GB2455486A - A sputtered film, solder spheres and solder paste formed from an Sn-Ag-Cu-In alloy - Google Patents
A sputtered film, solder spheres and solder paste formed from an Sn-Ag-Cu-In alloy Download PDFInfo
- Publication number
- GB2455486A GB2455486A GB0804138A GB0804138A GB2455486A GB 2455486 A GB2455486 A GB 2455486A GB 0804138 A GB0804138 A GB 0804138A GB 0804138 A GB0804138 A GB 0804138A GB 2455486 A GB2455486 A GB 2455486A
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- United Kingdom
- Prior art keywords
- solder
- balance
- paste
- sphere
- package
- Prior art date
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Links
- 229910000679 solder Inorganic materials 0.000 title claims abstract description 203
- 229910000846 In alloy Inorganic materials 0.000 title 1
- 229910052738 indium Inorganic materials 0.000 claims abstract description 73
- 229910052802 copper Inorganic materials 0.000 claims abstract description 71
- 229910052709 silver Inorganic materials 0.000 claims abstract description 67
- 238000000034 method Methods 0.000 claims abstract description 63
- 230000004907 flux Effects 0.000 claims abstract description 23
- 238000004544 sputter deposition Methods 0.000 claims abstract description 9
- 238000005476 soldering Methods 0.000 claims description 16
- 239000000843 powder Substances 0.000 claims description 8
- RSWGJHLUYNHPMX-UHFFFAOYSA-N Abietic-Saeure Natural products C12CCC(C(C)C)=CC2=CCC2C1(C)CCCC2(C)C(O)=O RSWGJHLUYNHPMX-UHFFFAOYSA-N 0.000 claims description 6
- KHPCPRHQVVSZAH-HUOMCSJISA-N Rosin Natural products O(C/C=C/c1ccccc1)[C@H]1[C@H](O)[C@@H](O)[C@@H](O)[C@@H](CO)O1 KHPCPRHQVVSZAH-HUOMCSJISA-N 0.000 claims description 6
- 239000012190 activator Substances 0.000 claims description 6
- KHPCPRHQVVSZAH-UHFFFAOYSA-N trans-cinnamyl beta-D-glucopyranoside Natural products OC1C(O)C(O)C(CO)OC1OCC=CC1=CC=CC=C1 KHPCPRHQVVSZAH-UHFFFAOYSA-N 0.000 claims description 6
- 229910045601 alloy Inorganic materials 0.000 abstract description 70
- 239000000956 alloy Substances 0.000 abstract description 70
- 239000010949 copper Substances 0.000 abstract description 53
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 abstract description 35
- 239000004332 silver Substances 0.000 abstract description 31
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 abstract description 13
- APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 abstract 3
- 239000011248 coating agent Substances 0.000 abstract 1
- 238000000576 coating method Methods 0.000 abstract 1
- 230000035882 stress Effects 0.000 description 26
- 229910000881 Cu alloy Inorganic materials 0.000 description 24
- 229910001316 Ag alloy Inorganic materials 0.000 description 22
- 230000008569 process Effects 0.000 description 20
- 239000000463 material Substances 0.000 description 10
- 230000007547 defect Effects 0.000 description 8
- 238000012360 testing method Methods 0.000 description 8
- 238000005137 deposition process Methods 0.000 description 6
- 238000002844 melting Methods 0.000 description 6
- 230000008018 melting Effects 0.000 description 6
- 239000000203 mixture Substances 0.000 description 6
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 230000001351 cycling effect Effects 0.000 description 4
- 238000000151 deposition Methods 0.000 description 4
- 230000008021 deposition Effects 0.000 description 4
- 238000006073 displacement reaction Methods 0.000 description 4
- 238000004090 dissolution Methods 0.000 description 4
- 230000007246 mechanism Effects 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 229910000978 Pb alloy Inorganic materials 0.000 description 2
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 2
- 229910001128 Sn alloy Inorganic materials 0.000 description 2
- 230000006978 adaptation Effects 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 230000001627 detrimental effect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000009661 fatigue test Methods 0.000 description 2
- 230000002349 favourable effect Effects 0.000 description 2
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 2
- 229910052737 gold Inorganic materials 0.000 description 2
- 239000010931 gold Substances 0.000 description 2
- 230000036541 health Effects 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 230000003647 oxidation Effects 0.000 description 2
- 238000007254 oxidation reaction Methods 0.000 description 2
- 238000004806 packaging method and process Methods 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 238000000926 separation method Methods 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 239000002904 solvent Substances 0.000 description 2
- 238000005477 sputtering target Methods 0.000 description 2
- 239000000758 substrate Substances 0.000 description 2
- 230000002459 sustained effect Effects 0.000 description 2
- 230000008646 thermal stress Effects 0.000 description 2
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/26—Selection of soldering or welding materials proper with the principal constituent melting at less than 400 degrees C
- B23K35/262—Sn as the principal constituent
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C13/00—Alloys based on tin
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
- C23C14/14—Metallic material, boron or silicon
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
- H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
- H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
- H05K3/3457—Solder materials or compositions; Methods of application thereof
- H05K3/3463—Solder compositions in relation to features of the printed circuit board or the mounting process
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electric Connection Of Electric Components To Printed Circuits (AREA)
Abstract
A solder sphere formed from a solder comprising: 3.0% to 4.5% silver, 0.3% to 0.7% copper, 3.4% to 7.5% indium and the balance tin. Also solder paste comprising a paste flux and an alloy comprising: 3.0% to 4.5% silver, 0.3% to 0.7% copper, 3.4% to 7.5% indium and the balance tin. Also a method of coating a target by sputtering an alloy comprising 3.0% to 4.5% silver, 0.3% to 0.7% copper, 3.4% to 7.5% indium and the balance tin onto the target.
