TW201700741A - Mixed alloy solder paste - Google Patents

Abstract

A solder paste consists of an amount of a first solder alloy powder between 44wt% to less than 60wt%; an amount of a second solder alloy powder between greater than 0wt% and 48wt%; and a flux; wherein the first solder alloy powder comprises a first solder alloy that has a solidus temperature above 260 DEG C; and wherein the second solder alloy powder comprises a second solder alloy that has a solidus temperature that is less than 250 DEG C. In another implementation, the solder paste consists of an amount of a first solder alloy powder between 44wt% and 87wt%; an amount of a second solder alloy powder between 13wt% and 48wt%; and flux.

Description

Mixed alloy solder paste Cross-reference to related applications

This application is part of the continuation of U.S. Patent Application Serial No. 14/629, 139, filed on Feb. The continuation of the case is incorporated herein by reference in its entirety.

The present invention relates generally to compositions of mixed alloy solder pastes, and more particularly, to certain compositions relating to each of the alloying compositions in solder pastes for high temperature solder joint applications.

Lead produced by the disposal of electronic assemblies is considered to be harmful to the environment and human health. Regulations increasingly prohibit the use of Pb solder in the electronic interconnect and electronic packaging industries. Pb-free solders have been extensively studied to replace conventional eutectic Pb-Sn. SnAg, SnCu, SnAgCu, and SnZn solders are becoming mainstream solders for interconnects in the semiconductor and electronics industries. However, the development of high-temperature Pb-free solders to replace conventional high-lead solders (ie, Pb-5Sn and Pb-5Sn-2.5Ag) is still in its infancy. The high temperature solder is used to maintain the internal connections within the assembly in the assembly while the assembly is being soldered to the printed wiring board (PWB).

A common use of high temperature solder is for die attach. In an exemplary procedure, an assembly is formed by soldering a germanium die to a leadframe using a high temperature solder. The germanium die/lead frame assembly (either encapsulated or unencapsulated) is then attached to the PWB by soldering or mechanical snapping on. The board can be exposed to a few more reflow procedures for surface mounting other electronic devices to the board. The internal connection between the germanium die and the leadframe should be well maintained during further soldering procedures. This requires high temperature solder to resist multiple backflows without any functional failure. Therefore, in order to be compatible with the solder backflow profile used in the industry, the main requirements for high temperature solders include (i) a melting temperature of about 260 ° C and above 260 ° C (according to a typical solder return flow curve), (ii) Good thermal fatigue resistance, (iii) high thermal conductivity/conductivity, and (iv) low cost.

Currently, there are no temporary lead-free alternatives available for use in industry. However, several lead-free solder candidates have recently been proposed for high temperature die attach applications such as (1) Sn-Sb, (2) Zn-based alloys, (3) Au-Sn/Si/Ge and (4) Bi-Ag.

The Sn-Sb alloy having less than 10% by weight of Sb maintains good mechanical properties without forming a large amount of intermetallic compounds. However, the solidus temperature is not higher than 250 ° C, which cannot meet the 260 ° C requirements for reflow resistance.

The Zn-based alloy (including eutectic Zn-Al, Zn-Al-Mg, and Zn-Al-Cu) has a melting temperature higher than 330 °C. However, the high affinity of Zn, Al, and Mg for oxygen results in extremely poor wetting on various metallized surface treatment layers. The Zn-(20-40 wt%) Sn solder alloy, which is proposed to be one of the high temperature lead-free solders, has a liquidus temperature higher than 300 ° C, but the solidus temperature is only about 200 ° C. The semi-solid state of the Zn-Sn solder at about 260 ° C should maintain good interconnection between the components during subsequent reflow. However, problems arise when the semi-solid solder is compressed inside the encapsulated package and forces the semi-solid solder to flow out. This creates a risk of unexpected functional failure. A Zn-based solder alloy will also form a large number of IMC layers between the metallized surface and the solder. The presence of the IMC layer and its intensive growth during subsequent reflow and operation also cause reliability concerns.

The eutectic Au-Sn composed of two intermetallic compounds has been experimentally demonstrated as a reliable high-temperature solder due to its melting temperature of 280 ° C, good mechanical properties, high electrical conductivity and thermal conductivity, and excellent corrosion resistance. However, extremely high cost limits its cost over reliability Applications in the field of consideration.

A Bi-Ag alloy having a solidus temperature of 262 ° C satisfies the melting temperature requirement for high temperature die attach solder. However, there are several major concerns: (1) poor wetting on various surface treatment layers and (2) associated weak joint interfaces from poor wetting.

The melting temperature requirements for high melting lead-free solders make Sn-Sb and Zn-Sn solders unsuitable. The extremely high cost of Au-rich solders limits their acceptance to the industry. Zn-Al and Bi-Ag meet the melting temperature requirements and are reasonably low cost. However, due to the high affinity for oxygen (in the Zn-Al solder system) or due to the adverse reaction chemistry between the solder and the substrate metallization (in Bi-Ag solder systems or even Pb-Cu and Pb) The poor wetting of certain lead-containing solders of the -Ag system makes these highly molten solders difficult to use in industry due to the weak joint strength caused by poor wetting. However, the high melting temperature of BiAg and ZnAl still makes it acceptable as a candidate for high temperature lead-free solder.

As explained above, poor wetting of the solder results from (1) adverse reaction chemistry or (2) oxidation of the solder. Weak joints are always associated with poor wetting. For example, poor wetting of Bi-based solder on different metallized surfaces is primarily caused by undesirable reaction chemistry between Bi and substrate material (ie, Cu) or oxidation of Bi. A Ge-doped BiAg has been developed to prevent excessive formation of scum on the surface of the alloy during melting. However, this doping will not alter the reaction chemistry between Bi and the metallized surface treated layer of the substrate. Bi and Cu will not form IMC at the Bi/Cu interface, which is the dominant cause of poor wetting and weak bonding interfaces. Bi and Ni will form an IMC layer between the Bi/Ni interfaces, but the fragile IMC (Bi3Ni or BiNi) weakens the joint strength because the crack is always along the interface between Bi3Ni and the solder substrate or between the BiNi and Ni substrates. The interface grows. Therefore, the reaction chemistry between Bi and the substrate material causes poor wetting and weak joint strength.

