JP3832151B2 - Lead-free solder connection structure - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a structure connecting solder having a different thermal expansion coefficient, such as a Si chip and a heat sink or an insulating substrate, a large structure sealing part connecting solder, and a connecting structure using the same.
[0002]
[Prior art]
Conventionally, the above purpose has been addressed by a combination of Sn—Pb solder compositions. Due to the various excellent characteristics of this solder, there are few problems in process, reliability, etc., and it has been almost solved. As the effects of lead on the human body are clarified, the fact that lead is easily dissolved in water is causing disasters and developing into global environmental problems. For this reason, Pb-free solder materials with superior characteristics that can replace Sn-Pb solder, especially heat fatigue resistance, soldering, and without damaging elements and components during temperature cycle tests, and their solderability and their A structure is desired.
[0003]
[Problems to be solved by the invention]
Although the development of solder products that do not use lead has been actively conducted for the above purpose, it is still in the process of being developed and not yet narrowed down. As a Sn-based lead-free solder that is a candidate for a solder that can meet these requirements, Sn-Sb (melting point: 236 to 240 ° C.) that has been used so far is known.
[0004]
No excellent alternative to Sn-Pb eutectic that can be soldered at 230 ° C or lower without melting Sn-Sb, that is, a temperature hierarchy with this solder, has not been found. Sn-9Zn (melting point: 199 ° C) is known as a material with excellent temperature characteristics, mechanical characteristics, and economical efficiency, but because of the remarkable oxidation of Zn, it is inferior in wettability and requires the use of a strong flux. Therefore, in the case of insulation characteristics, in the case of paste, there is a need for limited usage that cannot be easily used due to problems such as pot life and cleaning. For this reason, a system in which Cu and Bi are added to Sn-3.5Ag (melting point: 221 ° C.) is the favorite.
[0005]
Various connection experiments have been conducted so far, but due to the necessity of lowering the soldering temperature, investigations have been made to lower the melting point by adding Bi, but (1) insufficient strength at the joint interface, (2) final during solidification Low-temperature Bi phase segregates at the interface at the solidification part, reducing the reliability. (3) When Pb is present in the metallization, the low-temperature phase (Sn-Pb-Bi ternary eutectic: 97 ℃ )), The reliability is further lowered, and therefore it is not possible to substitute for Sn—Pb eutectic in terms of properties.
[0006]
The present proposal is to propose a solder that has a solder composition in which the joint does not easily peel off during soldering and can be joined with high reliability without causing damage to the elements, metallization, and the like. And the joint which can guarantee also about the conditions which enter into the severest category as reliability evaluation conditions, such as -55-125 degreeC and -65-150 degreeC, is proposed. Furthermore, the present invention proposes a solder capable of providing a temperature hierarchy with high-temperature Sn-Sb.
[0007]
[Means for Solving the Problems]
In order to clear the above-mentioned problems, it is necessary to secure a minimum joint temperature of 220 ° C. in order for the solder based on Sn—Ag—Cu to get wet normally. The temperature variation of the furnace is within about 10 ℃ at the joint of the central part and the joint of the end part. If the heat capacity of the parts is large, temperature variations are likely to occur. Therefore, it is necessary to be a solder that can be joined within a range of 220 to 230 ° C. and can ensure high reliability.
[0008]
As a potential candidate in this region, a system in which Bi is added to lower the soldering temperature to the Sn-Ag-Cu system is known. If the amount of Bi is small, it will not be different from Sn-Ag-Cu, and the temperature of the joint will be 235 ° C or higher during soldering. If Bi is around 5% Sn- (2-3.5) Ag- (0.2-0.8) Cu (4-6) Bi, it is almost the temperature of Sn-Pb eutectic substitution, and a representative example is Sn-3Ag-0.7 Cu5Bi (melting point: 195-215 ° C). If it is this range, the connection in 220-230 degreeC including the temperature variation in a furnace and the variation of a joint part is possible. However, when the amount of Bi is within this range, the strength of the bonding interface is small, so that a structure having a high rigidity cannot withstand the thermal stress due to the difference in thermal expansion during cooling, and may cause peeling failure. Furthermore, the target life may not be satisfied under the temperature cycle conditions of -55 to 125 ° C and -65 to 150 ° C.
