1.6 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.
General Information
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Standards Content (Sample)
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: B798 − 95 (Reapproved 2020)
Standard Test Method for
Porosity in Gold or Palladium Coatings on Metal Substrates
by Gel-Bulk Electrography
This standard is issued under the fixed designation B798; 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 techniques for
B374 Terminology Relating to Electroplating
determining porosity in noble metal coatings, particularly
B542 Terminology Relating to Electrical Contacts and Their
electrodeposits and clad metals used on electrical contacts.
Use
1.2 The test method is designed to show whether the
B735 Test Method for Porosity in Gold Coatings on Metal
porosity level is less or greater than some value which by
Substrates by Nitric Acid Vapor
experience is considered by the user to be acceptable for the
B741 Test Method for Porosity In Gold Coatings On Metal
intended application.
Substrates By Paper Electrography (Withdrawn 2005)
B765 Guide for Selection of Porosity and Gross Defect Tests
1.3 Other porosity testing methods are outlined in Guide
for Electrodeposits and Related Metallic Coatings
B765. Detailed critical reviews of porosity testing are also
B799 Test Method for Porosity in Gold and Palladium
available. Other porosity test methods are B735, B741, B799,
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
and B809.
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 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:
Material Safety Data Sheet (MSDS) for this product/material
3.2.1 decorations, n—those reaction products emanating
as provided by the manufacturer, to establish appropriate
from the pores that provide visual contrast with the gel
safety, health, and environmental practices, and determine the
medium.
applicability of regulatory limitations prior to use. For specific
hazard statements, see Sections 7 and 8. 3.2.2 measurement area (or “significant surface”), n—the
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 April 1, 2020. Published April 2020. Originally
approved in 1988. Last previous edition approved in 2014 as B798 – 95 (2014). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/B0798-95R20. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity Testing of Gold Standards volume information, refer to the standard’s Document Summary page on
Deposity,” Plating, Vol 52, 1965, p. 1001, and Krumbein S. J., “Porosity Testing of the ASTM website.
Contact Platings,” Proceedings, Connectors and Interconnection Technology The last approved version of this historical standard is referenced on
Symposium, October 1987, p. 47. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B798 − 95 (2020)
3.2.3 metallic coatings, n—include platings, claddings, or 5.3 Methods for determining pores on a contact surface are
other metallic layers applied to the substrate. The coatings can most suitable if they enable their precise location and numbers
comprise a single metallic layer or a combination of metallic to be determined. Contact surfaces are often curved or irregular
layers. in shape, and testing methods should be suitable for them. In
addition, the severity of porosity-determining tests may vary
3.2.4 porosity, n—the presence of any discontinuity, crack,
from procedures capable of detecting all porosity to procedures
or hole in the coating that exposes a different underlying metal.
that detect only gross defects. The test method in this document
3.2.5 underplate, n—a metallic coating layer between the
is generally regarded as severe.
substrate and the topmost layer or layers. The thickness of an
underplate is usually greater that 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 judgment. Thus, absence of
4.1 This test method is an electrographic technique, “gel-
porosity in the coating may be a requirement for some
bulk electrography.” The specimen is made the anode in a cell
applications, while a few pores in the contact zone may be
containing a solid or semisolid electrolyte of gelatin, conduct-
acceptable for others.
ing salts, and an indicator. Application of current to this cell
5.5 This test method is capable of detecting porosity or
results in the migration of base medal ions through continuous
other defects in gold or palladium coatings that could partici-
pores. Reaction of cations with an indicator gives rise to
pate in substrate corrosion reactions. In addition, it can be used
colored reaction products at pore sites which may be counted
on contacts having complex geometry such as pin-socket
through the clear gel. Individual spots are counted with the aid
contacts (although difficulty may be experienced in inspecting
of a loupe or low power stereomicroscope.
deep recesses).
4.2 This test method is suitable for coatings containing
75 % or more of gold on substrates of silver, nickel, copper,
6. Limitations
and its alloys, which are commonly used in electrical contacts.
6.1 This test is considered destructive in that it reveals the
This test method is also suitable for coatings of 95 % or more
presence of porosity by contaminating the surface with corro-
of palladium on nickel, copper and its alloys.
sion products and by under-cutting the corrodible metal at pore
4.3 These porosity tests involve corrosion reactions in
sites and at unplated areas. In addition, the surface is coated
which the products delineate defect sites in coatings. Since the
with a corrosive gel mixture which is difficult to remove
chemistry and properties of these products do not resemble
completely. Any parts exposed to the gel test shall not be
those found in natural or service environments, these tests are
placed in service.
not recommended for prediction of the electrical performance
6.2 The gel-bulk procedure is not as sensitive to small pores
of contacts unless correlation is first established with service
and is more complex than porosity tests involving gaseous
experience.
corrodants (see Test Methods B735 and B799). It also
5. Significance and Use
involves more chemicals, preparation, and auxiliary equip-
5.1 Noble metal coatings, particularly gold or palladium, are
ment.
often specified for the contacts of separable electrical connec-
6.3 This test is intended to be used for quantitative descrip-
tors and other devices. Electrodeposits are the form of gold or
tions of porosity (such as number of pores per unit area or per
palladium which is most used on contacts, although gold and
contact) only on measurement areas where coatings have pore
palladium are also employed as clad metal and as weldments
densities that are sufficiently low so that the corrosion sites are
on the contact surface. The intrinsic nobility of gold and to a
well separated and can be readily resolved. As a general
certain extent palladium enables them to resist the formation of
guideline this can be achieved for pore densities up to about
insulating films that could interfere with reliable contact
25/cm .
operation.