Attempts have been made to modify the reaction chemistry between the solder alloy and the metallized surface treated layer by fusing additional elements into the solder. However, fusion is often associated with some unexpected property loss. For example, Sn has a better reaction chemistry with the substrate than Bi. nature. However, direct fusion of Sn to BiAg (where Ag is intended to increase thermal conductivity/conductivity) can result in (1) a significant decrease in melting temperature or (2) formation of Ag3Sn IMC in the alloy. If there is not enough time for the Sn and the substrate metal to dissolve in the molten solder during reflow, this will not improve the reaction chemistry between Sn and the substrate metal. Thus, direct fusion of the elements into the solder, such as direct fusion of Sn into the Bi-Ag alloy, reveals minimal improvements.

The present invention is directed to new techniques for designing and preparing mixed alloy solder pastes that achieve the combined advantages of forming alloy powders. In certain embodiments, a mixed alloy solder paste is suitable for use in high temperature solder applications, such as die attach, as the composition provides the desired advantages, including improved reaction chemistry from the second alloy, which is well controlled. The IMC layer thickness and enhanced reliability as well as the high melting temperature and good thermal conductivity/conductivity from the first alloy. The present invention also presents a method of preparing a mixed alloy solder paste and a method of using a mixed alloy solder paste in combination with an electronic component or a mechanical component.

The inventive technique provides a method of designing a mixed alloy powder paste in which an additive powder is present in the paste to improve reaction chemistry at relatively low temperatures or in conjunction with melting of the first alloy solder powder. In some embodiments, the mixed alloy powder paste comprises two or more alloy powders and a flux. The alloy powder in the paste is composed of a solder alloy powder as a main component and an additive alloy powder as a secondary material. The additives provide excellent chemical properties for wetting on various metallized surface treatment layers of the substrate (i.e., conventional Cu and Ni surface treatment layers, etc.).

In certain embodiments, the additives will melt prior to melting of the main solder or with melting of the main solder. The molten additives will wet and bond to the substrate prior to or with the first alloy, partially or completely melting the first alloy. The additives are designed to dominate the formation of the IMC along the substrate metallization surface treatment layer and are completely converted to IMC during the reflow process. The thickness of the IMC layer will therefore be due to the addition of the paste Additives play a leading role in the formation of IMC and are well controlled by the amount of such additives. In certain embodiments, the first alloy solder will have a strong affinity for the IMC layer formed between the additive and the substrate. This strong affinity will enhance the bond strength between the solder body and the IMC. Thus, the desired chemistry of the IMC layer and the well controlled thickness not only improve wetting efficiency, but also enhance the bond strength associated with wetting performance.

Other features and aspects of the present invention will be apparent from the description of the accompanying drawings. The summary is not intended to limit the scope of the invention, which is defined by the scope of the appended claims.

100‧‧‧melting the first alloy/first alloy

103‧‧‧solution

106‧‧‧Mixture

109‧‧‧IMC layer/IMC

112‧‧‧Second alloy/fused solder alloy/melted second alloy

115‧‧‧Second alloy solder particles/second alloy

118‧‧‧First alloy solder particles / first alloy particles / first alloy

121‧‧‧ solder bumps

124‧‧‧Substrate

200‧‧‧Cu substrate

201‧‧‧ solder bumps

202‧‧‧de-wetting

205‧‧‧ alloy 42 substrate

206‧‧‧de-wetting

207‧‧‧ solder bumps

210‧‧‧Cu substrate

211‧‧‧ solder bumps

215‧‧‧ alloy 42 substrate

216‧‧‧ solder bumps

300‧‧‧Cu test piece/Cu

301‧‧‧IMC

302‧‧‧rich in Bi phase

303‧‧‧Ag

400‧‧‧Cu test piece

401‧‧‧Returned paste

402‧‧‧Returned paste

405‧‧‧ alloy 42 test piece

Tm(A)‧‧‧ melting temperature of the first alloy

Tm(B)‧‧‧ melting temperature of the second alloy

The invention is explained in detail with reference to the following drawings in accordance with one or more embodiments. The drawings are provided for illustrative purposes only and are merely illustrative of typical or exemplary embodiments of the invention. The drawings are provided to facilitate the reader's understanding of the invention and are not to be construed as limiting the scope, scope or applicability of the invention. It should be noted that the drawings are not necessarily to scale unless

Figure 1 illustrates a reflow process implemented in accordance with an embodiment of the present invention.

Figure 2 shows the wetting efficiency of an exemplary solder paste consisting of 90 wt% Bi10.02 Ag 3.74 Sn + 10 wt% flux on a Cu coupon and an Alloy 42 coupon.

3 shows the wetting efficiency of an example of a mixed alloy powder solder paste composed of 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux on a Cu test piece and an alloy 42 test piece.

4 shows the wetting efficiency of an example of a mixed alloy powder solder paste composed of 84 wt% Bi11Ag + 6 wt% 52 In48Sn + 10 wt% flux on a Cu test piece and an alloy 42 test piece.

Figure 5 is a mixed alloy composed of 84wt% Bi11Ag + 6wt% Sn15Sb + 10wt% flux DSC chart of powder solder paste.

Figure 6 is a DSC chart of a mixed alloy powder solder paste consisting of 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% flux.

Figure 7 is a DSC chart of a mixed alloy powder solder paste consisting of 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux.

8A and 8B are cross-sectional images of joints made of a mixed alloy powder solder paste on Cu and Ni test pieces. The mixed alloy powder paste consisted of 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% flux.

The figures are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It is to be understood that the invention may be practiced with modifications and variations, and the invention is only limited by the scope of the invention and its equivalents.

