Download Flowers of Sulfur Corrosion Test for Surface Mount Resistors

Revision 12 – 23 July 2015
Flowers-of-Sulfur Corrosion Test for Surface Mount Resistors Containing Silver
1. Scope
This test method evaluates the susceptibility of surface mount resistors to sulfur-based corrosion. This
test method is a modified form of ASTM B 809. The ASTM B 809 test method is intended to test the
porosity of metallic coatings; however, the method has been adapted such that it can be used to detect
exposed silver in a particular type of component (surface mount resistors) that have a known corrosion
failure mode due to the sulfidation of exposed silver when placed in a sulfur-containing atmosphere.
2. Applicable Documents
ASTM B 809 Standard Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (“Flowers-ofSulfur”)
3. Summary of Test Method
Specimens are suspended over powdered sulfur (“flowers of sulfur”) in an unvented container at
controlled elevated temperature such that the elemental sulfur vapor reacts with any exposed silver on
the resistor. Observable growths of silver sulfide crystals (Ag2S) indicate exposed silver.
4. Test Specimen
4.1. Component – The component types to be tested in this method are surface mount resistors,
surface mount resistor networks or arrays containing silver or silver alloys. The component
body type, size, value, and tolerance shall be recorded. If no resistance value is specified, then
10 Ohm resistors (any tolerance) shall be used. If electrical resistance measurements will be
used to complement visual observations at inspection (Section 7.3.1), components with
tolerances better than 1% should be used.
4.2. Test board – The preferred test configuration is for the resistors to be soldered to a printed
wiring board. An example of such a board is shown in the following figure. If an alternate
configuration is used, such as loose components, the surface(s) of interest on the components
must have an unobstructed exposure to the sulfur vapor. Due to the highly elevated test
temperatures (Section 6), most products will not be able to be tested in a functioning state. If
product printed circuit assemblies (PCAs) will be used as test boards, they should not be tested
in their product enclosures unless airflow within the enclosures will be equivalent to service
conditions during the test.
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4.3.
Figure 1: Flowers of sulfur test boards for surface mount resistors
4.4. Test specimen creation documentation – All relevant parameters from the component
placement and soldering operation shall be recorded, including but not limited to: solder and
flux materials, soldering temperature or profile, and component placement method.
4.5. Test specimen cleaning – After the components are soldered to the printed wiring board, the
test specimens may be cleaned in order to remove contamination that might confound the
corrosion resistance results of the component itself. Cleaning requirements and conditions shall
be AABUS.
4.6. Control specimens – A silver control specimen (4.5.1) must be suspended in the chamber by a
nylon line, or other non-corroding material, during the test to confirm the severity of the
environment.
4.6.1.Making control specimens from silver sheets – Control specimens may be created by
shearing a 0.508mm (0.020 in) thick silver sheet into smaller coupons that are
approximately 76.2mm (3 in) inches long and 12.7mm (0.5 in) wide. Abrade the silver
surface using 400 grit sand paper followed by 600 grit sand paper to achieve a consistent
surface finish and to remove any surface contaminants or oxides. A hole approximately
1.6mm (0.0625 in) in diameter may be drilled at the end of the coupon for ease of
suspending the control within the test apparatus. At the time of publication of this test
method, suitable silver sheet (5N purity, 0.020” x 3” x 12”) was available from ESPI Metals,
1050 Benson Way, Ashland, OR, 97520 (http://www.espimetals.com), part number
Knc2000.
4.6.2.The entire surface of the silver control specimen is expected to turn black within 24 hours
of the start of the test because of the formation of silver sulfide (Ag2S) tarnish through
sulfidation of the silver surface. The appearance of a properly tarnished (blackened)
control specimen is shown in Figure 2a, and a control specimen from an insufficient sulfur
environment is shown in Figure 2b. If control specimens do not completely blacken
(tarnish) in 24 hours at test conditions, additional surface area of sulfur or fresh reagent
may be needed (refer to Sections 5.1 and 5.2.2).
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Figure 2: Control specimens after 24 hour exposure in test conditions
4.6.3.Reuse of control specimens – Control specimens may be reused if they are re-abraded to
expose clean silver surfaces.
4.7. Handling – Non-contaminating gloves shall be worn while handling test specimens and control
samples.
5. Apparatus
5.1. Glass Container – A Petri or other shallow dish approximately 15 cm (6 inches) in diameter to
hold powdered sulfur. If a large test vessel is used and the silver control specimen does not
blacken (tarnish) from sulfidation within 24 hours (per the definition in Section 4.5), a larger
diameter shallow glass container may be needed to accommodate additional sulfur and to
increase the surface area available.
5.2. Reagent – Sulfur, Sublimed (“Flowers-of-Sulfur”), CAS No. 7704-34-9, N.F. or laboratory grade
or better, powder -100 mesh particle size.
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5.2.1.Sulfur placement in glass container – A layer of sulfur approximately 6.35 mm (0.25 in)
thick shall be placed in the shallow glass container such that it covers the entire bottom of
the container.
5.2.2.Reuse of reagent – It is recommended that the sulfur be replaced with fresh material each
test, however, the reagent may potentially be used for multiple tests by using the
corrosion of the silver control specimen as an indicator that the reagent must be replaced.
