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Experiment 2

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Experiment #2: Ongoing 
Characterize the Effectiveness of Arathane 5750 (formerly known as Uralane 5750) Conformal Coat Material in Prohibiting Tin Whisker Formation and/or Tin Whisker Penetration

Results To Date 
Overall Summary:

After >9 years, no whiskers have grown through the nominal 2 mil thick Arathane 5750 (formerly known as Uralane 5750).  
There are extensive (high # per area) whisker growths beneath the coating that are pushing/stretching the conformal coat, but none have managed to puncture the coating where the coating is a nominal 2 mils thick or greater..

 
"Metal Whiskers: A Discussion for CALCE Symposium", Symp. on Part Reprocessing, Tin Whisker Mitigation and Assembly Rework, Nov. 12, 2008
J. Brusse, H. Leidecker, L. Panashchenko

Tin whiskers have grown through ~0.25 mil thick Uralane 5750 after 2.5 years of room ambient storage
Conformal Coat applied over top of a whisker-prone surface will NOT prevent the formation of tin whiskers
Whisker growths INITIATED faster on specimens that were covered with conformal coat; HOWEVER, 
Conformal coat also REDUCED (but does not eliminate) the rate of growth of tin whiskers compared to an uncoated specimen
 
Current Observations/Photos:
"The Continuing Dangers of Tin Whiskers and Efforts to Control Them with Conformal Coat".  This interim report is published in the NASA EEE Links Newsletter July 2001
"Effect of Conformal Coat on Tin Whisker Growth": This interim report is published in the Proceedings of the 37th IMAPS Nordic Conference, September 2000.

Purpose:

To determine if Arathane 5750 (formerly known as Uralane 5750) conformal coat material over a bright tin plated surface provides a sufficient barrier to prevent tin whiskers from penetrating the protective coating.  If this material proves to be successful at stopping whisker penetration, then its use may be suitable as a workaround to protect systems that already have pure tin plated components that cannot be replaced with non-pure tin plated alternates.

Abstract: 

Most published literature on tin whiskers agree that brass with bright tin finish is a combination that promotes whisker formation.  For this experiment several brass substrates (some with copper flash) have been plated using a "bright" tin finish.  To further promote whisker formation some of the plated samples were scratched using a knife blade. 

The samples were segregated into two test groups.  Portions of each sample were coated with Uralane 5750 conformal coat material.   One test group has been placed in an oven at 50°C because the literature suggests that whisker growth is greatest at this temperature.  The other test group is being stored at room temperature.  All samples will be visually examined periodically to determine 
If conformal coat inhibits whisker formation
If whiskers are capable of growing outward through the conformal coat
As a side benefit
The incubation period for whisker formation
The rate of whisker growth

This experiment is ongoing.  If possible, some substrates with whiskers grown during this experiment may be used in Experiment #3

Sample Preparation:

The experiment team utilized the plating services of Alexandria Metal Finishers (Lorton, VA)**.  At the request of the experimenters AMF plated several brass substrates using "bright" tin processes ("bright" tin is considered to be susceptible to tin whisker formation).  Two separate plating processes were used:
1/2 of the samples were "bright" tin plated directly over the brass substrate
1/2 of the samples received a Copper flash over the brass substrate prior to "bright" tin plating
The application of a Copper flash is part of Alexandria's normal plating process.  Since this experiment desires the formation of whiskers, the non-Copper flash samples were ordered because most of the literature regarding tin whisker formation suggests that pure tin over brass is a combination that is highly susceptible to whisker growth.

The samples were intentionally scratched with the goal of encouraging whisker formation at the stressed locations.

The samples were then coated on one half of the substrate with Uralane 5750 to a nominal thickness of 1 to 2 mils.  Several samples (both Cu and non-Cu flashed) were placed in an oven maintained at 50°C (literature suggests that 50°C is the optimal temperature for whisker formation).  The remaining samples were maintained at room ambient conditions.

Inspection Procedure:

Periodic optical microscope inspections using 40X to 100X will be performed to look for any whisker growth with special emphasis on inspecting the conformally coated areas.  As necessary, Scanning Electron Microscopy (SEM) will be used to confirm and document findings.  Some whisker sites will be SEM examined again on a regular basis to determine growth rate.  Results of this experiment will be posted to this web site.

Observations/Findings
The links below provide an assortment of Scanning Electron Microscope (SEM) photographs taken to date:
Photographs Available 
Conformally Coated Side 
Non-Conformally Coated Side
Area between Not Coated Side and Coated Side

 
As of November, 2008:
"Metal Whiskers: A Discussion for CALCE Symposium", Symp. on Part Reprocessing, Tin Whisker Mitigation and Assembly Rework, Nov. 12, 2008
Slides 19 through 28

As of 3/1/02:

The latest inspection of conformal coating showed that more nodules have formed under the conformal coating (by the density increase of whisker domes). None of the nodules have penetrated through nominal 2 mils thickness of the coating after over 3 years of experiment. However, early observations did note that a whisker did penetrate ~0.25 mils of conformal coat.
Inspection of four specimens that were subjected to conformal coat removing process showed that the nodules from stripped side are still in nodule form after 6 months of additional storage (some at room temperature, some at 50C) since the removal of conformal coating.

