LOW MELTING TEMPERATURE COMPLIANT SOLDERS

FIELD OP THE DISCLOSURE

The present disclosure relates generally to solder compositions and, more particularly, to low melting temperature compliant solders.

BACKGROUND OF THE DISCLOSURE

As feature sizes of semiconductor devices continue to shrink, low dielectric constant (low K) materials are more frequently employed to replace conventional insulators (e.g., silicon oxide) in the manufacturing of semiconductor devices. Currently, carbon-doped silicon oxide (SiOC) (K ~ 2.5 - 3) is the industry' s primary choice for a low K material in the manufacturing of semiconductor devices.

Carbon-doped silicon oxide (SiOC) typically comprises numerous air pockets to improve low K performance. However, these air pockets make this low K material very brittle and susceptible to fracture. Consequently, during electronic packaging and assembly processes, this low K material is known to crack due to stresses generated during soldering processes. In particular, solder paste reflow processes require reflow temperatures approximately 20 - 30 ⁰ C above the liquidus temperatures of solder alloys. For example, for a conventional Sn63Pb37 solder paste, the reflow temperature is typically around 210 - 230 ⁰ C. However, the recent conversion to Sn-Ag-Cu lead free solder alloys has resulted in a great increase in reflow temperatures to typically around 235 - 260 ⁰ C. The liquidus temperatures and yield strengths of some of these Sn-Ag-Cu lead free solder alloys is summarized in the table of Figure 1.

Due to the higher liquidus temperatures ( > 218 ⁰ C) of the Sn-Ag-Cu lead free solder alloys and mismatches in

coefficients of thermal expansion between these Sn-Ag-Cu lead free solder alloys and low K materials, high stresses develop in low K materials during cooling from high temperature reflow processes and thus cause cracking and failures in the low K materials. In light of the above, solder alloys with lower melting temperatures are required.

In addition to the requirement for solder alloys with low liquidus temperatures, the ability of a solder to deform to accommodate possible stresses or impact loading is critical to the reliability of electronic devices employing low k materials. In general, solders with low yield strengths are softer and easier to deform so as to relieve stresses. Common low melting temperature solder alloys presently consist mainly of generic 91Sn9Zn solder alloy and patented Sn-Ag-In and Sn- Ag-Cu-In solder alloys. However, in comparison with Sn-Ag-Cu solder alloys, these common low melting temperature solder alloys are at least 50% greater in yield strength and rigidity. A brief summary of these common low melting temperature solder alloys is provided in the table of Figure 2.

As shown in Figure 2, 91Sn9Zn solder has a melting point of 199 ⁰ C, and this solder is very strong (yield strength of 9.1 ksi) and very rigid. As also shown in Figure 2, patented Sn-Ag-In and Sn-Ag-Cu-In solder alloys are also very strong and rigid. Specifically, U.S. Patent No. 5,580,520 discloses a solder alloy with (71.5-91.9) %Sn, (2.6-3.3) %Ag, and (4.8- 25.9)%In, which has a melting point below 213 ⁰ C, but is too strong for use in low K material embedded semiconductor devices. Also, U.S. Patent No. 6,176,947 discloses a solder alloy with (76-96) %Sn, (0.2-2.5) %Cu, (2.5-4.5) %Ag, and (6- 12)%In, which has a liquidus temperature below 215 °C, but has proven too rigid for use with low K material embedded semiconductor devices. Similarly, U.S. Patent No. 6,843,862 discloses an alloy composition with (88.5-93.5) %Sn, (3.5-

4.5) %Ag, (2-6) %In, (0.3-1) %Cu, and up to 0.5% of an antioxidant and anti-skinning additive. This alloy is also too strong and rigid for use in low K material embedded semiconductor devices. In addition, U.S. Patent No. 6,689,488 reveals a solder alloy with (l-3.5)%Ag, (0.1-0.7) %Cu, (0.1- 2)%In, balanced with Sn, but this alloy composition has shown to be either too high in melting temperature or too rigid for use in low K material embedded semiconductor devices.

In view of the foregoing, it would be desirable to provide low melting temperature compliant solders which overcome the above-described inadequacies and shortcomings.

SUMMARY OP THE DISCLOSURE

Low melting temperature compliant solders are disclosed. In one particular exemplary embodiment, a low melting temperature compliant solder alloy comprises from about 91.5% to about 97.998% by weight tin, from about 0.001% to about 3.5% by weight silver, from about 0.0% to about 1.0% by weight copper, and from about 2.001% to about 4.0% by weight indium.

In accordance with other aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may comprise at most about 3.0% by weight indium.