The present invention is directed to a solder paste comprising a mixture of different solder alloys in a flux. Two or more solder alloys or metals are incorporated into the flux material. The first solder alloy or metal ("first alloy") will form the body of the solder joint during reflow. The remaining second solder alloy or metal or further additional solder alloy or metal ("second alloy") is selected based on the reactive chemistry with the metal substrate or affinity for the first alloy. The melting temperature Tm (B) of the second alloy is lower than the melting temperature Tm (A) of the first alloy. During reflow, the second alloy first melts and spreads onto the substrate. The presence of the second alloy promotes melting of the first alloy on the substrate as the first alloy melts. The second alloy is designed to be completely converted to IMC, resulting in minimal or no low melting phase in the final joint.

The additives in the paste modify the reaction chemistry during reflow, improve wetting, control the thickness of the IMC and thus enhance the bond strength. In addition to the solder used for high-temperature lead-free soldering with the desired wetting and reliability, the design process can be extended to many other soldering applications regardless of where the poorly wet solder is used. For example, Pb-Cu alloys have high melting temperatures but have poor wetting on various metal substrates. Therefore, Pb-Cu alloy is difficult to weld Used in succession. With regard to the present invention, small additives such as Sn or a Sn-containing alloy will help Pb-Cu wet various metal surfaces. However, if Sn is only fused in Pb-Cu, Cu6Sn5 IMC formation will reduce the reaction chemistry from Sn. Fusion of a higher amount of Sn in the solder will significantly reduce the melting temperature of Pb-Cu, which is not desirable.

Figure 1 illustrates a reflow procedure using a mixed solder paste in accordance with an embodiment of the present invention. The mixed solder paste includes first alloy solder particles 118 and second alloy solder particles 115 suspended in a flux. In certain embodiments, the second alloy is selected based on its excellent reaction chemistry with the substrate or a range of common substrates. A mixed solder paste is applied to the substrate 124. (For illustration, the flux is omitted from the figure.)

During the reflow, the temperature of the assembly rises above the melting temperature Tm(B) of the second alloy. The second alloy 112 is melted and spread over the substrate 124 and around the still solid first alloy particles 118. The superior surface reaction chemistry of the second alloy will promote wetting of the molten solder alloy 112 on the substrate 124. This causes the IMC layer 109 to melt the formation between the second alloy 112 and the substrate 124. Therefore, the IMC is primarily controlled by the amount of the second alloy 115 in the initial paste.

Additionally, the second alloy is designed to have good affinity for the first alloy. This affinity can be determined by (1) a negative mixing enthalpy between the first alloy and the second alloy, or (2) formation of a eutectic phase composed of constituent elements from the first and second alloys. In certain embodiments, this affinity causes some of the first alloy 118 to dissolve into the molten second alloy 112 to form a mixture 106 of the first and second alloys.

When the temperature rises above the melting temperature Tm(A) of the first alloy, the first alloy is completely melted, thereby forming a solution 103 of the first and second alloys, and the solution 103 is wetted to the IMC layer 109. When the assembly is maintained above Tm(A), the second alloy is removed from solution 103, thereby increasing IMC layer 109 and leaving molten first alloy 100. In certain embodiments, in addition to forming the IMC layer 109, excess components from the second alloy may also be incorporated into the IMC along with the components from the first alloy. Affinity between the first alloy and the second alloy assists the improvement Wetting of an alloy 100 to the IMC layer 109, thereby enhancing joint strength.

When the assembly is cooled, the solder bumps 121 or joints are formed by a substrate 124 bonded to the IMC 109, which is bonded to cure the first alloy. After curing, a homogenous solder joint with a modified joint interface has been achieved.

The solder joint produced by the use of a mixed solder paste exhibits a significant improvement in the use of a solder paste containing a single solder alloy, even when a single solder alloy is composed of elements of the first and second solder alloys. 2 illustrates solder bumps 201 and 207 formed on Cu substrate 200 and alloy 42 substrate 205, respectively, using a solder paste consisting of 86.24 wt% Bi10.02 Ag 3.74 Sn + 10 wt% solder. Such results show that significant dewetting 202 and 206 occurs with a single solder alloy. In contrast, FIG. 3 illustrates solder bumps 211 and 216 formed on Cu substrate 210 and alloy 42 substrate 215, respectively, using a mixed solder paste consisting of 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux. Such results show that the use of a mixed solder paste exhibits almost invisible dewetting.

In one embodiment, the mixed solder paste includes BiAg as the first alloy and SnSb as the second alloy. In the second alloy, Sn is selected for its superior reaction chemistry with Bi and various substrates. SnSb has a melting temperature lower than that of BiAg. According to the binary phase diagram, Sn and Bi exhibit a negative mixing enthalpy and form a eutectic phase in a broad composition range. Sb and Bi also exhibit negative mixing and infinite solubility with each other. During reflow, SnSb first melts and forms a Sn-containing IMC layer on the surface of the substrate. When the temperature reaches above the melting temperature of BiAg, all of the alloy powder in the paste melts. The good affinity between Bi and Sn/Sb ensures good bonding of the Bi on the Sn-containing IMC layer. Additionally, the presence of Ag in the first alloy can convert any additional Sn into Ag3Sn IMC residing in the solder body. Therefore, little or no low melting BiSn phase is left, since Sn is completely consumed by forming (1) the IMC layer between the solder and the metal substrate and (2) the Ag3Sn inside the BiAg solder bump.

Figure 5 illustrates the use of 84wt% Bi11Ag + 6wt% Sn15Sb + 10wt% flux The DSC curve of the raw joint. The top curve illustrates the heat flow curve after reflow on the ceramic coupon. A spike at about 138 °C illustrates the presence of the second alloy. The bottom curve illustrates the heat flow curve of the paste after reflow on the Cu coupon. The presence of this spike in the bottom curve verifies the disappearance of the low melting phase in the BiAg+SnSb system. Figure 6 illustrates the disappearance of the low melting phase in the BiAg+SnAg system. The experiment of Figure 6 used 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% flux on ceramic and Cu coupons, as in Figure 5. Figure 7 illustrates the disappearance in the BiAg+BiSn system. The experiment of Figure 7 used 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux on ceramic and Cu coupons, as in Figures 5 and 6. In Figure 7, the top curve illustrating the heat flux curve after reflow of the solder on the ceramic shows the lack of a low melt phase. This is most likely due to the high affinity between the small amount of reactant Sn in the mixed solder paste and Sn and Ag, resulting in the incorporation of Sn of the second alloy with some Ag of the first alloy into the final solder bump. in.