If the control specimen does not blacken (tarnish) from sulfidation within 24 hours (per
the definition in Section 4.5), the reagent must be replaced.
5.3. Test Vessel – Any conveniently-sized vessel of glass, polycarbonate, stainless steel (or any other
material) that is not affected by sulfur vapor and elevated temperatures may be used as the
test vessel. The size of the test samples will dictate the required size of the chamber. For small
components such as chip resistors, a glass desiccator of 9 to 10L capacity will be sufficient. The
vessel does not need to be hermetically sealed and may have a small vent opening (1 mm
diameter hole) to allow equalization of pressure when elevated temperatures are used.
5.3.1.Large test vessels – Larger vessels may be required to accommodate test boards and PCAs.
If a large test vessel is used and the silver control specimen does not blacken (tarnish) from
sulfidation within 24 hours (per the definition in Section 4.5), a larger diameter shallow
glass container and additional sulfur may be needed to increase surface area available
(Sections 5.1 and 5.2).
5.4. Sample Fixture or Holders – Supports or hangers shall be made from materials such as glass,
Teflon, stainless steel that will not be affected by sulfur. The fixture shall not cover more than
20 percent of the vessel’s cross-sectional area so that air movement within the vessel will not
be restricted during the test.
5.5. Sample Placement – The samples shall be placed at least 75 mm (3 inches) away from the sulfur
powder, at least 25 mm (1 inch) from the vessel walls, and at least 10 mm (0.4 inch) from other
samples or surfaces.
5.6. Air Circulation – A low level of air circulation is needed to prevent temperature and sulfur
vapor stratification within the vessel. This level of air circulation is achieved by using one or two
1.5 in (3.81cm) 12V fans placed in corners of the chamber. Due to the corrosive environment in
which they will operate, the fans may need to be replaced periodically. It is recommended that
these fans run continuously throughout the test.
5.7. Vessel Temperature — It is recommended that the vessel temperature be controlled by placing
it in a larger convection oven. Thermocouples connected to the temperature controller should
be placed in close proximity to the test samples. If a convection oven is not available, properly
placed flat heating elements (pancake heaters) may be used on the exterior of the vessel if the
vessel temperature can be controlled to ±2°C consistently throughout the vessel (See Appendix
A for more information about temperature consistency requirements). It is highly
recommended that proportional temperature controllers with safety temperature limit shutoff
be used for heating larger vessels.
5.8. Vessel Humidity is not actively controlled because the formation of silver sulfide (Ag2S) occurs
independent of the relative humidity.
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6. Exposure
6.1. Uncoated samples and samples with non-silicone-based conformal coatings must be exposed
for 20 days in a test vessel maintained at 105°C±2°C (221°F±3.6°F).
6.2. Samples with silicone-based conformal coatings must be exposed for 20 days in a test vessel
maintained at 60°C±2°C (140°F±3.6°F).
6.3. Monitoring control specimens – The control specimens should be visually inspected (if possible,
from outside of the test vessel) 24 hours after a test is begun. The silver control specimen must
blacken (tarnish) from sulfidation within 24 hours (per the definition in Section 4.5). If the silver
control specimen does not completely blacken (tarnish) from sulfidation within 24 hours,
additional surface area of sulfur or fresh reagent may be needed (refer to Sections 5.1 and
5.2.2). It is recommended that the silver control specimens remain in the chamber for the
duration of the test to avoid disrupting the equilibrium within the test vessel.
6.4. Applied bias
6.4.1.If resistance measurements are not being used for test sample monitoring and/or
inspection, bias shall not be applied to any test sample during the test.
6.4.2.If resistance measurements are being used for sample monitoring and/or inspection
(Section 7.3.1), the applied voltage must not cause local heating of the test component
because of the strong effect of temperature on the sulfidation chemical reaction and local
sulfur vapor concentration (Appendix A). Calculations to determine the maximum voltage
that can be applied are given in Appendix B.
7. Sample Inspection
7.1. After exposure, components must be examined using optical microscopy using a microscope
with adjustable magnifications to 30X.
7.1.1.Higher magnifications may be necessary, including the use of scanning electron
microscope, for more critical applications.
7.2. Conformal coatings must be removed from components prior to inspection.
7.2.1.Coating removers are available from Dynaloy and Vericlean.
7.3. Any observable growths of silver sulfide crystals (Ag2S) must be recorded. Images of silver
sulfide crystals are shown in Error! Reference source not found..
7.3.1.Resistance measurement may be used to complement visual observations.
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Figure 3: Optical and SEM images of silver sulfide on surface mount resistor
Appendix A
The concentration of sulfur vapor is controlled by the temperature in the test vessel (elemental sulfur
vapor comes to equilibrium with sulfur powder in the closed system). Sulfur vapor concentration as a
function of temperature is shown in Figure 4.1 As can be seen in the figure, sulfur vapor concentration
can change significantly based on temperature gradients at the test conditions, therefore conditions
within the chamber must be as uniform as possible. The need for temperature consistency is the reason
convection ovens are recommended over pancake or other point source heaters for heating the test
vessel in most cases.
Figure 4: Sulfur vapor concentration as a function of temperature
1
Washburn, E.W. (1926 - 1930; 2003). International Critical Tables of Numerical Data, Physics, Chemistry and
Technology (1st Electronic Edition)
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