JP2017225979A - Pb-FREE Zn-BASED SOLDER ALLOY FOR HIGH TEMPERATURE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pb-free Zn-based solder alloy for high temperature which has a melting point of approximately 300-400°C suitable for assembly of a semiconductor element, can greatly improve and enhance workability and stress relaxation, and is also excellent in wettability and reliability.SOLUTION: A first Pb-free Zn-based solder alloy contains 4.0 mass% or more and 10.0 mass% or less of Al, contains more than 3.5 mass% and 5.0 mass% or less of Cu, and the balance Zn with inevitable impurities. A second Pb-free Zn-based solder alloy contains 4.0 mass% or more and 10.0 mass% or less of Al, contains more than 3.0 mass% and 7.0 mass% or less of Cu, contains one or more Ge and P, which contains 0.01 mass% or more and 1.00 mass% or less of Ge in the case of Ge and 0.005 mass% or more and 0.500 mass% or less of Ge in the case of P, and the balance Zn with inevitable impurities.SELECTED DRAWING: None

Description

  The present invention relates to a Pb-free solder alloy, and more particularly, to a Pb-free Zn-based solder alloy mainly composed of Zn suitable for high temperature use.

  In an assembly process of electronic parts such as various semiconductor elements including die bonding of power transistors, a high temperature solder alloy having a relatively high melting point of about 300 to 400 ° C. is used. Conventionally, Pb-based solder alloys represented by Pb-5 mass% Sn alloys have been mainly used as such high temperature solder alloys. However, in recent years, there has been a strong movement to limit the use of Pb due to consideration for environmental pollution. For example, Pb is a regulated substance in the RoHS directive. Corresponding to such a movement, a solder alloy containing no Pb, that is, a Pb-free solder alloy has been demanded also in the field of assembling semiconductor elements and the like.

  Under such circumstances, with regard to solder alloys for medium and low temperatures (about 140 to 230 ° C.), Pb-free solder alloys have already been put into practical use by using Sn as a main component. For example, in Patent Document 1, Sn is the main component, Ag is 1.0 to 4.0 mass%, Cu is 2.0 mass% or less, Ni is 1.0 mass% or less, and P is 0.2 mass%. Pb-free solder alloys and the like contained below are described.

  On the other hand, regarding solder alloys for high temperatures, Bi solder alloys and Zn solder alloys have been proposed as Pb-free solder alloys. For example, for a Bi-based solder alloy, Patent Document 2 describes a Bi / Ag-based brazing material containing 30 to 80% by mass of Bi and having a melting temperature of 350 to 500 ° C. On the other hand, for a Zn-based solder alloy, for example, Patent Document 3 discloses a Zn alloy containing 3.5 to 18% by mass of Al, with the balance being Zn and inevitable impurities. This Patent Document 3 also discloses a Zn alloy containing 1 to 3.5% by mass of Cu, and a Zn alloy containing Si and adding Cu and Si in a total of 1.8 to 4.3% by mass. .

  In Patent Document 4, Al is contained in an amount of 1.0% to 9.0% by mass, Cu is contained in an amount of 0.001% to 3.000% by mass, and the balance is made of Zn and inevitable impurities. A Pb-free Zn solder alloy is disclosed. Patent Document 4 further discloses a Pb-free Zn-based solder alloy containing Ag of 4.0% by mass or less and / or P of 0.50% by mass or less.

Japanese Patent Application Laid-Open No. 11-077366 JP 2002-160089 A JP 2012-180557 A JP2013-052433A

  In recent years, electronic parts including the above-described various semiconductor elements have been made thinner and smaller, and accordingly, miniaturization is required for solder alloys used as bonding materials. Therefore, solder alloys are required to have higher joint strength than before. In addition, the volume and area of solder alloys used for joining tend to decrease due to the miniaturization of solder alloys, and even when low-level voids and foreign matter that have not been a problem until now exist at the joint interface. There are cases where problems such as deterioration of the bondability may occur, and a solder alloy having less voids and foreign matters and high bondability is required.

  However, the actual situation is that a high-temperature Pb-free solder alloy suitable for fine bonding as described above, which can replace the conventional Pb-based solder alloy, has not yet been put into practical use. That is, the Bi / Ag brazing material disclosed in Patent Document 2 has a liquidus temperature as high as 400 to 700 ° C., so that the working temperature at the time of joining is also 400 to 700 ° C. or higher. Although it is applicable to the use of the hermetic terminal as described, it is considered that the melting temperature is too high for application to electronic components such as semiconductor elements and substrates.