6.4 For this purpose, the measurement area, or “significant
5.2 In order that the nobility of gold be assured, porosity,
surface,” shall be defined as those portions of the surface that
cracks, and other defects in the coating that expose base metal
are essential to the serviceability or function of the part, such
substrates and underplates must be minimal or absent, except
as its contact properties, or which can be the source of
in those cases where it is feasible to use the contacts in
corrosion products or tarnish films that interfere with the
structures that shield the surface from the environment or
function of the part. When necessary, the significant surfaces
where corrosion inhibiting surface treatments for the deposit
shall be indicated on the drawings of the parts, or by the
are employed. The level of porosity in the coating that may be
provision of suitably marked samples.
tolerable depends on the severity of the environment to the
underplate or substrate, design factors for the contact device
6.5 The test applicability to platings of varying thickness is
like the force with which it is mated, circuit parameters, and the a function of the quality of the plating.
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 or are outside of the
For example, Clarke, M., “Porosity and Porosity Tests,” in “Properties of
zone of contact of the mating surfaces, their presence can often
Electrodeposits,” edited by Sard, Leidheiser, and Ogburn, The Electrochemical
be tolerated. Society, 1975, p. 122.
B798 − 95 (2020)
6.6 The applicability of this test method to localized plat-
ings or claddings with adjacent exposed substrate is limited by
the efficacy of coatings applied to mask the non-noble areas to
prevent gross decoration of the surfaces under test. Users of
this method are required to develop their own techniques for
masking such exposed substrate areas.
7. Apparatus
7.1 Test Vessel may be made of glass, acrylic resin, or other
inert uncolored transparent material. It shall have thin-walled
flat sides, and be of a size appropriate to the sample to be
tested.
7.2 Power Supply, 0 to 1 A and 0 to 10 V dc, an
electronically-regulated, constant-current (65 %) apparatus is
preferred.
7.3 dc Milliammeter and Separate dc Voltmeter.
7.4 Cathode Material in the form of foil or wire made of
platinum or gold is required. The cathode and specimen
(anode) areas shall be approximately the same. Additionally,
gold or platinum wire for cathode and anode are needed for that
FIG. 2 Exploded View of Alternate Cell Design Incorporating
portion of the hook-up that is in the reagent solution. It may be
Cathode as Part of Cell Structure
convenient to use small alligator clips to secure the lead wires
to the cathode and anode. These clips must be heavily gold
plated so as to be entirely free of porosity. A variation of this
procedure, suitable for samples having relatively few pores, is
the sample surfaces need be in the gel.
to use a second identical test sample as the cathode. The test
7.5 Timer capable of indicating seconds. It is convenient to
can be run with current first in the forward, then in the reverse
use a timer switch to control the test current.
direction so that the porosity in both samples may be deter-
7.6 Stereomicroscope having 10× magnification and an
mined. Fig. 1 is a schematic of the test cell setup.
illuminator are required for sample inspection after test. An
eyepiece reticle is recommended for convenience in locating
the contact area or other significant measurement areas.
8. Reagent
8.1 Note that some of the indicating reagents are sensitive to
heat and light, particularly the rubeanic acid (dithio-oxamide).
The indicator solutions should be stored in the dark in
stoppered bottles. For rubeanic acid, do not store for more than
a month, and filter prior to use.
8.2 Food-Grade Gelatin—This type is preferred to USP
grade gelatin, because the latter may not give transparent
solutions. A 10 % solution is prepared by mixing 9 g of the
gelatin in 91 mL of distilled or deionized water, and slowly
heating to 60 to 65°C with stirring, until all the gelatin
dissolves.
NOTE 2—If the storage bottle is tightly capped, the plain gelatin
solution may be stored for up to 2 days in a refrigerator and kept at 5 to
10°C, discard it if mold appears on its surface.
9. Safety Hazards
9.1 Reagents identified in Table 1 have the potential to
cause injury or skin discoloring if improperly handled. Good
FIG. 1 Schematic of Typical Test-Cell Setup with Anode (Sample)
and Cathode Facing Each Other (Preferred Orientation) laboratory practice including the use of a fume hood and skin
and eye protection should be observed, especially during
solution preparative and the cleaning of the test samples.
NOTE 1—A commonly-used alternate cell design incorporates the
Proper precautions in the use of electrical power supplies and
cathode as part of the cell structure (as shown in Fig. 2). In addition, the
samples may be attached to a common carrier strip or holder, so that only electrical connections should also be scrupulously observed.
B798 − 95 (2020)
TABLE 1 Guide to Gel Porosity Testing Solutions
Tes
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