JP2011251330A - High-temperature lead-free solder paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Zn-based high-temperature lead-free solder paste which is suitable for die bonding of a power semiconductor element, assembling of various electronic components or the like, prevents crack or peeling of joined chip and can secure high joint reliability.SOLUTION: The Zn-based high-temperature lead-free solder paste comprises Zn-alloy solder powders containing Zn of ≥70 mass%, metal powders consisting mainly of Ni or Cu and the balance flux. The metal powder is an Ni or Cu powder having an average particle size of 1-100 μm, or preferably the coated Ni or Cu powder, on the surface of which a coat comprising Au or Ag having a thickness of ≤1 μm is provided. The total metal powder accounts for 5-65 mass% with respect to 100 mass% of the total of the metal powder and Zn-alloy solder powder.

Description

  The present invention relates to a high-temperature lead-free solder paste used for die bonding of power semiconductor elements.

  Solder materials used for die bonding of power semiconductor elements, assembly of various electronic components, etc., in addition to the properties required for ordinary solder materials such as wettability to the material to be joined, 1) at a temperature of about 380 ° C. or lower 2) Soldering is possible, 2) Soldered parts are not re-melted at a temperature of about 240 ° C. when mounted on a printed circuit board, and 3) Reliability of solder joints can be ensured, that is, use at a relatively high temperature. Performance such as no deterioration of the joints under the environment is required.

  As a solder material having these performances, high-temperature solder represented by Pb-5 mass% Sn has been conventionally used for applications such as die bonding of power semiconductor elements. However, in recent years, from the viewpoint of prevention of environmental pollution, there has been an effort to eliminate lead used in solder materials, and so-called lead-free solder that does not contain lead has been developed.

  As a result, low-temperature lead-free solder represented by Sn-Ag-Cu solder has been developed for Sn-40 mass% Pb solder used for mounting on a printed circuit board, and substitution is already progressing. On the other hand, no suitable material that satisfies all the above conditions 1) to 3) has been found in the region of high-temperature solder, and the substitution from lead-based solder to lead-free solder has hardly progressed. is the current situation.

  For example, Patent Document 1 proposes to use a Zn solder alloy containing Ge and Mg together with Al as high-temperature lead-free solder. Patent Document 2 also describes a Zn solder alloy containing Mg and Ga together with Al. However, the Zn alloy has a drawback that shrinkage occurs depending on conditions after soldering, and the joined chip is cracked or peeled off. The Zn alloys of Patent Documents 1 and 2 are not always satisfactory because the chip is easily cracked or peeled off.

  The cause of chip cracking in a Zn-Al alloy is not only the difference in thermal expansion coefficient between Cu and Si, but also the shrinkage rate during solidification (+ is shrinkage,-is expansion) is +4.9 to + 6.9% for Zn and One of the causes is considered that Al is +6.4 to + 6.8%. In addition, due to the phase transformation near 280 ° C. in the Zn—Al phase diagram, for example, after the die attach, phase transformation occurs near 280 ° C. during cooling, causing volume change and generating high residual stress. May also be the cause.

  On the other hand, Non-Patent Document 1 reports a chip cracking phenomenon at the time of die-attaching Zn—Al-based lead-free solder. That is, the cause of chip cracking is that the difference in thermal expansion coefficient between Cu as the substrate material and Si as the chip material is 5 times or more, so when cooling after die attach, stress is generated due to the difference in thermal expansion, and Pb− In the case of 5 mass% Sn, plastic deformation occurs, but it is reported that the Zn-Al solder is hard and cannot relieve stress.

  As a countermeasure against chip cracking in this Zn—Al-based lead-free solder, Non-Patent Document 1 proposes reducing the chip thickness. However, since there is a limit to how thin the chip can actually be, it is difficult to think that this measure can solve all cases. In particular, in a Zn-based Pb-free solder in which Zn exceeds 70% by mass, it is considered that it is often difficult to solve the problem of chip cracking and peeling only by adjusting the chip thickness.