  Although it is described that the Zn alloy disclosed in Patent Document 3 can also be used as a bonding material, the object is to provide a Zn alloy having excellent workability by controlling the eutectic structure. In some applications, such as electronic parts, it may be difficult to exhibit sufficient bondability for use in fine solder bonding. In addition, the Pb-free Zn solder alloy disclosed in Patent Document 4 has a sufficient joining performance obtained by evaluation with a solder alloy processed relatively large in a sheet shape, but in the joining of a micromachined solder alloy, In some cases, sufficient bondability cannot be exhibited.

  The present invention has been made in view of the problems of the above-described conventional high-temperature Pb-free solder alloys. In a fine solder alloy used for assembling semiconductor elements, etc., a suitable melting point of about 300 to 400 ° C. An object of the present invention is to provide a high-temperature Pb-free Zn-based solder alloy having good workability and stress relaxation properties, and excellent bondability such as wettability and reliability.

  In order to achieve the above object, the first Pb-free Zn-based solder alloy according to the present invention contains 4.0% by mass or more and 10.0% by mass or less of Al, and contains more than 3.5% by mass of Cu. The content is 0% by mass or less, and the balance is made of Zn and inevitable impurities. The second Pb-free Zn-based solder alloy according to the present invention contains Al in an amount of 4.0% by mass to 10.0% by mass and Cu in an amount of 3.0% by mass to 7.0% by mass. Further, it contains at least one of Ge and P, in the case of Ge, 0.01 mass% or more and 1.00 mass% or less, and in the case of P, 0.005 mass% or more and 0.500 mass% or less. Consists of Zn and inevitable impurities.

  According to the present invention, it is excellent in workability, mechanical properties, bondability and reliability, and can sufficiently withstand a reflow temperature of about 300 ° C., for example, an assembly process of electronic components such as die bonding of power transistor elements. It is possible to provide a high-temperature Pb-free Zn-based solder alloy suitable for soldering.

It is a longitudinal cross-sectional view which shows typically the state which soldered Cu board | substrate which has Ni plating layer, and Si chip with each solder alloy sample.

  The first Pb-free Zn-based solder alloy according to the present invention contains Al and Cu as additive elements, and the balance consists of Zn and an impurity element that is unavoidably included in production. Zn as the main component has a melting point of 419 ° C., which is higher than 300 to 400 ° C., which is the junction temperature of the semiconductor element. In the first Zn-based solder alloy of the present invention, the melting point of the solder alloy is lowered to 400 ° C. or less by containing Al and Cu within predetermined ranges, respectively, which makes it suitable as a high-temperature solder alloy. It is said. Since the melting point of the first Zn-based solder alloy of the present invention is lower than the melting point of the eutectic alloy of Zn and Al, the melting point is further lowered by eutectic alloying with the three elements of Zn, Al and Cu. It is thought that there is.

  Further, it has been confirmed that the first Zn-based solder alloy of the present invention is miniaturized in crystals, and it is considered that the refinement of the crystals contributes to the improvement of the workability of the solder alloy. Moreover, since the 1st Zn type solder alloy of this invention contains Cu, it is easy to react with the general Cu board | substrate with which a semiconductor element is joined, and, thereby, the outstanding wettability and bondability are obtained.

  On the other hand, the second Pb-free Zn-based solder alloy according to the present invention contains Al and Cu in Zn as a main component in the same manner as the first Zn-based solder alloy described above. 1 or more of these are contained. In this alloy composition, not only the effect of lowering the melting point and the effect of improving the workability by refining the crystal, but also the effect of suppressing the oxidation of the solder alloy is obtained, and the wettability and bondability are further improved. Is possible. However, since the second Zn-based solder alloy may be inferior in workability to the first Zn-based solder alloy, the second Zn-based solder alloy may be appropriately adjusted according to the purpose in consideration of various characteristics and applications of the objects to be joined. However, it is preferable to determine the content of Ge or P.

