ASTM B798-95(2005)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:B798–95(Reapproved2005)
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 B 798; 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 741 Test Method for Porosity In Gold Coatings On Metal
Substrates By Paper Electrography
1.1 This test method covers equipment and techniques for
B 765 Guide for Selection of Porosity and Gross Defect
determining porosity in noble metal coatings, particularly
Tests for Electrodeposits and Related Metallic Coatings
electrodeposits and clad metals used on electrical contacts.
B 799 Test Method for Porosity in Gold and Palladium
1.2 The test method is designed to show whether the
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
porosity level is less or greater than some value which by
B 809 Test Method for Porosity in Metallic Coatings by
experience is considered by the user to be acceptable for the
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
intended application.
1.3 Other porosity testing methods are outlined in Guide
3. Terminology
B 765. Detailed critical reviews of porosity testing are also
2 3.1 Definitions—Many terms used in this test method are
available. Other porosity test methods are B 735, B 741,
defined in Terminology B 542 and terms relating to metallic
B 799, and B 809.
coatings are defined in Terminology B 374.
1.4 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 decorations, n—those reaction products emanating
responsibility of the user of this standard to become familiar
from the pores that provide visual contrast with the gel
with all hazards including those identified in the appropriate
medium.
Material Safety Data Sheet (MSDS) for this product/material
3.2.2 measurement area (or “significant surface”), n—the
as provided by the manufacturer, to establish appropriate
surface that is examined for the presence of porosity. The
safety and health practices, and determine the applicability of
significant surfaces or measurement areas of the part to be
regulatory limitations prior to use. For specific hazard state-
tested shall be indicated on the drawing of the part or by
ments, see Sections 7 and 8.
provision of suitably marked samples.
1.5 The values stated in SI units are to be regarded as
3.2.2.1 Discussion—For specification purposes, the signifi-
standard. The values in parentheses are for information only.
cant surfaces or measurement areas are often defined as those
2. Referenced Documents portions of the surface that are essential to the serviceability or
3 functionofthepart,suchasitscontactproperties,orwhichcan
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
3.2.3 metallic coatings, n—include platings, claddings, or
Their Use
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.
3.2.4 porosity, n—the presence of any discontinuity, crack,
1 or hole in the coating that exposes a different underlying metal.
This test method is under the jurisdiction of ASTM Committee B02 on
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee 3.2.5 underplate, n—a metallic coating layer between the
B02.11 on Electrical Contact Test Methods.
substrate and the topmost layer or layers. The thickness of an
Current edition approved Nov. 1, 2005. Published February 2006. Originally
underplate is usually greater that 0.8 µm (30 µin.).
approved in 1988. Last previous edition approved in 2000 as B 798 – 95 (2000).
Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity Testing of Gold
4. Summary of Test Method
Deposity,” Plating, Vol 52, 1965, p. 1001, and Krumbein S. J., “Porosity Testing of
Contact Platings,” Proceedings, Connectors and Interconnection Technology Sym-
4.1 This test method is an electrographic technique, “gel-
posium, October 1987, p. 47.
bulk electrography.” The specimen is made the anode in a cell
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 ASTM website. Withdrawn.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B798–95 (2005)
containing a solid or semisolid electrolyte of gelatin, conduct- 5.5 This test method is capable of detecting porosity or
ing salts, and an indicator. Application of current to this cell other defects in gold or palladium coatings that could partici-
results in the migration of base medal ions through continuous pate in substrate corrosion reactions. In addition, it can be used
pores. Reaction of cations with an indicator gives rise to on contacts having complex geometry such as pin-socket
colored reaction products at pore sites which may be counted contacts (although difficulty may be experienced in inspecting
through the clear gel. Individual spots are counted with the aid deep recesses).
of a loupe or low power stereomicroscope.
4.2 This test method is suitable for coatings containing 6. Limitations
75 % or more of gold on substrates of silver, nickel, copper,
6.1 This test is considered destructive in that it reveals the
and its alloys, which are commonly used in electrical contacts.
presence of porosity by contaminating the surface with corro-
This test method is also suitable for coatings of 95 % or more
sion products and by under-cutting the corrodible metal at pore
of palladium on nickel, copper and its alloys.
sites and at unplated areas. In addition, the surface is coated
4.3 These porosity tests involve corrosion reactions in
with a corrosive gel mixture which is difficult to remove
which the products delineate defect sites in coatings. Since the
completely. Any parts exposed to the gel test shall not be
chemistry and properties of these products do not resemble
placed in service.
those found in natural or service environments, these tests are
6.2 The gel-bulk procedure is not as sensitive to small pores
not recommended for prediction of the electrical performance
and is more complex than porosity tests involving gaseous
of contacts unless correlation is first established with service 5
corrodants (see Test Methods B 735 and B 799). It also
experience.
involves more chemicals, preparation, and auxiliary equip-
ment.
