ASTM B798-95(2000)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:B798–95(Reapproved2000)
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 799 Test Method for Porosity in Gold or Palladium
Coatings by Sulfurous Acid/Sulfur-Dioxide Vapor
1.1 This test method covers equipment and techniques for
B 809 Test Method for Porosity in Metallic Coatings By
determining porosity in noble metal coatings, particularly
Humid Sulfur Vapor (“Flowers-of-Sulfur”)
electrodeposits and clad metals used on electrical contacts.
1.2 The test method is designed to show whether the
3. Terminology
porosity level is less or greater than some value which by
3.1 Definitions—Many terms used in this test method are
experience is considered by the user to be acceptable for the
defined in Terminology B 542 and terms relating to metallic
intended application.
coatings are defined in Terminology B 374.
1.3 Other porosity testing methods are outlined in Guide
3.2 Definitions of Terms Specific to This Standard:
B 765. Detailed critical reviews of porosity testing are also
2 3.2.1 decorations—those reaction products emanating from
available. Other porosity test methods are B 735, B 741,
the pores that provide visual contrast with the gel medium.
B 799, and B 809.
3.2.2 measurement area (or “significant surface”)—the
1.4 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. For specific hazard
3.2.3 Discussion—For specification purposes, the signifi-
statements, see Sections 7 and 8.
cant surfaces or measurement areas are often defined as those
1.5 The values stated in SI units are to be regarded as
portions of the surface that are essential to the serviceability or
standard. The values in parentheses are for information only.
functionofthepart,suchasitscontactproperties,orwhichcan
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 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 542 Terminology Relating to Electrical Contacts and
4 comprise a single metallic layer or a combination of metallic
Their Use
layers.
B 735 Test Method for Porosity in Gold Coatings on Metal
4 3.2.5 porosity—the presence of any discontinuity, crack, or
Substrates by Nitric Acid Vapor
hole in the coating that exposes a different underlying metal.
B 741 Test Methods for Porosity In Gold Coatings On
4 3.2.6 underplate—a metallic coating layer between the
Metal Substrates By Paper Electrography
substrate and the topmost layer or layers. The thickness of an
B 765 Guide for Selection of Porosity Tests for Electrode-
underplate is usually greater that 0.8 µm (30 µin.).
posits and Related Metallic Coatings
4. Summary of Test Method
This test method is under the jurisdiction of ASTM Committee B02 on 4.1 This test method is an electrographic technique, “gel-
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
bulk electrography.” The specimen is made the anode in a cell
B02.11 on Electrical Contact Test Methods.
containing a solid or semisolid electrolyte of gelatin, conduct-
Current edition approved April 15, 1995. Published June 1995. Originally
ing salts, and an indicator. Application of current to this cell
published as B 798 – 88. Last previous edition B 798 – 90.
Nobel, F. J., Ostrow, B. D., and Thompson, D. W., “Porosity Testing of Gold
results in the migration of base medal ions through continuous
Deposity,” Plating, Vol 52, 1965, p. 1001, and Krumbein S. J., “Porosity Testing of
pores. Reaction of cations with an indicator gives rise to
Contact Platings,” Proceedings, Connectors and Interconnection Technology Sym-
colored reaction products at pore sites which may be counted
posium, October 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.
B798–95 (2000)
through the clear gel. Individual spots are counted with the aid on contacts having complex geometry such as pin-socket
of a loupe or low power stereomicroscope. contacts (although difficulty may be experienced in inspecting
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 B 735 and B 799). It also
involves more chemicals, preparation, and auxiliary equip-
5. Significance and Use
ment.