Description
Improvements in or Relating to Lead-Free Solders DescriDtion of Invention THIS INVENTION relates to lead free solders, and in particular to lead free solders that may be used effectively in harsh environments.
Until relatively recently, a tin/lead alloy was used in most industria! so!dering applications. This simple solder alloy had advantageous physical and mechanical properties, and was used very widely.
More recently, concerns have been raised about the effects of large quantities of lead, both on human health and on the environment, particularly when items such as consumer electronics are discarded.
Since tin/lead solders have been used so widely, a direct "drop-in" replacement was sought, which had similar physical and mechanical properties and therefore could be used directly with existing techniques and machinery without the need for significant adaptations.
Recently, a tin/silver/copper alloy has been used, particularly in reflow soldering processes, with some success. However, in harsh environments this tin/silver/copper alloy suffers from defects. More demanding applications such as automotive, high-end electronics, aerospace, military, medical and navigation systems expose solders to wide ranges of temperature and also to high mechanical strain, and very high reliability is also demanded in many of these applications. In such cases, the tin/silver/copper lead free alloy has often proved to be inadequate.
The tin/silver/copper alloy has also faced difficulties in the packaging industry, for instance in the formation of ball grid array (BGA) packages. A BGA package has one face at least partly covered with balls of solder in a grid pattern. The package is placed on a printed circuit board (PCB) that is provided with a corresponding pattern of copper pads. The assembly is then heated, for instance by being placed in a reflow oven or by an infrared heater, to melt the solder balls and thus form conductive and mechanical connections between the package and the PCB.
Once they have cooled and solidified, the solder balls hold the package in coplanar alignment with the PCB. Previously, it was standard to have around to 300 input/output (I/O) connections, whereas in many applications the norm is now around 2000 to 7000 I/O counts. When forming packages of this type, it has been found that the tin/silver/copper alloy displays an unacceptably high rate of cracks due to stress and fatigue, and also does not maintain a desired high level of coplanarity between the package and the PCB.
It is an object of the present invention to seek to ameliorate one or more of the above problems.
Accordingly, one aspect of the present invention provides a solder sphere formed from a solder comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn.
Preferably, the solder comprises 3.5% to 4.5% In.
Conveniently, the solder comprises 6.5% to 7.5% In.
Advantageously, the solder comprises 4.1 % Ag, 0.5% Cu, 4.0% In and a balance of Sn.
Alternatively, the solder comprises 4.1 % Ag, 0.5% Cu, 7% In and a balance of Sn.
Another aspect of the present invention provides a ball grid array package comprising a plurality of solder spheres.
A further aspect of the present invention provides a method of forming a solder sphere comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn and forming a sphere from the solder.
Another aspect of the present invention provides a method of forming a BGA package, comprising the steps of fixing a grid of solder spheres according to the above to a package and fixing the package to a circuit board.
Advantageously, the fixing step comprises reflow soldering.
A further aspect of the present invention provides a solder paste comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and a paste flux.
Preferably, the solder comprises 3.5% to 4.5% In.
Alternatively, the solder comprises 6.5% to 7.5% In.
Conveniently, the solder comprises 4.1 % Ag, 0.5% Cu, 4.0% In and a balance of Sn.
Alternatively, the solder comprises 4.1 % Ag, 0.5% Cu, 7% In and a balance of Sn.
Advantageously, the flux is a rosin flux.
Preferably, the paste further comprises one or more activators.
Another aspect of the present invention provides a method of forming a solder paste, comprising the steps of: providing a solder comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and mixing the solder with a paste flux.
Conveniently, the solder is provided in a powder form.
Another aspect of the present invention provides a sputtering method comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and spluttering the solder onto a target for a predetermined time so that a layer of the solder is deposited on the target.
In order that the present invention may be more readily understood, embodiments thereof will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph showing properties of various alloys; Figure 2 is a graph of steady-state strain rate against stress for various alloys; Figure 3 is a graph showing displacement range against numbers of load cycles for various alloys; Figure 4 is a graph showing the relationship between inelastic strain range and fatigue life for various alloys; Figure 5 shows a graph of the temperatures at the top and undersides of a BGA package during a soldering process; Figure 6 shows a joint formed by a conventional technique; Figure 7 is a graph of temperatures at the top and undersides of a BGA package during a soldering process; Figure 8 shows a joint formed by a process embodying the present invention; Figure 9 shows the joint of figure 8 following temperature cycling; and Figure 10 shows a further joint formed by a method embodying the present invention.
