TWI681063B - 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℃; and wherein the second solder alloy powder comprises a second solder alloy that has a solidus temperature that is less than 250℃. 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 a partial continuation of US Patent Application No. 14/629,139 filed on February 23, 2015. This US Patent Application is US Patent Application No. 12/772,897 filed on May 3, 2010 The continuation of the case, the two US patent applications are incorporated by reference in their entirety.

The present invention relates generally to compositions of mixed alloy solder pastes, and more specifically, certain embodiments relate to compositions of each alloy component in solder pastes used in high temperature solder joint applications.

Lead generated by discarding electronic assemblies is considered harmful to the environment and human health. Regulations increasingly prohibit the use of Pb solder in the electronic interconnection and electronic packaging industries. Pb-free solder to replace traditional eutectic Pb-Sn has been extensively studied. SnAg, SnCu, SnAgCu and SnZn solders are becoming mainstream solders for interconnection 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 the initial stage. High temperature solder is used to maintain internal connections within components in the assembly while the assembly is being soldered to a printed wiring board (PWB).

The common use of high temperature solder is for die attach. In an exemplary procedure, the assembly is formed by soldering silicon die to the lead frame using high temperature solder. Then, attach the silicon die/lead frame assembly (encapsulated or unencapsulated) to the PWB by soldering or mechanical fastening on. The board can be exposed to a few more reflow procedures for surface mounting other electronic devices to the board. During the further soldering process, the internal connection between the silicon die and the lead frame should be well maintained. This requires high temperature solder to resist multiple reflows without any functional failure. Therefore, in order to be compatible with the solder reflow profile used in industry, the main requirements for high temperature solders include (i) a melting temperature of approximately 260°C and higher than 260°C (based on typical solder reflow profile), (ii ) Good thermal fatigue resistance, (iii) high thermal conductivity/conductivity, and (iv) low cost.

Currently, there are no temporary lead-free alternatives that can be used 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.

Sn-Sb alloys with less than 10wt% Sb maintain good mechanical properties without forming large amounts of intermetallic compounds. But its solidus temperature is not higher than 250℃, which cannot meet the requirement of 260℃ for reflow resistance.

Zn-based alloys (including eutectic Zn-Al, Zn-Al-Mg, and Zn-Al-Cu) have melting temperatures higher than 330°C. However, the high affinity of Zn, Al, and Mg for oxygen leads to extremely poor wetting on various metallized surface treatment layers. The Zn-(20-40wt%) Sn solder alloy proposed as one of the high-temperature lead-free replacement solders has a liquidus temperature higher than 300°C, but the solidus temperature is only about 200°C. The semi-solid state of Zn-Sn solder at about 260°C should maintain good interconnection between components during subsequent reflow. However, problems arise when the semi-solid solder is compressed inside the encapsulated package and forces the semi-solid solder out. This creates a risk of unexpected functional failure. Zn-based solder alloys will also form a large number of IMC layers between the metalized 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 and thermal conductivity, and excellent corrosion resistance. However, the extremely high cost limits its cost over reliability Application in the field of consideration.

The Bi-Ag alloy with a solidus temperature of 262°C meets the melting temperature requirements for high-temperature grain-attached solder. However, there are several major concerns: (1) poor wetting on various surface treatment layers and (2) the associated weak bonding interface resulting 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 solder restricts its acceptance by 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 between the solder and the substrate metallization chemistry (in the Bi-Ag solder system or even such as Pb-Cu and Pb -The poor wetting of certain lead-containing solders in the Ag system makes these high-melting solders difficult to use in industry due to the weak bonding strength caused by poor wetting. However, the high melting temperature of BiAg and ZnAl still makes it a qualified candidate for high-temperature lead-free solder.

