Conformal coating, composition and method for the mitigation of growth of metallic crystalline structures

Abstract

A nanocomposite coating composition for use in the mitigation of whisker growth from a metallic surface (82) includes a polymer matrix (86) comprising a base polymer and insulating material nanoplatelets (85), for example clay nanoplatelets, within the polymer matrix (86). A conformal coating (84) for application to a metal surface (82) is formed from the coating composition. The conformal coating mitigates the spontaneous growth of whiskers (83), in particular tin whiskers, from the coated surface (82), reducing the risk of short-circuits caused by such whiskers bridging gaps within electronic devices. Methods are provided for the preparation of coating compositions and coatings.

Claims

1. A method of mitigating whisker growth from a metallic surface, the method comprising: applying a coating composition to at least part of the metallic surface, wherein the coating composition comprises: a polymer matrix comprising a base polymer, wherein the base polymer is selected from one or more of polyethylene, polypropylene, polystyrene, polyacrylic acid, polyurethanes, acrylics, silicones, paralenes, epoxies, and polyamides, and insulating material nanoplatelets within the polymer matrix, wherein the insulating material nanoplatelets include a layered silicate clay that is a 2:1 clay, and the nanoplatelets have an average aspect ratio between 200 and 500.

2. The method according to claim 1, wherein the layered silicate clay is a smectite group clay.

3. The method according to claim 1, wherein the layered silicate clay comprises a clay selected from montmorillonite, hectorite and saponite.

4. The method according to claim 1, wherein the layered silicate clay is montmorillonite.

5. The method according to claim 1, wherein the insulating material nanoplatelets have a length of at least 50 nm.

6. The method according to claim 1, wherein the insulating material nanoplatelets have a length of up to 1000 nm.

7. The method according to claim 1, wherein the insulating material nanoplatelets have a length of up to 700 nm.

8. The method according to claim 7, wherein the insulating material nanoplatelets have a length of up to 400 nm.

9. The method according to claim 1, wherein the surface of the insulating material nanoplatelets is functionalised with one or more polar or non-polar functional groups.

10. The method according to claim 9, wherein the base polymer is a non-polar polymer and the insulating material nanoplatelet surfaces are functionalised with one or more non-polar functional groups.

11. The method according to claim 9, wherein the base polymer is selected from one or more of polyethylene, polypropylene and polystyrene.

12. The method according to claim 11, wherein the one or more non-polar functional groups are selected from one or more of linear or branched alkyl, cycloalkyl, linear or branched alkenyl, cycloalkenyl, linear or branched alkynyl, aryl or aralkyl.

13. The method according to claim 9, wherein the base polymer is a polar polymer and the insulating material nanoplatelet surfaces are functionalised with one or more polar functional groups.

14. The method according to claim 13, wherein the base polymer is selected from one or more of polyacrylic acid, polyurethanes, acrylics, silicones, paralenes, epoxies and polyamides.

15. The method according to claim 13, wherein the polar functional group is selected from one or more of OH, OR, NH.sub.2, NHR, NR.sub.2, NO.sub.2, F, Cl, Br, I, CN, COOH and COOR, wherein R is selected from linear or branched alkyl, phenyl or benzyl or tallow.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows SEM images of tin whiskers emerging from a metallic surface.

(2) FIG. 2 shows SEM images of tin whiskers growing through a conformal coating.

(3) FIG. 3 shows further SEM images of tin whiskers growing through a conformal coating.

(4) FIG. 4 shows a further SEM image of tin whiskers growing through a conformal coating.

(5) FIG. 5 shows further SEM images of tin whiskers growing through conformal coatings and demonstrates the effect of coating stiffness on the ability of a whisker to penetrate through it (a) very flexible, (b) moderate stiffness and (c) very rigid.

(6) FIG. 6 is a schematic diagram showing the intercalation and exfoliation processes for layered silicate clays.

(7) FIG. 7 is an idealized diagram of a substrate and metallic surface with a clay nanoplatelet containing conformal coating applied.

