JP4168735B2 - Solder paste and electronic circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solder paste used when a semiconductor element or a surface-mounted electronic component is mounted on a printed wiring board or the like, and an electronic circuit on which the electronic component is mounted.
[0002]
[Prior art]
Conventionally, Sn—Pb eutectic solder has been generally used to electrically bond semiconductor elements and electronic components to a substrate. A typical example is Sn-37Pb having a melting point of 183 ° C.
On the other hand, restrictions on the use of Pb have begun to be studied due to problems of environmental influences including Pb contamination of groundwater, and various solders not containing Pb have been proposed and put into practical use.
[0003]
In addition, Cu has been mainly used as an electrode of a circuit board on which these electronic components are mounted. Furthermore, in order to suppress a decrease in reliability due to the diffusion of the electrodes, conventionally, Ni by electrolytic plating has been used as a barrier layer of the electrodes of the semiconductor chip. However, in recent years, Ni by electroless plating, which can be formed at a low cost, is becoming mainstream due to demand for cost reduction.
[0004]
Among solders that do not contain Pb (hereinafter referred to as Pb-free solder), Pb-free solder composed of Sn at 90% or more of the composition diffuses Cu and Ni as electrode materials compared to Pb-Sn eutectic solder. Since it is easy, the problem of the liquid phase diffusion which arises at the time of solder melting has become obvious. This problem is further exacerbated by the need for electroless plating.
[0005]
For example, as a Pb-free solder paste, it is possible to respond to various demands by combining as simple and few alloy types as possible, and further reduce the burden on the manufacturer by preventing the risk of segregation in the manufacturing process. In addition, an object of the present invention is to provide a Pb-free solder paste having excellent characteristics that cannot be obtained with a multi-element alloy, and an eutectic composition binary alloy powder and an alloy powder containing In or In There is one that prevents the risk of segregation by mixing the flux and the flux (for example, see Patent Document 1).
[0006]
However, when this Pb-free solder paste is used, Cu or Ni as the electrode material is easily diffused, which causes a problem of liquid phase diffusion that occurs when the solder melts.
In addition, for the purpose of providing a Pb-free solder having good wettability and a low melting point, it is made of a metal powder containing a combination of two or more metals that can form an intermetallic compound, and before the intermetallic compound is formed. Pb-free solder powder containing an unreacted phase and an amorphous phase has been proposed.
[0007]
That is, since it does not contain harmful lead, it has excellent safety during soldering operations, can suppress adverse effects on the environment even when discarding components containing solder, and can form an intermetallic compound 2 Since the metal powder containing a combination of more than one kind of metal is contained in an unreacted phase and an amorphous phase, the melting point is lower than that of the intermetallic compound, so the soldering operation can be performed at a relatively low temperature. This has the effect of reducing the possibility of damage.
[0008]
The unreacted phase as used herein refers to an alloy powder containing at least one or more metal atoms as a raw metal powder, and a single metal or a second alloy that can form an intermetallic compound by fusing with the alloy powder. It is a combination of powder and means an alloy powder before forming an intermetallic compound.
In addition, the amorphous phase referred to here is an intermediate phase that appears when two kinds of metals are melted together, and means one having poor metallic properties (for example, see Patent Document 2).
[0009]
However, there is a problem of oxidation of the solder powder because it does not touch on the problem of electrode diffusion and further does not consider the oxidation of the single metal or alloy powder by heating.
On the other hand, this problem is that when a Pb-free solder mainly composed of Sn is bonded onto a Ni electrode, a Cu-Zn-Ni compound phase is formed on the electrode by bonding using Pb-free solder to which Cu and Zn are added. It has been clarified that the Ni electrode can be uniformly formed and diffusion of the Ni electrode can be suppressed (for example, see Patent Document 3).
[0010]
That is, at least one of the connection electrode on the circuit board and the connection terminal electrode on the electronic component that is in contact with the Pb-free solder is formed by electroless Ni plating, and the interface between the Ni portion and the Pb-free solder In this case, the presence of the Cu—Zn—Ni phase causes the effect of suppressing the diffusion of Ni.
However, when producing a Pb-free solder bump electrode on at least one of the connection electrode on the circuit board and the connection terminal electrode on the electronic component, it is active to integrate the Pb-free solder powder during formation. An agent is needed.
[0011]
Also, solder bumps formed with a solder paste using Pb-free solder powder have a large surface oxidation. For example, when joining with a connection electrode on a circuit board, a high active flux is used to promote wettability. is necessary.
By the way, since the surface is oxidized even if it is a solder bump formed with a solder paste using Pb-free solder powder to which Cu and Zn are added, when this solder bump is joined to the circuit board, In order to promote the wettability, a flux with high activity is required. In addition, wettability here is evaluated based on the standard prescribed | regulated to ASTM-B-545 as an evaluation method of solderability.