Figure 8A is a micrograph of a solder joint produced using a mixed solder paste consisting of 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% flux. In this example, a mixed solder paste is applied to the Cu coupon 300. IMC 301 is formed between Cu 300 and the second alloy. The size of this IMC 301 is primarily dependent on the amount of the second alloy in the phase. In the illustrated example, 6 wt% of the second alloy Sn3.5Ag produced an IMC that was only a few microns thick. The block of the solder joint is composed of Ag 303 rich in the Bi phase 302. Aging at 150 ° C for 2 weeks did not significantly increase the IMC thickness. In contrast, Bi and Cu do not form an intermetallic compound, and thus Bi11Ag alone forms a weak bond because there is no IMC layer between the solder and the substrate.

In one embodiment of the invention, a method of designing a hybrid solder paste includes selecting a first alloy based on the desired characteristics of the finished solder joint, and then selecting a second alloy based on the applicable substrate and affinity with the selected first alloy. The relative amounts of the first alloy, the second alloy, and the flux can be determined based on factors such as the thickness of the desired IMC layer, the desired application conditions, and the reflow procedure. The thickness of the IMC layer and the amount of the second alloy in the solder paste, the return flow curve and compliance Relevant to the aging conditions of the application. The acceptable thickness of the IMC layer can vary with different application conditions and different IMC compositions. For example, for a Cu6Sn5/Cu3Sn IMC layer, 10 microns can be approximately as thick as acceptable.

When the amount of the second alloy in the paste is increased, a low melt phase may remain in the final joint. If the amount of the second alloy is reduced in the solder paste, it may be difficult to achieve the desired wetting performance. When reducing the amount of the second alloy, good wetting requires the use of a larger amount of paste printed or dispensed onto the substrate. However, increasing the total amount of paste can interfere with the geometric constraints from the solder package.

For high temperature solder applications, the first alloy must be selected from a variety of high melting solder alloys. In certain embodiments, a Bi-rich alloy is used with a solidus temperature of about 258 ° C and above 258 ° C, namely Bi-Ag, Bi-Cu, and Bi-Ag-Cu. The second alloy (or additive) is selected from the alloys that have been shown to wet and bond to various metallized surface treatment layers on various metallized surface treatment layers and have good affinity for melting Bi.

In such embodiments, the second alloy will melt prior to or rich with the Bi alloy and then readily wet and bond to the substrate on the substrate. At the same time, good affinity between Bi and the second alloy will provide good wetting. Therefore, Sn, Sn alloy, In and In alloy were selected as the second alloy. Three groups have been classified based on the melting temperature of the selected second alloy. Group A comprises an additive alloy having a solidus temperature between about 230 ° C and 250 ° C, namely Sn, Sn-Sb, Sn-Sb-X (X = Ag, Al, Au, Bi, Co, Cu) , Ga, Ge, In, Mn, Ni, P, Pd, Pt and Zn) alloys. Group B contains a solder alloy having a solidus temperature between about 200 ° C and 230 ° C, including Sn-Ag, Sn-Cu, Sn-Ag-X (X = Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, and Zn), and Sn-Zn alloy. Group C comprises a solder alloy having a solidus temperature below 200 ° C, namely Sn-Bi, Sn-In, Bi-In, In-Cu, In-Ag, and n-Ag-X (X=Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb, Sn, and Zn), Sn-Bi-X (X=Ag, Al, Au, Co, Cu, Ga, Ge) , In, Mn, Ni, P, Pd, Pt, Sb or Zn), Sn-In-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn) and Bi-In-X (X = Ag, Al, Au, Co, Cu, Ga, Ge, Mn , Ni, P, Pd, Pt, Sb or Zn) alloys, etc. Among these alloys, the reactants in the Sn and/or In system.

In one embodiment of the invention, the first alloy is from an alloy of a Bi-Ag system and has a solidus temperature of about 260 ° C and above 260 ° C. In a particular embodiment, the first alloy comprises from 0 to 20 wt% Ag, with the remainder being Bi. In yet another embodiment, the first alloy comprises from 2.6 to 15 wt% Ag, with the remainder being Bi.

In a second embodiment of the invention, the first alloy is selected from the Bi-Cu system and has a solidus temperature of about 270 ° C and above 270 ° C. In a particular embodiment, the first alloy comprises from 0 to 5 wt% Cu, with the remainder being Bi. In yet another embodiment, the first alloy comprises from 0.2 to 1.5 wt% Cu, with the remainder being Bi.

In a third embodiment of the invention, the first alloy is selected from the Bi-Ag-Cu system and has a solidus temperature of about 258 ° C and above 258 ° C. In a particular embodiment, the first alloy comprises from 0 to 20 wt% Ag and from 0 to 5 wt% Cu, with the remainder being Bi. In yet another embodiment, the first alloy comprises from 2.6 to 15 wt% Ag and from 0.2 to 1.5 wt% Cu, with the remainder being Bi.

In a fourth embodiment of the invention, the second alloy is from the Sn-Sb system and has a solidus temperature between about 231 ° C and about 250 ° C. In a particular embodiment, the second alloy comprises from 0 to 20 wt% Sb, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 15 wt% Sb, with the remainder being Sn.

In a fifth embodiment of the present invention, the second alloy includes Sn-Sb-X (where X = Ag, Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt) And Zn) and have a solidus temperature between about 230 ° C and about 250 ° C. In a particular embodiment, the second alloy comprises from 0 to 20 wt% Sb and from 0 to 20 wt% X, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 10 wt% Sb and from 0 to 5 wt% X, with the remainder being Sn.

