New Generation Fluoropolymer Coatings for Electronic Applications
Steve Henly – Acota Limited
Fluoropolymer coatings are insulating, protective coatings that conform to the object
being coated and are composed of polymers that contain fluorine, either in the
backbone or side chains of the polymer. Currently several fluoropolymer coatings are
in the research stage of development. A commercial product for electronic
applications, Certonal FC-742 exists, however, and this will be discussed in this
article.
Commercialisation of fluoropolymer coatings has been limited by their high cost,
compared to analogous hydrocarbon acrylates, epoxies, silicones and other coating
types. The high cost of fluoropolymers is often due to the cost and availability of
monomers like hexafluoroacetone. In addition special equipment is required to
manufacture most of these fluorinated compounds.
The most well known fluoropolymers research program began at The Naval Research
Laboratory, Washington D.C., in 1968. The focus of this program was to establish the
chemistry and properties of fluoropolymers for naval use. The military service has the
most critical need for the properties that only fluoropolymers can deliver. Figure 1
depicts the basic fluorodiol developed at this laboratory. The fluorine content of this
diol can be varied at will during synthesis by choosing the size of the perfluoroalkyl
group.
Fluoropolymer surface modifier coatings are used in environments that require high
reliability; water oil and silicone resistance; resistance to microbiological attack; a
very low dielectric constant; a very low ion migration rate; and weatherability.
Fluoropolymer surface modifier coatings can withstand these environments while
maintaining their excellent electric and dielectric properties.
R1
|
CF3 | CF3
|
|
|
HO – C –
– C – OH
|
|
CF3
CF3
Fig. 1
1-perfluoroalkyl-3,5-bi(2-hydroxyhexafluoro-2-propyl) benzene where R1 = H-C6-fluorodiepoxide, or R1 = C6H13:C6fluorodiepoxide, or R1 = C8H17:C8-fluorodiepoxide.
Available materials
Certonal FC742 is a single component fluorochemical acrylic coating supplied in a
dilution of methyl nonafluorobutyl ether and methyl nonafluoroisobutyl ether
(C4F9OCH3) (3M Novec HFE-7100). They are ultraviolet (UV) transparent solutions
of an oleophobic, hydrophobic, fluorochemical polymer. FC-742 meets MIL-B-81744
requirements, which call for a lubricant-migration-deterring barrier coating solution.
The fluoropolymer in FC-742 is a fluorinated methacrylate. The polymer backbone is
methyl methacrylate and the side chain is fluorochemical C4F9 moiety. The fluorine
content of the polymer is approximately 45% of the total structure which is made up
of many fluorinated branches.
Specific Applications
Certonal FC-742 is used as a conformal coating, water repellent coating, and nonwetting barrier to prevent lubricants from migrating. FC-742 is used where reliability
requirements demand protection from condensed moisture and contaminants. Two
examples are circuit boards in extreme environments and ion-contaminated, high
relative humidity environments.
To prevent the spread of silicone fluids, Certonal FC-742 is applied to substrates in a
very thin film (dilution to 0.1% solids). If silicone is allowed to spread, electrical
relay and switch contacts may break down in arcs to form silicon-containing deposits.
Certonal FC-742 is used in industrial and military semiconductor fabrication. It is also
designed for use in military applications and has been used for in-dash automotive
environments. FC-742 is also used as a fluoropolymer mask release coating for both
emulsion and hard surface photolithographic masks to increase yields for large scale
integration (LSI) and very large scale integration (VLSI) devices.
Physical and Chemical Properties
Certonal FC742 has some of the most desirable physical and chemical properties of
all conformal coatings. It is stable at high temperatures, resistant to high-energy
radiation, resistant to chemicals and solvents, is water repellent, has low surface
energy, and excellent electrical properties. Table 1 details typical properties of
Certonal FC-742.