[0009]
Therefore, since it is necessary to lower the soldering temperature and to ensure high reliability, the relationship between the control of the addition amount of Bi and In, reliability, and process compatibility is important. When Bi is added, the melting point decreases (Fig. 1), but the mechanical properties are also significantly reduced (Fig. 2). Therefore, it became clear that the mechanical reliability of the joint is determined by the amount of Bi, and that the addition of In plays a role in lowering the melting point with almost no loss of mechanical properties (FIG. 3) (FIG. 4). Since the solid solubility limit of Bi in Sn-Ag-Cu solder to Sn crystals is about 1.5%, the temperature can be lowered if more is added, but Bi that cannot be dissolved precipitates and mechanical properties, particularly important elongation As a result, the strength of the bonding interface is reduced (FIG. 5).
[0010]
When the solid solution of Bi in Sn crystals is within 1.5%, the mechanical properties are slightly reduced compared to 0% Sn-Ag-Cu. The Bi amount that can lower the melting point without impairing the mechanical strength is set to 0.5 to 3.0%. It has been found that an amount of Bi higher than this leads to a decrease in the strength of the bonding interface, and it is experimentally affected beyond this. In particular, at Bi: 3%, Bi tends to segregate at the interface, but usually a Ni / Au metallized film is applied to the surface, so even if Bi is segregated to some extent, Bi and Ni form an intermetallic compound. Because it is easy to do, it can withstand it because it is familiar and good adhesion unlike Cu.
[0011]
Next, in order to lower the soldering temperature, that is, to lower the melting point, it was decided to add 3 to 5% of In. The reason for setting it to 3% is that it has an effect of being lowered by 5 to 10 ° C. with respect to the case where In is not added. Below this, the soldering temperature is too high. If it is 5% or more, In is easy to oxidize from the viewpoint of cost, it tends to be affected by paste storability and printability, and leads to instability on the solder structure.
[0012]
That is, when the solidus temperature is lowered and Pb is further contained, a low-temperature phase of Sn—In—Pb—Bi is likely to be produced, resulting in a problem with reliability. The composition range of Cu has the effect of lowering the melting point of the Sn—Ag crystal several times without impairing the mechanical properties. However, when the Cu content exceeds 0.8%, the elongation decreases (FIG. 6).
[0013]
When Cu is a bonding target, it is also effective in preventing Cu erosion. If the Ag content is 2% or less, the melting point increases and the mechanical strength properties such as elongation decrease. If it is 3.5% or more, the melting point is increased and the mechanical strength characteristics are lowered, and the cost is high. Therefore, the solder composition was determined to be Sn- (2-3.5) Ag- (0.2-0.8) Cu- (3-5) In- (0.5-3) Bi.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Since the liquidus temperature of the Sn—Ag—Cu system is 218 ° C., the temperature of the joint portion needs to be 220 ° C. or higher. When placed in about 300 x 300 in the furnace, considering the temperature variation of the joint (ΔT = 10 ° C), the joint has a temperature of 220 (min) to 230 (max) ° C. Temperature hierarchy connection is possible.
[0015]
In the Sn-Ag-Cu system, if the temperature is 220 ° C or higher, the terminal part is surely wetted, and even if the temperature rises, wetting spread does not increase, but the place where the solder is placed is definitely wet. confirmed. Therefore, the part on which the solder foil is placed is surely wetted.
[0016]
The Sn-Ag-Cu-Bi system is known as the composition of Pb-free solder that can be connected at this temperature. As a specific composition, for example, Sn-3Ag-0.7Cu temporarily fixed to a typical composition of Ag and Cu,
Bi: About 4 to 6% of solder is within the solderable range. When Bi is 6% or more, the melting point is lowered and soldering becomes easy. However, the elongation characteristic of the material is lowered and the strength (peel strength) of the joint interface is lowered (FIG. 5), so the reliability is lowered and Bi is reduced. There is no point in putting more. Therefore, the melting point becomes high, but the region with a small Bi in the direction of improving the reliability becomes important. The composition range of the Sn—Ag—Cu—Bi solder that can be soldered at 220 to 230 ° C. is about Bi: 5%.
[0017]
Using Sn-3Ag-0.5Cu solder with different Bi content, 42 alloy leads plated with Sn-10Pb and Cu foil were soldered, and the peel strength results at the joint interface (Fig. 7) It can be seen that when 5% Bi enters, the strength of the interface decreases. This tendency with respect to Bi shows a similar tendency with Ni plating although the absolute values are different. The cause of this strength reduction is segregation at the Bi interface. At present, when Bi is as low as 2-3%, an increase in Bi concentration is observed on the fracture surface, but the existence of a clear Bi phase has not been confirmed. In addition, the mechanical properties such as elongation are not abrupt changes with respect to Bi as in peel strength, but are somewhat gradual. Therefore, it is expected that 3Bi has excellent temperature cycle resistance.
