Patent Application
20070275262
For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
Embodiments of the present invention and its advantages are best understood by referring to
According to the illustrated example, plated substrate 50 includes a plating layer 64 disposed outwardly from a substrate 60, with an interface 66 disposed between substrate 60 and plating layer 64. Plating layer 64 includes an oxidized layer 68 and grains 72, with interstices 76 disposed between grains 72.
Substrate material of substrate 60 may move into plating material of plating layer 64 to form intermetallic compounds 80 at interface 66. Larger amounts of intermetallic compounds 80a may form proximate to interstices 76, and smaller amounts of intermetallic compounds 80b may form farther away from interstices 76. The increasing volume of compounds 80 yields stresses 84 that operate generally in a direction from substrate 60 towards plating layer 64. The plating material of plated layer 64 may also move in a direction substantially parallel to interface 66 in response to stresses 88. Stresses 84 and 88 may contribute to a stress 90 that may form a whisker 54.
Whisker 54 may generally be electrically conductive and comprise single crystals of the plating material. Whisker 54 may have any suitable size, for example, 1 to 2 millimeters in length and 1 to 3 micrometers in diameter. Whisker 54 may take several days, months, or years to grow.
Referring back to the illustrated embodiment of
Substrate 20 comprises any suitable substrate material, for example, a metal such as copper or brass. Plating layer 22 comprises any suitable plating material 32, for example, a metal such as tin.
A blocking particle 42 may represent a particle disposed within an interstice 38 between grains 34, and a plurality of blocking particles 42 may at least reduce the formation of whiskers 54. A blocking particle 42 may have any suitable shape and size to fit within an interstice 38. A blocking particle 42 may, for example, have a highly polygonized shape. A blocking particle 42 may, for example, have an average diameter in the range of less than 100 nanometers, less than 50 nanometers, less than 40 nanometers, or less than 30 nanometers, such as approximately 20 nanometers. An average diameter may refer to the average of the diameters of a substantially spherical shape. Blocking particles 42 of plating layer 22 may have substantially the same average diameter.
Plating layer 22 may comprise any proportion of blocking particles 42 suitable to at least reduce the formation of whiskers 54. For example, blocking particles 42 may comprise less than 5%, less than 3%, less than 1%, less than 0.5%, such as 0.25% blocking particles. Blocking particles 42 may comprise any suitable material, for example, a metal such as nickel, copper, iron, titanium dioxide, bismuth, other suitable material, or any combination of the preceding. Blocking particles 42 may comprise a material different from the material of grains 34.
Blocking particles 42 may inhibit or even prevent the formation of whiskers 54. According to one embodiment, blocking particles 42 may at least partially relieve one or more stresses 84 and 88 that may contribute to the formation of whiskers 54.
According to the embodiment, blocking particles 42 may form a boundary 44 at interface 26 and interstices 38. Boundary 44 may operate to at least partially relieve stresses that contribute to the formation of whiskers 54. Boundary 44 may or may not be continuous in order to at least partially relieve the stresses. That is, boundary 44 may include breaks between blocking particles 42.
The boundaries may at least partially relieve one or more stresses 84 and 88. For example, the boundaries may at least partially relieve stresses 84, which may reduce or prevent the formation of intermetallic compounds 80, and may at least partially relieve stresses 88, which may reduce or prevent the movement of plating material 32. Relieving these stresses may inhibit or even prevent the formation of whiskers.
Other parameters may be adjusted to reduce the growth of whiskers. As an example, certain materials of substrate 20 may be more prone to whisker formation. Brass, copper, and copper alloys may be more prone to whisker formation. As a second example, a thicker plating layer 22 may be less prone to whisker formation.
Plated substrate 10 may be used in any suitable application. For example, plated substrate 10 may be used in electronic components such as electromagnetic relays, fuses, leads, microcircuits, test points, terminal lugs, wiring boards, capacitors, resistors, or other components.
Modifications, additions, or omissions may be made to plated substrate 10 without departing from the scope of the invention. Plated substrate 10 may include more, fewer, or other layers. For example, another layer may be disposed outwardly from plated substrate 10, or plated substrate 10 may be disposed outwardly from another layer. As used in this document, “each” refers to each member of a set or each member of a subset of a set.
Cathode 120 may comprise any suitable substrate material, for example, a metal such as copper or brass. Anode 124 may comprise any suitable material that may be used to form plating layer 22 comprising plating material 32 and blocking particles 48. According to one embodiment, anode 124 comprises a composite anode in which blocking particles 48 are substantially uniformly disposed with plating material 32.
Anode 124 may be fabricated in any suitable manner, for example, according to a hot embossing process. Hot embossing involves softening a material by raising the temperature of the material just above the softening transition temperature, but below the melting point. A pattern is stamped into the softened material. The stamping may uniformly distribute blocking particles 48 throughout plating material 32.
Plating solution 130 may comprise any suitable chemical solution that transports grains 34 and blocking particles 48 at substantially the same rate to distribute blocking particles 42 substantially uniformly throughout plating material 32. Plating solution may include, for example, an acid such as sulfuric acid H2(SO4), a concentration of plating material such as a tin concentration, a brightener, and/or water, in any suitable proportion. Plating solution 130 may comprise any other suitable solution, for example, an acid fluoride-chloride solution or a pyrophosphate citrate solution.
Modifications, additions, or omissions may be made to system 110 without departing from the scope of the invention. The components of system 110 may be integrated or separated according to particular needs. Moreover, the operations of system 110 may be performed by more, fewer, or other components.
According to the illustrated embodiment, a particle 142, such as a blocking particle 42 or grain 34, released from anode 124 travels through plating solution 130 and layers 140 and 144 towards cathode 120 to form plating layer 22. Diffusion layer 144 represents a diffusion double layer, and boundary layer 140 represents a hydrodynamic boundary layer. Particles 142 may be suspended in plating solution 130 during travel using any suitable method, for example, using mechanical or air agitation.
Particle 142 proceeds stages 150 through 158 during the codeposition process. At stage 150, an ionic cloud 160 forms around blocking particle 42 by adsorption of the ionic species upon the particle surface. Clouds 160 may be created by adding particles 142 to plating solution 130 or by pre-treating particles 42 in ionic solutions.
A convection force moves particle 142 towards boundary layer 140 at stage 152. Particle 142 diffuses through diffusion layer 144 at stage 154. Particle 142 is adsorbed at cathode 120 at stage 156. The ionic species of ionic cloud 160 is reduced at stage 158 to incorporate particle 142 into the matrix of plating layer 22.
Blocking particles 42 may be codeposited with grains 34 by any suitable mechanism, for example, electrophoretic movement of positively charged particles 142 towards cathode 120, adsorption of particles 142 at the electrode surface by Van der Waals forces, or mechanical inclusion of particles 142 at layer 22.
Modifications, additions, or omissions may be made to the method without departing from the scope of the invention. The method may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order without departing from the scope of the invention.
Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that blocking particles may form boundaries that at least partially relieve stresses that contribute to the growth of intermetallic compounds formed from the substrate material and the plating material. Relieving these stresses may inhibit or even prevent the formation of whiskers.
Another technical advantage of one embodiment may be that the boundaries formed by the blocking particles may at least partially relieve stresses associated with movement of the plating material towards whisker seeds. Relieving these stresses may inhibit or even prevent the formation of whiskers.
Another technical advantage of one embodiment may be that the formation of whiskers may be inhibited or even prevented without increasing the thickness of the plating layer. Another technical advantage of one embodiment may be that the formation of whiskers may be inhibited or even prevented without the addition of lead.
While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.
