The current invention is directed to processes for joining materials together utilizing bulk metallic glasses; and more particularly to processes for joining materials at low temperature utilizing such bulk metallic glasses.
Lead (Pb) is widely recognized as a toxic substance, and the health and environmental issues related to the use of lead have been well documented over many decades. Lead poisoning is a serious health threat which usually occurs after a prolong exposure to lead and lead compounds. As a result, in the United States, the use of lead and lead compounds has already been banned from many consumer products. For example, tetraethyl-lead was formerly used as an “anti-knock” additive in gasoline, lead solder was used in plumbing applications, and of course in past decades lead was commonly found in paint.
Despite the concerted efforts of a number of different industries to eliminate the use of lead, it is still found in many consumer goods, including storage batteries, ammunition and electronic products. While storage batteries account for approximately 80% of lead consumption, its recycling program is very effective and therefore raises few health concerns. However, lead solder usage in electronic materials is of particular concern because these devices are rarely recycled resulting in the contamination of landfills when the products are discarded. More alarmingly, once in the landfill lead solder from electronic circuit boards can leach into the ground water system and also contaminate the soil.
Many countries around the world have taken steps to eliminate lead contamination from electronic products over the past two decades. The global electronic industry is under a lot of pressure from the European Union (EU) to completely phase out lead from many electronic products. Specifically, the EU enacted the directive called ROHS (Official Journal of the European Union, L37 19-23, Feb. 13, 2003,), or the Restriction of Hazardous Substances, which as of Jul. 1, 2006, banned lead (and a few other hazardous substances) from most consumer electronic products. Many countries in Asia including China, Korea, and Japan have come up with their own version of ROHS legislation. In the US, California's SB20 prohibits the sale of electronic devices which are prohibited under the EU's ROHS after Jan. 1, 2007.
As a result, there has been an ongoing research effort to find a substitute for Pb solder, but, until now, there has been no clear solution to the problem. In the current transition stage, the commercial Pb-free solders for reflow application in electronics packaging include a few varieties of near ternary eutectic of tin (Sn), silver (Ag) and/or copper (Cu) alloys with possible minute additions of elements such as bismuth (Bi), indium (In), zinc (Zn), and antimony (Sb). However, these Sn—Ag—Cu (SAC) and Sn—Ag-Bi (SAB) solders are only band-aid solutions to comply with ROHS. SAC solders are inferior to Pb-Sn solder in terms of solderability (wetting, spreading and low melting) and reliability. Each of these technical drawbacks can limit the effectiveness and applicability of these materials. For example, higher processing temperatures create a serious problem in a system with multiple joining processes, such as flip-chip packaging. The temperature of the last reflow process dictates the temperature of prior reflow processes. Specifically, in the case of electronics, replacing the traditional Pb-Sn solder with Sn—Ag raises the soldering temperature from 180° C. to 215-250° C. This in turn elevates the required melting temperature of prior reflow processes to above the 300° C. range to avoid subsequent remelting. Unfortunately, there are only a limited number of solders that satisfy these conditions. Moreover, there are other potential problems regarding the stability of substrates and other features on the chip that are not designed to withstand processing at these higher temperatures.
Likewise, molten lead has a very low surface tension, which contributes to its excellent wettability and spreading. Indeed, it has long been observed that the wetting characteristic of Pb/Sn solder far exceeds those of lead-free alternatives. At the interconnect interface, Pb/Sn solder forms chemical bond by creating a stable pure Sn compound. The replacement SAC solders would have three competing phases competing: β—Sn, Ag3Sn and Cu6Sn5. The two latter phases are non-equilibrium intermetallic compounds, which nucleate and grow with minimal undercooling. Adequate undercoating usually translates to the reduction of residual stresses. There are numerous studies showing the poor mechanical, thermal and electrical reliability of these two intermetallics.
Specifically, mechanical connection, electrical conduction and thermal pathway are three main functions of solder joints, particularly in the electronics industry. Mechanical attachment problems usually stem from the mismatch of the coefficients of thermal expansion between the materials attached to both ends of the solder joints and mismatch between the solder joint and attached substrate materials. During thermal cycling, the joint experiences shear stresses. Pb-solder joints release thermally induced stress by plastic deformation, which is not possible with SAC solder. For example,
The current invention is directed to methods for joining materials at low temperature using bulk metallic glasses.