It has been found that alloys comprising 3.0 to 4.5% Ag, 0.3 to 0.7% Cu, 3.5 to 7.5% In and a balance of Sn perform exceptionally well when used to make solder spheres (for instance in BGA applications), solder pastes, and also in deposition processes such as sputtering, particularly in harsh environments.
Two particularly favoured alloys have concentrations of In from 3.5% to 4.5% and from 6.5% to 7.5%.
Even more preferably, solders having the compositions 4.1 % Ag, 0.5% Cu, 4% In and a balance of Sn, and 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn have particularly advantageous properties.
One particularly effective property of these alloys relates to solder joints.
Clearly, when using solder joints to connect components, the mechanical properties of the joint are important. Moreover, if a small joint is formed then a different microstructure will arise than if a larger joint is created. The microstructure will determine not only the mechanical properties of the joint, but also the fatigue life and creep properties of the solder.
Solders embodying the present invention have been tested using a relatively recently-devised microjoint testing method. The method uses two pieces of round copper bar, which are joined together with a joint made from an alloy under test. The temperature at which the test is conducted is set to be 398 K, which is usually the peak temperature at which temperature cycle tests are conducted on alloys to determine the low cycle fatigue (i.e. fatigue determined using less than about 1000 stress cycles).
Loads are applied cylindrically to the joint to determine the maximum load that can be sustained. When the maximum load drops by 20%, Coffin Manson plots from the fatigue life and the inelastoplastic creep analysis were made. The material constant number of the solder can then be obtained from a stress relaxation method. The method was carried out on pure copper, a conventional lead-free solder comprising 3.0% Ag, 0.5% Cu and a balance of Sn, and the two lead-free solders embodying the present invention discussed above. The material constant numbers for each material are shown in table 1.
In conducting this analysis, it was assumed that the creep properties follow the Norton Creep law: cr = Aa (1) 1/a1l_1/al =(n-1)AEt (2) where t means steady creep speed, A is the material constant number, a is stress, n is the stress exponent, o is initial stress, E is the Young's modulus, and t is time.
The stress exponent n is generally related to deformation of the joint. With reference to figure 2, a steady-state mechanism creep rate as function of stress for each solder is shown.
The vertical axis of figure 2 is steady-state strain rate and the horizontal axis is the stress. In this figure, the further line is to the right side of the plot, the better the creep strength of the alloy. The slope of the line shows the stress exponent. It can be seen that the solders embodying the present invention display superior creep strengths at high stresses, and lower stress exponents than the conventional tin/silver/copper alloy.
Turning to figure 3, a double logarithmic graph is shown, which displays displacement range and fatigue life obtained from the above-described fatigue test. The alloys embodying the present invention are shown to have superior fatigue lives as compared to the tin/silver/copper alloy -the fatigue life of the solder having 4% In is shown to be around five times as long as that of the tin/solder/copper alloy, whereas the alloy having 7% In is shown to have a fatigue life around two and a half times as long as that of the tin/silver/copper alloy.
Figure 4 shows the relationship between inelastic strain range and fatigue life based on the Coffin Manson formula: = C (3) where, Ac is nonlinear strain range, Nf is fatigue life, a is the fatigue ductility index, and Cis the fatigue ductility coefficient.
It is found that the slopes and fatigue indices of the tin/silver/copper alloy and the two alloys embodying the present invention are similar, but on comparison of the fatigue lives, the results show that the fatigue lives of the solders embodying the present invention are superior to that of the tin/silver/copper alloy. In particular, the alloy comprising 5% In has a fatigue life four times greater than that of the tin/silver/copper alloy, whereas the solder comprising 7% In has a fatigue life which is approximately twice that of the tin/silver/copper alloy.
Alloys embodying the present invention also have favourable dissolution properties, which assist in lowering the soldering temperature that is necessary to melt the alloy. This can be understood as a similar mechanism to the melting of sugar -to melt solid sugar requires a relatively high temperature. However, when sugar is placed into water, the sugar will readily dissolve in the water at a relatively low temperature. The reflow temperature of a conventional lead-free solder comprising 3% Ag, 0.5% Cu and a balance of Sn is around 235° C. By contrast, reflow temperatures of alloys embodying the present invention are around 2200 C. Clearly, if solders embodying the present invention are to be used with delicate components and relatively thin circuit boards, the thermal stress that is placed on these other components will be reduced since the solder can be worked at a lower temperature.
Uses of solders embodying the present invention will now be described.
A first advantageous use of the solder is for creating solder spheres, for instance for use in BGA packages. Presently, a lead-free solder such as 4% Ag, 0.5% Cu and a balance Sn, or 1 % Ag, 0.5% Cu and a balance of Sn is used to make solder balls which are placed on packages. The packages are reflowed with a solder paste comprising, for example, 3% Ag, 0.5% Cu and a balance of Sn, or 3.8% Ag, 0.7% Cu and a balance of Sn. The melting temperature of all of these alloys ranges from 218° to 227° C, depending on the composition of the alloys. In the reflow process, if a microjoint is to be formed, it is generally necessary that the joint peak temperature has to be least 230° C, which is 12°C above the liquidus temperature of the alloys.