In accordance with further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may comprise at most about 2.5% by weight indium.

In accordance with still further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may further comprise traces of impurities .

In accordance with still further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy does not comprise traces of impurities.

In accordance with additional aspects of this particular exemplary embodiment, the low melting temperature compliant

solder alloy may further comprise from about 0.01% to about 3.0% by weight at least one dopant selected from the group consisting of zinc (Zn), nickel (Ni), iron (Fe), cobalt (Co), germanium (Ge) , phosphorus (P) , aluminum (Al) , antimony (Sb) , cadmium (Cd) , tellurium (Te) , bismuth (Bi) , platinum (Pt) , rare earth elements, and combinations thereof to improve oxidation resistance and increase physical properties and thermal fatigue resistance.

In accordance with still additional aspects of this particular exemplary embodiment, the rare earth elements may be selected from the group consisting of cerium (Ce) , lanthanum (La) , praseodymium (Pr) , neodymium (Nd) , promethium (Pm) , samarium (Sm) , europium (Eu) , gadolinium (Gd) , terbium (Tb) , dysprosium (Dy) , holmium (Ho) , erbium (Er) , thulium (Tm) , ytterbium (Yb) , lutetium (Lu) , actinium (Ac) , thorium (Th), protactinium (Pa), and combinations thereof.

In another particular exemplary embodiment, a low melting temperature compliant solder alloy comprises from about 89.7% to about 94.499% by weight tin, from about 3.5% to about 6.0% by weight silver, from about 0.0% to about 0.3% by weight copper, and from about 2.001% to about 4.0% by weight indium.

In accordance with other aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may comprise at most about 3.0% by weight indium.

In accordance with further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may comprise at most about 2.5% by weight indium.

In accordance with still further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may further comprise traces of impurities .

In accordance with still further aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy does not comprise traces of impurities.

In accordance with additional aspects of this particular exemplary embodiment, the low melting temperature compliant solder alloy may further comprise from about 0.01% to about 3.0% by weight at least one dopant selected from the group consisting of zinc (Zn) , nickel (Ni) , iron (Fe) , cobalt (Co) , germanium (Ge), phosphorus (P), aluminum (Al), antimony (Sb), cadmium (Cd) , tellurium (Te) , bismuth (Bi) , platinum (Pt) , rare earth elements, and combinations thereof to improve oxidation resistance and increase physical properties and thermal fatigue resistance.

In accordance with still additional aspects of this particular exemplary embodiment, the rare earth elements may be selected from the group consisting of cerium (Ce) , lanthanum (La) , praseodymium (Pr) , neodymium (Nd) , promethium (Pm) , samarium (Sm) , europium (Eu) , gadolinium (Gd) , terbium (Tb) , dysprosium (Dy) , holmium (Ho) , erbium (Er) , thulium (Tm) , ytterbium (Yb) , lutetium (Lu) , actinium (Ac) , thorium (Th), protactinium (Pa), and combinations thereof.

The present disclosure will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings . While the present disclosure is described below with reference to exemplary embodiments, it should be understood that the present disclosure is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein, and with respect to which the present disclosure may be of significant utility.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like

numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be exemplary only.

Figure 1 is a table showing the liquidus temperatures and yield strengths of several Sn-Ag-Cu lead free solder alloys.

Figure 2 is a table showing the liquidus temperatures and yield strengths of several common low melting temperature solder alloys.

Figure 3 is a graph showing the effect of adding indium (In) to standard Sn-Ag-Cu (SAC) alloys.

Figure 4 is a table showing the liquidus temperatures and yield strengths of indium (In) added Sn-IAg-O.5Cu alloy compositions with respect to the concentration of indium (In) .

Figure 5 is a table showing the liquidus temperatures and yield strengths of indium (In) added Sn-2Ag-0.5Cu alloy compositions with respect to the concentration of indium (In) .

Figure 6 is a table showing the liquidus temperatures and yield strengths of indium (In) added Sn-2.5Ag-O .5Cu alloy compositions with respect to the concentration of indium (In) .

Figure 7 is a table showing the liquidus temperatures and yield strengths of indium (In) added Sn-3Ag-0.5Cu alloy compositions with respect to the concentration of indium (In) .

Figure 8 is a table showing the liquidus temperatures and yield strengths of indium (In) added Sn-4Ag-0.2Cu alloy compositions with respect to the concentration of indium (In) .