In a sixth embodiment of the invention, the second alloy comprises Sn-Ag and has a solidus temperature of about 221 ° C and above 221 ° C. In a particular embodiment, the second alloy comprises from 0 to 10 wt% Ag, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 5 wt% Ag, with the remainder being Sn.

In a seventh embodiment of the invention, the second alloy comprises Sn-Cu and has a solidus temperature of about 227 ° C and above 227 ° C. In a particular embodiment, the second alloy comprises from 0 to 5 wt% Cu, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 2 wt% Cu, with the remainder being Sn.

In an eighth embodiment of the present invention, the second alloy includes Sn-Ag-X (wherein X = Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb) And Zn) and have a solidus temperature of about 216 ° C and above 216 ° C. In a particular embodiment, the second alloy comprises from 0 to 10 wt% Ag and from 0 to 20 wt% X, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 5 wt% Ag and from 0 to 5 wt% X, with the remainder being Sn.

In a ninth embodiment of the invention, the second alloy comprises Sn-Zn and has a solidus temperature of about 200 ° C and above 200 ° C. In a particular embodiment, the second alloy comprises from 0 to 20 wt% Zn, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 0 to 9 wt% Zn, with the remainder being Sn.

In a tenth embodiment of the invention, the second alloy comprises a Bi-Sn alloy having a solidus temperature of about 139 ° C and above 139 ° C. In a particular embodiment, the second alloy comprises from 8 to 80 wt% Sn, with the remainder being Bi. In yet another embodiment, the second alloy comprises from 30 to 60 wt% Sn, with the remainder being Bi.

In an eleventh embodiment of the invention, the second alloy comprises a Sn-In alloy having a solidus temperature of about 120 ° C and above 120 ° C. In a particular embodiment, the second alloy comprises from 0 to 80 wt% In, with the remainder being Sn. In yet another embodiment, the second alloy comprises from 30 to 50 wt% In, with the remainder being Sn.

In a twelfth embodiment of the invention, the second alloy comprises a Bi-In alloy having a solidus temperature between about 100 ° C and about 200 ° C. In a particular embodiment, the second alloy comprises from 0 to 50 wt% In, with the remainder being Bi. In a particular embodiment, the second alloy comprises from 20 to 40 wt% In, with the remainder being Bi.

In a thirteenth embodiment of the invention, the second alloy comprises an In-Cu alloy having a solidus temperature between about 100 ° C and about 200 ° C. In a particular embodiment, the second alloy comprises from 0 to 10 wt% Cu, with the remainder being In. In a particular embodiment, the second alloy comprises from 0 to 5 wt% Cu, with the remainder being In.

In a fourteenth embodiment of the invention, the second alloy comprises an In-Ag alloy having a solidus temperature between about 100 ° C and about 200 ° C. In a particular embodiment, the second alloy comprises from 0 to 30 wt% Ag, with the remainder being In. In yet another embodiment, the second alloy comprises from 0 to 10 wt% Ag, with the remainder being In.

In the fifteenth embodiment, the second alloy is In-Ag-X (X = Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb, Sn, and Zn) The alloy has a solidus temperature between about 100 ° C and about 200 ° C. In yet another embodiment, the second alloy comprises from 0 to 20 wt% Ag, 0 to 20 wt% X, with the remainder being In. In a particular embodiment, the second alloy comprises from 0 to 10 wt% Ag, 0 to 5 wt% X, with the remainder being In.

A further embodiment of the invention provides a method for making a mixed solder paste. In certain embodiments, particles of the first alloy are formed and particles of the second alloy are formed. The particles of the first and second alloys are then mixed with a flux to form a solder paste. The final paste includes a first alloy powder, a second alloy powder, and the remaining flux. In certain embodiments, the first alloy particles are alloys having a solidus temperature of at least about 260 °C.

In a further embodiment, the second alloy comprises an alloy having a solidus temperature between about 230 ° C and about 250 ° C, an alloy having a solidus temperature between about 200 ° C and about 230 ° C or having a low An alloy at a solidus temperature of about 200 °C. In some In an embodiment, the paste is comprised of a first alloy powder between about 60 wt% and about 92 wt%, a second alloy powder having a quantity greater than 0% but less than or equal to about 12 wt%, with the balance being flux. In a further embodiment, the second alloy powder is between 2 wt% and 10 wt% of the mixed solder paste.

In a particular embodiment, the first alloy comprises a Bi-Ag alloy, a Bi-Cu alloy, or a Bi-Ag-Cu alloy. In yet another embodiment, the alloy having a solidus temperature between about 230 ° C and about 250 ° C comprises a Sn alloy, a Sn-Sb alloy, or a Sn-Sb-X (where X = Ag, Al, Au, Bi) , Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt and Zn) alloys. In another embodiment, the alloy having a solidus temperature between about 200 ° C and about 230 ° C includes a Sn-Ag alloy, a Sn-Cu alloy, and Sn-Ag-X (where X = Al, Au, Bi) , Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, and Zn) alloys or Sn-Zn alloys. In still another embodiment, the alloy having a solidus temperature below about 200 ° C comprises a Sn-Bi alloy, a Sn-In alloy, or a Bi-In alloy.

In a further embodiment, the second alloy powder comprises a powder comprised of a plurality of alloy powders. For example, the second alloy powder can comprise a mixture of different alloys selected from the alloys set forth herein. In some embodiments, the relative amounts of the first and second alloys in the mixed solder paste are determined based on the solder application. In some cases, when the amount of the second solder alloy in the paste increases beyond a certain threshold, the chance of holding a certain low melt phase in the final solder joint can be increased. In some cases, wetting of the substrate can be reduced when the amount of the second solder alloy in the paste is below a certain threshold. In one embodiment, the amount of the second solder alloy in the paste is determined such that the low melt phase can be completely converted to a highly molten IMC after reflow. In yet another embodiment, the second alloy in the paste varies between greater than 0 wt% but less than about 12 wt%. In a particular embodiment, the second alloy in the paste is greater than about 2 wt% but less than about 10 wt%.