(8) FIG. 8 is a schematic diagram showing the structure of montmorillonite, a layered silicate clay.

(9) FIG. 9 is an X-ray diffraction pattern for 2 wt % Cloisite 15A-urethane polymer composite material formed by direct clay addition to the polymer.

(10) FIG. 10 is a transmission electron micrograph showing well-dispersed Cloisite 15A nanoclay platelets in a 2 wt % 15A-urethane polymer composite.

(11) FIG. 11 is a TEM image showing the distribution and orientation of nanoplatelets in a 2 wt % 15A-acrylic polymer nanocomposite film prepared by dip coating.

(12) FIG. 12 is a graph comparing whisker growth for unmodified and 3 wt % 15A modified acrylic polymer coatings after oven storage at 55 C. and 85% relative humidity.

(13) FIG. 13 shows scanning electron micrographs comparing the typical whisker size and distribution on the surface of coated 2 m Sn deposits on brass (a) a 3 wt % 15A modified acrylic polymer conformal coating and (b) an unmodified acrylic polymer conformal coating. Samples tilted at an angle of 70 for observation.

(14) FIG. 14 is a bar chart comparing whisker density for 2 m Sn deposits on brass coated with unmodified acrylic polymer (hatched bars) and 3 wt % 15A modified (solid bars) acrylic polymer conformal coatings after storage at a combination of room temperature and 55 C./85% humidity for approximately 15 weeks. Estimated coating thickness is also shown for (a) the unmodified acrylic polymer coating and (b) the 3 wt % 15A modified acrylic polymer coating.

DETAILED DESCRIPTION

(15) Any sub-titles herein are included for convenience only, and are not to be construed as limiting the disclosure in any way.

(16) The invention will now be further described with reference to the following non-limiting Figures and Examples. Other embodiments of the invention will occur to those skilled in the art in the light of these.

(17) The disclosure of all references cited herein, inasmuch as it may be used by those skilled in the art to carry out the invention, is hereby specifically incorporated herein by cross-reference.

(18) Method of Producing a Coating Composition

(19) FIG. 6 depicts a schematic representation of a process according to the present invention for the production of a coating composition which is suitable for use as a conformal coating on a surface prone to whisker growth.

(20) A layered silicate clay 71 is made up of individual layers 72. The clay may be a 1:1 clay or a 2:1 clay. Where the clay is a 1:1 clay, each layer 72 includes a tetrahedral sheet and an octahedral sheet. Where the clay is a 2:1 clay, each layer 72 includes an octahedral sheet sandwiched between two tetrahedral sheets.

(21) The layered clay in FIG. 6 consists of individual bundles or tactoids, of which three are shown in FIG. 6. The layers 72 within each individual tactoid are all parallel with each other. However, the tactoids themselves may not be parallel with each other, as shown in the figure.

(22) Counter cations (not shown) are intercalated between layers 72. These may be any suitable cation, for example alkali metal cations, alkaline earth metal cations or transition metal cations.

(23) The surfaces of the clay layers, especially those internal surfaces adjacent to the cations, may be functionalised with organic functional groups. These may be polar or non-polar.

(24) The first step of the process involves mixing the layered clay, in particulate form, into a solution of base polymer 73. The base polymer 73 may be a single polymer or copolymer, or a mixture of polymers and/or copolymers. This step provides an initial phase-separated microcomposite as shown in FIG. 6(a). At this stage, polymer chains have not yet moved into the space between clay layers.

(25) Once the clay 71 has been added to the polymer solution, shearing forces are applied with a mixer using a suitable blade or paddle to promote the interactions between the clay 71 and the polymer 73, primarily through hydrogen bonding. To further promote such interactions, a compatible clay/polymer combination is chosen. For example, where a clay functionalised with polar organic groups is used, a corresponding polar polymer or mixture of polar polymers will be used. Alternatively a clay functionalised with non-polar organic functional groups may be used, in which case a corresponding non-polar polymer or mixture of non-polar polymers is selected.