As explained above, the poor wetting of solder originates from (1) adverse reaction chemistry or (2) oxidation of solder. Weak bonding is always associated with poor wetting. For example, the poor wetting of Bi-based solder on different metallized surfaces is mainly caused by the bad reaction chemistry between Bi and the substrate material (ie, Cu) during reflow or the oxidation of Bi. Ge-doped BiAg has been developed to prevent excessive formation of scum on the alloy surface during melting. However, this doping will not change the reaction chemistry between Bi and the metalized surface treatment layer of the substrate. Bi and Cu will not form IMC at the Bi/Cu interface, which is the leading cause of poor wetting and weak bonding interface. Bi and Ni will form an IMC layer between the Bi/Ni interface, but the fragile IMC (Bi3Ni or BiNi) weakens the joint strength, because the crack always runs along the interface between Bi3Ni and solder matrix or between BiNi and Ni substrate Interface growth. 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 metalized surface treatment layer by fusing additional elements into the solder. However, fusion is usually 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 into BiAg (where Ag aims to increase thermal conductivity/conductivity) can lead to (1) a significant reduction in melting temperature or (2) the formation of Ag3Sn IMC in the alloy. If there is not enough time for 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. Therefore, fusing the elements directly into the solder (such as Sn directly into the Bi-Ag alloy) reveals the smallest improvement.

The present invention advocates a new technology for designing and preparing mixed alloy solder pastes that realize the combined advantages from the constituent alloy powders. In some embodiments, the mixed alloy solder paste is suitable for high temperature solder applications (such as grain attachment) because the composition provides the desired advantages, including correspondingly improved reaction chemistry from the second alloy, well controlled The thickness and enhanced reliability of the IMC layer and the high melting temperature and good thermal conductivity/conductivity from the first alloy. The invention also presents a method of preparing a mixed alloy solder paste and a method of using the mixed alloy solder paste to combine electronic components or mechanical parts.

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

In some embodiments, the additives will melt before or together with the melting of the main solder. The melting additives will wet and adhere to the substrate before or together with the first alloy that is partially or completely melted. These additives are designed to govern the formation of the IMC metallization surface treatment layer along the substrate and are fully converted to IMC during the reflow process. The thickness of the IMC layer will therefore be added due to the paste The leading role of additives in the formation of IMC is well controlled by the amount of these additives. In some 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 bonding strength between the solder body and the IMC. Therefore, the desired reaction chemistry and well-controlled thickness of the IMC layer not only improve the wetting performance, but also enhance the bonding strength associated with the wetting performance.

The following detailed description in conjunction with the accompanying drawings will illustrate other features and aspects of the present invention. The accompanying drawings illustrate the features of the embodiments according to the present invention by way of examples. The summary of the invention is not intended to limit the scope of the invention, which is defined solely by the scope of the patent application attached to the invention.

100‧‧‧Melted first alloy/first alloy

103‧‧‧ solution

106‧‧‧Mixture

109‧‧‧IMC layer/IMC

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

115‧‧‧Second alloy solder particles/second alloy

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

121‧‧‧Solder bump

124‧‧‧ substrate

200‧‧‧Cu substrate

201‧‧‧Solder bump

202‧‧‧Dewetting

205‧‧‧Alloy 42 substrate

206‧‧‧Dewetting

207‧‧‧Solder bump

210‧‧‧Cu substrate

211‧‧‧Solder bump

215‧‧‧Alloy 42 substrate

216‧‧‧Solder bump

300‧‧‧Cu test piece/Cu

301‧‧‧IMC

302‧‧‧ rich in Bi phase

303‧‧‧Ag

400‧‧‧Cu test piece

401‧‧‧Ointment

402‧‧‧Ointment

405‧‧‧Alloy 42 test piece

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

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

According to one or more various embodiments, the present invention is explained in detail with reference to the following figures. The drawings are provided for illustrative purposes only and only show typical or exemplary embodiments of the invention. These drawings are provided to promote the reader's understanding of the present invention and should not be considered as limiting the breadth, scope, or applicability of the present invention. It should be noted that for clarity and convenience of illustration, these drawings are not necessarily to scale.

FIG. 1 illustrates a reflow soldering process implemented according to an embodiment of the present invention.

Figure 2 shows the wetting performance of an exemplary solder paste composed of 90wt%Bi10.02Ag3.74Sn+10wt% flux on Cu test piece and Alloy 42 test piece.

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

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

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

Fig. 6 is a DSC chart of a mixed alloy powder solder paste composed of 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux.

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

8A and 8B are cross-sectional images of joints made of mixed alloy powder solder paste on Cu and Ni test pieces. The mixed alloy powder paste is composed of 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux.

The drawings are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It should be understood that the present invention can be practiced with modifications and alterations, and the present invention is limited only by the scope of patent applications and their equivalents.