In addition to various normal impurities or small amounts of different elements, other elements may be added or incorporated into such alloys as long as the reactive nature of Sn is maintained.

In some embodiments, the return flow curve for welding by means of a mixed solder paste is designed to rapidly heat up above the melting temperature of the first alloy. In such cases, a shorter soaking time at low temperatures may allow the reactants (such as Sn) to rapidly flow toward the substrate and react with the substrate in a fully molten pool rather than a semi-solid melt pool. The melting of both the first and second alloys will promote diffusion of the second alloying element from the molten solder toward the substrate and portions and "grooves" to the surface to form the IMC layer.

Instance

Various mixed alloy powder solder pastes spanning the range described herein for solder potency testing.

Table 1 illustrates the formulation of an exemplary hybrid solder paste made using a first alloy comprising Bi11Ag or Bi2.6Ag, a second alloy comprising Sn10Ag25Sb or Sn10Ag10Sb, and a flux.

Table 2 illustrates the formulation of an exemplary hybrid solder paste made using a first alloy comprising Bi11Ag, a second alloy comprising Sn3.8Ag0.7Cu, Sn3.5Ag, Sn0.7Cu or Sn9Zn, and a flux.

Table 2 Weight percentage of mixed solder alloy using Group B second alloy

Table 3 illustrates the formulation of an exemplary hybrid solder paste made using a first alloy comprising Bi11Ag, a second alloy comprising Bi42Sn, Bi33In or In48Sn, and a flux.

Each paste described in Tables 1, 2, and 3 was prepared and printed onto the test piece using a three-hole stainless steel stencil. Cu, Ni, alloy 42 and alumina test pieces were used. Each paste was printed onto each test piece. The diameter of the hole is 1/4 inch. The printed test piece was passed through a reflow oven to be refluxed by a quantitative curve designed to mix the alloy powder solder paste. The reflux was carried out under a N ₂ atmosphere at a belt speed of 13"/min in a three-zone reflow furnace at 380 ° C, 400 ° C and 420 ° C.

The wetting efficiency on Cu and Ni coupons was visually examined. All mixed solder alloys are in Improved wetting is shown when compared to a single BiAg solder paste. Figures 3 and 4 are pictures representing typical results. Figure 3 shows the wetting efficiency of an example of a mixed alloy powder solder paste consisting of 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux. The left image shows the reflow paste on the Cu coupon; the right image shows the reflow paste on the Alloy 42 coupon. Figure 4 shows the wetting efficiency of an example of a mixed alloy powder solder paste consisting of 84 wt% Bi11Ag + 6 wt% 52 In48Sn + 10 wt% flux. The left image shows the paste 401 reflowed on the Cu coupon 400; the right image shows the paste 402 reflowed over the Alloy 42 coupon 405.

The reflowed solder balls were peeled from the alumina coupon for DSC testing. Solder bumps formed on the Cu test piece and the Ni test piece were also punched out for DSC testing. DSC measurements were performed using a differential scanning calorimeter at a heating rate of 10 ° C/min. Representative DSC curves are shown in Figures 5-7. Figure 5 illustrates the DSC curve of a joint resulting from the use of 84 wt% Bi11Ag + 6 wt% Sn 15 Sb + 10 wt% flux. The top curve illustrates the heat flow curve after reflow on the ceramic coupon. A spike at about 138 °C illustrates the presence of the second alloy. The bottom curve illustrates the heat flow curve of the paste after reflow on the Cu coupon. The presence of this spike in the bottom curve verifies the disappearance of the low melting phase in the BiAg+SnSb system. Figure 6 illustrates the disappearance of the low melting phase in the BiAg+SnAg system. The experiment of Figure 6 used 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% flux on ceramic and Cu coupons, as in Figure 5. Figure 7 illustrates the disappearance in the BiAg+BiSn system. The experiment of Figure 7 used 84 wt% Bi11Ag + 6 wt% Bi42Sn + 10 wt% flux on ceramic and Cu coupons, as in Figures 5 and 6. In Figure 7, the top curve illustrating the heat flux curve after solder reflow on the ceramic shows the lack of a low melt phase. This is likely due to the high affinity between Sn and Ag, resulting in the incorporation of Sn of the second alloy into the IMC in the final solder bump.

The cross section of the sample is imaged to determine the IMC thickness at the interface between the solder bump and the Cu or Ni coupon. A representative image is shown in FIG. Figure 8a is a cross section of a solder bump using 84 wt% Bi11Ag + 6 wt% Sn 3.5 Ag + 10 wt% solder on a Cu coupon. Figure 8b is a cross section of a solder bump using the same solder paste on a Ni test strip. Such results show that The IMC layer thickness is limited to a few microns on both coupons.

In an embodiment, the second solder alloy is a Bi-Sb-Sn alloy composed of greater than 0 wt% to 40 wt% Sb, greater than 0 wt% to 40 wt% Sn, and Bi.

In an embodiment, the second solder alloy is a Bi-Sb-Sn-X alloy, wherein X = Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt or Zn. In embodiments of these embodiments, the Bi-Sb-Sn-X alloy is comprised of greater than 0 wt% to 40 wt% Sb, greater than 0 wt% to 40 wt% Sn, greater than 0 wt% to 5 wt% X, and Bi.

In an embodiment, the first solder alloy is a Bi-Ag-Y alloy, a Bi-Cu-Y alloy or a Bi-Ag-Cu-Y alloy, wherein the Y-based Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn. In embodiments of these embodiments, Y is in a range from greater than 0 wt% to 5 wt% of the first solder alloy.

In a further embodiment, the solder paste is comprised of a higher weight percentage of the second alloy powder, a lower weight percentage of the first alloy powder, or some combination thereof. In one such embodiment, the solder paste consists of a certain amount of the first solder alloy powder between 44 wt% and less than 60 wt% of the first alloy powder, a second alloy of greater than 0 wt% and less than 48 wt% The powder is composed of a certain amount of flux between 0 wt% and 15 wt%. In a particular embodiment of this embodiment, the amount of the second alloy powder can be greater than 2 wt% and less than 40 wt%.