(26) The polymer 73 then becomes intercalated between layers 72 of the clay tactoid, in a process known as intercalation. The inter-layer spacing of the clay (the distance between two adjacent layers) increases and the gallery 74 widens. At this stage the individual layers 75 are still associated with each other and are not yet distinct. The tactoid may become intercalated equally and evenly, or in a random arrangement, as shown in FIG. 6(b). The result in each case is an intercalated nanocomposite.

(27) Prior to, or as an alternative to, attaining a fully exfoliated morphology a nanocomposite comprising fully intercalated nanoplatelets 76, partially intercalated nanoplatelets 77 and discrete exfoliated nanoplatelets 78 may be achieved, as shown in FIG. 6(c).

(28) As intercalation by the polymer continues, the forces holding adjacent layers of clay together weaken and the gallery 74 widens further. Eventually, exfoliation of the clay occurs and the layers become completely separate forming discrete nanoplatelets comprised of a single clay layer 79 dispersed within a polymer matrix 80. A fully exfoliated nanocomposite is produced, as shown in FIG. 6(d).

(29) Once the nanoplatelets are fully exfoliated they lose their regular structure that is apparent following intercalation. Depending on the application method used, they will retain some orientation or alignment. However, more importantly the nanoplatelets will present a significant degree of overlap meaning that whisker mitigation is more likely.

(30) Conformal Coating

(31) FIG. 7 shows the coating composition from FIG. 6(c) applied as a conformal coating to a substrate.

(32) The substrate 81 may be any part of an electrical device upon which a metallic layer is deposited. In FIG. 7, PCB 81 has a tin outer layer 82, which is magnified in the upper part of the Figure. Tin whiskers 83 grow spontaneously from the outer surface of the tin layer, and may cause problems such as short circuits and device failure.

(33) The conformal coating 84 is applied to the tin outer layer 82. The conformal coating is made from the nanocomposite shown in FIG. 6 including the polymer matrix 86 and clay nanoplatelets 85 embedded therein. The coating is applied to the tin outer surface by dip (immersion) coating, but may also be applied by spray coating or brush coating. Ideally the nanoplatelets are fully exfoliated and oriented within planes substantially parallel to the surface of the conformal coating (and therefore also parallel with the surface of tin outer layer 82). The nanoplatelets 85 therefore form a highly effective physical barrier to growth of tin whiskers 83 through the conformal coating 84. As a result, tin whisker growth is mitigated and the device is effectively protected. Furthermore, the physical characteristics required of a conformal coating are not compromised.

(34) As shown in FIG. 7, the orientation of the nanoplatelets in the coating 84 is somewhat random in that not all nanoplatelets lie parallel to the same plane. The nanoplatelets retain some orientation from the method of coating application. The nanoplatelets present an overlapping structure which presents an effective barrier to the tin whiskers.

(35) FIG. 8(a) shows the chemical structure of montmorillonite, a layered silicate clay which may be used to form the nanocomposite coating composition of the present invention.

(36) Montmorillonite is a 2:1 clay consisting of clay layers 90 separated by the inter-layer gallery 91. Each layer is comprised of two silica tetrahedra fused to an edge-shared octahedral sheet of either aluminium or magnesium hydroxide. Isomorphic substitution within the layers generates negative charges that are normally counterbalanced by sodium ions 94 intercalated between any two adjacent layers, within the gallery.

(37) The inter-layer spacing 95 is 1.17 nm. The clay is also characterised by layer thickness 96 and basal spacing 97.

(38) The thickness of the 2:1 layer 96 is approximately 0.94-0.96 nm. The size of the inter-layer space 95 is dependent on the radius of the counterbalancing cation, the negative charge density in the layers and hydration of the counterbalancing cation, which is in the range of 0.02-0.25 nm. Hence, the basal spacing 97 is between 0.96 nm and 1.21 nm.