The present invention is directed to solder paste including a mixture of different solder alloys in the 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 reaction chemistry with the metal substrate or the 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. When the first alloy melts, the presence of the second alloy promotes melting the placement of the first alloy on the substrate. 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 bonding strength. In addition to solders used for high-temperature lead-free soldering with desired wetting and reliability, the design process can also be extended to many other soldering applications regardless of where the poorly wet solder is used. For example, Pb-Cu alloy has a high melting temperature, but has poor wetting on various metal substrates. Therefore, Pb-Cu alloy is difficult to weld Used in connection. Regarding the present invention, small additives such as Sn or Sn-containing alloys will help Pb-Cu to wet various metal surfaces. However, if Sn is only fused in Pb-Cu, the formation of Cu6Sn5 IMC will reduce the reaction chemistry from Sn. Fusion of a higher amount of Sn in solder will significantly lower the melting temperature of Pb-Cu, which is not desirable.

FIG. 1 illustrates a reflow process using mixed solder paste according to an embodiment of the present invention. The mixed solder paste includes first alloy solder particles 118 and second alloy solder particles 115 suspended in the flux. In some embodiments, the second alloy is selected based on its excellent reaction chemistry with the substrate or a range of common substrates. The mixed solder paste is applied to the substrate 124. (For illustration, 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 melts and spreads over the substrate 124 and around the first alloy particles 118 that are still solid. The excellent surface reaction chemistry of the second alloy will promote wetting of the molten solder alloy 112 on the substrate 124. This results in the formation of the IMC layer 109 between the molten second alloy 112 and the substrate 124. Therefore, the IMC is mainly controlled by the amount of the second alloy 115 in the initial paste.

In addition, the second alloy is designed to have a good affinity for the first alloy. This affinity can be determined by: (1) the negative mixing enthalpy between the first alloy and the second alloy, or (2) the 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 the solution 103, thereby increasing the IMC layer 109, and leaving the first alloy 100 molten. In some embodiments, in addition to forming the IMC layer 109, excess components from the second alloy may also be incorporated into the IMC along with components from the first alloy. The affinity between the first alloy and the second alloy assists in improving the Wetting of an alloy 100 onto the IMC layer 109, thereby enhancing the bonding strength.

When cooling the assembly, the solder bump 121 or the joint is formed by the substrate 124 bonded to the IMC 109, which is bonded to the solidified first alloy. After curing, a homogeneous solder joint with an improved bonding interface has been achieved.

The solder joints produced by the use of mixed solder paste demonstrate a substantial improvement in the use of solder paste containing a single solder alloy, even when the single solder alloy is composed of elements of the first and second solder alloys. 2 illustrates solder bumps 201 and 207 formed on a Cu substrate 200 and an alloy 42 substrate 205 using solder paste composed of 86.24wt%Bi10.02Ag3.74Sn+10wt% flux, respectively. These 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 the Cu substrate 210 and the alloy 42 substrate 215 using a mixed solder paste composed of 84wt%Bi11Ag+6wt%Bi42Sn+10wt% flux, respectively. Such results show that the use of mixed solder paste shows 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 reaction chemistry with various substrates that is superior to Bi. SnSb has a melting temperature lower than BiAg. According to the binary phase diagram, Sn and Bi exhibit a negative mixing enthalpy and form a eutectic phase in a wide range of compositions. Sb and Bi also exhibit negative mixing enthalpy and infinite solubility with each other. During reflow, SnSb first melts and forms an Sn-containing IMC layer on the substrate surface. When the temperature reaches above the melting temperature of BiAg, all alloy powders in the paste melt. The good affinity between Bi and Sn/Sb ensures good adhesion of molten Bi on the Sn-containing IMC layer. In addition, the presence of Ag in the first alloy can convert any additional Sn into Ag3Sn IMC that resides in the solder body. Therefore, a minimum or no remaining low-melting BiSn phase is left because Sn is completely consumed by forming (1) an IMC layer between the solder and the metal substrate and (2) Ag3Sn inside the BiAg solder bump.