  The first and second Zn-based solder alloys of the present invention described above may be used after being processed into pellets or punched products having a predetermined shape, or may be used after being formed into a wire shape or a ball shape. . Also, a solder alloy processed into a ball shape or the like can be mixed with a flux and used as a solder paste. Next, each element contained in the first Zn-based solder alloy and the second Zn-based solder alloy of the present invention will be described in detail.

<Al>
Al is an essential element constituting the first and second Zn-based solder alloys of the present invention. By containing Al, the melting point can be lowered to about 380 ° C. Since the Al content in the first and second Zn solder alloys of the present invention is close to the Zn—Al eutectic alloy composition, the first and second Zn solder alloys of the present invention are also eutectic alloys. It is considered that the melting point is lowered by forming. Moreover, since the formed crystal is fine and the solder alloy becomes relatively soft, workability and stress relaxation properties are improved. However, in order to reduce the melting point, improve workability and obtain high stress relaxation properties as a high temperature solder alloy, it is necessary to contain Cu described later.

  In the first and second Zn-based solder alloys of the present invention, the Al content is 4.0% by mass or more and 10.0% by mass or less. When the Al content is less than 4.0% by mass, the melting point does not sufficiently decrease even when other elements described later are added, and the bondability decreases. In other words, if the melting point is not sufficiently lowered, when the solder alloy is melted at a predetermined joining temperature when assembling an electronic component or the like, the solder alloy is not sufficiently melted and a partially unmelted portion is generated. A separation phenomenon occurs, and bubbles are entrained in the unmelted portion to increase the void generation rate, or the unmelted portion prevents alloying of the joint. In addition, when the separation phenomenon occurs, the composition of the undissolved part and the part that has been once melted and re-solidified is often different, and an alloy having a uniform composition cannot be produced, and sufficient bonding strength may not be obtained. There is.

  On the other hand, when the Al content is more than 10.0% by mass, the melting point is not sufficiently lowered in the same manner as above, and the same problem as described above is caused. If the Al content is 5.5% by mass or more and 8.5% by mass or less, the melting point can be easily lowered to about 380 ° C., the crystal becomes finer, the workability is improved, and the use is further improved. Since it can be set as an easy solder alloy, it is preferable.

<Cu>
Cu is an essential element together with Al in the first and second Zn-based solder alloys of the present invention. Cu functions to improve wettability, stress relaxation, workability, etc., and lower the melting point. A Zn—Al alloy, which is a eutectic alloy, cannot be said to have sufficient workability as it is, and for example, when it is processed into a ribbon material by rolling, burrs may be generated or bending may occur. On the other hand, the inclusion of Cu in addition to Al makes the crystals finer and improves the workability, so that the above problems can be solved.

  Further, by using a ternary alloy in which Al and Cu are contained in Zn as a main component, the melting point can be made lower than the eutectic point temperature of Zn—Al. By lowering the melting point in this manner, the melting point of the high-temperature Pb solder alloy is further approached, and the material becomes easy to use as a Pb solder substitute. In addition, since a substrate to which a semiconductor element is bonded generally uses a Cu substrate, the substrate and the solder alloy are easily alloyed and excellent wettability by containing Cu having the same composition as this substrate. High bonding strength can be obtained. As described above, the first and second Zn-based solder alloys of the present invention contain Cu in addition to Al, thereby having a fine metal structure with good workability and generating a uniform and strong joint interface. Therefore, very high bonding reliability can be obtained.

  In the first Zn-based solder alloy of the present invention, the Cu content is more than 3.5 mass% and not more than 5.0 mass%. By setting the Cu content within this range, the crystal becomes finer, and even when thermal stress or the like is applied, cracks are not easily generated or propagated, and the melting point can be lowered. If the Cu content is 3.5% by mass or less, the strength at the time of finely processing the solder alloy may not be sufficiently obtained. On the other hand, when the Cu content is 5.0% by mass or more, the crystal grains are coarsened or the phenomenon of melting and separating occurs, and good bonding cannot be performed.

  On the other hand, since the second Zn-based solder alloy of the present invention contains one or more of Ge and P as described later, the Cu content is more than 3.0% by mass and 7.0% by mass. % Or less. However, even in the second Zn-based solder alloy, if the Cu content exceeds 3.5 mass% and is equal to or less than 5.0 mass%, the melting point can be easily increased to about 380 ° C., and the stress relaxation property In addition, it is preferable because the effect of improving the workability and bondability appears more remarkably.