(39) FIG. 8(b) shows an exfoliated nanoplatelet formed from a montmorillonite clay by exfoliation with a polymer. The nanoplatelet in FIG. 8(b) has a thickness of 1 nm. The length of the nanoplatelet depends upon clay selection. FIG. 8(c) shows another representation of the layered clay structure. The clay surfaces are functionalised with hydroxy groups, some of which are deprotonated. To balance the overall charge of the clay, sodium ions are present within the gallery between the layers. The functionalisation allows for dispersion into and miscibility with the polymer matrix.

(40) As a result, the nanoplatelets produced from the clay are also functionalised with polar hydroxy functional groups. This is achieved by using a pre-functionalised clay.

EXAMPLES

(41) Clay Compositions

(42) A number of clays were used to produce nanocomposite materials. Table 1 below summarises the clays used.

(43) TABLE-US-00001 TABLE 1 Typical Dry d.sub.001 (X-ray Particle Size Packed Bulk Density/ results)/ Clay Moisture/% (d.sub.50)/m Colour Density/g/l g/cm nm Cloisite <2 <10 Off 299 1.66 3.15 15A white Cloisite <3 <10 Off 365 1.98 1.85 30B white

(44) The clays were obtained from BYK Additives & Instruments.

(45) Cloisite 15A is dimethyl, dihydrogenated tallow quaternary ammonium with bentonite. Cloisite 30B is alkyl quaternary ammonium salt bentonite.

(46) The clays are surface modified with an organic chemistry to allow complete dispersion into and provide miscibility with the polymer matrix systems such as Cloisite 15A: 2M2HT, dimethyl dihydrogenated tallow onium ion, d-spacing: 3.15 nm, where HT is Hydrogenated Tallow (65% C18; 30% C16; 5% C14) and Cloisite 30B: MT2EtOH methyl tallow bis-(2-hydroxyethyl) alkyl quaternary ammonium chloride. D-spacing=1.85 nm, where T is Tallow (65% C18; 30% C16; 5% C14).

(47) Depending on the characteristics of the polymers used (e.g. polar or non-polar, the types of polar groups in the polymer chains), different nanoclays and organic modified clays were added into the polymer solutions (coatings) to achieve the exfoliation by applying shearing forces to promote the interactions between the clays and polymers mainly through hydrogen bonds.

Example 1: Direct Addition of Nanoclay (15A) to Urethane Based Polymer

(48) 0.6 g of Cloisite 15A nanoclay was added to 50 g of urethane based polymer, with a solids content of 40%, and mixed using an overhead stirrer fitted with a high shear mixing blade for 90 min at 750 rpm. The resultant cured polymer-nanoclay composite had a nanoclay content of 3 wt %.

Example 1A: Direct Addition of Nanoclay (15A) to Urethane Based Polymer

(49) 0.4 g of Cloisite 15A nanoclay was added to 50 g of urethane based polymer, with a solids content of 40%, and mixed using an overhead stirrer fitted with a high shear mixing blade for 90 min at 750 rpm. After curing at 80 C. in an oven for 1 hour, the resultant cured polymer-nanoclay composite had a nanoclay content of approximately 2 wt %.

(50) X-ray diffraction (XRD) measurements on the cured composite material (an example of which is shown in FIG. 9) demonstrated that the interlayer spacing of the nanoclay had increased in the composite relative to that of the clay (4.07 nm compared with 3.153 nm), which indicates that intercalation of the clay by the polymer has occurred and a nanocomposite material has been formed. This was confirmed by transmission electron microscope (TEM) investigations (FIG. 10) which also show that the clay nanoplatelets are well dispersed in the composite material.

(51) Although stable polymer-clay mixes can be achieved for loadings up to approximately 2 wt %, mixes with higher clay loadings were shown to be unstable and liable to sedimentation. The stability of the polymer-nanoclay mixture can be improved for higher clay loadings by pre-swelling in a suitable solvent.