Advantageously, the diameters of the spheres formed are in the region of 8Ojim to 0.8mm.
Once this temperature has been achieved on the underside of a BGA package, the temperature on the other side (i.e. the top side) of the package will be at least 100 C higher, and will therefore be around 239° to 240° C. Referring to figure 5, a graph of the temperature at the top side of a BGA package, and also on the underside thereof, is shown with respect to time during a typical process of this nature.
Though a temperature of 239° to 240° C will not be sufficient to damage certain components, this temperature is likely to have a detrimental effect on most types of circuit board, and to reduce the reliability of both the circuit board and components mounted thereon.
This situation is worsened as larger BGA packages are formed, as the temperature gradient will inevitably be larger.
Further, once such a BGA package has been manufactured, the joints are often ill-formed. Ideally, the joints that are formed should be symmetrical, and should not be prone to cracking or other physical defects. Referring to figure 6, however, a cross-section of a joint formed by the above method is shown. It can clearly be seen that the joint is non-symmetrical, and if many of the joints display these properties then the heights of the various joints will not be consistent. This will lead to undue strains being placed on particular joints, and may also affect the coplanarity of the package and the PCB.
By contrast, if such joints are made with alloys embodying the present invention, the lower melting temperatures of the alloys (217° C for the alloy comprising 6.5% to 7.5% In, and 210° C for the alloy comprising 3.5 to 4.5% In) will mean that the reflow peak temperatures will be in the range of 2200 C to 225° C. With the temperature gradient described above giving rise to a temperature difference of around 100 to 15° C between the top and bottom sides of the package, the temperature at the top side of the package will reach 230°, which is within the permissible ranges for most components.
Coupled with the dissolution property of the alloys, the type of paste used will be of little importance.
Referring to figure 7, a further graph of the temperature at the topside of a BGA package, and also on the underside thereof, is shown with respect to time during a process using an alloy embodying the present invention. The maximum temperatures reached are significantly lower.
Figure 8 shows a reflowed joint formed using the method described above, along with a commonly-available paste comprising 3.0% Ag, 0.5% Cu, and a balance Sn employing a solder embodying the present invention. It can be seen that the joint is well-formed and highly symmetrical, and there is no phase difference between the paste and the solder. The joint was tested under temperature cycling between 25° C and 125° for 250 cycles, and the resulting joint is shown in figure 9. It can be seen that no phase changes are observed, and that no cracks are formed at the intermetallic boundaries either at the package or board area.
Solders embodying the present invention may also be used to form solder pastes. To do so, a solder embodying the present invention may be made into powder form, with particles preferably having sizes ranging from around 10tm to 45jim and having at least 90% sphericity, and mixed with a formulated rosin paste flux, with appropriate activators and solvents. This mixture is blended into paste form. Preferably, the paste comprises at least 88% alloy powder, and if the components are mixed in appropriate proportions then there should be no separation between the relatively heavy alloy and the lighter flux components. The paste is applied through a stencil onto a PCB with circuitry, before attaching a BGA package including spheres made from other lead free solders (for instance, 3.0% Ag, 0.5% Cu, and a balance tin) to the PCB. The resulting package was then subjected to reflow soldering at 2200 C, and was able to form a sound joint. The topside temperature during the process was 226° C, which is comfortably within the maximum temperature range permissible in most applications. The resulting joint is shown in figure 10, and it can be seen that the joint has a high level of symmetry and few defects.
A further use for the solders embodying the present invention is in deposition processes, such as sputtering. For instance, solder bumps could be formed through a deposition process, in which an alloy embodying the present invention is sputtered onto a target for a predetermined length of time. The thickness of the bump can be determined by the deposition rate, and the time for which the deposition takes place. Solder bumps are used, for instance, in "flip chip" mounting of integrated circuits.
As an example, the sputtering target could be any base material for a printed circuit board, for instance copper, nickel or silver. It is then necessary to create an imprint of the solder onto the land area of the PCB, to generate the necessary mass of solder for joint formation. Printing of a solder paste is one option, although for finally-pitched joints this technique does have limitations.
Advantageously, oxidation of alloys embodying the present invention is significantly lower than those of gold or silver, which are currently used to form conductive bumps for many applications.
The soldering of a bump formed in this way onto a PCB substrate to form a full interconnect may be done either with a normal solder paste, or with a paste flux. As discussed above, this process may be carried out at a relatively low temperature, thus reducing the possibility of warpage or other defects in a circuit board or package.
In certain embodiments of the invention, the alloy consists essentially of the elements listed above. In other words, the alloy comprises only the recited elements, aside from unavoidable impurities.
When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
Improvements in or Relating to Lead-Free Solders DescriDtion of Invention THIS INVENTION relates to lead free solders, and in particular to lead free solders that may be used effectively in harsh environments.
Until relatively recently, a tin/lead alloy was used in most industria! so!dering applications. This simple solder alloy had advantageous physical and mechanical properties, and was used very widely.