Figure 5 illustrates the production of 84wt%Bi11Ag+6wt%Sn15Sb+10wt% flux DSC curve of the raw joint. The top graph illustrates the heat flux curve after reflow on the ceramic test piece. The peak at about 138°C illustrates the presence of the second alloy. The bottom curve illustrates the heat flux curve of the paste after reflow on the Cu test piece. 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 in FIG. 6 uses 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux on the ceramic and Cu test pieces, as shown in FIG. 5. Figure 7 illustrates the disappearance in the BiAg+BiSn system. The experiment of FIG. 7 uses 84wt%Bi11Ag+6wt%Bi42Sn+10wt% flux on ceramic and Cu test pieces, as shown in FIGS. 5 and 6. In Figure 7, the top curve of the heat flux curve after solder reflow on the ceramic is illustrated to show the lack of low melting phase. This is probably due to the small amount of Sn in the mixed solder paste and the high affinity between Sn and Ag, resulting in the Sn of the second alloy being incorporated into the IMC of the final solder bump together with some Ag of the first alloy in.

8A is a micrograph of a solder joint produced by using a mixed solder paste (consisting of 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux). In this example, mixed solder paste is applied to the Cu test piece 300. IMC 301 is formed between Cu 300 and the second alloy. The size of this IMC 301 mainly depends on the amount of the second alloy in the phase. In the illustrated example, 6wt% of the second alloy Sn3.5Ag produces an IMC that is only a few microns thick. The bulk of the solder joint is composed of Ag 303 rich in 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 intermetallic compounds, so Bi11Ag alone forms a weak junction, because there is no IMC layer between the solder and the substrate.

In one embodiment of the invention, the method of designing a mixed solder paste includes selecting a first alloy based on the desired characteristics of the completed solder joint, and then selecting a second alloy based on the applicable substrate and the 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 desired IMC layer thickness, required application conditions, and reflow procedures. The thickness of the IMC layer, the amount of the second alloy in the solder paste, the return flow curve and the compliance Related 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 the Cu6Sn5/Cu3Sn IMC layer, 10 microns may be approximately as thick as acceptable.

When increasing the amount of the second alloy in the paste, a low melting 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 total amount of paste printed or applied on 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 various high-melting solder alloys. In certain embodiments, a Bi-rich alloy is used, the solidus temperature of which is approximately 258°C and above, that is, Bi-Ag, Bi-Cu, and Bi-Ag-Cu. The second alloy (or additive) is selected from alloys that have been shown to have excellent chemical properties for wetting and adhering to various metallized surface treatment layers and good affinity for molten Bi.

In such embodiments, the second alloy will melt before or together with the Bi-rich alloy and then easily wet and adhere to the substrate on the substrate. At the same time, a 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. Based on the melting temperature of the selected second alloy, three groups have been classified. Group A contains additive alloys with solidus temperatures between approximately 230°C and 250°C, ie Sn, Sn-Sb, Sn-Sb-X (X=Ag, Al, Au, Bi, Co, Cu , Ga, Ge, In, Mn, Ni, P, Pd, Pt and Zn) alloys, etc. Group B contains solder alloys with solidus temperatures between approximately 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), Sn-Zn alloy, etc. Group C contains solder alloys with a solidus temperature below 200°C, ie 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) alloy, etc. In these alloys, Sn and/or In system reactants.

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

In the second embodiment of the present invention, the first alloy is selected from the Bi-Cu system and has a solidus temperature of about 270°C and above. In certain embodiments, the first alloy includes from 0 to 5 wt% Cu, with the remainder being Bi. In yet another embodiment, the first alloy includes from 0.2 to 1.5 wt% Cu, where the remainder is 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. In certain embodiments, the first alloy includes 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 includes 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 certain embodiments, the second alloy includes from 0 to 20 wt% Sb, with the remainder being Sn. In yet another embodiment, the second alloy includes from 0 to 15 wt% Sb, with the remainder being Sn.

In the 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 specific embodiment, the second alloy includes from 0 to 20 wt% Sb and from 0 to 20 wt% X, where the remainder is Sn. In yet another embodiment, the second alloy includes from 0 to 10 wt% Sb and from 0 to 5 wt% X, where the remainder is Sn.

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

In the seventh embodiment of the present invention, the second alloy includes Sn-Cu and has a solidus temperature of about 227°C and above. In certain embodiments, the second alloy includes from 0 to 5 wt% Cu, with the remainder being Sn. In yet another embodiment, the second alloy includes from 0 to 2 wt% Cu, wherein the remaining part is Sn.

