Multi-phase coatings for inhibiting tin whisker growth and methods of making and using the same

Description of equivalent WO 2008027856 A2

0001MULTI-PHASE COATINGS FOR INHIBITING TIN WHISKER GROWTH AND METHODS OF MAKING AND USING THE SAME

CROSS-REFERENCES TO RELATED APPLICATIONS

0003[0001] This application claims the benefit of U.S. Provisional Application No. 60/811,609, filed June 7, 2006.

TECHNICAL FIELD

0005[0002] The present invention relates to new or refurbished electronic assemblies or assembly components that may have a metal plating or finish, and more particularly to such assemblies or components having a tin plating or finish.

BACKGROUND

0007[0003] Electronic assemblies or assembly components are often plated or finished with a metal. Printed wiring boards and electrical leads are just some examples of many components that typically have a metal finish. Perhaps the most abundant metal composition for a plating or a finish has been lead/tin (PbSn). However, laws and directives recently passed in several countries encourage or require the elimination of lead by those procuring, designing, building, or repairing electronic assemblies. The restriction of lead use has generated a transition by many piece part and board suppliers from PbSn surface finishes to lead-free finishes such as pure tin.

0008[0004] Tin finishes may be susceptible to spontaneous growth of single crystal structures known as tin whiskers. Tin whiskers are cylindrical, needle-like crystals that may grow either straight or kinked, and usually have a longitudinally striated surface. Growth rates for tin whiskers vary, although rates from 0.03 to 9 mm/yr have been reported. Interrelated factors including substrate materials, grain structure, plating chemistry, and plating thickness may influence growth rate. Although the whisker length depends on growth rate and sustained periods of growth, in experimental tests most measure between 0.5 and 5.0 mm although whiskers having a length of more than to 10 mm have been reported. The growth mechanisms for tin whiskers are largely unknown, although it is widely believed that whisker formation and growth are correlated with stresses such as localized compressive forces and environmental stresses on the tin plating or finish. Additional factors that may influence tin whisker growth include the materials constituting the substrate underlying the tin, and specifically a significant difference in the coefficients of thermal expansion between tin and the underlying substrate material since such a difference may stress the tin.

0009[0005] Tin whiskers may cause electrical failures ranging from performance degradation to short circuits. In some cases, the elongate structures have interfered with sensitive optical surfaces or the movement of micro-electromechanical systems (MEMS). Thus, tin whiskers are a potential reliability hazard. It is therefore desirable to provide materials and manufacturing procedures that mitigate the tendencies of pure tin and tin-containing solders, platings, and finishes to form tin whiskers. It is also desirable to provide such materials and methods that minimize the use of lead-containing compositions such as Pb/Sn solder.

BRIEF SUMMARY

0011[0006] The present invention provides an electrical component, comprising a conductive substrate, a tin layer formed on the substrate, and a multi-phase coating formed on the tin layer to impede tin whisker growth. The multi-phase coating comprises a polymer matrix having pores dispersed therethrough, with the pores constituting at least 30% by volume of the coating. The pores may be empty, or filled with a material that is different than the matrix material

0012[0007] The present invention also provides a method for impeding tin whisker growth from a tin plating or finish formed over an electrical component using the multi-phase coating. The tin plating or finish is covered with a coating comprising a polymer matrix having a second material mixed therein. The second material is subsequently removed from the polymer matrix to produce a porous coating having pores in the matrix where the second material was disposed before being removed. The pores may remain empty, or a material that is different than the matrix material may be used to fill the pores.

BRIEF DESCRIPTION OF THE DRAWINGS

0014[0008] FIG. 1 is a cross-sectional view illustrating an electrical component having a tin plating or finish with a porous coating formed thereon to impede tin whisker growth according to an exemplary embodiment of the invention;

0015[0009] FIG. 2 is a cross-sectional view illustrating an electrical component having a tin plating or finish with a multi-phase coating formed thereon according to an exemplary method for forming a porous coating;

0016[0010] FIG. 3 is a cross-sectional view illustrating the electrical component having a tin plating or finish with a porous coating formed thereon after removing one of the phases of a multiphase coating and thereby forming a porous coating; and

0017[0011] FIG. 4 is a cross-sectional view illustrating an electrical component having a tin plating or finish with a multi-phase coating formed thereon to impede tin whisker growth according to an exemplary embodiment of the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

0018[0012] The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.