More recently, concerns have been raised about the effects of large quantities of lead, both on human health and on the environment, particularly when items such as consumer electronics are discarded.
Since tin/lead solders have been used so widely, a direct "drop-in" replacement was sought, which had similar physical and mechanical properties and therefore could be used directly with existing techniques and machinery without the need for significant adaptations.
Recently, a tin/silver/copper alloy has been used, particularly in reflow soldering processes, with some success. However, in harsh environments this tin/silver/copper alloy suffers from defects. More demanding applications such as automotive, high-end electronics, aerospace, military, medical and navigation systems expose solders to wide ranges of temperature and also to high mechanical strain, and very high reliability is also demanded in many of these applications. In such cases, the tin/silver/copper lead free alloy has often proved to be inadequate.
The tin/silver/copper alloy has also faced difficulties in the packaging industry, for instance in the formation of ball grid array (BGA) packages. A BGA package has one face at least partly covered with balls of solder in a grid pattern. The package is placed on a printed circuit board (PCB) that is provided with a corresponding pattern of copper pads. The assembly is then heated, for instance by being placed in a reflow oven or by an infrared heater, to melt the solder balls and thus form conductive and mechanical connections between the package and the PCB.
Once they have cooled and solidified, the solder balls hold the package in coplanar alignment with the PCB. Previously, it was standard to have around to 300 input/output (I/O) connections, whereas in many applications the norm is now around 2000 to 7000 I/O counts. When forming packages of this type, it has been found that the tin/silver/copper alloy displays an unacceptably high rate of cracks due to stress and fatigue, and also does not maintain a desired high level of coplanarity between the package and the PCB.
It is an object of the present invention to seek to ameliorate one or more of the above problems.
Accordingly, one aspect of the present invention provides a solder sphere formed from a solder comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn.
Preferably, the solder comprises 3.5% to 4.5% In.
Conveniently, the solder comprises 6.5% to 7.5% In.
Advantageously, the solder comprises 4.1 % Ag, 0.5% Cu, 4.0% In and a balance of Sn.
Alternatively, the solder comprises 4.1 % Ag, 0.5% Cu, 7% In and a balance of Sn.
Another aspect of the present invention provides a ball grid array package comprising a plurality of solder spheres.
A further aspect of the present invention provides a method of forming a solder sphere comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn and forming a sphere from the solder.
Another aspect of the present invention provides a method of forming a BGA package, comprising the steps of fixing a grid of solder spheres according to the above to a package and fixing the package to a circuit board.
Advantageously, the fixing step comprises reflow soldering.
A further aspect of the present invention provides a solder paste comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and a paste flux.
Preferably, the solder comprises 3.5% to 4.5% In.
Alternatively, the solder comprises 6.5% to 7.5% In.
Conveniently, the solder comprises 4.1 % Ag, 0.5% Cu, 4.0% In and a balance of Sn.
Alternatively, the solder comprises 4.1 % Ag, 0.5% Cu, 7% In and a balance of Sn.
Advantageously, the flux is a rosin flux.
Preferably, the paste further comprises one or more activators.
Another aspect of the present invention provides a method of forming a solder paste, comprising the steps of: providing a solder comprising 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and mixing the solder with a paste flux.
Conveniently, the solder is provided in a powder form.
Another aspect of the present invention provides a sputtering method comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and spluttering the solder onto a target for a predetermined time so that a layer of the solder is deposited on the target.
In order that the present invention may be more readily understood, embodiments thereof will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph showing properties of various alloys; Figure 2 is a graph of steady-state strain rate against stress for various alloys; Figure 3 is a graph showing displacement range against numbers of load cycles for various alloys; Figure 4 is a graph showing the relationship between inelastic strain range and fatigue life for various alloys; Figure 5 shows a graph of the temperatures at the top and undersides of a BGA package during a soldering process; Figure 6 shows a joint formed by a conventional technique; Figure 7 is a graph of temperatures at the top and undersides of a BGA package during a soldering process; Figure 8 shows a joint formed by a process embodying the present invention; Figure 9 shows the joint of figure 8 following temperature cycling; and Figure 10 shows a further joint formed by a method embodying the present invention.
It has been found that alloys comprising 3.0 to 4.5% Ag, 0.3 to 0.7% Cu, 3.5 to 7.5% In and a balance of Sn perform exceptionally well when used to make solder spheres (for instance in BGA applications), solder pastes, and also in deposition processes such as sputtering, particularly in harsh environments.
Two particularly favoured alloys have concentrations of In from 3.5% to 4.5% and from 6.5% to 7.5%.
Even more preferably, solders having the compositions 4.1 % Ag, 0.5% Cu, 4% In and a balance of Sn, and 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn have particularly advantageous properties.
One particularly effective property of these alloys relates to solder joints.
Clearly, when using solder joints to connect components, the mechanical properties of the joint are important. Moreover, if a small joint is formed then a different microstructure will arise than if a larger joint is created. The microstructure will determine not only the mechanical properties of the joint, but also the fatigue life and creep properties of the solder.