The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:
a and 8b provide schematics of an exemplary metal-to-metal joining process;
a provides schematics of different channel designs for metal joining using BMGs;
b provides a micrograph of a joint interface surface having mechanical interlock channels formed therein in accordance with the current invention;
The current invention is directed to methods and compositions for a novel metal-to-metal or material-to-material joining technique using bulk metallic glasses. The method of the current invention relies on the superior mechanical properties of bulk metallic glasses and/or softening behavior of metallic glasses in the undercooled liquid region of temperature-time process space, enabling joining of a variety of materials at a much lower temperature than typical ranges used for soldering, brazing or welding. For example, by selection of the appropriate bulk metallic glasses can be used in the semiconductor industry, e.g., copper, copper-aluminum, gold, to allow for the replacement of lead and lead alloy solders.
A bulk metallic glass (BMG), also referred to as an amorphous alloy or a metallic glass, is a new class of metallic material that does not have crystalline structure. Various alloy families of BMG have been discovered during the past two decades. Overview of bulk metallic glasses and their properties could be found in a number of references, including, W. L. Johnson, MRS Bull. 24(10), 42 (19991); A. Inoue, Acta Materialia 48, 279-306 (2000); and A. L. Greer, Science 267, 1947-1953 (1995), the disclosures of which are incorporated herein by reference. One of the important characteristics of BMGs is that they may be processed like plastics or conventional silicate glasses when heated above their glass transition temperature, Tg. It has now been discovered that these properties allow the viscous BMG liquid to be used as a low temperature replacement for conventional joining materials, such as, for example, Pb-Sn and Sn-based solders. More specifically, the current invention recognizes that using bulk metallic glasses it is possible to join materials together at low temperatures and with high reliability by maintaining specific heating and cooling profiles for the BMG materials during the joining process.
The flowchart of the basic joining process in accordance with the current invention is set forth in
Although the above-discussion has focused on some preferred methods for preparing the amorphous joining materials prior to forming the joint, it should be understood that any process capable of producing a suitable joining material precursor may be used, such as, for example, atomization, etc.
Turning to preparation of the materials to be joined (Step B in
Finally, once the joining material and the materials to be joined are sufficiently prepared the joint itself is formed as shown by Step C in
Description of Thermoplastic Joining Process
In a first exemplary joining methodology, a thermoplastic joining process is described. This “thermoplastic joining” process is based on the unique rheological behavior and pattern-replication ability of Bulk Metallic Glass. More specifically, the method relies on three unique properties of these materials: that an amorphous solid BMG specimen may be processed as a thermoplastic when heated above its glass transition temperature (Tg), that the Tg of these BMGs is typically substantially below the melting temperature (Tm) of the material, and that the viscosity of these BMG materials continues to decrease with increasing temperature. The temperature profile of the thermoplastic joining process is labeled as “Method 1” in temperature curve shown in
As shown in
During the operation of the thermoplastic joining process the BMG is positioned on the area of the solder joint, along with an optional flux. Typically fluxes are applied to reduce oxides and other impurities on the substrate surface. In the current invention any flux may be applied that is compatible with the BMG materials. The assembly is then heated to a temperature above glass transition temperature, into the supercooled Liquid region. The preferred processing temperature is usually much lower than the alloy's melting temperature and the crystallization kinetics are slow. As a result, the part can be held in the amorphous, supercooled liquid for a few minutes up to hours depending upon the particular amorphous alloy being used. Optionally this heating may be followed by mechanically pressing the parts to help the flow of the BMG joining materials over the parts to be joined, as necessary. The assembly is then cooled to room temperature following soldering.