[0012]
[Patent Document 1]
JP-A-11-138292 (pages 4-5, Table 1-1)
[0013]
[Patent Document 2]
JP 2000-246483 A (pages 2 and 3, FIG. 1)
[0014]
[Patent Document 3]
JP 2002-185130 A (page 3, table 1)
[0015]
[Problems to be solved by the invention]
As described above, when the Sn—Ag solder with Cu and Zn added is used to join the electrode, the Cu—Zn—Ni compound layer can be uniformly formed on the electrode, thereby suppressing the diffusion of the Ni electrode. can do.
However, the surface of the solder bump formed using the solder added with Cu and Zn is oxidized. Therefore, when this solder bump is joined to the circuit board, a flux with high activity is required to promote wettability with the electrodes of the circuit board, and the use of flux with high activity increases the amount of use. As a highly corrosive residue is generated, the electrode is corroded, and there is a problem that reliability is lowered.
[0016]
Therefore, the present invention can uniformly form a stable Cu—Zn—Ni compound phase on a Ni electrode, and suppresses oxidation of solder powder to improve soldering reliability and an electronic circuit. The purpose is to provide.
[0017]
[Means for Solving the Problems]
According to claim 1 of the present invention, the above-described problem is obtained from a solder powder composed of at least one of Sn-Ag or Sn-Ag-Cu and at least one of Cu-Zn or Cu-Zn-Sn. This is achieved by forming a solder paste containing a diffusion suppressing alloy powder and a flux.
[0018]
That is, a solder paste containing a diffusion-suppressing alloy powder consisting of at least one of Cu-Zn or Cu-Zn-Sn and a flux in a solder powder consisting of at least one of Sn-Ag or Sn-Ag-Cu. Therefore, when using solder in which Cu, Zn, Sn powder is added to Sn-Ag solder powder, it is possible to prevent surface oxidation of Zn powder at the time of joining, and to reduce the amount of flux used. The attachment reliability can be improved.
[0019]
Further, according to claim 2 of the present invention, the above-described problem is to provide the solder paste according to claim 1, wherein the respective particle sizes of the solder powder and the diffusion suppressing alloy powder do not exceed 20 μm. To be achieved.
That is, by making the particle size of the solder powder and the diffusion suppressing alloy powder 20 μm or less, the wettability has been improved by using a large amount of flux in the past. This produces an effect that the wettability can be maintained by reducing the amount of use.
[0020]
On the other hand, when the solder powder and the diffusion suppressing alloy powder are larger than 20 μm, the mutual powders are hardly melted.
According to claim 3 of the present invention, the above-described problem is achieved by providing the solder paste according to claim 1 wherein the particle size of the diffusion suppressing alloy powder is smaller than the particle size of the solder powder. Is done.
[0021]
That is, by setting the particle size of the diffusion-inhibiting alloy powder to be the same as or smaller than the particle size of the solder powder, there is an effect that both powders can be uniformly mixed. On the other hand, if the diffusion suppressing alloy powder is larger, the diffusion suppressing effect of both mixed powders is not preferable because it varies depending on the region.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these.
[Example 1]
FIG. 1 is an explanatory diagram of the principle of the present invention. FIG. 2 is a diagram showing problems in the prior art.
[0023]
Here, reference numeral 1 is a semiconductor element as an example of an electronic component, 2 is an electroless Ni plating electrode, 31 is a solder paste of the present invention, 32 is a conventional solder paste, 311 is solder powder, and 312 is diffusion. Inhibitory alloy powder, 313 indicates a flux, 4 indicates a Cu—Zn—Ni compound phase formed on an electroless Ni plating electrode, 5 indicates a solder bump, and 6 indicates an oxide film.
[0024]
Electroless Ni-plated electrode 2 having a diameter of 0.7 mm formed on semiconductor element 1, Sn-3.5Ag solder powder (average particle size of 20 μm or less), 60Cu-40Zn diffusion suppression alloy powder (average particle size of 15 μm or less) 2% by weight, and the solder paste 31 of the present invention mixed with the flux at a weight ratio of 90:10 was screen-printed, and then soldered by nitrogen reflow at a peak temperature of 250 ° C.
[0025]
When the diffusion amount of the sample thus prepared was heated for 15 minutes on a 250 ° C. hot plate, the Ni electrode was hardly diffused. Thereby, it was found that the liquid phase diffusion of the electroless Ni-plated electrode 2, which is a concern with Sn-based solder, is equivalent to that of Sn—Pb eutectic solder and can maintain a good joint.