<Ge>
Ge is an element having an effect of improving weather resistance and oxidation resistance in the second Zn-based solder alloy of the present invention. When Ge is contained in a Zn-based solder alloy, Ge is segregated near the surface of the Zn solder alloy and has an effect of suppressing oxidation of Zn as a main component. This effect is particularly prominent in an environment where the temperature and humidity are high, and a solder alloy having very excellent weather resistance can be obtained.

  In the second Zn-based solder alloy of the present invention, the Ge content is 0.01 mass% or more and 1.00 mass% or less. If the Ge content is less than 0.01% by mass, the content is too small and the effect of inclusion is not sufficiently exhibited. Conversely, if the Ge content exceeds 1.00% by mass, the content is too large and the solder alloy is hard. Since it becomes weak, it is not preferable.

<P>
P is an element that improves wettability in the second Zn-based solder alloy of the present invention. That is, P is highly reducing and suppresses oxidation of the solder alloy surface by oxidizing itself. The second Zn-based solder alloy of the present invention contains Zn that is easily oxidized and further contains Al that is more easily oxidized than Zn. Inclusion of wettability can be improved.

  In addition, the inclusion of P also has the effect of reducing the generation of voids during bonding. That is, as already described, since P easily oxidizes itself, oxidation proceeds preferentially over Zn and Al, which are the main components of the solder alloy, at the time of bonding. As a result, it is possible to prevent the solder mother phase from being oxidized and reduce the bonding surface of the semiconductor element or the substrate to ensure wettability. At the time of this joining, since there is no oxide film on the solder surface and the surface of the joining surface of the semiconductor element, etc., which cause voids, voids are less likely to occur, thereby making it possible to bond and improve reliability. Can be improved. Note that P is excellent in that it does not substantially remain on the surface of the solder, the substrate or the like because it is almost vaporized when it is reduced to the oxide by reducing the joint surface of the solder alloy or the substrate.

  In the second Zn-based solder alloy of the present invention, the P content is 0.005 mass% or more and 0.500 mass% or less. As described above, since P is very reducible, the effect of improving the wettability can be obtained if a trace amount is contained, but the effect of improving the wettability is more than that even if contained exceeding 0.5% by mass. On the contrary, a large amount of P or P oxide gas generated by excessive inclusion is entrained in the solder alloy, increasing the void generation rate, or remaining P segregates and becomes brittle It is not preferable because a solder phase may be embrittled due to the formation of a short phase and the reliability may be lowered. In particular, it has been confirmed that wire breakage is likely to be caused when processing into an elongated shape such as a wire. If the content of P is 0.300% by weight or less, the reduction effect is exhibited and the possibility of generating a brittle P compound is reduced, which is more preferable. When the P content is less than 0.005% by mass, the content is too small and the effect of inclusion is not substantially exhibited.

  As raw materials, Zn, Al, Cu, Ge and P having a purity of 99.99% by weight or more were prepared. Large flakes and bulk-shaped raw materials were finely cut to a size of 3 mm or less by cutting and pulverizing while paying attention to the uniformity of the composition of the alloy after melting without variation in the sampling location. Next, a predetermined amount of each of these raw materials was weighed and placed in a graphite crucible for a high-frequency melting furnace.

  The crucible containing the raw material was placed in a high-frequency melting furnace, and nitrogen was flowed at a flow rate of 0.7 liter / min or more per kg of the raw material in order to suppress oxidation. In this state, the melting furnace was turned on to heat and melt the raw material. When the metal began to melt, it was stirred well with a mixing rod and mixed uniformly so as not to cause local compositional variations. After confirming that all the raw materials were sufficiently melted, the high frequency power supply was turned off, the crucible was quickly taken out, and the molten metal in the crucible was poured into the mold of the solder mother alloy. The mold used was a solder mother alloy having a width of 50 mm, a length of 200 mm, and a thickness of 5 mm.

  In this way, Pb-free Zn-based solder mother alloys of Samples 1 to 30 having different compositions were produced by changing the mixing ratio of the raw materials. The composition of each of the solder mother alloys of Samples 1 to 30 was analyzed using an ICP emission spectroscopic analyzer (SHIMADZU S-8100). The analysis results are shown in Table 1 below.