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

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

In the tenth embodiment of the present invention, the second alloy includes a Bi-Sn alloy having a solidus temperature of about 139°C and above. In a specific embodiment, the second alloy includes from 8 to 80 wt% Sn, where the remainder is Bi. In yet another embodiment, the second alloy includes from 30 to 60 wt% Sn, where the remainder is Bi.

In an eleventh embodiment of the present invention, the second alloy includes a Sn-In alloy and has a solidus temperature of about 120°C and above. In certain embodiments, the second alloy includes from 0 to 80 wt% In, with the remainder being Sn. In yet another embodiment, the second alloy includes from 30 to 50 wt% In, where the remainder is Sn.

In a twelfth embodiment of the invention, the second alloy includes a Bi-In alloy with a solidus temperature between about 100°C and about 200°C. In a specific embodiment, the second alloy includes from 0 to 50 wt% In, where the remainder is Bi. In certain embodiments, the second alloy includes from 20 to 40 wt% In, with the remainder being Bi.

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

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

In the fifteenth embodiment, the second alloy system 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 includes from 0 to 20 wt% Ag, 0 to 20 wt% X, where the remainder is In. In a specific embodiment, the second alloy includes from 0 to 10 wt% Ag, 0 to 5 wt% X, where the remainder is In.

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

In a further embodiment, the second alloy includes 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 Alloy at a solidus temperature of about 200°C. In some In an embodiment, the paste is composed of a first alloy powder between about 60% and about 92% by weight, a second alloy powder in a quantity greater than 0% but less than or equal to about 12% by weight, and the rest is flux. In a further embodiment, the second alloy powder is a mixed solder paste between 2 wt% and 10 wt%.

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

In a further embodiment, the second alloy powder includes powder composed of a plurality of alloy powders. For example, the second alloy powder may include 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 maintaining a low melting phase in the final solder joint can be increased. In some cases, when the amount of the second solder alloy in the paste is below a certain threshold, wetting of the substrate can be reduced. In one embodiment, the amount of the second solder alloy in the paste is determined so that the low melting phase can be completely converted into a high melting IMC after reflow. In yet another embodiment, the second alloy in the paste varies between an amount greater than 0 wt% but less than about 12 wt%. In certain embodiments, 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 can also be added or incorporated into these alloys, as long as the reactive properties of Sn are maintained.

In some embodiments, the reflow profile used for soldering with mixed solder paste is designed to quickly heat up above the melting temperature of the first alloy. In such cases, the shorter soaking time at low temperature may allow reactants (such as Sn) to flow rapidly toward the substrate and react with the substrate in a complete melting pool rather than a semi-solid melting pool. The melting of both the first and second alloys will promote the diffusion of the second alloy element from the molten solder toward the substrate and parts and "grooves" onto the surface to form an IMC layer.

Examples

For solder performance testing of various mixed alloy powder solder pastes that span the range described in this article.

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

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

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

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

Each paste described in Table 1, Table 2 and Table 3 was prepared and printed on the test piece using a three-hole stainless steel template. Cu, Ni, alloy 42 and alumina test pieces were used. Print each paste on each test piece. The diameter of the hole is 1/4 inch. The printed test piece was passed through a reflow oven to reflow using a quantity curve designed for mixed alloy powder solder paste. In at 380 ℃, three-zone reflow oven deg.] C and 400 to 420 ℃ belt speed of 13 "/ minute at reflux under an atmosphere of performing N _2.

Visually inspect the wetting efficiency of Cu and Ni test pieces. All mixed solder alloys It shows improved wetting when compared to a single BiAg solder paste. Figures 3 and 4 are pictures representing typical results. Figure 3 shows the wetting performance of an example of a mixed alloy powder solder paste consisting of 84wt%Bi11Ag+6wt%Bi42Sn+10wt% flux. The left image shows the reflowed paste on the Cu test piece; the right image shows the reflowed paste on the Alloy 42 test piece. Figure 4 shows the wetting performance of an example of a mixed alloy powder solder paste composed of 84wt%Bi11Ag+6wt%52In48Sn+10wt% flux. The left image shows the reflowed paste 401 on the Cu test piece 400; the right image shows the reflowed paste 402 on the Alloy 42 test piece 405.