[0026]
For comparison, screen printing was performed using a conventional solder paste 32 in which only Sn-3.5Ag solder powder (average particle size of 20 μm or less) was mixed with flux at a weight ratio of 90:10, followed by a peak temperature of 250 ° C. When the sample soldered by nitrogen reflow was heated on a hot plate at 250 ° C. for 15 minutes and the diffusion amount of the Ni electrode was investigated, the diffusion amount was compared with that added with Cu—Zn powder. More than twice.
[0027]
Further, on the electrodes having a diameter of 110 μm and a pitch of 250 μm formed on a 10 mm square semiconductor element, the solder paste 31 of the present invention used in Example 1 was screen-printed by screen printing to form solder bump electrodes. This semiconductor element was joined to the build-up substrate by nitrogen reflow with a peak temperature of 250 ° C.
The sample was subjected to a temperature cycle test from −25 ° C. to 125 ° C. up to 1000 cycles, and operated normally in the same manner as the Sn—Pb eutectic solder.
[0028]
Incidentally, in 2000 cycles, a malfunction occurred in part in the Sn—Pb eutectic solder, but no malfunction occurred in this sample.
[Example 2]
The Sn-3.5Ag solder powder (average particle size 20 μm or less) is mixed with 2% by weight of 60Cu-40Zn diffusion suppression alloy powder (average particle size 15 μm or less) and mixed with the flux at a weight ratio of 90:10. The solder paste 31 was screen-printed on a Si substrate in which a recess was formed by etching, and then a solder ball was produced by nitrogen reflow at a peak temperature of 250 ° C.
[0029]
This solder ball was placed on a Cu substrate onto which flux was dropped, and the sample was soldered by heating to 250 ° C. on a hot plate, and wettability was evaluated in accordance with ASTM-B-545.
As a result, the wettability of the Sn-3.5Ag solder was 78%, whereas the wettability of the solder paste 31 of the present invention to which the diffusion suppressing alloy powder was added was almost equal to 72%.
[0030]
For comparison, a solder ball produced using the Sn-3.5Ag-0.5Cu-1Zn solder powder used in Patent Document 3 showed a low value of 61%.
This result revealed the effect of adding Cu—Zn diffusion suppressing alloy powder.
(Supplementary Note 1) Diffusion-inhibiting alloy comprising at least one of Sn-Ag or Sn-Ag-Cu and 2 weight percent of the solder powder, Cu-Zn or Cu-Zn-Sn A solder paste comprising a powder and a flux.
(Additional remark 2) The solder paste of Additional remark 1 characterized by each particle size of the said solder powder and the said diffusion suppression alloy powder not exceeding 20 micrometers.
(Additional remark 3) The solder paste of Additional remark 1 characterized by the particle size of the said diffusion suppression alloy powder being smaller than the particle size of the said solder powder.
(Additional remark 4) It has a protruding electrode formed using the solder paste of Additional remark 1, The electronic circuit characterized by the above-mentioned.
(Additional remark 5) The said protrusion electrode is formed on Ni electrode , The electronic circuit of Additional remark 4 characterized by the above-mentioned.
(Additional remark 6) The solder paste of Additional remark 1 characterized by the said flux consisting of rosin.
[0031]
【The invention's effect】
By using the solder paste of the present invention, the liquid phase diffusion of electroless Ni plating, which is a concern with Pb-free Sn-based solder, can be made equivalent to that of conventional Sn-Pb eutectic solder, and the amount of flux used By suppressing this, it is possible to ensure highly reliable solderability.
[0032]
In addition, it becomes clear that the reliability of the joining is ensured, and there is an effect that it is possible to use inexpensive electroless Ni plating, which greatly contributes to high reliability of the product.
[Brief description of the drawings]
FIG. 1 illustrates the principle of the present invention.
FIG. 2 is a diagram showing problems in the conventional technique.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor element 2 Electroless Ni plating electrode 31 Solder paste 311 Solder powder 312 Diffusion suppression alloy powder 313 Flux 32 Sn-Ag-Cu-Zn solder paste 321 Sn-Ag-Cu-Zn solder powder 4 Cu- Zn-Ni compound phase 5 Solder bump 6 Oxide film

Claims (5)

A solder powder comprising at least one of Sn-Ag or Sn-Ag-Cu;
2 weight percent of the solder powder, a diffusion-inhibiting alloy powder comprising at least one of Cu-Zn or Cu-Zn-Sn;
A solder paste characterized by containing flux. 2. The solder paste according to claim 1, wherein each of the solder powder and the diffusion-inhibiting alloy powder has a particle size not exceeding 20 μm. The solder paste according to claim 1, wherein a particle size of the diffusion suppressing alloy powder is smaller than a particle size of the solder powder. An electronic circuit comprising a protruding electrode formed using the solder paste according to claim 1. 5. The electronic circuit according to claim 4 , wherein the protruding electrode is formed on a Ni electrode .

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