An investigation on the outgassing of a pigment employed as a fluorescent medium in conformal coatings has been performed. The conformal coatings in question are used to protect printed wiring boards from environmental hazards such as dust and moisture. The pigment is included in the coating at low concentration to allow visual inspection of the conformal coating for flaw detection. Calcofluor, the fluorescent pigment has been found to be a significant outgasser under vacuum conditions and a potential source of contamination to flight hardware. A minimum acceptable concentration of Calcofluor for flaw detection is desirable. Tests have been carried out using a series of Solithane(TM) conformal coating samples, with progressively lower Calcofluor concentrations, to determine the minimum required concentration of Calcofluor. It was found that the concentration of Calcofluor could be reduced from 0.115% to 0.0135% without significant loss in the ability to detect flaws, while at the same time significant reductions in Calcofluor outgassing and possible contamination of systems could be realized

Miller, Michael K.

English

NASA Technical Reports Server

/REDUCTION OF CALCOFLUOR IN SOLITHANE CONFORMAL COATINGS
OF PRINTED WIRING BOARDS
ABSTRACT
Michael K. Miller I Lt. USAF
Goddard Space Flight Center, NASA
Greenbelt, MD 20771
©
c:J c__ <;'._3
An investigation on the outgassing of a pigment employed as a fluorescent medium in conformal
coatings has been performed. The conformal coatings in question are used to protect printed wiring
boards from environmental hazards such as dust and moisture. The pigment is included in the coating at
low concentration to allow visual inspection of the conformal coating for flaw detection. Calcofluor, the
fluorescent pigment has been found to be a significant outgasser under vacuum conditions and a potential
source of contamination to flight hardware. A minimum acceptable concentration of Calcofluor for flaw
detection is desirable. Tests have been carried out using a series of Solithane TM conformal coating
samples, with progressively lower Calcofluor concentrations, to determine the minimum required
concentration of Calcofluor. It was found that the concentration of Calcofluor could be reduced from
0.115% to 0.0135% without significant loss in the ability to detect flaws, while at the same time
significant reductions in Calcofluor outgassing and possible contamination of systems could be realized.
INTRODUCTION
Calcofluor[1] is mixed into Solithane[2] to provide detection of flaws when the Solithane-
Calcofluor conformal coating is exposed to UV light. It has been found that the use of Calcofluor in
Solithane conformal coatings for circuit boards may be a large source of contamination. Because of this,
interest has been expressed in developing a specification for an acceptable minimum percentage
Calcofluor mixed with Solithane which is still adequate to detect flaws in the conformal coatings
under UV. The normal amount of Calcofluor used in a Solithane compound has been 100g of part
A(Solithane 113), 74g of part B(Solithane 113-300), and 0.2g Calcofluor. These weights are referred to
as a 0.2 pbw (parts by weight) Calcofluor mixture. Likewise the same mixture of Solithane components
with 0.1g Calcofluor would correspond to the 0.1 pbw mixture. In order to determine the amount of
Calcofluor required to detect defects in the Solithane conformal coatings, a set of 4 mil thick coatings
composed of different percentages of Calcofluor were applied to aluminum test plates. The plates were
visually inspected for defects and fluorescence. The set of plates included the following amounts of
Calcofluor mixed with 20.0 g of Solithane 113 and 14.8 g of Solithane 113-300:
Plate Number Calcofluor (_) Calcofluor (pbw)* %Calcofluor
1,2 0.0 0.0 0.0
3,4 0.04 0.2 0.115
5,6 0.02 0.1 0.0574
7,8 0.01 0.05 0.0287
9,10 0.01 0.05 0.0287
11,12 0.005 0.025 0.0144
13,14 0.0027 0.0135 0.0078
* The units pbw are a common designation used locally within GSFC, they refer to the weight in grams
used per 100g of the A component of Sollthane and 74g of the B component.
Mr. Miller, a graduate of the Air Force Academy, carried out this research at GSFC as a participant of the Program
for Research and Education in Space Technology (PREST). His advisers were Dr. J. Scialdone, Code 313 and Dr. P.