5.1 Noblemetalcoatings,particularlygoldorpalladium,are
6.3 This test is intended to be used for quantitative descrip-
often specified for the contacts of separable electrical connec-
tions of porosity (such as number of pores per unit area or per
tors and other devices. Electrodeposits are the form of gold or
contact) only on measurement areas where coatings have pore
palladium which is most used on contacts, although gold and
densities that are sufficiently low so that the corrosion sites are
palladium are also employed as clad metal and as weldments
well separated and can be readily resolved. As a general
on the contact surface. The intrinsic nobility of gold and to a
guideline this can be achieved for pore densities up to about
certain extent palladium enables them to resist the formation of
25/cm .
insulating films that could interfere with reliable contact
6.4 For this purpose, the measurement area, or “significant
operation.
surface,’’ shall be defined as those portions of the surface that
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
7.4 Cathode Material in the form of foil or wire made of
teststocontactbehaviormustbemadebytheuserofthesetests
platinum or gold is required. The cathode and specimen
through practical experience or judgment. Thus, absence of
(anode) areas shall be approximately the same. Additionally,
porosity in the coating may be a requirement for some
goldorplatinumwireforcathodeandanodeareneededforthat
applications, while a few pores in the contact zone may be
acceptable for others.
5.5 This test method is capable of detecting porosity or
For example, Clarke, M., “Porosity and Porosity Tests,” ’ in “Properties of
other defects in gold or palladium coatings that could partici-
Electrodeposits,” edited by Sard, Leidheiser, and Ogburn, The Electrochemical
pate in substrate corrosion reactions. In addition, it can be used Society, 1975, p. 122.
B798–95 (2000)
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 gelatine
dissolves.
NOTE 2—If the storage bottle is tightly capped, the plain gelatine
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
FIG. 1 Schematic of Typical Test-Cell Setup with Anode (Sample) 9.1 ReagentsidentifiedinTable1havethepotentialtocause
and Cathode Facing Each Other (Preferred Orientation)
injury or skin discoloring if improperly handled. Good labora-
torypracticeincludingtheuseofafumehoodandskinandeye
protection should be observed, especially during solution
NOTE 1—A commonly-used alternate cell design incorporates the
preparative and the cleaning of the test samples. Proper
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
precautions in the use of electrical power supplies and electri-
the sample surfaces need be in the gel.
cal connections should also be scrupulously observed.
7.5 Timer capable of indicating seconds. It is convenient to
10. Procedure
use a timer switch to control the test current.
10.1 This test is suitable for gold coated on silver, nickel, or
7.6 Stereomicroscope having 103 magnification and an
illuminator are required for sample inspection after test. An copper and its alloys, and palladium coated on nickel, copper
and its alloys either as underlayers or substrates, in accordance
eyepiece reticle is recommended for convenience in locating
with the reagents chosen in Table 1.
the contact area or other significant measurement areas.
10.2 Sequence of Operations:
8. Reagent
10.2.1 Solution Preparation:
10.2.1.1 Electrolyte.
8.1 Notethatsomeoftheindicatingreagentsaresensitiveto
10.2.1.2 Indicator.
heat and light, particularly the rubeanic acid (dithio-oxamide).
10.2.2 Calculate the current to be used.
The indicator solutions should be stored in the dark in
10.2.3 Prepare the samples prior to cleaning.
stoppered bottles. For rubeanic acid, do not store for more than
10.2.4 Clean the samples.
a month, and filter prior to use.
10.2.5 Prepare the gel while the samples are cleaning.
8.2 Food-Grade Gelatine, —This type is preferred to USP
Remove from heat when dissolved.
grade gelatine, because the latter may not give transparent
10.2.6 Dry the samples.
solutions. A10 % solution is prepared by mixing9gofthe
10.2.7 Suspend the samples in the test cells.
10.2.8 Prepare the composite gel solution and add to the
cells.
Knox brand packed by Knox Gelatine, Inc., Englewood Cliffs, NJ 07632 has
been found satisfactory or its equivalent may also be used. 10.2.9 Solidify the gel thoroughly.
B798–95 (2000)
TABLE 1 Guide to Gel Porosity Testing Solutions
Test for Electrolyte (Aqueous) Indicator Indicating Color Comments
A
Copper 4 % sodium carbonate+1%so- saturated solution of rubeanic dark olive green also detects nickel, cobalt
dium nitrate acid in ethanol
Copper 4 % sodium carbonate+1%so- 7.5 % potassium ferrocyanide in brown -------
dium nitrate
...