[0018]
In the power device, as shown in FIG. 8, a Mo plate 2 (low thermal expansion thermal diffusion plate, Ni plating 5) and a Cu plate heat sink 9 (Ni plating) are sandwiched between 0.15 mmt Sn-7Sb so as to sandwich the alumina substrate 3 therebetween. (Melting point: 235 to 244 ° C.) Soldering was performed using solder foil 8 at a maximum of 270 ° C. in an inert atmosphere. Then, Si chip 1 (metalized 4 with W / Ni, etc.) is placed on the back of the bonded Mo (Ni plating) using Sn-3Ag-0.7Cu-3In-3Bi solder foil 7 (0.15mmt). Similarly, soldering was performed at 230 ° C. in an inert atmosphere. At this time, as a matter of course, Sn-7Sb does not melt, so that hierarchical connection can be realized. 6 is W-Ni plating.
[0019]
FIG. 9 shows an example in which this solder is applied to a large sealing structure. A W / Ni plated ceramic container 9 and a Ni plated Cu plate 10 are sealed with Sn-3Ag-0.7Cu-3In-2Bi solder foil 12. It was confirmed that there was no problem in the helium leak test. 11 is ceramic.
[0020]
On the other hand, for comparison, the Si chip was similarly joined with Sn-3Ag-0.7Cu5Bi (melting point: 195 to 215 ° C.) solder and the reliability was evaluated. As a result of cutting out the soldered sample and polishing the cross section, a concentrated layer in which Bi was contained in the solder in a content ratio (5%) or more was found by EDX analysis on the side where final solidification occurred. Since it is a joint with Ni, it was not extremely weak like Cu, but as long as this segregation occurs, it cannot be said to be a normal joint. In a temperature cycle test at -55 to 125 ° C, it was found that the crack growth at the interface of the concentrated layer of Bi was faster than that of Sn-3Ag-0.7Cu-3In-3Bi solder, which was a problem.
[0021]
As a result, Sn-3Ag-0.7Cu-3In-3Bi has better thermal fatigue resistance than Sn-Pb eutectic, while Sn-3Ag-0.7Cu-5Bi has better resistance to Sn-Pb eutectic. It was confirmed that the heat fatigue resistance was inferior. Creep resistance is Sn-based, Sn-3Ag-0.7Cu-3In-3Bi and Sn-3Ag-0.7Cu-5Bi are both Sn-based, and since Bi is contained, the strength is high, so it is compared with Sn-Pb eutectic. And several times better. In addition, the melting point is 195 ° C or higher, which is higher than 183 ° C of Sn-Pb eutectic and is a collection of strong and hard Sn crystals.
[0022]
Fig. 10 shows Sn-3Ag-0.7Cu solder as an example. The left half of the horizontal axis contains 0 to 5% Bi and the right half contains 0 to 5% In. The vertical axis shows temperature. is there. In FIG. 11, the horizontal axis is the same, and the vertical axis is elongated. The amount of Bi, the liquidus and the solidus of the solder are shown in the left half, and 2% Bi is contained in Sn-3Ag-0.7Cu, and the liquidus and solidus of the solder containing In are contained in the right half. Indicated. Sn-3Ag-0.7Cu-2Bi-3In has a melting point close to Sn-3Ag-0.7Cu-5Bi and lower than Sn-3Ag-0.7Cu-2Bi. Thus, the soldering temperature of Sn-3Ag-0.7Cu-2Bi-3In can be as high as Sn-3Ag-0.7Cu-5Bi. Similarly, a combination of Bi: 0.5 to 3% and In: 3 to 5% is possible at a soldering temperature comparable to Sn-3Ag-0.7Cu-5Bi.
[0023]
As a result, the melting point and soldering temperature were similar to Sn-3Ag-0.7Cu-5Bi, and a highly reliable solder composition could be obtained. In the case of Sn-3Ag-0.7Cu-2Bi-5In, whose melting point was lowered with In, the process and reliability were cleared in the same way. Furthermore, the inclusion of In is not desirable because it tends to lower the melting point and has problems of cost, printability (lifetime), and structural stability (formation of a low temperature phase). Although the solid solution of Bi in the Sn matrix is 1.2%, the bulk material is brittle in proportion to the amount of Bi, but it can be said that the Bi is excellent up to 2%.