Chen, Code 724. He wishes to acknowledge the help of several members of the Materials Engineering Branch.
59
https://ntrs.nasa.gov/search.jsp?R=19970019966 2020-06-18T00:16:51+00:00Z
METHOD
The study consisted of mixtures including 0.2, 0.1, 0.05, 0.025, and 0.01 pbw. The goal of the study
was to determine the minimum amount of Calcofluor required for effective flaw detection in conformal
coatings of circuit boards prior to exposure to vacuum conditions. The second step was to determine if
this reduction in the quantity of Calcofluor would actually reduce the level of contaminant present
under vacuum conditions. To determine if this was so a series of tests was carried out in a Vacuum
Industries test chamber at pressures on the order of 10 -7 and 10 -8 torr. The 0.2, 0.1, 0.025 and 0.013 pbw
test plates as well as the pure Solithane test plates were exposed to vacuum for a period of 24 hours at a
temperature of 75°C. During the test, condensates were collected on a cold plate at a temperature of -
25°C. After completion of the 24 hours cycle the cold plate was rinsed and condensed material was
weighed to determine the collected volatile condensible mass (CVCM). Then the residue was analyzed
by both Infrared and Gas Chrornatograph-Quadrupole Mass Spectometry. The IR and GS/MS analysis
was used to obtain the recommendation on the level of Calcofluor to be used.
A diagram of the instrumentation is shown in the appendix. The Vacuum Industries chamber
includes a two stage pumping system with a mechanical backing pump and a diffusion pump which can
achieve pressures down to approximately 10 -8 torr. Backstreaming from the diffusion pump was
controlled using liquid nitrogen cooled cold trap and a liquid nitrogen controller.
The chamber itself is an 18 inch diameter bell jar which contains a hot plate, cold plate and
thermocouples. All of the hardware within the bell jar was chosen with the fewest number of
outgassants possible, so as to hold contamination to a minimum. The hot plate rested on an adjustable
scissors jack and was raised so that there was approximately 1 inch spacing between the sample and
the cold plate. The hot plate itself was a simple resistance coil hot plate controlled by an external
controller. The controller ran through a high limit controller which detected the hot plate
temperature and would open an alarm condition switch if the temperature went above a preset value.
The controller signal was sent to a 110 volt, 10 ampere solid state relay which would open and close the
switch from the power source to the hot plate depending on the temperature of the hot plate.
Temperatures were measured on the hot plate, the cold plate and the sample itself using a series of
type T thermocouples.
The 8x6 inch cold plate consisted of a simple aluminum plate with copper tubing coiled at 1 inch
intervals across the top surface. These coils carried ethylene glycol based antifreeze which was cooled
by a Neslab chiller to temperatures of -25°C. This plate was covered with an aluminum foil wrap
which had been cleaned using a series of isopropyl alcohol and chloroform rinses. The foil served as a
surface for condensation which provided and easy method to collect and rinse the contaminant
Once the series of plates were made the first step in analysis was visual inspection of the samples
for defects. Upon visual inspection of the conformal coatings under UV illumination it was apparent
that as little as 0.0135 parts by weight was sufficient for detection of flaws in the conformal coating.
Each coating was prepared with included flaws (nylon fibers and brush fibers) to simulate possible
defects. The nylon filament was 2 mil in diameter and cut into lengths of approximately 1-2 mm. The
brush hair was approximately 1 mil in diameter and was cut to lengths of less than I mm. In addition a
certain amount of defects were created during the preparation technique. It was found that although
the level of fluorescence was reduced somewhat in both the 0.025 and 0.0135 pbw Calcofluor, the
fluorescence was sufficient to detect both the included flaws as well as naturally occurring defects in the
coating. This demonstrated that the level Calcofluor could be reduced to as little as 0.0078% rather
than the 0.115% currently used.