5. Significance and Use
6.3 This test is intended to be used for quantitative descrip-
5.1 Noblemetalcoatings,particularlygoldorpalladium,are
tions of porosity (such as number of pores per unit area or per
often specified for the contacts of separable electrical connec-
contact) only on measurement areas where coatings have pore
tors and other devices. Electrodeposits are the form of gold or
densities that are sufficiently low so that the corrosion sites are
palladium which is most used on contacts, although gold and
well separated and can be readily resolved. As a general
palladium are also employed as clad metal and as weldments
guideline this can be achieved for pore densities up to about
on the contact surface. The intrinsic nobility of gold and to a
25/cm .
certainextentpalladiumenablesthemtoresisttheformationof
6.4 For this purpose, the measurement area, or “significant
insulating films that could interfere with reliable contact
surface,’’ shall be defined as those portions of the surface that
operation.
are essential to the serviceability or function of the part, such
5.2 In order that the nobility of gold be assured, porosity,
as its contact properties, or which can be the source of
cracks, and other defects in the coating that expose base metal
corrosion products or tarnish films that interfere with the
substrates and underplates must be minimal or absent, except
function of the part. When necessary, the significant surfaces
in those cases where it is feasible to use the contacts in
shall be indicated on the drawings of the parts, or by the
structures that shield the surface from the environment or
provision of suitably marked samples.
where corrosion inhibiting surface treatments for the deposit
6.5 The test applicability to platings of varying thickness is
are employed. The level of porosity in the coating that may be
a function of the quality of the plating.
tolerable depends on the severity of the environment to the
6.6 The applicability of this test method to localized plat-
underplate or substrate, design factors for the contact device
ings or claddings with adjacent exposed substrate is limited by
liketheforcewithwhichitismated,circuitparameters,andthe
the efficacy of coatings applied to mask the non-noble areas to
reliability of contact operation that it is necessary to maintain.
prevent gross decoration of the surfaces under test. Users of
Also, when present, the location of pores on the surface is
this method are required to develop their own techniques for
important. If the pores are few in number or are outside of the
masking such exposed substrate areas.
zone of contact of the mating surfaces, their presence can often
be tolerated.
7. Apparatus
5.3 Methods for determining pores on a contact surface are
7.1 Test Vessel maybemadeofglass,acrylicresin,orother
most suitable if they enable their precise location and numbers
inert uncolored transparent material. It shall have thin-walled
tobedetermined.Contactsurfacesareoftencurvedorirregular
flat sides, and be of a size appropriate to the sample to be
in shape, and testing methods should be suitable for them. In
tested.
addition, the severity of porosity-determining tests may vary
7.2 Power Supply,0to1 A and0to10 V dc, an
fromprocedurescapableofdetectingallporositytoprocedures
electronically-regulated, constant-current (65 %) apparatus is
thatdetectonlygrossdefects.Thetestmethodinthisdocument
preferred.
is generally regarded as severe.
7.3 dc Milliammeter and Separate dc Voltmeter.
5.4 Therelationshipofporositylevelsrevealedbyparticular
teststocontactbehaviormustbemadebytheuserofthesetests
through practical experience or judgment. Thus, absence of
porosity in the coating may be a requirement for some
For example, Clarke, M., “Porosity and Porosity Tests,” in “Properties of
applications, while a few pores in the contact zone may be
Electrodeposits,” edited by Sard, Leidheiser, and Ogburn, The Electrochemical
acceptable for others. Society, 1975, p. 122.
B798–95 (2005)
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,
goldorplatinumwireforcathodeandanodeareneededforthat
portion of the hook-up that is in the reagent solution. It may be
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
to use a second identical test sample as the cathode. The test
can be run with current first in the forward, then in the reverse
direction so that the porosity in both samples may be deter-
mined. Fig. 1 is a schematic of the test cell setup.
FIG. 2 Exploded View of Alternate Cell Design Incorporating
Cathode as Part of Cell Structure
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 ReagentsidentifiedinTable1havethepotentialtocause
injury or skin discoloring if improperly handled. Good labora-
torypracticeincludingtheuseofafumehoodandskinandeye
protection should be observed, especially during solution
FIG. 1 Schematic of Typical Test-Cell Setup with Anode (Sample)
preparative and the cleaning of the test samples. Proper
and Cathode Facing Each Other (Preferred Orientation)
precautions in the use of electrical power supplies and electri-
cal connections should also be scrupulously observed.
NOTE 1—A commonly-used alternate cell design incorporates the
10. Procedure
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
10.1 This test is suitable for gold coated on silver, nickel, or
the sample surfaces need be in the gel.
copper and its alloys, and palladium coated on nickel, copper
7.5 Timer capable of indicating seconds. It is convenient to
and its alloys either as underlayers or substrates, in accordance
use a timer switch to control the test current. with the reagents chosen in Table 1.
7.6 Stereomicroscope having 103 magnification and an
10.2 Sequence of Operations:
illuminator are required for sample inspection after test. An 10.2.1 Solution Preparation:
eyepiece reticle is recommended for convenience in locating
10.2.1.1 Electrolyte.
the contact area or other significant measurement areas. 10.2.1.2 Indicator.
10.2.2 Calculate the current to be used.
8. Reagent
10.2.3 Prepare the samples prior to cleaning.
8.1 Notethatsomeoftheindicatingreagentsaresensitiveto 10.2.4 Clean the samples.
heat and light, particularly the rubeanic acid (dithio-oxamide). 10.2.5 Prepare the gel while the samples are cleaning.
The indicator solutions should be stored in the dark in Remove from heat when dissolved.
stoppered bottles. For rubeanic acid, do not store for more than 10.2.6 Dry the samples.
a month, and filter prior to use. 10.2.7 Suspend the samples in the test cells.
8.2 Food-Grade Gelatin—This type is preferred to USP 10.2.8 Prepare the composite gel solution and add to the
grade gelatin, because the latter may not give transparent cells.
solutions. A 10 % solution is prepared by mixing9gofthe 10.2.9 Solidify the gel thoroughly.
B798–95 (2005)
TABLE 1 Guide to Gel Porosity Testing Solutions
Test for Electrolyte (Aqueous) Indicator Indicating Color Comment
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