As of 6/18/01:

Four specimens that were stored in oven at 50C for 2.5 years have been subjected to conformal coat removing process to compare nodule formation in shape and size with the nodules from non-coated side. The specimens were soaked in Uralane stripper for hours until all the coating was removed. Subsequent SEM inspection revealed most of the nodules that were growing under the conformal coat were irregular shapes. Some of these nodules are 2 or 3 times bigger in size than nodules found on non-coated side.
Two of the four specimens were returned to the 50C oven for storage. The other two specimens were not returned to the oven in favor of keeping them stored in room ambient conditions because GSFC observations to date suggest that whisker formation occurs faster at room temperature compared to 50C storage.

As of 3/1/01:

A whisker is breaking through the conformal coating (see photo).  At this point, only one whisker has penetrated through the coating.  The thickness where the whisker penetrate through is confirmed to be very thin (1/10 mil). 
The pure tin plated samples without Cu flash stored at room temperature (25 °C) show a faster whisker growth rate (10x) compared to samples stored at 50 °C.  The pure tin with Cu flash sample comparison has not yet been made. 

As of 09/15/00:
Still none of these domes made complete penetration through the conformal coat, but the number of domes in the conformal coat has increased since the last observation.
The whiskers from the non-conformally coated side are now long enough that they are visible without the microscope.
The interim report titled "Effect of Conformal Coat on Tin Whisker Growth [pdf]" is published in the Proceedings of the 37th IMAPS Nordic Conference, September 2000.

As of 03/20/00:
The latest observation indicates that the domes observed in the conformal coat from the last inspection become larger and narrower at the top, which suggests that the whisker tips are beginning to penetrate the conformal coat.  To date no  whiskers have been observed piercing through from underneath the conformal coat.
Two random samples (sn#1T and sn#7C) have been coated with gold (vapor deposition, 200 angstrom) to reduce electronic charging effect during SEM inspection on coated side.  This approach made the inspection of conformal coated side easier.
The longest whisker observed on the non-conformally coated area is 800 um (31 mil).

As of 09/15/99:
Picture sn2T_04_0830.jpg (new) shows possibility of whisker penetrating through conformal coating.  It is still difficult to predict whether whiskers are breaking through the conformal coating based on just one picture.  However, this picture shows strong evidence that conformal coating may not be the solution for preventing whisker growth.
As of 07/13/99:
To date it is still not known if the observed tin whiskers will be able to penetrate the 1 to 2 mil thick Uralane coating.  Domes observed in the conformal coating are indicative of whisker growth beneath the coating.  However, we have not yet confirmed that whiskers are breaking through the conformal coating.
The longest whisker observed from the non-conformally coated area is 85.5 um (3.3 mil).
The estimated average rate of whisker growth during 6 months is 0.098 mm/year.
The whisker shapes can be categorized into 3 shapes; pyramid, toothpaste, and triangular.  In addition, several irregular shaped whiskers have been observed.
As of 05/10/99:
To date it is still not known if the observed tin whiskers will be able to penetrate the 1 to 2 mil thick Uralane coating.
The longest whisker observed from non-conformally coated area is 38 um (1.5 mil) which is the nominal thickness of the conformal coating.
The estimated rate of whisker growth during 4 months is 0.08 mm/year.
Regular re-examination will be continued.

As of 03/17/99:
It has been observed by optical microscope inspection that there are more tin whisker nodules growing under the conformal coat than there are growing on the non-conformally coated side.  One hypothesis for this observation is that the uncoated portion of the sample oxidizes due to exposure to the atmosphere.  The tin oxide that forms may slow down the rate of new whisker formation.  On the conformally coated side of the test samples the conformal coat may limit oxidation of the tin leaving a surface that could be more susceptible to whisker initiation.
Some of the whiskers photographed previously were re-examined and photographed to identify changes in growth.
Future examinations will include repeated observations of selected areas to determine the rate of new whisker formation.

As of 02/17/99:
Initiation of whisker growth was noted within the first 3 weeks after plating.
Observation has confirmed the initiation of whiskers on areas coated with Uralane 5750.
It is not yet known if the whiskers will penetrate the 1 to 2 mil thick coating.
** The GSFC Code 562 experiment team commends Alexandria Metal Finishers for their quick turnaround in preparing the test samples utilized in Experiment 2 and for their attentiveness to the specific plating requirements imposed by the team. 
 
Responsible NASA Officials:

   Michael Sampson/NASA GSFC Code 306
   Dr. Henning Leidecker/NASA GSFC Code 562
Additional Researchers: 

   Jay Brusse/Perot Systems
Jong Kim/Orbital Sciences Corp.

Last Updated:

June 16, 2009

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