JP2004047550A - Joining method and structure of electronic component and substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a joining method which can give sufficient joining strength to a joining portion and can also make the drop impact resistance of electronic component improved, even when leadfree solder is used. <P>SOLUTION: A solder bump 2 of an LSI package 1 is joined to an electrode pad 4 of a printed circuit board 3. In the printed circuit board 3 before the joining, a gold-plated layer 5 is formed with a reduction type gold plating method on the electrode pad 4 formed of copper. The solder bump 2 is joined with the electrode pad 4, by making the solder bump 2 of the LSI package 1 to be contact with the metal plated layer 5 and then reflowing the same. The solder bump 2 is formed of the leadfree solder, including silver. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a surface mounting technology for electronic components, and more particularly to a bonding method for mounting an electronic component on a gold-plated electrode of a circuit board using a conductive bonding member such as solder.
[0002]
[Prior art]
With the recent demand for smaller and lighter electronic devices and the development of high integration technology for electronic components such as ICs, LSIs, and VLSIs, the number of input / output terminals (electrode terminals) of electronic components has increased from hundreds to thousands. That's a huge number.
[0003]
As a method for mounting a multi-terminal electronic component on a circuit board at a high density, there is a method in which conductive bumps are formed on a package surface and mounted on a circuit board (printed wiring board) via the bumps. Among such connection methods, flip-chip connection, in which a bare chip is directly mounted on a substrate by bump connection, is a mounting technology that can realize a highly integrated semiconductor device because it has the smallest mounting area and the shortest wiring length.
[0004]
In addition, the technical development of circuit boards has been progressing, and in recent years, a technology that can perform high-density mounting in a small area for portable electronic devices has been proposed. As one of such proposals, there is known a circuit board having a surface treated with electroless nickel and electroless gold flash plating (electroless Ni-P / Au flash plating) for mounting a multi-terminal electronic component. .
[0005]
Conventionally, in electroless plating as a surface treatment of a substrate, it has been considered impossible to apply gold plating directly on copper. Accordingly, nickel is widely used as an under barrier metal for preventing oxidation of copper and elution of copper. However, nickel itself is also oxidized and has poor wettability with solder, so it is common to further apply gold plating to the nickel surface. In particular, as a surface treatment of a printed circuit board on which a multi-terminal electronic component such as a portable device is mounted, a process of performing electroless gold plating on electroless nickel-phosphorus plating has been used.
[0006]
The principle of electroless nickel plating is to deposit nickel ions as nickel metal with a reducing agent. Sodium hypophosphite is widely used as a reducing agent. Since sodium hypophosphite contains phosphorus in its composition, the resulting nickel deposition film usually contains 5 to 10% of phosphorus (practical plating by the Japan Plating Association). I supplement, Maki Shoten, 1997, p. 515).
[0007]
Conventionally, bumps of a semiconductor chip used for mounting a bare chip on a printed wiring board include Sn-Pb solder and Au bumps. However, since Sn-Pb solder contains lead which is harmful to the environment, it does not satisfy the recent demand for lead-free. For this reason, use of a lead-free solder such as Sn-Ag based solder that does not contain lead has been studied as a solder used for mounting on a printed circuit board.
[0008]
[Problems to be solved by the invention]
The mounting reliability of an electronic component using a lead-free solder is evaluated by a temperature cycle test, a board repeated bending test, a drop impact test, and the like, as compared with a conventional Sn-Pb solder. Among them, it has been pointed out that the characteristics of lead-free solder are relatively deteriorated, especially with respect to the drop impact.
[0009]
One of the causes has been pointed out by the influence of phosphorus described above (for example, Sugizaki: Effect of electroless Ni / Au treatment on BGA solder joint, Journal of Japan Institute of Electronics Packaging, Vol. 4, No. 2, pp. 124-127, 2001). This document describes that a high-concentration phosphorous layer is formed at the interface in the joining made of Sn-Ag-based lead-free solder and electroless Ni-P / Au flash plating. Since the bonding strength of the high-concentration phosphorus layer is low, it is considered that the high-concentration phosphorus layer is broken from the interface by a drop impact.