Example 2: Pre-Swell Nanoclay (15A) in Solvent (Xylene) Prior to Adding to Urethane Based Polymer

(52) The first stage was to pre-swell the nanoclay in solvent. 0.9 g of Cloisite 15A nanoclay was added gradually to 20 g of xylene at a rate of 0.1 g/30 min. Subsequently, the pre-swelled clay mixture was added to 60 g of urethane polymer, with a solids content of 50%, and mixed using an overhead stirrer fitted with a high shear mixing blade for 90 min at 750 rpm. The resultant cured polymer-nanoclay composite had a nanoclay content of 3 wt %.

Example 2A: Pre-Swell Nanoclay (15A) in Solvent (Xylene) Prior to Adding to Urethane Based Polymer

(53) The first stage was to pre-swell the nanoclay in solvent. 0.9 g of Cloisite 15A nanoclay was added gradually to 20 g of xylene at a rate of 0.1 g/30 min. Subsequently, the pre-swelled clay mixture was added to 60 g of urethane-based polymer, with a solids content of 50%, and mixed using an overhead stirrer fitted with a high shear mixing blade for 90 min at 750 rpm. After curing at 80 C. in an oven for 1 hour, the resultant cured polymer-nanoclay composite had a nanoclay content of approximately 3 wt %.

(54) After mixing the composite mixture was cast into approximately 10 cm10 cm sheets and test samples were prepared to investigate the mechanical properties of the 15A nanoclay modified composite material. Dynamic mechanical analysis (DMA) was performed on the cured material using a TA Instruments Q800 DMA system operating in dual cantilever mode. Storage modulus values at a range of temperatures are shown in Table 2 for a 3 wt % 15A nanoclay composite material and compared with values obtained from unmodified urethane control samples. The results clearly demonstrate that the clay additions bring about a significant increase in the storage modulus of the material.

(55) TABLE-US-00002 TABLE 2 Comparison of storage modulus values obtained from unmodified and 3 wt % 15A modified urethane materials at different temperatures Test Average Storage modulus (MPa) temperature Unmodified Urethane modified % change in ( C.) urethane with 3 wt % 15A modulus 30 581 41 886 122 +52% 40 280 9 466 46 +66% 50 78 5 174 13 +123%

(56) Further improvements in mechanical properties were achieved by increasing the clay content in the composite. This is demonstrated in Table 3, which shows data obtained for a 7 wt % 15A nanoclay-urethane composite material, fabricated in the same manner as the 3 wt % 15A material described above.

(57) TABLE-US-00003 TABLE 3 Comparison of storage modulus values obtained from unmodified and 7 wt % 15A modified urethane materials at different temperatures Test Average Storage modulus (MPa) temperature Unmodified Urethane modified % change in ( C.) urethane with 7 wt % 15A modulus 30 769 188 1836 361 +139% 40 451 122 1123 164 +149% 50 160 39 479 61 +199% 60 42 11 151 55 +260%

Example 3: Addition of Nanoclay (30B) to Acrylic Based Polymer with the Incorporation of Ultrasound

(58) In this method, sonication was used to improve dispersion of the nanoclay, break up agglomerated particulates and potentially enhance intercalation/exfoliation. 0.9 g of Cloisite 30B nanoclay was added to 100 g of an acrylic based polymer with a solids content of 30%. This was then mixed for 30 min at 500 rpm using an overhead stirrer fitted with a high shear mixing blade to uniformly distribute the clay within the polymer. After mixing, sonication was then applied using a Misonix Sonicator 4000. Sonication was performed in pulsed mode using an amplitude of 25% with a 10 s off and 10 s on pulse duration. The total sonication time was 20 min. After sonication, the polymer-nanoclay mixture was further mixed using an overhead stirrer fitted with a high shear mixing blade for 30 min at 500 rpm. After processing, the solids content of the polymer-nanoclay mixture had increased to 34% due to solvent evaporating during mixing and sonication. The resultant cured polymer-nanoclay composite has a nanoclay content of 3 wt %.