Japanese Patent No. 3850135 Japanese Patent No. 3945915

6th on “Microjoining and Assembly Technology in Electronics” February 3-4, 2000, Yokohama p.339-p.344

  In view of the above-described conventional circumstances, the present invention provides a high-temperature lead-free solder material suitable for die bonding of power semiconductor elements, assembly of various electronic components, and the like. An object of the present invention is to provide a Zn-based high-temperature lead-free solder paste that eliminates peeling and ensures high bonding reliability.

  The present invention solves problems such as chip cracking and peeling after bonding by mixing a metal alloy powder of Ni or Cu into a Zn alloy. However, since it is necessary to uniformly disperse the metal powder in the Zn alloy, it has become industrially usable by taking the form of a solder paste.

  That is, the high-temperature lead-free solder paste of the present invention comprises a Zn alloy solder powder containing 70% by mass or more of Zn, a metal powder mainly composed of Ni or Cu, and the remaining flux, and the metal powder is an average grain. Ni powder having a diameter of 1 to 100 μm, coated Ni powder having a film having a thickness of 1 μm or less composed of at least one element of Au and Ag on the surface of the Ni powder, Cu powder having an average particle diameter of 1 to 100 μm, and the Cu powder And at least one selected from coated Cu powder provided with a film having a film thickness of 1 μm or less composed of at least one element of Au and Ag, and the total of the metal powder and the Zn alloy solder powder is 100% by mass In this case, the total amount of the metal powder is 5 to 65% by mass.

  In the high-temperature lead-free solder paste of the present invention, the Zn alloy solder powder includes (1) a Zn alloy containing 1.0 to 7.0% by mass of Al, with the balance being Zn and inevitable impurities, (2) Al Any of Zn alloys containing 1.0 to 7.0% by mass of Ge, 1.5% by mass or less of Ge and / or 6.0% by mass or less of Mg, with the balance being Zn and inevitable impurities. preferable.

  According to the present invention, it is possible to provide a Zn-based high-temperature lead-free solder paste that not only has high thermal conductivity but also can eliminate cracking and peeling of the bonded chip and ensure high bonding reliability. it can. Therefore, the high-temperature lead-free solder paste of the present invention can achieve lead-free Pb-based solder used in assembling various electronic components and can be suitably used particularly for die bonding of power semiconductor elements.

  The high-temperature lead-free solder paste of the present invention is composed of a Zn alloy solder powder containing 70% by mass or more of Zn, a metal powder mainly composed of Ni or Cu, and the remaining flux. In the high-temperature lead-free solder paste of the present invention, the bonding temperature in die bonding of power semiconductor elements and assembly of various electronic components is 450 ° C. or lower.

  The metal powder mainly composed of Ni or Cu used in the present invention may be Ni powder or Cu powder as it is, but coated Ni powder or coated Cu provided with a film made of at least one element of Ag and Au on the surface. More preferably, powder is used. By applying the Au or Ag film, the Ni powder or Cu powder is prevented from being oxidized, and at the same time, the wettability with the Zn solder alloy is increased and the metal powder is uniformly dispersed in the molten Zn alloy. Can be increased.

  The film thickness of the coated Ni powder and the coated Cu powder of Au or Ag is 1 μm or less. The reason why the film thickness is 1 μm or less is that when it exceeds 1 μm, not only the above-mentioned effects of preventing oxidation and improving wettability are saturated, but also the amount of film material used increases and the cost increases. Further, the coated Ni powder and the coated Cu powder are obtained by subjecting the surface of the Ni powder or Cu powder to electrolytic plating or electroless plating, and also include a salt of Au or Ag in the final step of metal powder production by a wet method. It can also be produced by a method in which an aqueous solution is added and reduction is performed using a reducing agent such as hydrazine.

  The average particle diameter of the said Ni powder and Cu powder shall be the range of 1-100 micrometers. If the average particle size of the Ni powder and Cu powder is less than 1 μm, the Ni powder and Cu powder are difficult to uniformly disperse in the Zn alloy. Conversely, if the average particle size exceeds 100 μm, the bonding is made of Zn alloy solder. It acts as a starting point for layer failure, and the reliability of the joint is reduced. In the present invention, the maximum diameter of the powder is used as the particle diameter. Further, the purity and shape of the Ni powder and Cu powder are not particularly limited, but the purity is preferably close to that of a pure metal or eutectic alloy rather than an alloy because it should be easily plastically deformed. The shape may be any of a spherical shape such as atomized powder, an irregular shape such as electrolytic powder, and a flat shape such as flake powder.