[0010]
The influence of nickel diffusion is considered as a cause of the high-concentration phosphorus layer. That is, when heat is applied to the substrate-solder joint by reflow or the like, nickel in the nickel-phosphorous film on the substrate surface diffuses into the solder, so that the nickel concentration on the film surface decreases, and the phosphorus concentration becomes relatively high. It is thought to be.
[0011]
As a measure to prevent such an increase in the phosphorus concentration, it has been proposed to use a compound other than phosphorus such as boron as a reducing agent instead of sodium hypophosphite serving as a phosphorus supply source. However, when boron-based dimethylaminoborane or the like is used as the reducing agent, the plating treatment itself is good, but the bonding reliability is significantly inferior to conventional sodium hypophosphite.
[0012]
In recent years, the use of boron used in nickel plating baths has been evaded by the establishment of environmental standards and wastewater standards based on the Water Pollution Control Law in recent years, and research and development of alternative substances has been progressing. . Furthermore, nickel itself has been studied as a substance allergic to the human body by local governments in Europe and Japan (for example, Kanagawa Prefecture).
[0013]
In the future, in particular, the PRTR (Pollutant Release and Transfer Register: Environmental Pollutant Release and Transfer Register), etc., will determine how many and various kinds of harmful chemical substances have been released into the environment and from what sources. There is a system in place to grasp, aggregate, and publish data on whether or not the data has been transported out of business sites, and regulations have become even stronger. And nickel is taken up as one of the chemical substances subject to the PRTR regulation.
[0014]
Under such stricter environmental regulations, it has become increasingly difficult to guarantee the bonding reliability of electronic components that have been bonded to conventional copper electrode substrates using Sn-Ag lead-free solder. However, it has been extremely difficult to improve the drop impact resistance.
[0015]
The present invention has been made in view of the above points, and can provide a solder joint with a sufficient joining strength even when a Sn-Ag-Cu-based lead-free solder is used, and can withstand drop impact of an electronic component. It is an object of the present invention to provide a joining method capable of improving the property.
[0016]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is characterized by taking the following means.
[0017]
The invention according to claim 1 is a bonding method for bonding an electrode of an electronic component to an electrode pad of a substrate or another electronic component, wherein a gold plating layer is formed on a copper electrode pad by a reduction gold plating method. Contacting the electrode and the electrode pad via the conductive bonding member by bringing a conductive bonding member made of an alloy containing silver into contact with the gold plating layer and performing reflow. .
[0018]
According to the first aspect of the present invention, it is possible to improve the bonding strength between the conductive bonding member and the electrode pad by directly performing gold plating on the electrode pad made of copper without using nickel plating. The reliability of the joint can be increased. Thereby, sufficient joining reliability can be ensured even if lead-free solder is used as the conductive joining member.
[0019]
The invention according to claim 2 is the bonding method according to claim 1, wherein the gold plating layer is formed such that the thickness of the gold plating layer is 0.1 μm or less.
[0020]
According to the second aspect of the present invention, the tensile strength of the joining portion can be maintained at a high value, and the joining reliability can be further improved.
[0021]
The invention according to claim 3 is the bonding method according to claim 1 or 2, wherein the conductive bonding member is reflowed three times or more.
[0022]
According to the third aspect of the invention, by increasing the number of times of reflow, the joining reliability of the joining portion can be further improved.
[0023]
The invention according to claim 4 is a connection structure for an electronic component, comprising an electrode pad made of copper, a gold plating layer formed on the electrode pad, and an alloy containing silver, and joined to the gold plating layer. And a conductive bonding member provided.
[0024]
According to the fourth aspect of the present invention, by directly applying gold plating on the electrode pad made of copper without using nickel plating, the bonding strength between the conductive bonding member and the electrode pad can be improved. The reliability of the joint can be increased.