Example 3A: Addition of Pre-Swollen Nanoclay (30B) to Acrylic Based Polymer with the Incorporation of Ultrasound

(59) In this method, sonication was used to improve dispersion of the nanoclay, break up agglomerated particulates and potentially enhance intercalation/exfoliation. 0.9 g of Cloisite 30B nanoclay was initially pre-swollen in 10 g of solvent in 0.1 g steps. This was subsequently added to 100 g of an acrylic based polymer with a solids content of 30%. This was then mixed for 30 min at 500 rpm using an overhead stirrer fitted with a high shear mixing blade to uniformly distribute the clay within the polymer. After mixing, sonication was then applied using a Misonix Sonicator 4000. Sonication was performed in pulsed mode using an amplitude of 25% with a 10 s off and 10 s on pulse duration. The total sonication time was 20 min. After sonication, the polymer-nanoclay mixture was further mixed using an overhead stirrer fitted with a high shear mixing blade for 30 min at 500 rpm. After processing, the solids content of the polymer-nanoclay mixture had increased to 34% due to solvent evaporating during mixing and sonication. After curing at 80 C. in an oven for 1 hour, the resultant cured polymer-nanoclay composite has a nanoclay content of approximately 3 wt %.

(60) Composite materials prepared using this method demonstrated significantly improved properties compared with the unmodified base material. Table 4 shows DMA test data for a 3 wt % 30B modified acrylic material. It is evident that the addition of the nanoclay resulted in a significant improvement in storage modulus.

(61) TABLE-US-00004 TABLE 4 Comparison of storage modulus values obtained from unmodified and 3 wt % 30B modified acrylic materials at different temperatures Test Average Storage modulus (MPa) temperature Unmodified Acrylic modified % change in ( C.) acrylic with 3 wt % 30B modulus 30 1222 166 2067 202 +69% 40 1282 138 2248 94 +75% 50 599 107 1117 58 +86%

Example 4: Orientation of Nanoplatelets within the Nanocomposite Film

(62) The Orientation of the Nanoplatelets with Respect to the Coating Surface was Investigated using transmission electron microscopy (TEM). A nanocomposite-acrylic mixture having a content of 2 wt % 15A was initially prepared by pre-swelling the clay in xylene and subsequently incorporating into an acrylic polymer using an overhead stirrer, equipped with a high shear mixing blade, at 750 rpm for 1 h. A nanocomposite film was then prepared using a dip coating method. To facilitate identification of the surface of the coating in the TEM, a thin film of Au/Pd was sputter deposited onto the surface of the nanocomposite. Samples of the resultant film were then cross-sectioned using an ultra-microtome to produce samples suitable for TEM analysis. Results show that the 15A clay is well dispersed within the polymer matrix (FIG. 11). Furthermore, analyses suggest that the clay nanoplatelets are, to a certain extent, oriented in a direction that is noticeably aligned relative to the surface of the coating (the orientation of which is indicated in FIG. 11). Table 5 shows the percentage of clay nanoplatelets that deviate by a given angle () from the surface of the nanocomposite film. Results indicate that approximately 60% of the nanoplatelets are aligned at an angle of less than 10 relative to the surface of the coating and over 80% of the nanoplatelets are aligned at an angle of less than 30 relative to the surface of the coating.

(63) TABLE-US-00005 TABLE 5 The percentage of clay nanoplatelets that deviate by a given angle () from the surface of the nanocomposite film Deviation angle from plane of coating surface () Nanoplatelets (%) <10 61 <20 75 <30 83 <40 86 <50 90 <60 94 <70 96 <80 97 <90 100

Example 5: Addition of Pre-Swollen Nanoclay (15A) to Acrylic Based Polymer

(64) 15A nanoclay-acrylic polymers were fabricated utilising the pre-swelling technique outlined in Examples 2 and 2A. For example, 0.9 g of 15A nanoclay was pre-swollen in 40 g of xylene and subsequently added to 60 g of acrylic polymer with a solids content of 50%. The mixture was then stirred using an overhead mixer fitted with a high shear mixing blade at 750 rpm for 1 h. After curing at 80 C. in an oven for 1 hour, the resultant cured polymer-nanoclay composite has a nanoclay content of approximately 3 wt %.