  On the other hand, the Zn alloy solder powder melts and joins metal powders (Ni powder or coated Ni powder, Cu powder or coated Cu powder), and at the same time, functions as a normal solder. The reason for selecting Zn as the main component of solder is due to the high thermal conductivity of Zn. In other words, Zn has a thermal conductivity approximately three times that of Pb, and a large current flows. Therefore, Zn is suitable for bonding power semiconductor elements that have a high temperature. However, since pure Zn does not have workability and wettability as a practical solder, it is necessary to use a Zn alloy containing 70% by mass or more of Zn. In addition, although it does not specifically limit about the particle size of Zn alloy solder powder, The powder of about 10-50 micrometers is often used from the ease of manufacture.

  As the Zn alloy solder powder, conventionally used or proposed Zn-based Pb-free solder material powder can be used. Preferred Zn alloy solder powders include those composed of the following Zn alloys (1) and (2). That is, the Zn alloy (1) is a Zn alloy containing 1.0 to 7.0% by mass of Al, with the balance being Zn and inevitable impurities. By adding Al to Zn, Al can be dissolved in Zn to improve brittleness. In particular, the Zn-Al eutectic point is obtained when the Zn content is 95.0% by mass, but in this vicinity, the effect of improving brittleness is particularly great, and the workability can be remarkably improved.

  The Zn alloy (2) contains 1.0 to 7.0% by mass of Al, 1.5% by mass or less of Ge and / or 6.0% by mass or less of Mg, with the balance being Zn and inevitable impurities. Zn alloy consisting of The addition of Mg is effective in improving workability as well as Al, and has an effect of lowering the melting point. Ge is an element added particularly for the purpose of adjusting the melting point, and is added to lower the melting point of Zn having a relatively high melting point of 419 ° C. in the solder field.

  The ratio of the metal powder and the Zn solder alloy powder in the present invention is such that the total of the metal powder is in the range of 5 to 65 mass% when the total of both is 100 mass%. If the content of the metal powder is less than 5% by mass, the amount of addition is too small, which is insufficient to alleviate the shrinkage of the Zn alloy solder. On the other hand, if it exceeds 65% by mass, the amount of Zn alloy solder becomes too small, and the electronic component and the substrate cannot be joined with sufficient strength, resulting in problems such as chip peeling.

  The high-temperature lead-free solder paste of the present invention can be produced by adding a liquid flux to the metal powder and the Zn alloy solder powder and kneading them. As for the flux, those used for Sn-based or Pb-based solder pastes may be used as they are, and those for high temperatures are more preferable. As a preferable flux, for example, a rosin compound, an organic acid, an organic amine compound and the like dissolved in a solvent such as alcohols, ethylene glycols and glycerins can be exemplified.

  The addition amount of the flux is preferably in the range of 7 to 13% by mass when the total of the metal powder and the Zn alloy solder powder is 100% by mass. If the added amount of the flux is less than 7% by mass, the flux amount is insufficient and sufficient reducing property cannot be obtained, and the viscosity of the paste is too high, so that handling at the time of joining becomes difficult. On the other hand, if it exceeds 13% by mass, the void ratio becomes high and the required bonding strength cannot be obtained, the thermal conductivity is extremely lowered, and the viscosity becomes too low, which makes the handling inconvenient.

[Example 1]
Using Zn having a purity of 99.9% by mass or more and Al having a purity of 99.95% or more as metal raw materials, each Zn alloy solder powder of Samples 1 to 20 shown in Table 1 below was manufactured. That is, in order to prevent oxidation, these metal raw materials were melted in a high-frequency melting furnace while flowing nitrogen at a flow rate of 700 ml / min or more per 1 kg of the metal raw material to obtain a Zn-Al alloy ingot. The compositions of these Zn—Al based ingots were measured using an ICP emission spectroscopic analyzer (SHIMAZU S-8100). Each Zn—Al alloy ingot was pulverized with a stamp mill and then sieved to obtain a Zn alloy solder powder having an average particle size of about 25 μm. The average particle size was measured with a laser diffraction particle size distribution meter (manufactured by Nippon Laser Co., Ltd.) (hereinafter, the average particle size was measured with the same apparatus). On the other hand, about the metal powder, the Ni powder and Cu powder of the average particle diameter shown in following Table 2 were manufactured by the atomizing method.

