Here is a lead test with a long history. (Write or call off-line if you have
questions).
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Date:
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Subject: Surface Lead Contaminant Detection Spot Test
1.0 Scope
1.1 This simple and rapid non-destructive colorimetric spot test method is
for use in determining if non-ionic lead (primarily lead oxide) contaminant is
present on bare printed wiring board and completed assembly surfaces.
1.2 The test will not identify the particular compound of lead present on
the surface tested nor separate mixtures of various lead compounds on the
surface. After a functional problem is found with boards or assemblies, usually
failure at elevated temperature and humidity, this test can be used to confirm
or eliminate non-ionic lead contaminant as a root cause.
1.3 The test can detect lead levels easily in the 0.1 microgram range.
Lower levels in the presence of interferring metals can be detected by a skilled
operator using accurate standards.
2.0 Applicable Documents
IPC-TP-135 "Insulation Resistance Degradation by Non-Ionic Surface
Contamination-- Its Cause & Cure."
Feigl , Fritz, and Suter, Hans A., “Analytical Use of Sodium Rhodiazonate”,
Industrial and Engineering Chemistry, Vol. 14, No. 10, 840-2, (1942).
3.0 Test Specimen
3.1 A bare printed wiring board or completed assembly at least 50 mm x 50 mm
x thickness.
4.0 Apparatus & Materials
4.1 Sodium rhodizonate
4.2 Tartaric acid
4.3 Sodium bicarbonate
4.4 Cotton tip swabs
4.5 Indicator dropper bottle
4.6 Balance capable of weighing to 0.001 g.
5.0 Procedure
5.1 Preparation
5.1.1 Prepare the stock buffer solution by dissolving 15 grams of tartaric
acid and 19 grams of sodium bicarbonate in one liter of distilled water. The
pH of the resultant buffer solution should be 2.79. This solution is
stable, and can be stored for an extended time in the laboratory.
5.1.2 Prepare the 0.2% sodium rhodizonate indicator solution by dissolving
0.01 g. of sodium rhodizonate in 5 ml. of water. Place the solution in the
indicator dropper bottle. [Note: the useful life of the indicator solution is
approximately 8 hrs.]
5.2 Test
5.2.1 Using a clean cotton swab, place 2 drops of buffer solution on the
cotton swab.
5.2.2 Rub the buffer soaked swab on a dielectric area (non-plated area) of
the printed wiring board or assembly surface, being sure not to touch the
traces.
5.2.3 Using a second clean cotton swab, place 3 drops of the indicator on
the cotton swab.
5.2.4 Rub the indicator soaked swab in the area coated by the buffer swab.
5.3 Evaluation
5.3.1 Any color change from yellow to red on the indicator swab indicates a
positive test for lead contaminant.
5.4 Action
5.4.1 The boards or assemblies should also be tested for the presence of
organic non-ionic contaminant using the test methods cited in 2.0.
5.4.2 The lead contaminated boards or assemblies can be recovered by
applying and curing the lacquer described in 2.0, then washing it off using a
suitable cleaning agent.
5.4.3 The organic non-ionic contaminated boards or assemblies can be
recovered per methods cited in 2.0
6.0 Notes
6.1 The tartaric acid, sodium bicarbonate and sodium rhodizonate used in
this method can be obtained from standard chemical supply houses. Reagent grade
chemicals are preferred.
6.2 The literature claims this test can be used to detect the presence of
stannous tin (+2 oxidation state) by a color change to violet, however, stannic
tin (+3 oxidation state) gives no change.
Originally Drafted by: W. G. Kenyon April, 1998
Revised: W. G. Kenyon March, 2005
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526
Tel: 970.207.9586 Cell: 970.980.6373
email: [log in to unmask]
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