The reflowed solder ball was peeled from the alumina test piece for DSC test. The solder bumps formed on the Cu test piece and the Ni test piece were also punched out for DSC test. A differential scanning calorimeter was used to perform DSC measurement at a heating rate of 10°C/min. Representative DSC curves are shown in Figures 5-7. Figure 5 illustrates the DSC curve of the joint produced by the use of 84wt%Bi11Ag+6wt%Sn15Sb+10wt% flux. The top graph illustrates the heat flux curve after reflow on the ceramic test piece. The peak at about 138°C illustrates the presence of the second alloy. The bottom curve illustrates the heat flux curve of the paste after reflow on the Cu test piece. 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 in FIG. 6 uses 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux on the ceramic and Cu test pieces, as shown in FIG. 5. Figure 7 illustrates the disappearance in the BiAg+BiSn system. The experiment of FIG. 7 uses 84wt%Bi11Ag+6wt%Bi42Sn+10wt% flux on ceramic and Cu test pieces, as shown in FIGS. 5 and 6. In Fig. 7, the top curve of the heat flux curve after solder reflow on the ceramic is illustrated to show the lack of low melting phase. This is likely due to the high affinity between Sn and Ag, which leads to the incorporation of Sn of the second alloy into the IMC in the final solder bump.

The cross-section of the sample was imaged to determine the IMC thickness at the interface between the solder bump and the Cu or Ni test piece. Representative images are shown in Figure 8. Fig. 8a is a cross section of a solder bump using 84wt%Bi11Ag+6wt%Sn3.5Ag+10wt% flux on a Cu test piece. Fig. 8b is a cross section of a solder bump using the same solder paste on a Ni test piece. Such results show that The thickness of the IMC layer is limited to a few microns on the two test pieces.

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

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

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

In a further embodiment, the solder paste is composed 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 first solder alloy powder between 44 wt% and less than 60 wt% of the first alloy powder, a second alloy of a certain amount greater than 0 wt% and less than 48 wt% Powder and a certain amount of flux between 0wt% and 15wt%. In a specific implementation of this embodiment, the amount of the second alloy powder may be greater than 2wt% and less than 40wt%.

In another such embodiment, the solder paste consists of a certain amount of first solder alloy powder between 44wt% and 87wt%, a certain amount of second solder alloy powder between 13wt% and 48wt%, and between 0wt A certain amount of flux composition between% and 15wt%. In a specific implementation of this example, the amount of second alloy powder may be between 26 wt% and 40 wt%.

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

Although various embodiments of the present invention have been described above, it should be understood that the various embodiments have been presented by way of example only and not limitation. Likewise, various drawings may illustrate exemplary architectures or other configurations of the invention, thereby assisting in understanding the features and functionality that may be included in the invention. The invention is not limited to the illustrated example architecture or configuration, but various alternative architectures and configurations may 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, many different component module names other than those shown in this document can be applied to various partitions. In addition, with regard to flowcharts, operating instructions, and method request items, the order of steps presented herein should not authorize the various embodiments to be implemented in the same order to perform the stated functionality unless the context of the content specifies otherwise.

Although the invention has been described above in terms of various exemplary embodiments and implementations, 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 the specific embodiments in which these features, aspects, and functionality are described, they may alternatively be applied to one or more of the other embodiments of the present invention, singly or in various combinations, whether or not such embodiments are described and Regardless of whether such features are present as part of the illustrated embodiment. Therefore, the breadth and scope of the present invention should not be limited by any of the exemplary embodiments set forth above.

Terms and phrases used in this document and their variations should be interpreted as open-ended and not restrictive, unless expressly stated otherwise. As an example of the foregoing: the term "including" should be interpreted to mean "including but not limited to" or the like; the term "instance" is used to provide illustrative examples of the item in question, rather than its exhaustiveness or limitation Checklist; the term "a (an or an)" should be interpreted to mean "at least one", "one or more" or the like; and such as "conventional", "traditional", "normal", "standard", Adjectives of "known" and similarly-meaning terms should not be interpreted as limiting the items described to a given period of time or available from a given time, but should instead be read to encompass the present or the future Any time can be available or known custom, traditional, normal or standard technology. Likewise, where this document mentions technologies that will be known or known by those skilled in the art, these technologies include those technologies that are known or known or known at any time in the future by those skilled in the art.