  The obtained Zn alloy solder powder and metal powder (Ni powder or Cu powder) are mixed in the addition amounts shown in Table 2 below, and a flux made of alcohol and a reducing organic material (Aoki Metal Co., Ltd.) is added. Each solder paste of Samples 1 to 20 shown in Table 2 below was obtained by stirring and mixing using a mixer (SR-500 manufactured by Shinky Corp.). In all samples 1 to 20, the amount of flux added was 8% by mass with respect to 100% by mass in total of the Zn alloy solder powder and the metal powder.

  Next, for each of the solder pastes of Samples 1 to 20, the die bonding property, the high temperature solder property, and the bonding reliability were evaluated by the following methods. The obtained results are shown in Table 3 below. Note that the evaluation of bonding reliability is an alternative to the evaluation of stress relaxation, which is difficult to evaluate directly.

(Die bonding)
Using a solder die bonder (manufactured by dage, EDB-200), on a Cu lead frame subjected to Ag plating, a dummy chip in which Ni and Ag were sequentially deposited on the surface was within a temperature range of 420 ± 3 ° C. It joined using the solder paste of said each sample. The solder paste was supplied by dropping a suitable amount of solder paste in advance on the Ag plating of the lead frame. The joint part of the obtained lead frame and the dummy chip was observed using an X-ray transmission device (TUX-3000W, manufactured by Token X-ray Inspection Co., Ltd.). % Or more and less than 8% were evaluated as “Δ”, and 8% or more were evaluated as “x”. The void ratio was calculated by the following equation by observing the solder joint from a direction perpendicular to the joint surface with an X-ray transmission device, obtaining the void area and the joint area.
Void ratio (%) = void area / (void area + joint area) × 100

(High temperature solder properties)
The suitability for use as a high temperature solder was evaluated by a high temperature shear test according to JIS Z 3198-7: 2003. That is, the sample after die bonding was set in a bond tester (manufactured by Techno Alpha Co., Ltd.), reheated to 240 ° C. while flowing nitrogen, and in that state, shear force was applied to the chip and the solder joint. Specifically, a shear test jig was horizontally hooked on the solder / dummy chip joint surface, and a shearing force of 90 N was applied. “○” indicates that the chip has cracked, but there is no crack or deformation in the joint or solder, and “chip” indicates that the chip has moved or shifted, or if there is a crack or deformation in the joint or solder. “×”.

(Joint reliability)
As the reliability evaluation 1, after performing the above-mentioned die bonding, a sample obtained by molding an epoxy resin (manufactured by Sumitomo Bakelite Co., Ltd., EME-6300) with a transfer mold molding machine was used at a temperature of 80 ° C. and a humidity of 80%. After performing a constant temperature and humidity test for 1000 hours, the resin was unsealed and the joint was observed. If there was no crack or peeling at the chip or joint interface, there was a crack or peeling. The case was evaluated as “x”.

  Further, as reliability evaluation 2 in bonding reliability, for a sample molded with an epoxy resin similar to the above, cooling and heating at −50 ° C. and 125 ° C. were set as one cycle, and after repeating this cycle 400 times, The joint was observed after opening, and the case where there was no crack at the chip or the interface of the joint was evaluated as “◯”, and the case where there was a crack or peeling was evaluated as “x”.

  As is apparent from these results, each of the solder pastes of Samples 1 to 12 according to the present invention has excellent die bonding properties and high temperature solder characteristics as high temperature solder, and has excellent bonding reliability. On the other hand, each solder paste of Comparative Samples 13 to 20 in which the addition amount or average particle diameter of Ni powder or Cu powder, which is a metal powder, is out of the scope of the present invention is insufficient in one or more of each evaluation. As a result.