0019[0013] Electrical assemblies and components of the present invention have a tin plating or finish, and a porous coating formed around the tin plating or finish. The porous coating is electrically nonconductive, and includes pores that are produced by removing one or more materials or phases from a multi-phase coating. Growth of tin whiskers through the porous coating is inhibited due to the coating's discontinuous structure. More particularly, the tin whiskers have interrupted lateral support within the pores, which causes the tin whiskers to buckle and consequently fail to exit the porous coating.

0020[0014] Turning now to FIG. 1, an electrical component substrate 10 having a tin finish 12 is depicted, with a porous coating 14 formed over the tin finish 12. Just some examples of the substrate 10 include a circuit card assembly, a wiring board, one or more components printed on a wiring board, and one or more conductive leads. The porous coating 14 includes a relatively soft matrix material. Exemplary matrix materials are polymers including urethane, silicone, acrylic, paralenes, and polymers having an epoxy group in the molecule thereof. As previously discussed, conventional conformal coatings that consist of the same matrix materials may be somewhat susceptible to penetration by tin whiskers 18 as illustrated in FIG. 1. For this reason, the coating 14 includes a dispersion of pores 20, inside of which the tin whiskers 18 will buckle instead of growing through the conformal coating 14.

0021[0015] The pores 20 have a sufficient abundance and distribution for the tin whiskers 18 to have a high probability of encountering and entering at least one pore 20 while growing through the conformal coating matrix 15. In other words, any tin whisker 18 growing perpendicularly from any point along the tin finish surface interfacing with the conformal coating 14 has a high probability of entering at least one pore 20. For this reason, an exemplary conformal coating 14 includes multiple "layers" of pores 20, and preferably more than two layers, although the layers simply constitute numerous pores at various depths rather than discrete pore-containing levels. According to an exemplary embodiment, the pores 20 constitute at least 30% of the total coating volume. A conformal coating 14 having a thickness as small as 50 microns may include five to ten layers of pores 20 along any particular cross-section. Depending on the overall coating thickness, relatively large or small pores 20 may be included to provide a high probability for a tin whisker to encounter a pore 20 before pushing through the entire conformal coating thickness. For example, thicker coatings may include pores having an average diameter of up to 40 microns, while thinner coatings may include pores having an average diameter of 5 to 10 microns. As depicted in FIG. 1, one tin whisker 18 may enter a pore 20 that is situated close to the tin finish 12 and will consequently buckle when, upon traversing the pore 20, colliding with the conformal coating matrix 15. Other tin whiskers may grow between some pores disposed closest to the tin finish 12, but will eventually enter a more outwardly disposed pore 20. The pores 20 may be randomly dispersed, and are preferably substantially homogenously dispersed, at a sufficient concentration to provide a high probability for a tin whisker 18 to enter at least one pore 20.

0022[0016] As illustrated in FIG. 1, when a tin whisker 22 enters and traverses a pore 20, the tin whisker 22 buckles and continues growing in a different direction, often folding back on itself, instead of reentering the conformal coating matrix 15. Buckling occurs because the tin whisker portion that is inside a pore 20 lacks sufficient lateral support for the whisker 22 to push back into the conformal coating matrix 15. As a tin whisker 22 collides with the coating matrix after traversing a pore 20, the tin whisker 22 begins to bend and continues to do so until the bending stress produces sufficient counter force to allow the tin whisker 22 to reenter the conformal coating matrix 15. Almost invariably, the tin whisker portion that reenters the conformal coating matrix 15 will be growing in a different direction than that at which the tin whisker 22 entered the pore 20. According to an exemplary embodiment, a significant differential between the lateral support inside and outside the pore 20 is created by selecting a relatively hard conformal coating matrix material. For example, epoxies and paralenes are exemplary relatively hard polymer materials that may be used as the coating matrix 15.