In summary, in a thermoplastic joining process the joining temperature (˜Tg) is “decoupled” from the melting temperature of the joining material (Tm). As a consequence, low temperature thermoplastic joining can be achieved without lowering the melting temperatures of the joining material, allowing for joints with superior reliability. Moreover, after bonding, a wide variety of nano/microstructures from fully amorphous, partially-crystallized to fully-crystallized structures can be obtained as a final state through controlled crystallization via post-bonding annealing for optimum electrical conductivity, creep and fatigue properties tailored to a given application. It has been surprisingly discovered that this technique posts significant advantage over conventional joining methods, such as soldering, because the glass transition temperatures of the BMG alloys are much lower than melting point. Indeed, the amorphous joining technique of the current invention typically requires a processing temperature range at a few hundred degrees (Celsius) below those required by conventional joining methods such as soldering, welding or brazing. As a result the deleterious effects of heat-effected zones, brittle oxide layers and unstable intermetallics typically found in conventional joining techniques will be reduced or eliminated in the joints formed in accordance with the current methodology.
As such, by judiciously choosing the amorphous alloy system, the amorphous joining technique of the current invention may be used for a wide variety of metal-to-metal joints using thermoplastic processing, not limited to the applications found in any specific industry. For example, the technique could be applied to metal-to-BMG joining, or BMG-to-BMG joining, fasteners, etc. Ideal processing conditions will obviously depend on different alloy family and composition, a fuller description of which is provided below. For an example, a processing temperature may be 30-60° C. above Tg for gold and platinum based BMG solder. Tg for one particular gold BMG is 130° C. (J. Schroers, B. Lohwongwatana, W. L. Johnson and A. Peker, Applied Physics Letters 87 061912 (2005), the disclosure of which is incorporated herein by reference), which means the thermoplastic soldering process could be conducted at 160-170° C., which is significantly below the 210-230° C. processing temperature window for a conventional Sn-based solder.
Description of Deeply Undercooled Joining Method
In a second exemplary joining method a deep undercoating process may be used. This processing technique utilizes the deeply undercoating characteristic of metallic glasses to form a liquid joining material that can be used to create joints that can be amorphous, crystalline or partially crystalline. Two potential processing paths labeled as “Method 2.1 and 2.2” on
As shown in
It is not a coincidence that good glass forming liquids deeply undercoat before crystallization takes place. Indeed, this is an important requirement for a material to be considered a BMG. In other words, the liquid metal needs to undercoat deeply enough so that the temperature is low, the atomic mobility is restricted, and the atoms become “frozen” before they form crystals. Such a deep undercoating process also improves the chance that the joining material will solidify as an amorphous metal. Path C in
In summary,
In the second “undercooled” process, labeled as Method 2.2 in
Again,
Description of Molten Plastic Processing Method
In another alternative joining method, plastic processing of the joint material from the molten state is utilized. The plastic processing method is explained schematically in
Description of Joint Surfaces
Although the above discussion has focused on the appropriate heating and cooling temperature profiles to be used with the joining method of the current invention to take advantage of the unique rheological properties of the amorphous materials, it should also be understood that the invention is also directed to engineering the joint surfaces to take advantage of the unique properties of the amorphous alloy materials. For example, another clear advantage of using these amorphous processing techniques is that the material can flow and fill up spaces (or volumes) of various shapes and geometries. As a result, a mechanical interlocking feature could be introduced on metal surfaces to increase the mechanical reliability of a joint. Having these surface features can dramatically improve the strength of the joint, because it allows the joint to utilize the mechanical property of both the BMG and the joint materials instead of relying solely on the wetting properties of the joining material.
For example, the process of forming an exemplary joint is shown in
Although interlocking features are shown in
Although on relatively simply features are shown and described above, it should be understood that more complicated features could be introduced to the materials to be joined, such as, for example, hollowed chambers inside the material to be joined so that metallic glass could flow inside and fill up the chamber. Such a mechanical interlock could be a desirable alternative to current conventional fasteners such as rivets and fasteners.
Description of Alloys
Although the above description has focused on specific joining methodologies in accordance with the current invention and not on the amorphous joining materials themselves, it should be understood that the inventive methodologies may be used with any amorphous alloy material. The only limitation for the suitability of any particular amorphous material is that it must have a temperature profile, i.e., melting, glass transition, and crystallization temperatures suitable for use in joining the materials of interest. The rheological properties of some exemplary amorphous alloys are provided in Table 1, below.