  Next, workability was evaluated by processing each of the Zn-based solder mother alloys of Samples 1 to 30 into a ribbon shape with a rolling mill. Further, the mechanical properties were evaluated by measuring the tensile strength and elongation of the obtained ribbon-shaped solder alloy. Further, a ribbon-shaped solder alloy is punched out into a square shape of 2.0 mm × 2.0 mm with a press machine to produce a punched product, and the obtained punched product is used to evaluate the bondability (wetting property) and heat. Reliability was evaluated by a cycle test. Hereinafter, each evaluation will be described in detail.

<Evaluation of workability (appearance of ribbon-like solder alloy)>
While supplying rolling oil and adjusting the feed speed of the ingot, each of the solder mother alloys (plate ingots of 5 mm thickness) of Samples 1 to 30 is rolled to a thickness of 0.05 mm by using a rolling mill. A solder alloy was obtained. Then, it cut | judged to the width of 25 mm by slitter process, and was set as the ribbon-shaped Zn type solder alloy for workability evaluation. The solder alloy thus processed into a ribbon shape was visually inspected according to the following criteria to evaluate the workability. That is, the case where there was no scratch or crack in the solder alloy was evaluated as “◯”, the case where there were 1 to 3 cracks or cracks per 10 m length was evaluated as “Δ”, and the case where there were 4 or more points was evaluated as “X”. .

<Evaluation of mechanical properties (tensile strength, elongation)>
Each of the sheet-like Zn-based solder alloys of Samples 1 to 30 rolled to a thickness of 0.05 mm was processed into a width of 3 mm with a slitter in the same manner as described above for evaluation of workability, and the length Was cut to about 15 cm to obtain a ribbon-like Zn-based solder alloy for evaluation of mechanical properties. The tensile strength and the elongation rate were measured with a tensile tester (Tensilon universal tester) using the evaluation sample thus prepared. The mechanical properties were evaluated by the relative values of the tensile strength and elongation of each sample when the measured value of Sample 1 was 100%.

<Evaluation of bondability (wetting)>
Each sample processed into the sheet shape was punched into a square shape of 2.0 mm × 2.0 mm by using a press machine to produce a punched product (hereinafter, □ 2 mm product). The wettability of each solder alloy sample was evaluated with a wettability tester (device name: atmosphere control type wettability tester) using the □ 2 mm product thus produced. Specifically, the heater part of the wettability tester is first covered with a double cover, and the heater set temperature is determined from the melting point of each sample while flowing nitrogen at a flow rate of 12 liters / minute from four locations around the heater part. About 10 ° C. was set at a high temperature and heated. After the inside of the wettability tester was replaced with a nitrogen atmosphere and the heater temperature was stabilized at a set value, a Cu substrate (plate thickness: about 0.70 mm) was set in the heater part and heated in a nitrogen atmosphere for 25 seconds.

  Next, a 2 mm product of each solder alloy sample was placed on the Cu substrate and heated for 25 seconds. Thereafter, the Cu substrate was gently picked up from the heater part so as not to affect the molten solder alloy, and was placed on a holding table in which the nitrogen atmosphere was maintained and cooled. After the solder alloy was solidified and sufficiently cooled, it was taken out into the atmosphere and the joint portion between the solder alloy and the Cu substrate was inspected according to the following criteria. That is, it is “x” when it is hardly joined and easily peeled off, and “△” when it is joined but the wetting spread is poor (when the solder does not spread), it is joined and the wetting spread is good. In the case (a state where the solder was thin and spread), it was evaluated as “◯”.

<Reliability evaluation (heat cycle test)>
Reliability evaluation was performed by producing a joined body using the above-mentioned □ 2 mm product. In order to produce a bonded body for evaluation of reliability, first, a die bonder (manufactured by Westbond, MODEL: 7327C) is started, a heater is covered and nitrogen is flown (nitrogen flow rate: 8 L / min in total), The heater part was a nitrogen atmosphere. Thereafter, the heater set temperature was heated to 50 ° C. higher than the melting point of each solder sample.