The presence of broad terms such as "one or more", "at least", "but not limited to" and phrases or other similar phrases in certain instances should not be interpreted as meaning that such broad terms may not exist Narrow cases are expected or needed in the language examples. The use of the term "module" does not imply that all 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) can be combined in a single package or maintained separately and can be further distributed in multiple groups or packages or across multiple locations .

In addition, the various embodiments set forth herein are set forth in terms of exemplary block diagrams, flowcharts, and other illustrations. As those skilled in the art will understand after reading this document, the illustrated embodiments and their various alternatives may be implemented without being limited to the illustrated examples. For example, the block diagram and accompanying description should not be interpreted as authorizing a specific architecture or configuration.

100‧‧‧Melted first alloy/first alloy

103‧‧‧ solution

106‧‧‧Mixture

109‧‧‧IMC layer/IMC

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

115‧‧‧Second alloy solder particles/second alloy

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

121‧‧‧Solder bump

124‧‧‧ substrate

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

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

Claims (15)

A solder paste consisting of: a first solder alloy powder in an amount between 44wt% and 74wt%; a second solder alloy powder in an amount between 26wt% and 40wt%; and a flux; wherein the The first solder alloy powder is composed of a first solder alloy having a solidus temperature above 260°C, wherein the first solder alloy is Bi-Ag alloy, Bi-Ag-Y alloy, Bi-Cu alloy, Bi -Cu-Y alloy, Bi-Ag-Cu alloy or Bi-Ag-Cu-Y alloy, where Y series Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn Or Zn; and wherein the second solder alloy powder is composed of a second solder alloy or solder having a solidus temperature of less than 250°C, wherein the second solder alloy or solder is Sn, Sn alloy, In, In alloy Or Bi alloy. The solder paste of claim 1, wherein the second solder alloy or solder has a solidus temperature between 230°C and 250°C. The solder paste according to claim 2, wherein the second solder alloy is Sn alloy, Sn-Sb alloy or Sn-Sb-X (X=Ag, Al, Au, Bi, Co, Cu, Ga, Ge, In, Mn , Ni, P, Pd, Pt or Zn) alloy. The solder paste according to claim 1, wherein the second solder alloy or solder has a solidus temperature between 200°C and 230°C. The solder paste according to claim 4, wherein the second solder alloy is Sn-Ag alloy, Sn-Cu alloy, 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 according to claim 1, wherein the second solder alloy or solder has a solidus temperature below 200°C. The solder paste according to claim 6, wherein the second solder alloy is Sn-Bi alloy, Sn-In alloy, Bi-In 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) 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 according to claim 1, 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 the remainder being Bi. The solder paste according to claim 1, wherein the second solder alloy is Bi-Sb- composed of greater than 0wt% to 40wt% Sb, greater than 0wt% to 40wt% Sn, greater than 0wt% to 5wt% X, and the remainder being Bi Sn-X (X=Ag, Al, Au, Co, Cu, Ga, Ge, In, Mn, Ni, P, Pd, Pt or Zn) alloy. The solder paste according to claim 1, wherein the first solder alloy is Bi-Ag alloy, Bi-Cu alloy or Bi-Ag-Cu alloy. The solder paste according to claim 1, wherein the first solder alloy is Bi-Ag-Y alloy, Bi-Cu-Y alloy or Bi-Ag-Cu-Y alloy, wherein Y is composed of Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn or Zn, and wherein Y is in the range from more than 0wt% to 5wt%. The solder paste according to claim 1, wherein the first solder alloy includes: from greater than 0wt% to 20wt% Ag, wherein the remaining portion is Bi; from greater than 0wt% to 5wt% Cu, wherein the remaining portion is Bi; or from greater than 0wt % To 20wt% Ag and from 0 to 5wt% Cu, the remainder of which is Bi. The solder paste according to claim 1, wherein the first solder alloy includes: from 2.6 wt% to 15 wt% Ag, where the remaining portion is Bi; from 0.2 wt% to 1.5 wt% Cu, where the remaining portion is Bi; or from 2.6 wt% to 15wt% Ag and from 0.2wt% to 1.5wt% Cu, The remaining part is Bi. The solder paste according to claim 1, wherein the solidus temperature of the first solder alloy is lower than 320°C. The solder paste according to claim 1, wherein: the first solder alloy is Bi-Ag alloy, Bi-Cu alloy, Bi-Ag-Cu alloy or 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) 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) alloy.

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