As shown in Table 1, the Tg and Tx temperatures of amorphous alloys can range from as low as 130° C. to well over 400° C. As a result, amorphous alloy materials can be used in low temperature joining processes such as soldering where joining temperatures are typically below 200° C. to welding and braizing where joining temperatures typically exceed 300 or 400° C. Although only a few exemplary amorphous alloys are set forth above, it should be understood that any suitable amorphous alloy may be used. A more detailed description of some of the well-known amorphous alloy families is provided below, although it should be understood that this listing of alloys is only meant to describe some exemplary alloys, and any alloy having rheological properties suitable for use in the joining methods of the current invention may be used.
The following discussion will follow a few rules that that will be used to classify families (or systems) of bulk metallic glasses (BMGs):
Turning to the truncated periodic table provided in
In the following discussion the alloy compositions will be explained using simple form A100-x-yBxCy. Each composition may consist of one or more elements chosen from respective group. The value of x and y represent atomic percent of each group. For example, Zr65Al10Ni10Cu15 could be regrouped as (Zr100-10-25)(Al10)(Ni10Cu15), which can be represented by the form ETM100-10-25M10LTM25.
Noble Metal Alloys
Typical noble metal alloy compositions take the form of the following:
Lanthanide Metal Alloys
Typical lanthanide metal alloy compositions take the form of the following:
Aluminum Metal Alloys
Typical aluminum metal alloy compositions take the two basic forms, first those based on LM and those based on other materials. In the first instance these Al-based systems emerge from LM-based systems by simply introducing more Al into the composition so that the Al content exceeds LM content. Hence, the systems become Al rich and take the following forms:
Other types of Al amorphous alloy take the forms:
Description of Simple Metal Alloys
The simple metal group includes AM (e.g. Li, Na), AEM (e.g. Mg, Ca) and simple metals in groups IIIV-VIA (e.g. Al, Bi). The most studied compositions are Al-based and Mg-based. Because Al-based systems are discussed above, this section will focus on M metals other than Al. Although there is a wide variety of such materials, in general alloy compositions of this group take the following forms:
Description of Late Transition Metal Alloys
Typically, late transition metal alloy compositions take the following forms:
Another set of LTM bulk-solidifying amorphous alloys are ferrous metal based compositions (Fe, Ni, Co). Examples of such compositions are disclosed in U.S. Pat. No. 6,325,868, and publications to (A. Inoue et. al., Appl. Phys. Lett., Volume 71, p 464 (1997)), (Shen et. al., Mater. Trans., JIM, Volume 42, p 2136 (2001)), and Japanese patent application No. 2000126277 (Publ. No. 0.2001.303218 A). One exemplary composition of such alloys is Fe72Al5Ga2P11C6B4. Another exemplary composition of such alloys is Fe72Al7Zr10Mo5W2B15.
Description of Early Transition Metal Alloys
This section includes all ETM-based systems other than Zr-based. For example, there have been recent developments on Cu-based, Ti-based, Mo-based, Nb-based, etc. Typically, alloy compositions of the ETM group take the following forms:
Description of Zr-Based Metal Alloys
This section includes all Zr-based systems, which is, arguably, the largest family of bulk glass formers known to date. There have been numerous patents and alloy composition discoveries. Instead of listing out all of the possible compositions, a listing of relevant patents, the disclosures of each of which are incorporated herein by reference is included below. The most relevant patents are U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975 each of which disclose Zr-based bulk solidifying amorphous alloys.
One exemplary family of bulk solidifying amorphous alloys can be described by the formula (Zr,Ti)a(Ni,Cu,Fe)b (Be,Al,Si,B)c, where a is in the range of from 30 to 75, b is in the range of from 5 to 60, and c in the range of from 0 to 50 in atomic percentages. A preferable alloy family is (Zr,Ti)a(Ni,Cu)b (Be)c, where a is in the range of from 40 to 75, b is in the range of from 5 to 50, and c in the range of from 5 to 50 in atomic percentages. A still, a more preferable composition is (Zr,Ti)a(Ni,Cu)b (Be)c, where a is in the range of from 45 to 65, b is in the range of from 7.5 to 35, and c in the range of from 10 to 37.5 in atomic percentages. Another preferable alloy family is (Zr)a (Nb,Ti)b (Ni,Cu)c (Al)d, where a is in the range of from 45 to 65, b is in the range of from 0 to 10, c is in the range of from 20 to 40 and d in the range of from 7.5 to 15 in atomic percentages. Furthermore, those alloys can accommodate substantial amounts of other transition metals up to 20% atomic, and more preferably metals such as Nb, Cr, V, Co.