(65) Table 6 shows the effect on average storage modulus of 3 wt % 15A additions to an acrylic coating material. The results show that storage modulus is significantly improved by the addition of 3 wt % 15A nanoclay.

(66) TABLE-US-00006 TABLE 6 Comparison of storage modulus values obtained from unmodified and 3 wt % 15A modified acrylic materials at different temperatures Test Average Storage modulus (MPa) temperature Unmodified Acrylic modified % change in ( C.) acrylic with 3 wt % 15A modulus 30 572 45 876 105 +53% 40 152 28 255 30 +67% 50 51 9 85 11 +65%

(67) The effect of clay content on the mechanical properties of nanoclay modified coating material has been further investigated by a programme of tensile testing. For these trials acrylic polymers with clay loadings of 3 wt %, 5 wt % and 7 wt % were investigated. Average values for Young's modulus, yield stress and % elongation at failure as a function of clay content are shown in Table 7.

(68) TABLE-US-00007 TABLE 7 The effect of clay content on the mechanical properties of 15A modified acrylic coatings 3 wt % 15A 5 wt % 15A 7 wt % 15A Unmodified modified modified modified Property acrylic acrylic acrylic acrylic Young's modulus 149 17 165 31 187 30 216 13 (MPa) Yield stress 2.63 0.15 3.02 0.14 3.45 0.14 4.03 0.17 (MPa) % elongation 422 8 417 16 425 5 399 6 at failure

(69) Results show that there is a gradual increase in both Young's modulus and yield stress as the clay content is increased. Importantly, results indicate that there is no significant loss in ductility even for a clay loading of 7 wt %.

(70) Evaluation of Whisker Mitigation

(71) To evaluate the effect of the clay additions on whisker growth, test samples were prepared by spray coating unmodified and 3 wt % 15A acrylic polymer modified coatings onto brass substrates electrodeposited with 2 m of bright tin. After coating, the samples were placed into an oven at 55 C. and 85% relative humidity to promote whisker growth. The extent of whisker growth has been evaluated after different periods of storage at elevated temperature and humidity. Whisker growth was first evaluated after 90 days in the oven. Test samples were analysed using a binocular microscope with the samples tilted at a steep angle to observe features protruding from the deposit surface. Further analyses were subsequently carried out at approximately monthly intervals. The result of these analyses is shown in FIG. 12.

(72) The data clearly demonstrates that whisker growth/tenting is considerably reduced for the samples coated with the 3 wt % 15A acrylic polymer modified material, i.e. measured whisker growth for the samples coated with the nanoclay modified conformal coating is reduced to a quarter of that of the unmodified material. Following the initial analysis whisker density does not appear to increase for either the modified or unmodified coatings.

(73) These results are supported by scanning electron microscope (SEM) observations (FIGS. 13(a) and (b)), which demonstrate that whisker growth is generally reduced for samples with the modified coating (SEM analysis was carried out after approximately 14 weeks storage at 55 C./85% relative humidity).

(74) Subsequent trials, carried out on a second batch of test samples to substantiate these results, also demonstrated reduced whisker growth for 3 wt % 15A acrylic polymer modified conformal coatings. In this instance whisker counting was carried out twice for each sample and an average value obtained for whisker density. Three test samples were evaluated for the modified and unmodified coatings. The results of this analysis are shown in FIG. 14. The graph also shows an estimate of the average coating thickness in the middle of the test sample that has been evaluated from two equivalent test samples, marked as (a) for the unmodified coating and (b) for the modified coating (this demonstrates that the thickness of the 3 wt % 15A and modified and unmodified acrylic polymer coatings are comparable, at around 46 m and 42 m respectively).

(75) Results from the second batch of whisker growth samples confirm that the addition of 3 wt % 15A nanoclay to the acrylic polymer results in a significant reduction in whisker growth.