The following observations were made during visual inspection of the test plates under UV:
6O
0.2 pbw Calcofluor: The entire plate demonstrated signs of fuorescence. All
the included flaws (nylon fibers and brush hair) could be seen with
relative ease. The plates also exhibited numerous spots which
demonstrated fluorescence. These were attributed to areas of higher
Calcoflouor concentrations.
0.1 pbw Calcofouor: The same observations were seen as in the 0.2 pbw
Calcofluor plates. In additon a dark, pin sized hole was observed
penetrating the entire thickness of the coat in plate #6. However,
when the edge of a probe was used to scratch the surface of the plate, an
increase in fluorescence at the scratch was observed.
0.05 pbw Calcofluor: All the included flaws could be seen with relative ease.
In additon bright spots of Calcofluor were visible.
0.025 pbw Calcofluor: The bright spots due to undissolved Calcofluor were less
visible, and the fluorescence was slightly less intense, but the flaws
were clearly visible.
0.0135 pbw Calcofluor: The level of fuorescence was similar i_ intensity to the
0.025 pbw. All flaws were visible.
0.0 pbw Calcofluor: Demonstrated no signs of fluorescence from the Solithane
itself. Nylon fibers could be seen because of their own fluorescence.
Once the visual' inspection was completed each plate was placed in the vacuum chamber and
heated to a temperature of 75°C for 24 hours at pressures ranging from 10-6 to 10 -8 torr. The actual
pressure was somewhat time dependent and also depended to some degree on the amount of material
outgassing from the Solithane sample. Condensables were collected on the aluminum wrapped cold
finger at -25°C. These deposits were rinsed from the aluminum after the test was completed with
alcohol and chloroform. The collected material was weighed and then analyzed by both IR and
GC/MS analysis. The results of this analysis are discussed in detail in the Results section.
RESULTS
The following data on the collected deposits and the related CVCM
(mass collected/initial mass of Calco + Solithane) were obtained.
Sample Plate Initial Conditions and Results
% Calcofluor Wt. Solithane(g)
0.115 3.34
0.057 1.18
0.014 1.13
0.008 2.34
0.0 0.00
Wt. Calcofluor(mg) Collected Wt.(mg) %CVCM
3.834 7.0 2.10E-03
0.678 3.7 3.14E-03
0.162 2.8 2.48E-03
0.182 4.8 2.05E-03
0.000 3.8 1.05E-03
pbw Calcofluor % Calcofluor
0.2 0.115
0.1 0.057
0.025 0.014
0.0135 0.(]08
61
0.0 0.0
%CVCM vs. Calcofluor Content
0 0.008 0.014 0.057 0.115
%Calcofluor in Solithane
-4.00E-03
1.00E-03 =_
O.OOE+O0
Figure 1
This results indicate that the highest concentration of Calcofluor produces by far the greatest
accumulation of contamination. However there was not a linear trend towards less contamination as
the concentration of Calcofluor was decreased. One possible explanation was that below a certain
percentage of Calcofluor, the contamination was mainly produced by the Solithane, and that
variations in the thickness of the Solithane coating itself lead to variations in the weight of the
outgassants collected.
The information illustrated in Figure 1 indicates that as the Calcofluor content is increased the
level of contamination also increases. However the highest Calcofluor concentration did not follow
this trend. An explanation may be that the conformal coating was somewhat thicker on the sample
plate for the 0.2 pbw(0.115%) sample. The thicker coating could limit the diffusion of Calcofluor out of
the material leading to lower collected weights. Additional analyses were carried out to provide
correlation between increased Calcofluor and CVCM.
The contaminant was analyzed by IR and GC/MS to determine the composition of the released
products and quantify the amount of those materials present. Upon completion of the 24 hour test the
collected deposits were rinsed into an evaporation dish and weighed once dry. The residue was then
analyzed by FTIR and GC/MS. FTIR analysis primarily focused on detection of absorption peaks at
1711 and 1612 cm -1. These wavelengths are indicative of the presence of Calcofluor, but 1711 cm -1
proved to be less useful because the several common esters cause absorption at that frequency as well.
By comparing the height of the Calcofluor peaks at 1612 cm -1 relative to the peaks caused by silicones
in the same spectra, the following charts (fig. 2) were obtained.