  After the heater temperature was stabilized at the set value, a Cu substrate in which a Ni plating layer having a thickness of 3.0 μm was formed on a Cu base material having a thickness of 0.3 mm was set in the heater portion and heated for 25 seconds. Next, a □ 2 mm product of each solder alloy sample was placed on the Cu substrate and heated for 25 seconds, and after 25 seconds, a Si chip was placed and scrubbed for 3 seconds. After scrubbing was completed, the Cu substrate was gently picked up from the heater part together with the Si chip, placed on a holding table maintaining a nitrogen atmosphere next to the Cu substrate, cooled sufficiently, and taken out into the atmosphere. As a result, the evaluation bonded body in which the Cu substrate having the Ni plating layer 2 formed on the surface of the Cu base 1 as shown in FIG. 1 is bonded to the Si chip 4 via each solder alloy sample 3 is obtained. Obtained.

  A heat cycle test was performed on the joined bodies of the solder alloy samples thus produced to evaluate the joining reliability. In addition, this heat cycle test was performed using two said joined bodies with respect to each solder alloy sample. That is, 300 cycles of a heat cycle in which one of the two joined bodies is cooled to −40 ° C. and heated to + 150 ° C. for one cycle, and this heat cycle is repeated for 500 cycles. Repeated.

  Thereafter, both bonded bodies obtained by repeating this heat cycle for 300 cycles and 500 cycles were embedded in resin and solidified, and then cross-section polishing was performed. This joint section was observed using SEM (device name: HITACHI S-4800), and reliability was evaluated according to the following criteria. That is, the case where peeling occurs on the joint surface or the solder part has cracks is evaluated as “X”, and the case where there is no such defect and the joint state similar to the initial state is evaluated as “◯”. did. The evaluation results are shown in Table 2 below together with the evaluation of the workability, the evaluation of mechanical properties, and the evaluation of bondability.

  As can be seen from Table 2 above, the Zn-based solder alloys of Samples 1 to 20 as examples of the present invention all showed good characteristics in all evaluation items. That is, the tensile strength and elongation rate are higher than those of the sample 1 as a comparative object, and because of excellent workability, there is no generation of scratches or cracks even when processed into a ribbon shape, and excellent wettability. Even in the heat cycle test, defects such as peeling and cracking did not occur at all up to 500 times.

  On the other hand, each of the Zn-based solder alloys of Samples 21 to 30 as the comparative example had an undesirable evaluation result because the content of any one of Al, Cu, Ge, and P was not appropriate. Specifically, in the evaluation of workability, the number of scratches and cracks was relatively small in the samples 23, 24 and 27, but the occurrence of scratches and cracks was confirmed in all the samples. Further, in the evaluation of mechanical properties, both the tensile strength and the elongation rate did not reach the samples 1 to 20 of the examples of the present invention. The sample 24 with the highest tensile strength showed 97% of the tensile strength of the sample 1, but the elongation at that time was 72% of that of the sample 1, and it was inferior in workability, so that sufficient bondability and reliability were not obtained. It was.

  In the evaluation of bondability, samples 21 to 27 and 29 to 30 were inferior in wettability and spreadability, but sample 28 could not be bonded. Therefore, in all samples, wettability was inferior to samples 1 to 20. . In the reliability evaluation, the samples 23 and 27 were not defective until 300 cycles, but the other samples were confirmed to be defective at 300 cycles. In the 500 cycle heat cycle test, the samples 23 and 27 were not defective. The occurrence of defects was confirmed in all of the samples including 27.

1 Cu base material 2 Ni plating layer 3 Each solder alloy sample 4 Si chip

Claims (3)

  Pb-free, characterized by containing Al 4.0% to 10.0% by weight, Cu containing more than 3.5% by weight and 5.0% by weight with the balance being Zn and inevitable impurities Zn solder alloy.   In the case where Al is contained in an amount of 4.0% by mass or more and 10.0% by mass or less, Cu is contained in an amount of more than 3.0% by mass and 7.0% by mass or less, and at least one of Ge and P is Ge. Is Pb-free Zn-based, characterized by containing 0.01 mass% or more and 1.00 mass% or less, and in the case of P, 0.005 mass% or more and 0.500 mass% or less, with the balance being Zn and inevitable impurities Solder alloy.   3. The Pb-free material according to claim 1, comprising Al in an amount of 5.5% by mass to 8.5% by mass and Cu in an amount of more than 3.5% by mass and 5.0% by mass or less. Zn solder alloy.

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