In another such embodiment, the solder paste consists of a certain amount of the first solder alloy powder between 44 wt% and 87 wt%, a certain amount of the second solder alloy powder between 13 wt% and 48 wt%, and at 0 wt. A certain amount of flux between % and 15% by weight. In a particular embodiment of this embodiment, the amount of the second alloy powder can be between 26 wt% and 40 wt%.

In these further embodiments, a higher weight percentage of the second alloy powder, a lower weight percent of the first solder alloy powder, or a combination enhances the homogeneity of the solder paste. This improved homogeneity permits mixed alloy solder pastes to be applied to smaller joint formations, which provides both wetting and high temperature performance even for joints having a joint diameter at least as small as 0.2 mm. Furthermore, this combination of solder alloy powders permits even higher temperature solder joint temperatures after reflow. For example, in one such embodiment, the joint remelting temperature formed can be above 270 °C. In another such embodiment, the joint remelting temperature can be as high as 280 ° C or even higher.

While the various embodiments of the invention have been described, Likewise, the various figures may illustrate an example architecture or other configuration of the present invention, thereby assisting in understanding the features and functionality that may be included in the present invention. The invention is not limited to the illustrated exemplary architecture or configuration, but various alternative architectures and configurations can be used to implement the desired features. In fact, those skilled in the art will understand how alternative functions, logical or physical partitions and configurations can be implemented to implement the desired features of the present invention. Moreover, a number of different constituent module names other than those constituting the module names described herein can be applied to various partitions. In addition, with regard to the flowcharts, the operation descriptions, and the method claims, the order of the steps presented herein should not be construed as being limited to the various embodiments.

Although the invention has been described above in terms of various exemplary embodiments and embodiments, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability. With regard to specific embodiments in which such features, aspects, and functionality are set forth, the invention can be applied to one or more of the other embodiments of the invention, either individually or in various combinations, whether or not such embodiments are Whether or not such features are presented as part of the illustrated embodiment. Therefore, the breadth and scope of the invention should not be limited to any of the illustrative embodiments set forth above.

The terms and phrases used in this document, as well as variations thereof, are intended to be construed as an As an example of the foregoing, the term "comprising" is to be interpreted as meaning "including but not limited to" or the like; the term "example" is used to provide an illustrative item of the item in question, rather than its exhaustive or limiting. List; the term "a" or "an" should be interpreted to mean "at least one", "one or more" or the like; and such as "custom", "traditional", "normal", "standard", An adjective of a term "known" and similar terms should not be construed as limiting the stated item to a given time period or item that is available from a given time, but instead should be interpreted as encompassing the present or future Any time may be a customary, conventional, normal or standard technique that is available or known. Likewise, where the document refers to techniques that will be apparent or known to those skilled in the art, such technology encompasses those skilled in the art, either now or in the future.

The existence of broad terms such as "one or more", "at least", "but not limited to" and the use of phrases or other similar phrases in certain instances should not be construed as meaning that there may be no such A narrower case is expected or required in the term. The use of the term "module" does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. In fact, any or all of the various components of the module (whether control logic or other components) may be combined in a single package or separately maintained and may be further distributed in multiple packets or packages or across multiple locations. .

In addition, the various embodiments set forth herein are set forth in the description of the exemplary figures, FIG. The illustrated embodiments and their various alternatives can be implemented without limitation to the illustrated examples, as will be apparent to those skilled in the art. For example, the block diagrams and their accompanying description should not be construed as an admission of a particular architecture or configuration.

100‧‧‧melting the first alloy/first alloy

103‧‧‧solution

106‧‧‧Mixture

109‧‧‧IMC layer/IMC

112‧‧‧Second alloy/fused solder alloy/melted second alloy

115‧‧‧Second alloy solder particles/second alloy

118‧‧‧First alloy solder particles / first alloy particles / first alloy

121‧‧‧ solder bumps

124‧‧‧Substrate

Tm(A)‧‧‧ melting temperature of the first alloy

Tm(B)‧‧‧ melting temperature of the second alloy

Claims (32)