Solders embodying the present invention have been tested using a relatively recently-devised microjoint testing method. The method uses two pieces of round copper bar, which are joined together with a joint made from an alloy under test. The temperature at which the test is conducted is set to be 398 K, which is usually the peak temperature at which temperature cycle tests are conducted on alloys to determine the low cycle fatigue (i.e. fatigue determined using less than about 1000 stress cycles).
Loads are applied cylindrically to the joint to determine the maximum load that can be sustained. When the maximum load drops by 20%, Coffin Manson plots from the fatigue life and the inelastoplastic creep analysis were made. The material constant number of the solder can then be obtained from a stress relaxation method. The method was carried out on pure copper, a conventional lead-free solder comprising 3.0% Ag, 0.5% Cu and a balance of Sn, and the two lead-free solders embodying the present invention discussed above. The material constant numbers for each material are shown in table 1.
In conducting this analysis, it was assumed that the creep properties follow the Norton Creep law: cr = Aa (1) 1/a1l_1/al =(n-1)AEt (2) where t means steady creep speed, A is the material constant number, a is stress, n is the stress exponent, o is initial stress, E is the Young's modulus, and t is time.
The stress exponent n is generally related to deformation of the joint. With reference to figure 2, a steady-state mechanism creep rate as function of stress for each solder is shown.
The vertical axis of figure 2 is steady-state strain rate and the horizontal axis is the stress. In this figure, the further line is to the right side of the plot, the better the creep strength of the alloy. The slope of the line shows the stress exponent. It can be seen that the solders embodying the present invention display superior creep strengths at high stresses, and lower stress exponents than the conventional tin/silver/copper alloy.
Turning to figure 3, a double logarithmic graph is shown, which displays displacement range and fatigue life obtained from the above-described fatigue test. The alloys embodying the present invention are shown to have superior fatigue lives as compared to the tin/silver/copper alloy -the fatigue life of the solder having 4% In is shown to be around five times as long as that of the tin/solder/copper alloy, whereas the alloy having 7% In is shown to have a fatigue life around two and a half times as long as that of the tin/silver/copper alloy.
Figure 4 shows the relationship between inelastic strain range and fatigue life based on the Coffin Manson formula: = C (3) where, Ac is nonlinear strain range, Nf is fatigue life, a is the fatigue ductility index, and Cis the fatigue ductility coefficient.
It is found that the slopes and fatigue indices of the tin/silver/copper alloy and the two alloys embodying the present invention are similar, but on comparison of the fatigue lives, the results show that the fatigue lives of the solders embodying the present invention are superior to that of the tin/silver/copper alloy. In particular, the alloy comprising 5% In has a fatigue life four times greater than that of the tin/silver/copper alloy, whereas the solder comprising 7% In has a fatigue life which is approximately twice that of the tin/silver/copper alloy.
Alloys embodying the present invention also have favourable dissolution properties, which assist in lowering the soldering temperature that is necessary to melt the alloy. This can be understood as a similar mechanism to the melting of sugar -to melt solid sugar requires a relatively high temperature. However, when sugar is placed into water, the sugar will readily dissolve in the water at a relatively low temperature. The reflow temperature of a conventional lead-free solder comprising 3% Ag, 0.5% Cu and a balance of Sn is around 235° C. By contrast, reflow temperatures of alloys embodying the present invention are around 2200 C. Clearly, if solders embodying the present invention are to be used with delicate components and relatively thin circuit boards, the thermal stress that is placed on these other components will be reduced since the solder can be worked at a lower temperature.
Uses of solders embodying the present invention will now be described.
A first advantageous use of the solder is for creating solder spheres, for instance for use in BGA packages. Presently, a lead-free solder such as 4% Ag, 0.5% Cu and a balance Sn, or 1 % Ag, 0.5% Cu and a balance of Sn is used to make solder balls which are placed on packages. The packages are reflowed with a solder paste comprising, for example, 3% Ag, 0.5% Cu and a balance of Sn, or 3.8% Ag, 0.7% Cu and a balance of Sn. The melting temperature of all of these alloys ranges from 218° to 227° C, depending on the composition of the alloys. In the reflow process, if a microjoint is to be formed, it is generally necessary that the joint peak temperature has to be least 230° C, which is 12°C above the liquidus temperature of the alloys.
Advantageously, the diameters of the spheres formed are in the region of 8Ojim to 0.8mm.
Once this temperature has been achieved on the underside of a BGA package, the temperature on the other side (i.e. the top side) of the package will be at least 100 C higher, and will therefore be around 239° to 240° C. Referring to figure 5, a graph of the temperature at the top side of a BGA package, and also on the underside thereof, is shown with respect to time during a typical process of this nature.
Though a temperature of 239° to 240° C will not be sufficient to damage certain components, this temperature is likely to have a detrimental effect on most types of circuit board, and to reduce the reliability of both the circuit board and components mounted thereon.
This situation is worsened as larger BGA packages are formed, as the temperature gradient will inevitably be larger.
Further, once such a BGA package has been manufactured, the joints are often ill-formed. Ideally, the joints that are formed should be symmetrical, and should not be prone to cracking or other physical defects. Referring to figure 6, however, a cross-section of a joint formed by the above method is shown. It can clearly be seen that the joint is non-symmetrical, and if many of the joints display these properties then the heights of the various joints will not be consistent. This will lead to undue strains being placed on particular joints, and may also affect the coplanarity of the package and the PCB.