Table 1
Typical properties of Certonal FC-742
Property – As supplied
Value
Form
Appearance
Specific gravity at 25°C (77°F)
Flashpoint (by Setaflash closed cup), °C (°F)
Thermal stability of dry film
Thin liquid (2% solids)
Clear to light coloured liquid
1.5
None
Repellent after 24h at 175°C
(350°F)
11 dynes/cm
~ 1 micron
1.36
3.1
0.0089
1.1kV (25.4 microns)
4.6 x 1012 40v DC (ohm-cm)
Surface energy of dry film
Thickness of dry film, dip coated,
Refractive Index
Dielectric constant @ 25°C, 30% RH
Dissipation Factor at 1kHz 25°C, 30% RH
Dielectric strength, 25°C, 35% RH
Volume resistivity, 25°C, 35% RH
Fluorine has the highest electronegativity of all the elements. When incorporated into
a polymer chain, the fluorine atoms tend to stiffen the polymer chain because the
carbon-fluorine bond distance is quite short and also quite strong, at approximately
477kJ/mol (450Btu/mol). These factors account for the chemical resistance and
thermal stability of fluoropolymers.
Table 2
Surface Energies
Material
Fluorocarbon acrylates
Polytetrafluoroethylene (Teflon)
Silicone
Polyethylene
Epoxide
Urethane
Acrylate
Typical values N/m
0.011-0.014
0.018-0.020
0.024
0.031
0.040
0.040
0.040
Typical values
Dynes/cm
11-14
18-20
24
31
40
40
40
It is the fluoroaliphatic branches on the FC-742 fluoropolymer that are exposed on the
surface of coatings and films (Fig. 2). FC-742 has the lowest surface energy
(0.0106N/m, or 10.6 dynes/cm) of any available conformal coating. This value
approaches the theoretical low of 0.006N/m (6 dynes/cm) for a perfluoromethyl
surface. FC-742 prevents the spread of all known lubricants at room temperature.
Table 2 compares the critical surface tension of FC-742 with that of other polymers.
CF3
|
CF2
|
CF2
|
C=O
|
O
|
(~C―CH2~)n
|
CH3
Fig. 2
Certonal FC-742 Polymer structure
Critical surface tension defines the wettability of a solid surface by noting the lowest
surface tension a liquid can have and still exhibit a contact angle greater than 0° on
that solid. This is an inverse measure of spreadability or wettability: The larger the
angle, the less the surface is wetted. Certonal FC-742 has a contact angle of 112° with
water and 74° with hexadecane.
When in contact with a substrate, and especially after an annealing treatment at 100°C
(212°F) for 15 min, certain fluoropolymers rearrange their microstructures to
minimise the difference between the surface energy of the coating (at the substrate
interface) and the surface energy of the substrate (at the coating interface). The
decrease in the surface energy difference allows good adhesion with such substrates
as copper, glass, epoxy, and polytetrafluoroethylene (PTFE)-impregnated woven
fibreglass.
The solubility parameter of fluoropolymers is reported to be 11.4(J/cm3)0.5, or
5.59(cal/cm3)0.5. The closest solubility parameter, at 11.6(J/cm3)0.5, or 5.68(cal/cm3)0.5,
is achieved with the following formulation: 2% fluoropolymer and 98% HFE-7100.
The same formulation gives a smooth, uniform, highly oil repellent coating. Oil
repellency has been tested against polyol ester-diester instrument oil, chlorophenyl
polysiloxane, and bis(2-ethylhexyl) sebacate, in 19-h immersion tests at 100°C
(212°F). After testing, pitting and roughening of the surface of the coating could be
detected but only with a scanning electron microscope.
A study performed by Christou, Griffith and Wilkins in 1978 (ref 4) utilised
interdigitated test structures in order to determine the migrative resistance short
phenomena in fluoropolymer encapsulated hybrids. The results showed that for a
surface ion contamination level of 1014 Na+/cm2 on the encapsulant surface, a relative
humidity of 95% and a temperature cycle from 100 to -10°C (212 to 14°F), migrative
resistance shorts were not observed after 100 cycles of a total duration of 4-h per
cycle. Their similar test with silicone materials resulted in migrative resistance short
formation after eight cycles.