0023[0017] In order for the tin whisker 22 to buckle inside a pore 20 without substantial resistance, the pores 20 preferably have a width that is at least ten times the tin whisker width. For example, if a tin whisker has a width of 3 microns, the pores 20 should have an average width of at least about 30 microns. Since tin whiskers typically have widths of up to about 5 microns, exemplary pores 20 have average widths of at least about 50 microns, although smaller pores may be formed if it is found that the tin whiskers are particularly thin growths. The tin whisker 22 becomes more bendable as it lengthens inside the pore 20. If the tin whisker 22 is too short the coating matrix 15, at the point where the tin whisker 22 entered the pore 20, will provide sufficient lateral support to enable the tin whisker 22 to re-penetrate the coating matrix 15 without buckling.

0024[0018] Turning now to FIGs. 2 to 4, an exemplary method is illustrated for forming the previously-described porous coating. The method is applicable generally to various electrical components that include a tin plating or finish. First, a multiphase coating 14 is formed over a tin plating or finish 12 previously formed on an electrical component substrate 10 such as a circuit card assembly, a wiring board, a component printed on a wiring board, or a conductive lead. The coating 14 may be applied by a variety of application methods, including screen printing, chemical deposition, rapid prototyping, extrusion, and thermal or cold physical deposition methods, to name a few, although preferred application methods include spraying the coating 14 onto the tin finish 12, and/or dipping the tin finished component into a liquid coating material. [0019] The coating 14 includes a relatively soft matrix material 15. Exemplary conformal coating matrices are polymers including urethane, silicone, acrylic, paralenes, and polymers having an epoxy group in the molecule thereof. Combined with the matrix material 15 is a second material 18, although as depicted in FIGs. 3 and 4 the second material 18 will subsequently be removed from the coating 14.

0025[0020] The second material 18 may be an entirely different chemical composition than that of the matrix material 15, or it may be the same composition having a different crystal structure. An important difference between the second material 18 and the matrix material 15 is that the second material is easily removable from the coating 14 using a selected process such as exposure to heat, radiation, or a chemical composition such as an acid or a solvent. The matrix material 15 is stable during the selected process, while the second composition decomposes or dissolves as a result of the process. Some exemplary second materials include materials readily soluble in a solvent for which the matrix material is insoluble. For example, starch is readily soluble in water and is well suited for use as a second material when the matrix material is one that is insoluble in water. Other exemplary second materials are polymers having melting points that are significantly lower than that of a matrix material. According to this embodiment, the electrical component is heated to the second material melting temperature to cause the second material to melt out of the coating 14. Similarly, other exemplary second materials are compounds that degrade or disintegrate when exposed to a particular radiation under which the matrix material is unaffected. In a preferred embodiment, the second material 18 is any material that is soluble in or otherwise removable when exposed to a solvent that is not harmful to the matrix material or to the overall electronic assembly.

0026[0021] After applying the coating 14 onto the tin plating or finish 12, the second material 18 is removed from the coating using one or more of the previously- described methods that will effectively melt, dissolve, disintegrate, or otherwise remove the second material 18. As illustrated in FIGs. 3 and 4, the selected method does not affect the coating matrix structure, and consequently creates pores 20 in the coating matrix 15 where the second material 18 was previously disposed in the coating 14.

0027[0022] According to one exemplary method, starch is removed from a urethane matrix by simply contacting the coating 14 with water until the starch dissolves and erodes out of the coating matrix 15 to create the pores 20 therein. The remaining urethane forms a spongy matrix 15 protects the underlying electrical component 10 from the surrounding environment.

0028[0023] In some cases, it may be desirable for the pores 20 to be filled. Additional thermal or environmental protection may be provided by a non-porous coating 14. In such a case, as illustrated in FIG. 4, the pores 20 are filled in with a new material 24 that is different from the coating matrix 15, and also different from the second material 18 previously removed from the coating 14. In order to provide discontinuous support to tin whiskers that may enter the pores 20, the new material 24 is substantially softer than the surrounding coating matrix 15.

0029[0024] The several methods and coating materials therefore provide electrical assemblies and components having a tin plating or finish, and a porous or multi-phase coating around the tin plating or finish. The empty or filled-in pores in the coating inhibit growth of any tin whiskers from the tin plating or finish. While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.