Figure 9 is a graph showing the yield strengths of Sn-Ag- Cu-In alloys with respect to the concentration of indium (In) . Figure 10 shows a scanning electron microscopy (SEM) snapshot where energy dispersive spectrometry (EDS) is used to identify major strengthening particles in an indium (In) added Sn-Ag-Cu alloy composition.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to Figure 3, there is shown a graph showing the effect of adding indium (In) to standard Sn-Ag-Cu (SAC)

alloys. As shown in Figure 3, the addition of indium (In) to the standard Sn-Ag-Cu (SAC) alloys results in a decrease of liquidus temperature. Specifically, when indium (In) is added to the standard Sn-Ag-Cu (SAC) alloys in an amount greater than 2%, the liquidus temperatures of the resultant Sn-Ag-Cu- In alloys are reduced to below the liquidus temperatures of the standard Sn-Ag-Cu (SAC) alloys. Thus, it may be advantageous to utilize Sn-Ag-Cu-In alloys with indium (In) concentrations greater than 2% in semiconductor devices using low K materials.

However, adding indium (In) to the standard Sn-Ag-Cu (SAC) alloys also results in a rapid increase of the yield strength due to solution hardening, and high strength Sn-Ag- Cu-In alloys may cause high stresses and unacceptable high defects. Thus, it would be beneficial to determine compositional ranges for Sn-Ag-Cu-In alloys that result in low liquidus temperatures, low yield strength, and low rigidity. Indeed, the present disclosure is directed to Sn-Ag-Cu-In alloy compositions exhibiting low liquidus temperatures, low yield strength, and low rigidity. Such Sn-Ag-Cu-In alloy compositions include Ag(O.001 - 3.5)%, Cu(O - 1)%, In (2.001 - 4)%, balanced with Sn, and Ag (3.5 - 6)%, Cu(O - 0.3)%, In (2.001 - 4)%, balanced with Sn. These Sn-Ag-Cu-In alloy compositions were derived through a series of multiple experimentations as exemplified below.

Example 1

The liquidus temperatures and yield strengths of indium (In) added Sn-IAg-O.5Cu alloy compositions with respect to the concentration of indium (In) are shown in the table of Figure 4. The yield strengths of the resultant alloy compositions increased rapidly as the concentration of indium (In) increased.

Example 2

The liquidus temperatures and yield strengths of indium (In) added Sn-2Ag-0.5Cu alloy compositions with respect to the concentration of indium (In) are shown in the table of Figure 5. The yield strengths of the resultant alloy compositions remained about constant as the concentration of indium (In) increased up to 2.5%. However, when the concentration of indium (In) exceeded 2.5%, the yield strengths increased as the concentration of indium (In) increased.

Example 3

The liquidus temperatures and yield strengths of indium (In) added Sn-2.5Ag-O .5Cu alloy compositions with respect to the concentration of indium (In) are shown in the table of Figure 6. The yield strengths of the resultant alloy compositions remained approximately constant as the concentration of indium (In) increased up to about 2.5%. However, when the concentration of indium (In) exceeded 2.5%, the yield strengths increased as the concentration of indium (In) increased.

Example 4

The liquidus temperatures and yield strengths of indium (In) added Sn-3Ag-0.5Cu alloy compositions with respect to the concentration of indium (In) are shown in the table of Figure 7. The yield strengths of the resultant alloy compositions decreased slightly as the concentration of indium (In) increased up to about 2.5%. However, when the concentration of indium (In) exceeded 2.5%, the yield strengths increased as the concentration of indium (In) increased.

Example 5

The liquidus temperatures and yield strengths of indium (In) added Sn-4Ag-0.2Cu alloy compositions with respect to the

concentration of indium (In) are shown in the table of Figure 8. Due to a high yield strength (> 6 ksi) developed because of a high silver (Ag) concentration (> 3.5%), a lower copper (Cu) concentration (0.2%) with respect to standard Sn-Ag-Cu (SAC) alloys (i.e., 0.5%) was employed. The yield strengths of the resultant alloy compositions decreased (approximately 20%) as the concentration of indium (In) increased up to about 2.5%. However, when the concentration of indium (In) exceeded 2.5%, the yield strengths increased as the concentration of indium (In) increased.

