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 that contain , either in the backbone or side chains of the . 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 | CF 3 | CF 3 | | | HO – C – – C – OH | | CF 3 CF 3

Fig. 1 1-perfluoroalkyl-3,5-bi(2-hydroxyhexafluoro-2-propyl) benzene where R1 = H-C6-fluorodiepoxide, or R1 = C 6H13 :C 6- fluorodiepoxide, or R1 = C 8H17 :C 8-fluorodiepoxide.

Available materials

Certonal FC742 is a single component fluorochemical acrylic coating supplied in a dilution of methyl nonafluorobutyl ether and methyl nonafluoroisobutyl ether (C 4F9OCH 3) (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 C 4F9 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 non- wetting 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 Thin liquid (2% solids) Appearance Clear to light coloured liquid Specific gravity at 25°C (77°F) 1.5 Flashpoint (by Setaflash closed cup), °C (°F) None Thermal stability of dry film Repellent after 24h at 175°C (350°F) Surface energy of dry film 11 dynes/cm Thickness of dry film, dip coated, ~ 1 micron Refractive Index 1.36 Dielectric constant @ 25°C, 30% RH 3.1 Dissipation Factor at 1kHz 25°C, 30% RH 0.0089 Dielectric strength, 25°C, 35% RH 1.1kV (25.4 microns) Volume resistivity, 25°C, 35% RH 4.6 x 10 12 40v DC (ohm-cm)

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

Typical values Material Typical values N/m Dynes/cm acrylates 0.011-0.014 11-14 (Teflon) 0.018-0.020 18-20 Silicone 0.024 24 Polyethylene 0.031 31 Epoxide 0.040 40 Urethane 0.040 40 Acrylate 0.040 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.

CF 3 | CF 2 | CF 2 | C=O | O | (~C ―CH 2~) n | CH 3

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/cm 3)0.5 , or 5.59(cal/cm 3)0.5 . The closest solubility parameter, at 11.6(J/cm 3)0.5 , or 5.68(cal/cm 3)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 10 14 Na +/cm 2 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 CF 3 group towards F displacement -1 or hydrolysis. The CF 3-CF 3 bond is 10kcal mol stronger than the CH 3-CH 3 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 pre- requisite. 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 Pd- Ag 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, “ – Principles and Commercial Applications” 1994. 7. C. Kreider, “Fluoropolymer Coatings – Electronic Materials Handbook – Volume 1” 1991.