62
Relative IR Peak Intensity vs. % Calcofluor
(1612 1/cm)
2.5
G) >, 2
='>e"_ 1.51
=.
•"$ 0.5
0" I I I I I
0 0.02 0.04 0.06 0.08 0.1 0.12
% Calcofluor
Figure 2
Figure 2 displays the Calcofluor peak at 1612 cm -1 relative to the peak caused by silicone pump oil at
1216 cm -1 as a function of the concentration of Calcofluor in the conformal coating. A second plot Figure
3 was produced by comparing the peaks due to Calcofluor relative to a peak at 1177 cm -1 from silicone
pump oil, as a function of the Calcofluor concentration. This diagram is shown below.
Relative Peak Intensity vs. % Calcofluor
(1612 llcm)
O" I I I I I I
0 0.02 0.04 0.06 0.08 0.1 0.12
% Calcofluor in Solithane
4
3.5 _=
3
2.s
2
_ _ 115
r" •
O.S
Figure 3
GC/MS analysis of the residue showed similar results. Each sample was normalized to a silicone
peak caused by backstreamed diffusion pump oil present in each sample. The silicone peaks were
63
assumed to be relatively constant and thus could be used to compare with the peak areas for Calcofluor.
This resulted in the followin_ curves (fi_. 4 and 5):
Outgassed Calcofluor normalized with
Silicone oil vs. %Calcofluor in Solithane
1
o 0.75
rr
0.5
¢
_. 0.25
=
0 ""-= I I I I
0 0.02 0.04 0.06 0.08 0.1 0.12
% Calcofluor in Solithane
Figure 4
Calcofluor peak area normalized to diffusion pump oil
GC/MS Peak Area/Initial Wt. vs.
%Calcofluor
1.6
= 1.4
"E 1.2
P_ 0.8
< 0.6
0.4
_. 0.2
0 m--'= = I I I I I I
0 0.02 0.04 0.06 0.08 0.1 0.12
%Calcofluor in Solithane
Figure 5
Calcofluor peak area normalized to initial weight
64
This method of analyzing the contamination is perhaps the most accurate and yet shows the most
dramatic increase in the level of contamination as the concentration of Calcofluor was increased.
CONCLUSIONS
After investigating the fluorescence of different concentrations of Calcofluor in Solithane conformal
coatings, as well as the contamination caused by outgassing Calcofluor in these coatings, one can
recommend that the concentration of Calcofluor be reduced from 0.115% Calcofluor (0.2pbw) to 0.0078%
Calcofluor (0.0135pbw). Using only 0.0135 pbw Calcofluor significantly reduced the contamination
caused by the fluorescent pigment yet proved to be sufficient for flaw detection in Solithane conformal
coatings.
The investigation has also indicated the methods to be followed in evaluating outgassing
originating from compounds. The peak area indications of GC/MS with respect to peak areas of some
reference compound can be helpful in establishing the amount of a specific contaminant over a series of
initial sample concentrations.
REFERENCE
[1.] Calcofluor is a trade name of American Cyanamid Co. for a fluorescent powder consisting of
4-methyl-7-diethyl amino coumarin (MDAC).
[2.] Solithane is a trade name of Thiokol Chemical Corp. for a polyurethane coating.
APPENDIX
VACUUM CHAMBER & TEST SET-UP
COLD PLATE
HOT PLATE
COOLANT
(TO -25"C) _
HI-VAC VALVE_
LN2 TRAP_
DIFFUSION PUNP_
18" DIA
BELL JAR THERMOCOUPLE
-GAUGETC_
_\ VENT VALVE
ROUGFI ] NG
VALVE
/-ADJUSIABLE JACK I
CONTROLLER I
_-ll0 VAC I THERMOCOUPLE
L_ GAUGE TC£
\
FOPELINEJ / \
VALVE _"\ MECHANICAL
PUMP
65



Reduction of Calcofluor in Solithane Conformal Coatings of Printed Wiring Boards

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