Patent Application
20060196579
This invention relates generally to melting point depression of small metal particles. More particularly, this invention relates to a soldering composition having high-energy metal particles that have a depressed melting point.
The phenomena of melting point depression of nanoscale metal particles has been studied since the 1950's, when it was noticed that these extremely small particles of metal have a lower melting point than the bulk material. This results from the increasingly important role of the surface as the size of the nanostructures decreases. As the size decreases, an increased proportion of atoms occupy the surface or interfacial sites as opposed to the interior. These interfacial atoms possess higher energy than bulk atoms, which facilitates the melting of the nanoparticle. However, this mechanism is not fully understood to this day. Initially, x-ray diffraction (XRD) was used to determine if these very small solid particles changed from ordered to a disordered phase, later followed by transmission electron microscopy (TEM) to monitor the loss of crystalline structure. More recently, alternate experimental methods such as calorimetry measured the heat capacity and latent heat of fusion as a function of the temperature. A new calorimetric technique known as nano-calorimetry has been developed where nano-Joules of heat are measured. A simple expression was developed in 2002 by Dr. Leslie Allen at the University of Illinois that relates melting point to particle size:
Tm(r)=156.6-(220/r)
where Tm(r) is the melting temperature in degrees Centigrade and r is the radius of the particle in nanometers. Inspection of this equation reveals that significant melting point suppression happens only when the particle radius approaches the 5 to 10 nanometer range, and no appreciable melting point suppression occurs when particle sizes exceed 50 nanometers in diameter. Further, all prior studies have focused on pure metals, not mixtures of metals or alloys. A need exists to depress the melting point of metal and metal alloy particles in the size range greater than the 1-50 nanometer range studied to date.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:
While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding elements in the several views of the drawings. A low temperature, high energy soldering composition for joining metals together contains a fluxing agent and high energy metal particles suspended in the fluxing agent, such that the melting point of the high energy metal particles is depressed by at least three degrees Celsius below the normal bulk melting temperature of metal. A solder joint is effected by placing the high energy metal particles in contact with one or more of the metal surfaces and heating the high energy metal particles in the presence of a fluxing agent to melt the high energy metal particles and fuse them to the metal surface.
The melting point of a solid has been classically defined as that temperature at which the vapor pressure of the solid is the same as the vapor pressure of the liquid formed when the material melts. The relationship between melting point and particle size has previously been studied by a number of researchers using nanoscale particles of tin, gold, and indium. All of these studies focused on materials with diameters less than 50 nanometers produced by evaporation in a vacuum, and most literature indicates that the melting point ceases to be significantly altered when particle size exceeds this level. While we are interested in this size range, we address here the generally larger size ranges in order to make the application of this phenomena more practical. It should be noted that these larger particles are not produced by conventional methods used to make solder used in solder pastes. Our work shows that melting point suppression is exhibited in solids greater than 50 nanometer diameter that possess energies higher than the thermodynamically most stable bulk phase(s) for a metal or metal alloy. We define ‘high energy particles’ as those particles having a vapor pressure greater than that of the thermodynamically lowest energy bulk phase, or multiplicity of phases, at equal temperatures and pressures. ‘Bulk’ is understood to mean a substantially sufficient quantity of material that resides as a single bound entity such that the material can assume the lowest achievable thermodynamic state without regard to specific external influences (e.g. placed in tension or compression or other mechanical working) or inducement (e.g. held in an electric or magnetic field), but providing no further requirements to preserve the lowest thermodynamically attained state.
There are two ways to make these higher energy solids. One way is to produce them in a manner that causes the solid to form in a higher energy state by manipulating the kinetics of the formation process. These solids form in metastable energy states which annealing or melting may cause to relax to the thermodynamically preferred energy state. The other way is to force the solid, by virtue of its environment, to assume a thermodynamically stable structure that is different from the bulk structure. Annealing and melting of the solid does not necessarily form the thermodynamically preferred energy due to the disposition of the solid. We have identified four methods to produce high-energy solid metal and metal alloys:
Traditional methods to produce metal and metal alloy spheres for solder paste typically are: 1) dispersion of molten solder alloy by impacting a stream of the molten metal with a jet of gas that disperses the molten stream into tiny droplets; 2) milling of bulk metals; and 3) melt dispersions in hot oil to make particles. None of these processes produce high-energy metal particles. Published literature indicates that the nanoscale melting point is generally only sensitive to particle sizes less than 10 nanometers in diameter, with dramatic lowering seen at less than 5 nanometers. In contrast,
These principles can be used for both pure metals and alloys of metals to form interconnect materials that may be used to form electrical interconnects in electronics products. For example, a low temperature solder interconnect material can be created by using combinations of higher energy metals, metal alloys or bulk materials, as shown, for example, in
There are, of course, other combinations of these four types of materials that will occur to the reader, and the examples listed above are presented by way of illustration and not by way of limitation. In order to form a high energy soldering composition to solder electronic components together, the high energy particles are suspended in a matrix of a conventional fluxing agent. The high energy soldering composition is then placed in contact with one or more metal surfaces, for example, an electronic component on a printed circuit board, and the metal surfaces and the high energy soldering composition are heated to melt the high energy metal particles and fuse them to the metal surface. The fluxing agent, removes any oxides on the metal surfaces and/or the high energy metal particles to facilitate soldering. The fluxing agent can also serve as an oxygen barrier to prevent re-oxidation of the metal surfaces and the particles. Since the high energy metal particles melt at a temperature that is lower than the normal melting temperature of the ‘bulk’ metal or metal alloy, soldering can be effected at a temperature that is substantially less than would normally be expected. Metals that can be used to form the high energy particles are aluminum, antimony, beryllium, boron, bismuth, cadmium, chrome, cobalt, copper, gold, indium, iron, lead, lithium, magnesium, manganese, nickel, phosphorous, platinum, silver, tin, titanium, and zinc. Alloys of two or more of these metals can also be used, singly, or in combination with the metal or with additional metal alloys. High energy particles need not be 10 nm or less nor does this preclude them from being substantially comprised of particles less than or equal to 10 nm. It is to be understood that while the process for forming the particles may produce particles that approximate spheres, they need not necessarily be perfectly spherical in shape, but can be other shapes. Additionally, the high energy particles should be of the size, shape, and energy state such that the melting point of the particles is at least 3 degrees Celsius less than the melting point of a comparable composition of ‘bulk’ material.
Another embodiment of the invention finds particles of ‘bulk’ metal or metal alloys mixed with the high energy particles, and suspended in the fluxing agent matrix. Referring now to
In summary, without intending to limit the scope of the invention, the use of high energy solid metal and metal alloy particles is a novel way to create a soldering composition that will reduce the reflow temperature of solder interconnects by depressing the melting point. Reduced temperatures facilitate the use of existing manufacturing lines and electronic components, minimizing the cost impact of transition to a no-lead solder, and one does not need to substitute electronic components that can withstand higher temperatures and/or retrofit manufacturing lines with higher operating temperature ovens.
While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.