An exemplary embodiment of the thermoplastic joining method of the current invention is demonstrated. In this approach, the bulk metallic glass is heated to the supercooled liquid region of the amorphous material and a small force is applied to the joint, resulting in good wetting and a strong bond. Complete wetting between a copper substrate and a platinum based bulk metallic glass is demonstrated and leads to atomistically intimate void-free interface, which is devoid of any reaction phase (e.g., intermetallic compounds). A joint produced by this method exhibits tensile strength up to 50 MPa, which meets or exceeds that of conventional Sn-based solders.
In order to demonstrate this novel joining process, a platinum based BMG was selected because of its oxygen inertness and low Tg comparable to the solder reflow temperatures in microelectronics applications. A fully amorphous strip of Pt57.5Cu14.7Ni5.3P22.5 with thickness of about 0.5-mm was prepared to have Tm, Tg, Tx of 499, 226.1, 299.2° C., respectively (measured by a Netzsch 404C DSC at a scan rate of 20° C.-min−1.). Copper cylinders with 6.35-mm diameter and 6.35-mm length of 99.996% purity (produced by Alfa Aesar) were used as substrates. Machining reduced the diameter at the bonding surface to 3-mm, as shown in the inset in
Three different process loads were used for joining: 5.4, 14.2 and 36.5N, respectively. With the 7.065-mm2 contact area, this correlates to an applied pressure of 0.76, 2.0 and 5.2 MPa. A stable, homogeneous joint was not formed at the lowest load, 5.4N. Thus, it is evident that some process load is necessary for joint formation. However, once the process load exceeds a critical threshold, the effect of process load on joint integrity seems insignificant. Failure stress test of joints generated at process loads of 14.2 and 36.5 N are shown in
The failure surfaces examined using SEM back-scattered images of separated joints processed with 36.5N load are shown in
The final thickness of the BMG solders range from 50 to 80 μm due to the significant flow under pressure and resultant electrical resistance of the joints are reasonably small ranging from 21 to 27μΩ for the joints formed at 290° C. and from 13 to 15μΩ for 300° C. For the joints formed at 300° C., ideal resistance estimated with 50 μm thick BMG solder is 13.1μΩ based on the resistivity of the platinum based glass, 1850 nΩ.m, indicating that the measured resistance values are close enough to claim the existence of intimate interface. TEM was used to confirm the existence of an intimate interface.
Cross-sectional TEM observation of the interface shown in
In summary, a novel amorphous joining process has been disclosed and demonstrated using the supercooled liquid region of BMG and the unique rheological properties of these materials to. With assistance of small load, the glassy solder wets a copper surface to form an atomistically-intimate interface. A joint thermoplastically formed between BMG and copper exhibits up to 50 MPa tensile strength. In contrast with conventional soldering the thermoplastically-formed interface shows absence of interfacial reaction products.
Those skilled in the art will appreciate that the foregoing examples and descriptions of various preferred embodiments of the present invention are merely illustrative of the invention as a whole, and that variations in the relative composition of the various components of the present invention may be made within the spirit and scope of the invention. For example, it will be clear to one skilled in the art that typical impurities and/or additives may be included in the compositions discussed above that would not affect the improved properties of the alloys of the current invention nor render the alloys unsuitable for their intended purpose. Accordingly, the present invention is not limited to the specific embodiments described herein but, rather, is defined by the scope of the appended claims.
The current application claims priority to U.S. Provisional Application Nos. 60/921,807, filed Apr. 4, 2007 and 60/921,805, filed Apr. 4, 2007, the disclosures of which are incorporated herein by reference.
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