The yield strengths of the Sn-Ag-Cu-In alloys with respect to the concentration of indium (In) are shown in the graph of Figure 9. As shown in Figure 9, it is clear that the yield strengths of the indium (In) added Sn-IAg-O.5Cu alloy compositions increased very rapidly as the concentration of indium (In) increased, and thus these alloy compositions are unacceptable for use in low K material embedded semiconductor devices. However, with higher silver (Ag) concentrations, the yield strengths of the indium (In) added Sn-Ag-Cu alloy compositions either remained about constant or decreased slightly as the concentration of indium (In) increased up to about 2.5%, after which the yield strengths increased as the concentration of indium (In) increased. For example, the yield strengths of the indium (In) added Sn-2Ag-0.5Cu, Sn- 2.5Ag-O.5Cu and Sn-3Ag-0.5Cu alloy compositions resulted in a slight decrease in yield strength as the concentration of indium (In) .increased up to about 2.5-3%. However, as the silver (Ag) concentration increased to 4% and the copper (Cu) concentration decreased to 0.2% (i.e., Sn-4Ag-0.2Cu) , the reduction in yield strength was very significant (approximately 20%) , although this low yield strength compositional range was shortened very significantly. By the same token, it is reasonable to expect that as the silver (Ag)

concentration becomes greater than 4% (e.g., Sn-6Ag-O.2Cu) , an even more significant reduction in yield strength would be produced, but the low yield strength compositional range would become even shorter. These results indicate that the yield strengths of indium (In) added Sn- (0-2) %Ag-0.5Cu alloy compositions increase as the concentration of indium (In) increases, but the yield strengths of indium (In) added Sn- (2- 3.5) %Ag-0.5Cu alloy compositions decrease as the concentration of indium (In) increases (i.e., (2.001-4) %In) . The latter alloy compositions give rise to the low melting temperature compliant solders of the present disclosure for use in low K material embedded semiconductor devices . In addition, when the copper (Cu) concentration is further reduced to 0.2%, the yield strengths of indium (In) added Sn- (3.5-6) %Ag-0.2Cu alloy compositions are most significantly reduced.

In order to obtain a better understanding of the above results, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were performed on the above mentioned alloys. For example, Figure 10 shows an SEM snapshot where EDS is used to identify major strengthening particles in an indium (In) added Sn-Ag-Cu alloy composition. As shown in Figure 10, the major strengthening particles of this indium (In) added Sn-Ag-Cu alloy composition is identified using EDS to be Sn66. ₆ Ag ₂₉ . ₄ In ₄ . Specifically, the bright domains may be identified as Sn-Ag-In within the composition Sn ₆₆ . ₆ Ag ₂₉ . ₄ In ₄ , and the dark grey matrix may be identified as a solid solution of indium (In) in tin (Sn) . This is in contrast to the well established microstructure of the standard Sn-Ag-Cu (SAC) alloys where the major strengthening Ag ₃ ≤n particles (the minor strengthening particles are CUeSn ₅ due to copper (Cu) ) are homogeneously distributed in the tine (Sn) matrix. That is, because of the addition of indium (In) to the stoichiometric Ag ₃ Sn, the indium (In) doped Sn _6S-6 Ag _2S-4 In ₄ particles are disordered and off-

stoichiometric. More specifically, these off-stoichiometric Sn ₆₆ . ₆ Ag ₂₉ . ₄ In4 particles do not strengthen the solder as much as Ag ₃ Sn particles do due to a softer nature of the off- stoichiometric compounds and a loss of coherency in the tin (Sn) matrix.

In addition, it has been discovered that solution hardening of indium was typically the main mechanism for strengthening Sn-Ag-Cu-In solder alloys. However, in the Sn- Ag-Cu-In compositions of the present disclosure, indium (In) is removed from the solution, thus reducing the solution hardening effect, and instead forms the off-stoichiometric Sn ₆₆ . ₆ Ag ₂₉ . ₄ In ₄ particles, which did not strengthen the alloy as much as the replaced stoichiometric Ag ₃ Sn particles . As a result of the above-mentioned effects, the yield strengths of the presently disclosed indium (In) added Sn-Ag-Cu alloy- compositions decrease as the concentration of indium (In) increases (i.e., between (2.001-4) %In) .

Figure 10 also reveals that as the concentration of silver (Ag) decreases below 2%, Sn ₆₆ . ₆ Ag ₂₉ . ₄ In ₄ particles are found to be sparsely distributed because less indium (In) is removed from the solution, and the softening effect is negligible. In contrast, as the concentration of silver (Ag) exceeds 6%, indium (In) available to form Sn ₆₆ . ₆ Ag ₂₉ . ₄ I.i ₄ particles is exhausted. Nevertheless, the number of Ag ₃ Sn particles continues to increase due to the increasing amount of available silver (Ag) , rendering the softening effect less conspicuous and the low strength compositional range shorter. In accordance with the present disclosure, further reduction of yield strength is achieved by reducing the number of the minor strengthening particles of Cu ₆ Sns by reducing the copper (Cu) concentration, thereby resulting in even more advantageous alloy compositions.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other

various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the ¹ foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.