By contrast, if such joints are made with alloys embodying the present invention, the lower melting temperatures of the alloys (217° C for the alloy comprising 6.5% to 7.5% In, and 210° C for the alloy comprising 3.5 to 4.5% In) will mean that the reflow peak temperatures will be in the range of 2200 C to 225° C. With the temperature gradient described above giving rise to a temperature difference of around 100 to 15° C between the top and bottom sides of the package, the temperature at the top side of the package will reach 230°, which is within the permissible ranges for most components.
Coupled with the dissolution property of the alloys, the type of paste used will be of little importance.
Referring to figure 7, a further graph of the temperature at the topside of a BGA package, and also on the underside thereof, is shown with respect to time during a process using an alloy embodying the present invention. The maximum temperatures reached are significantly lower.
Figure 8 shows a reflowed joint formed using the method described above, along with a commonly-available paste comprising 3.0% Ag, 0.5% Cu, and a balance Sn employing a solder embodying the present invention. It can be seen that the joint is well-formed and highly symmetrical, and there is no phase difference between the paste and the solder. The joint was tested under temperature cycling between 25° C and 125° for 250 cycles, and the resulting joint is shown in figure 9. It can be seen that no phase changes are observed, and that no cracks are formed at the intermetallic boundaries either at the package or board area.
Solders embodying the present invention may also be used to form solder pastes. To do so, a solder embodying the present invention may be made into powder form, with particles preferably having sizes ranging from around 10tm to 45jim and having at least 90% sphericity, and mixed with a formulated rosin paste flux, with appropriate activators and solvents. This mixture is blended into paste form. Preferably, the paste comprises at least 88% alloy powder, and if the components are mixed in appropriate proportions then there should be no separation between the relatively heavy alloy and the lighter flux components. The paste is applied through a stencil onto a PCB with circuitry, before attaching a BGA package including spheres made from other lead free solders (for instance, 3.0% Ag, 0.5% Cu, and a balance tin) to the PCB. The resulting package was then subjected to reflow soldering at 2200 C, and was able to form a sound joint. The topside temperature during the process was 226° C, which is comfortably within the maximum temperature range permissible in most applications. The resulting joint is shown in figure 10, and it can be seen that the joint has a high level of symmetry and few defects.
A further use for the solders embodying the present invention is in deposition processes, such as sputtering. For instance, solder bumps could be formed through a deposition process, in which an alloy embodying the present invention is sputtered onto a target for a predetermined length of time. The thickness of the bump can be determined by the deposition rate, and the time for which the deposition takes place. Solder bumps are used, for instance, in "flip chip" mounting of integrated circuits.
As an example, the sputtering target could be any base material for a printed circuit board, for instance copper, nickel or silver. It is then necessary to create an imprint of the solder onto the land area of the PCB, to generate the necessary mass of solder for joint formation. Printing of a solder paste is one option, although for finally-pitched joints this technique does have limitations.
Advantageously, oxidation of alloys embodying the present invention is significantly lower than those of gold or silver, which are currently used to form conductive bumps for many applications.
The soldering of a bump formed in this way onto a PCB substrate to form a full interconnect may be done either with a normal solder paste, or with a paste flux. As discussed above, this process may be carried out at a relatively low temperature, thus reducing the possibility of warpage or other defects in a circuit board or package.
In certain embodiments of the invention, the alloy consists essentially of the elements listed above. In other words, the alloy comprises only the recited elements, aside from unavoidable impurities.
When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
Claims (23)
- CLAIMS: 1. A solder sphere formed from a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn.
- 2. A solder sphere according to claim 1, wherein the solder comprises 3.5% to 4.5% In.
- 3. A solder sphere according to claim 1, wherein the solder comprises 6.5% to 7.5% In.
- 4. A solder sphere according to claim 1, wherein the solder comprises 4.1% Ag, 0.5% Cu, 4.0% In and a balance of Sn.
- 5. A solder sphere according to claim 1, wherein the solder comprises 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn.
- 6. A ball grid array package comprising a plurality of solder spheres according to any preceding claim.
- 7. A method of forming a solder sphere, comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and forming a sphere from the solder.
- 8. A method of forming a BGA package, comprising the steps of: fixing a grid of solder spheres according to any one of claims 1 to 5 to a package; and fixing the package to a circuit board.
- 9. A method according to claim 8, wherein the fixing step comprises reflow soldering.
- 10. A solder paste comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and a paste flux.
- 11. A paste according to claim 10, wherein the solder comprises 3.5% to 4.5% In.
- 12. A method according to claim 10, wherein the solder comprises 6.5% to 7.5% In.
- 13. A method according claim 10, wherein the solder comprises 4.1% Ag, 0.5% Cu, 4.0% In and a balance of Sn.
- 14. A method according to claim 9, wherein the solder comprises 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn.
- 15. A paste according to any one of claim 10 to 14, wherein the flux is a rosin flux.