A solder paste comprising: a certain amount of a first solder alloy powder between 44 wt% and less than 60 wt%; a certain amount of a second solder alloy powder between greater than 0 wt% and 48 wt%; a flux; wherein the first solder alloy powder comprises a first solder alloy having a solidus temperature above 260 ° C; and wherein the second solder alloy powder comprises a second solder alloy having a solidus temperature of less than 250 ° C. A solder paste according to claim 1, wherein the second solder alloy has a solidus temperature between 230 ° C and 250 ° C. The solder paste of claim 2, wherein the second solder alloy comprises a Sn alloy, a Sn-Sb alloy or a Sn-Sb-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn , Ni, P, Pd, Pt or Zn) alloys. A solder paste according to claim 1, wherein the second solder alloy has a solidus temperature between 200 ° C and 230 ° C. The solder paste of claim 4, wherein the second solder alloy comprises a Sn-Ag alloy, a Sn-Cu alloy, and a Sn-Ag-X (X=Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn , Ni, P, Pd, Pt, Sb or Zn) alloy or Sn-Zn alloy. The solder paste of claim 1, wherein the second solder alloy has a solidus temperature below 200 °C. The solder paste of claim 6, wherein the second solder alloy comprises a Sn-Bi alloy, a Sn-In alloy, a Bi-In alloy, and a Sn-Bi-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy, Sn-In-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy or Bi-In-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy. The solder paste of claim 1, wherein the amount of the second solder alloy powder is between 2% by weight and 40% by weight. The solder paste of claim 1, wherein the second solder alloy is a Bi-Sb-Sn alloy composed of more than 0 wt% to 40 wt% Sb, more than 0 wt% to 40 wt% Sn, and Bi. The solder paste of claim 1, wherein the second solder alloy is Bi-Sb-Sn-X composed of more than 0 wt% to 40 wt% Sb, more than 0 wt% to 40 wt% Sn, more than 0 wt% to 5 wt% X, and Bi composition. (X = Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt or Zn) alloy. The solder paste of claim 1, wherein the first solder alloy comprises a Bi-Ag alloy, a Bi-Cu alloy, or a Bi-Ag-Cu alloy. The solder paste of claim 1, wherein the first solder alloy comprises a Bi-Ag-Y alloy, a Bi-Cu-Y alloy or a Bi-Ag-Cu-Y alloy, wherein the Y system is composed of Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn, and wherein the Y system is in a range from more than 0 wt% to 5 wt%. The solder paste of claim 1, wherein the first solder alloy comprises: from greater than 0 wt% to 20 wt% Ag, wherein the remainder is Bi; from greater than 0 wt% to 5 wt% Cu, wherein the remainder is Bi; or from greater than 0 wt % to 20 wt% Ag and from 0 to 5 wt% Cu, with the remainder being Bi. The solder paste of claim 1, wherein the first solder alloy comprises: from 2.6 wt% to 15 wt% Ag, wherein the remainder is Bi; from 0.2 wt% to 1.5 wt% Cu, wherein the remainder is Bi; or from 2.6 From wt% to 15 wt% Ag and from 0.2 wt% to 1.5 wt% Cu, with the remainder being Bi. A solder paste comprising: a certain amount of a first solder alloy powder between 44 wt% and 87 wt%; a quantity of the second solder alloy powder between 13 wt% and 48 wt%; and a flux; wherein the first solder alloy powder comprises a first solder alloy having a solidus temperature above 260 ° C; and wherein the second The solder alloy powder includes a second solder alloy having a solidus temperature of less than 250 °C. The solder paste of claim 15 wherein the second solder alloy has a solidus temperature between 230 ° C and 250 ° C. The solder paste of claim 16, wherein the second solder alloy comprises a Sn alloy, a Sn-Sb alloy or a Sn-Sb-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn). , Ni, P, Pd, Pt or Zn) alloys. The solder paste of claim 15, wherein the second solder alloy has a solidus temperature between 200 ° C and 230 ° C. The solder paste of claim 18, wherein the second solder alloy comprises a Sn-Ag alloy, a Sn-Cu alloy, and a Sn-Ag-X (X=Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn). , Ni, P, Pd, Pt, Sb or Zn) alloy or Sn-Zn alloy. The solder paste of claim 15 wherein the second solder alloy has a solidus temperature below 200 °C. The solder paste of claim 20, wherein the second solder alloy comprises a Sn-Bi alloy, a Sn-In alloy, a Bi-In alloy, and a Sn-Bi-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy, Sn-In-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, An alloy of Pd, Pt, Sb or Zn) or Bi-In-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn). The solder paste of claim 15, wherein the amount of the second solder alloy powder is between 26 wt% and 40 wt%. The solder paste of claim 15, wherein the second solder alloy is a Bi-Sb-Sn alloy composed of greater than 0 wt% to 40 wt% Sb, greater than 0 wt% to 40 wt% Sn, and Bi. The solder paste of claim 15, wherein the second solder alloy is Bi-Sb-Sn-X composed of more than 0 wt% to 40 wt% Sb, more than 0 wt% to 40 wt% Sn, more than 0 wt% to 5 wt% X, and Bi composition. (X = Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt or Zn) alloy. The solder paste of claim 15, wherein the first solder alloy comprises a Bi-Ag alloy, a Bi-Cu alloy, or a Bi-Ag-Cu alloy. The solder paste of claim 15, wherein the first solder alloy comprises a Bi-Ag-Y alloy, a Bi-Cu-Y alloy or a Bi-Ag-Cu-Y alloy, wherein the Y system is composed of Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn, and wherein the Y system is in a range from more than 0 wt% to 5 wt%. The solder paste of claim 15 wherein the first solder alloy comprises: from greater than 0 wt% to 20 wt% Ag, wherein the remainder is Bi; from greater than 0 wt% to 5 wt% Cu, wherein the remainder is Bi; or from greater than 0 wt % to 20 wt% Ag and from 0 to 5 wt% Cu, with the remainder being Bi. The solder paste of claim 15, wherein the first solder alloy comprises: from 2.6 wt% to 15 wt% Ag, wherein the remainder is Bi; from 0.2 wt% to 1.5 wt% Cu, wherein the remainder is Bi; or from 2.6 From wt% to 15 wt% Ag and from 0.2 wt% to 1.5 wt% Cu, with the remainder being Bi. The solder paste of claim 1, wherein the solidus temperature of the first solder alloy is less than 320 °C. The solder paste of claim 1, wherein: the first solder alloy is a Bi-Ag alloy, a Bi-Cu alloy, a Bi-Ag-Cu alloy or a Bi-Ag-Cu-Y alloy, wherein the Y system is composed of Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn; The second solder alloy is a Bi-Sn alloy, a Bi-In alloy, a Bi-Sb-Sn alloy, and a Sn-Bi-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni). , P, Pd, Pt, Sb or Zn) alloy, Sn-In-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn Alloy, Bi-In-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy or Bi-Sb-Sn-X (X =Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt or Zn) alloys. The solder paste of claim 15, wherein the solidus temperature of the first solder alloy is less than 320 °C. The solder paste of claim 15 wherein: the first solder alloy is a Bi-Ag alloy, a Bi-Cu alloy, a Bi-Ag-Cu alloy or a Bi-Ag-Cu-Y (Y=Al, Au, Co, Ga , Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn) alloy; and the second solder alloy is Bi-Sn alloy, Bi-In alloy, Bi-Sb-Sn alloy, Sn-Bi -X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb or Zn) alloy, Sn-In-X (X=Ag, Al, Au) , Bi, Co, Cu, Ga, Ge, Mn, Ni, P, Pd, Pt, Sb or Zn) alloys, Bi-In-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, Mn , Ni, P, Pd, Pt, Sb or Zn) alloy or Bi-Sb-Sn-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt Or Zn) alloy.