[Example 2]
As the Zn alloy solder powder, each Zn solder alloy powder of Samples 21 to 28 having the composition shown in Table 4 below was manufactured by the same method as in Example 1 above. On the other hand, as a metal powder, a Cu powder is produced using a normal atomization method in the same manner as in Example 1 above, and has a film made of Au or Ag having a film thickness shown in Table 5 below by electroless treatment. Manufactured. This Cu powder is embedded in a resin, polished using a polishing machine in order from coarse abrasive paper, and finally polished with buff, and finally buffed, followed by line analysis using EPMA (device name: SHIMADZU EPMA-1600). The film thickness of the film was measured.

  The obtained Zn alloy solder powder and coated Cu powder were mixed in the addition amounts shown in Table 5 below, and the flux composed of alcohol and a reducing organic material was added and mixed in the same manner as in Example 1 to obtain Table 5 below. Each solder paste of Samples 21 to 28 according to the present invention shown in FIG. In all the samples 21 to 28, the addition amount of the flux was 8% by mass with respect to 100% by mass in total of the Zn alloy solder powder and the metal powder.

  About each solder paste by this invention of the said samples 21-28, the die bonding property, the high temperature solder characteristic, and joining reliability were evaluated by the method similar to the said Example 1. FIG. The obtained results are shown in Table 6 below.

  As is clear from these results, each solder paste using the coated Cu powder according to the present invention has excellent die bonding properties and high temperature solder characteristics as a high temperature solder, and has excellent bonding reliability.

[Example 3]
Samples having the compositions shown in Table 7 below were used in the same manner as in Example 1, using Zn, Al, Mg having a purity of 99.9% by mass or more and Ge having a purity of 99.95% or more as metal raw materials. Each Zn alloy solder powder of 29-37 was manufactured. On the other hand, as a metal powder, Cu powder was manufactured by the atomizing method in the same manner as in Example 2 above, and then an Ag film was applied to the surface of the Cu powder to manufacture a coated Cu powder.

  The obtained coated Cu powder and Zn alloy solder powder are mixed so that the coated Cu powder is 30% by mass, and a flux composed of alcohol and a reducing organic material is added and mixed as in Example 1 above. The solder pastes of Samples 29 to 37 shown in Table 8 below were manufactured. In all the samples 29 to 37, the addition amount of the flux was 8% by mass with respect to the total of 100% by mass of the Zn alloy solder powder and the metal powder.

  About each solder paste of the said samples 29-37, the die bonding property, the high temperature solder characteristic, and the joining reliability were evaluated by the method similar to the said Example 1. FIG. The obtained results are shown in Table 9 below.

  As can be seen from these results, it can be seen that the solder pastes of Samples 29 to 34 are all excellent in die bonding property, high-temperature solder characteristics, and bonding reliability, and are extremely good as high-temperature solder. On the other hand, since the composition of the Zn alloy solder powder is not appropriate for each of the solder pastes of Samples 35 to 37, even when the coated Cu powder is used, it is insufficient in each evaluation of die bonding property, high-temperature solder characteristics, and bonding reliability. It is the result.

Claims (2)

  It consists of Zn alloy solder powder containing 70 mass% or more of Zn, metal powder mainly composed of Ni or Cu, and the remaining flux, and the metal powder is Ni powder having an average particle diameter of 1 to 100 μm, Coated Ni powder provided with a film having a film thickness of 1 μm or less composed of at least one element of Au and Ag on the surface, Cu powder having an average particle diameter of 1 to 100 μm, and composed of at least one element of Au and Ag on the surface of the Cu powder It is at least one selected from coated Cu powder provided with a film having a thickness of 1 μm or less, and when the total of the metal powder and the Zn alloy solder powder is 100 mass%, the total of the metal powder is 5 to 65 mass%. A high-temperature lead-free solder paste characterized by 2. The high-temperature lead-free solder paste according to claim 1, wherein the Zn alloy solder powder is composed of the following (1) or (2) Zn alloy.
(1) A Zn alloy containing 1.0 to 7.0% by mass of Al with the balance being Zn and inevitable impurities.
(2) A Zn alloy containing 1.0 to 7.0% by mass of Al and containing 1.5% by mass or less of Ge and / or 6.0% by mass or less of Mg, with the balance being Zn and inevitable impurities. .