- 16. A paste according to any one of claims 10 to 14, further comprising one or more activators.
- 17. A method of forming a solder paste, comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and mixing the solder with a paste flux.
- 18. A method according to claim 17, wherein the solder is provided in a powder form.
- 19. A sputtering method comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and spluttering the solder onto a target for a predetermined time so that a layer of the solder is deposited on the target.
- 20. A solder sphere substantially as hereinbefore described, with reference to the accompanying drawings.
- 21. A solder paste substantially as hereinbefore described, with reference to the accompanying drawings.
- 22. A method substantially as hereinbefore described, with reference to the accompanying drawings.
- 23. Any novel feature or combination of features disclosed herein.23. Any novel feature or combination of features disclosed herein.CLAIMS: 1. A solder sphere formed from a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn.2. A solder sphere according to claim 1, wherein the solder comprises 3.5% to 4.5% In.3. A solder sphere according to claim 1, wherein the solder comprises 6.5% to 7.5% In.4. A solder sphere according to claim 1, wherein the solder comprises 4.1% Ag, 0.5% Cu, 4.0% In and a balance of Sn.5. A solder sphere according to claim 1, wherein the solder comprises 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn.6. A ball grid array package comprising a plurality of solder spheres according to any preceding claim.7. A method of forming a solder sphere, comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and forming a sphere from the solder.8. A method of forming a BGA package, comprising the steps of: fixing a grid of solder spheres according to any one of claims 1 to 5 to a package; and fixing the package to a circuit board.9. A method according to claim 8, wherein the fixing step comprises reflow soldering.10. A solder paste comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and a paste flux.11. A paste according to claim 10, wherein the solder comprises 3.5% to 4.5% In.12. A method according to claim 10, wherein the solder comprises 6.5% to 7.5% In.13. A method according claim 10, wherein the solder comprises 4.1% Ag, 0.5% Cu, 4.0% In and a balance of Sn.14. A method according to claim 9, wherein the solder comprises 4.1% Ag, 0.5% Cu, 7% In and a balance of Sn.15. A paste according to any one of claim 10 to 14, wherein the flux is a rosin flux.16. A paste according to any one of claims 10 to 14, further comprising one or more activators.17. A method of forming a solder paste, comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and mixing the solder with a paste flux.18. A method according to claim 17, wherein the solder is provided in a powder form.19. A sputtering method comprising the steps of: providing a solder comprising: 3.0% to 4.5% Ag, 0.3% to 0.7% Cu, 3.4% to 7.5% In and a balance Sn; and spluttering the solder onto a target for a predetermined time so that a layer of the solder is deposited on the target.20. A solder sphere substantially as hereinbefore described, with reference to the accompanying drawings.21. A solder paste substantially as hereinbefore described, with reference to the accompanying drawings.22. A method substantially as hereinbefore described, with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0804138A GB2455486A (en) | 2008-03-05 | 2008-03-05 | A sputtered film, solder spheres and solder paste formed from an Sn-Ag-Cu-In alloy |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0804138A GB2455486A (en) | 2008-03-05 | 2008-03-05 | A sputtered film, solder spheres and solder paste formed from an Sn-Ag-Cu-In alloy |
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| GB0804138D0 GB0804138D0 (en) | 2008-04-09 |
| GB2455486A true GB2455486A (en) | 2009-06-17 |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| EP3031567A4 (en) * | 2013-08-05 | 2017-04-19 | Senju Metal Industry Co., Ltd | Lead-free solder alloy |
| CN109483081A (en) * | 2018-12-03 | 2019-03-19 | 先导薄膜材料（广东）有限公司 | Metal targets bind solder and preparation method thereof |
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| US6176947B1 (en) * | 1998-12-31 | 2001-01-23 | H-Technologies Group, Incorporated | Lead-free solders |
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| US20070071634A1 (en) * | 2005-09-26 | 2007-03-29 | Indium Corporation Of America | Low melting temperature compliant solders |
| US20070178007A1 (en) * | 2005-12-27 | 2007-08-02 | Kabushiki Kaisha Toshiba | Lead-free solder, solder joint product and electronic component |
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| WO1997009455A1 (en) * | 1995-09-01 | 1997-03-13 | Sarnoff Corporation | Soldering composition |
| US6176947B1 (en) * | 1998-12-31 | 2001-01-23 | H-Technologies Group, Incorporated | Lead-free solders |
| WO2003006200A1 (en) * | 2001-07-09 | 2003-01-23 | Quantum Chemical Technologies (S'pore) Pte Ltd. | Improvements in or relating to solders |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| EP3031567A4 (en) * | 2013-08-05 | 2017-04-19 | Senju Metal Industry Co., Ltd | Lead-free solder alloy |
| US10076808B2 (en) | 2013-08-05 | 2018-09-18 | Senju Metal Industry Co., Ltd. | Lead-free solder alloy |
| CN109483081A (en) * | 2018-12-03 | 2019-03-19 | 先导薄膜材料（广东）有限公司 | Metal targets bind solder and preparation method thereof |
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| Publication number | Publication date |
|---|---|
| GB0804138D0 (en) | 2008-04-09 |
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