[0024]
Next, Sn-3Ag-0.7Cu-3In (melting point: 207 to 216 ° C.) and Sn-3Ag-0.7Cu (melting point: 216 to 220 ° C.), which were not added with Bi, were evaluated. The melting point of Sn-3Ag-0.7Cu-3In was not much different from Sn-3Ag-0.7Cu-5Bi, and connection was possible at 230 ° C. Although the joint part temperature of Sn-3Ag-0.7Cu exceeds max 235 ° C, it cannot be used, but the joint reliability was achieved. There is no peeling of the joint during soldering. The reason for this is thought to be that the strength of the bonding interface is high because Bi is not included.
[0025]
Furthermore, as a result of performing the temperature cycle test in the same manner, both of them cleared the target 2000 cycles in the temperature cycle test of −55 to 125 ° C., and crack progress is small. However, in the case of Sn-3Ag-0.7Cu, fracture occurred in the metallized part at a severely stressed position. Since such a phenomenon is not caused in other compositions, it is considered to be a phenomenon peculiar to Sn-3Ag-0.7Cu. The cause is that the melting point is high, the residual stress and strain are high when cooled to room temperature, and the strength of the solder is high. This is especially true when the rigidity is strong.
[0026]
From these results, it was confirmed that Sn-3Ag-0.7Cu- (0.5-3) Bi- (3-5) In has an excellent balanced composition. The base Sn-3Ag-0.7Cu can be Sn- (2-3.5) Ag- (0.2-0.8) Cu as the range of Ag and Cu, so the Pb-free solder composition that satisfies these conditions is Sn- (2-3.5) Ag- (0.2-0.8) Cu- (0.5-3) Bi- (3-5) In.
[0027]
【The invention's effect】
As described above, according to the present invention, environmentally friendly products can be supplied because lead is not used.
[0028]
The conventional method can be applied as it is in the current furnace. On the other hand, it is superior to conventional Sn—Pb eutectic solder in heat fatigue resistance, creep resistance and high temperature resistance, and a high yield can be expected for high density mounting.
[Brief description of the drawings]
FIG. 1 is a characteristic diagram showing a melting point when Bi is added to Sn—Ag—Cu.
FIG. 2 is a characteristic diagram showing elongation when Bi is added to Sn—Ag—Cu.
FIG. 3 is a characteristic diagram showing elongation when In is added to Sn—Ag—Cu.
FIG. 4 is a characteristic diagram showing a melting point when In is added to Sn—Ag—Cu.
FIG. 5 is a characteristic diagram showing the connection strength with a 42 alloy lead plated with Sn when Bi is added to Sn—Ag—Cu.
FIG. 6 is a characteristic diagram showing elongation when Cu is added to Sn-Ag.
FIG. 7 is a characteristic diagram showing the connection strength with Cu when Bi is added to Sn—Ag—Cu.
FIG. 8 is a cross-sectional view showing a power device using the present invention.
FIG. 9 is a cross-sectional view showing a large sealing structure using the present invention.
FIG. 10 is a characteristic diagram showing a melting point when Bi and In are added to Sn—Ag—Cu.
FIG. 11 is a characteristic diagram showing elongation when Bi and In are added to Sn—Ag—Cu.
[Explanation of symbols]
1 ... Si chip, 2 ... Mo plate, 3 ... alumina, 4 ... metallized film, 5 ... Ni plating, 6 ... W-Ni plating, 7 ... Sn-3Ag-0.7Cu-3Bi-3In, 8 ... Sn-Sb, 9 ... Cu plate, 10 ... Ceramic, 11 ... Cu plate, 12 ... Sn-3Ag-0.7Cu-2Bi-3In.

Claims (2)

In a hierarchical connection structure in which a chip such as Si having a built-in element is connected to an insulating substrate or a heat sink material with a first lead-free solder, and further a heat sink material or an insulating substrate is connected to the lower layer with a second lead-free solder,
It said second lead-free solder is an Sn-Sb (236~240 ℃),
The first lead-free solder is obtained by addition of Sn- (2~3.5) Ag- (0.2~0.8) Cu- (3~5) In- (0.5~3) Bi or other trace elements to it A lead-free solder connection structure characterized by that. In a hierarchical connection structure in which a chip such as Si having a built-in element is connected to an insulating substrate or a heat sink material with a first lead-free solder, and further a heat sink material or an insulating substrate is connected to the lower layer with a second lead-free solder,
The second of the lead-free solder is a Sn-Sb (236 ~ 240 ℃ ),
The first lead-free solder is Sn- (2-3.5) Ag- (0.2-0.8) Cu- (3-5) In- (0.5-3) Bi- (0.2-1) Sb or other trace amounts. lead-free solder connection structure characterized by elemental is obtained by adding.

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