Stability
As can be seen from the above properties, fluoropolymers are extremely benign
materials exhibiting strong thermal stability and chemical resistance. Their high
dielectric strengths and resistance makes them superior insulating properties. The
stability of these materials lies in the strength of the C-F bond, discussed previously,
which is the strongest single bond with carbon. The strength of the bond is attributed
to the effective shielding of the carbon centre by fluorine, but the strong C-F bond
also contributes to the unreactiveness of the alkyl CF3 group towards F- displacement
or hydrolysis. The CF3-CF3 bond is 10kcal mol-1 stronger than the CH3-CH3 bond.
The thermal stability of fluoropolymers is only decreased by the relative strengths of
their C-C bonds, which decrease with chain length.
These properties make fluoropolymers highly suited to conformal coating applications
where long term stability in humid, temperature cycling environments is a prerequisite. Due to the strength of the C-F bond present in fluoropolymers, such as
Certonal FC-742, and their electrical characteristics they are highly suited to long
term protection of circuit boards.
Chemical Reactions
In general, fluoropolymer conformal coatings are unreactive, and the details of the
manufacture of specific fluoropolymers are considered trade secrets.
Thermal degradation studies performed on Certonal FC-742 concluded that the
degradation is a two-stage, thermal bond breaking phenomena not requiring additional
reactants such as oxygen or water vapour. One stage of the two-stage degradation
begins at approximately 140°C (285°F) and is almost complete at 200°C (390°F). The
other stage begins at approximately 220°C (430°F) and is complete at 320°C (610°F).
The product of degradation is assumed to be the monomer from which the polymer
was made.
Application Methods
Fluoroacrylate solution polymers can be used as received, or diluted to a 0.1 to 5%
solids content with fluorochemical or other suitable solvents. A part to be coated can
be dipped into the solution and withdrawn at constant velocities, ranging from 1 to
55mm/s (0.04 to 2.2in./s) to achieve uniform coverage. For example, the critical speed
for a concentration of 0.2% is approximately 13mm/s (0.50 in./s) for rectangular PdAg coupons.
It should be noted that different substrates will result in different values of adhesion of
the coating fluoropolymer. Conformal coatings adhere best to clean, moisture free
surfaces.
The coating can also be applied by using a small brush to paint the fluoropolymer on
surfaces or by pouring the coating onto a horizontal substrate and allowing the solvent
to evaporate.
Certonal FC-742 can be removed by immersion for 5 min in Fluorinert FC-77 vapour
at 97°C (205°F). Agitation may be required to remove aged films. In addition a
component may be removed by applying a hot soldering iron through the coating until
the solder flows. The circuits may be recoated after repair.
Acknowledgements
1. S.Y. Lee and J.R. Griffith, Fluoroepoxy Resin for Moisture Vapour Barrier
Coating and Other Applications, Ind. Eng. Chem. Prod. Res. Dev., Dec 1986,
p 572-577
2. B.J. Kinzig and H. Ravner, “Surface Chemistry of Fluoropolymer Barrier Film
Relation of Solution Variables to Film Properties,” Report 8156, U.S. Naval
Research Laboratory, 15 Nov 1977
3. A. Christou, J.R. Griffith, and B.R. Wilkins, Reliability of Hybrid
Encapsulation Based on Fluorinated Polymeric Materials, IEEE Trans,
Electron Devices, Vol ED-26 (No. 1), Jan 1979, p 77-83
4. H.G. Tomkins, Degradation of Thin Films of a Fluoropolymer, J. Appl.
Polymer Sci., Vol 17, 1973, p 3795-3807
5. H.G. Tomkins and S.P. Sharma, On the Thickness and Spatial Distribution of
a Fluoropolymer Film Deposited by Solutions Dipping, J. Appl. Polymer Sci.,
Vol 25 (No. 2), Feb 1980, p 211-222
6. R.E.Banks et al, “Organofluorine Chemistry – Principles and Commercial
Applications” 1994.
7. C. Kreider, “Fluoropolymer Coatings – Electronic Materials Handbook –
Volume 1” 1991.