[0025]
The invention according to claim 5 is a substrate on which an electronic component is mounted, comprising: an electrode pad made of copper; and a gold plating layer formed on the electrode pad, wherein the gold plating layer is a reduction mold. It is a gold plating layer having a thickness of 0.1 μm or less formed by a plating method, and is a layer that comes into contact with a conductive bonding member.
[0026]
According to the fifth aspect of the present invention, the gold plating having a thickness of 0.1 μm or less is directly applied on the electrode pad made of copper without nickel plating, thereby joining the conductive bonding member and the electrode pad. The strength can be improved, and the reliability of the joint can be increased.
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[0027]
The present inventor has conducted intensive studies in order to solve the problem of reduced bonding strength when using lead-free solder, and as a result, directly on the copper electrode without using nickel, on a substrate on which a gold film was formed, It has been found that bonding strength can be improved by mounting lead-free solder.
[0028]
In recent years, the technical development of electroless gold plating has progressed, and it has become possible to perform gold plating directly on a copper surface which has been impossible in the past. Based on this, the present inventor has once again reviewed the surface treatment of copper electrodes using nickel conventionally, and found that eliminating nickel plating is effective in improving the strength of the joint. . Therefore, when a verification experiment was performed using lead-free solder as the solder used for mounting the surface-treated printed circuit board and the electronic component, good joining reliability was obtained.
[0029]
In particular, the inventor of the present invention has proposed a method in which a solder alloy has a composition containing silver (Ag) and the surface treatment of a printed circuit board to be mounted is a configuration in which gold (Au) plating is directly performed on a copper (Cu) electrode. It was found that drop impact resistance was significantly improved.
[0030]
As the composition of the lead-free solder, there are various alloy compositions such as a tin-zinc (Sn-Zn) system. Among these, the present invention is also effective when copper (Cu) is contained, such as a tin-silver-copper (Sn-Ag-Cu) based lead-free solder which has recently attracted attention.
[0031]
Generally, Sn-Ag-Cu-based solder has a high melting point, but Sn-3Ag-0.5Cu, which is recommended by the Japan Electronics and Information Technology Industries Association (JEITA), has a melting point (solidus 218 ° C) and a conventional lead-based solder. (Sn-37Pb: melting point: 183 ° C) is higher by about 30 ° C. Experiments have confirmed that Sn-3Ag-0.5Cu having a melting point of 200 ° C. or more is effective.
[0032]
The bonding reliability, especially the drop impact resistance, is greatly affected by the Young's modulus, which is a measure of the solder stiffness. The Young's modulus of the Sn-Ag-Cu-based solder is larger than that of the conventional lead-based solder (22 GPa) (≥25 GPa). However, when the bonding method according to the present invention was used, good drop impact resistance was exhibited even when the Sn-Ag-Cu-based solder was used.
[0033]
On the other hand, as for the film thickness of gold (Au) on the surface of the printed circuit board, a result that the thicker the film was, the better the bonding reliability was, but the thinner the better. Although the reason is not clear, it is considered that the adhesion strength between the gold plating layer and the electrode is weakened. For this reason, it has been found that the thickness of the gold film on the substrate surface is preferably 0.1 μm or less, which is equivalent to the thickness of the electroless gold (Au) plating formed on the electroless nickel (Ni) plating.
[0034]
Here, an experiment performed using a semiconductor device mounted on a printed circuit board by the bonding method according to the present invention will be described. FIG. 1 is a diagram showing a semiconductor device mounted on a printed circuit board by a bonding method according to the present invention.
[0035]
The semiconductor device 1 is a so-called ball grid array type (BGA type) LSI package, and is mounted on a printed board 3 made of glass epoxy via solder bumps 2. The semiconductor device 1 is a quadrilateral having a side of 35 mm, and the solder bumps 2 are arranged in a grid on the bottom surface. The printed board 3 was a quadrilateral having a side of 110 mm, and the electrode pads 4 were arranged in the same arrangement as the arrangement of the solder bumps 2. In the example shown in FIG. 1, the diameter of the electrode pad was 760 μm, and the distance between the electrode pitches was 1.27 mm.
[0036]
The LSI package 1 and the printed circuit board 3 were designed so that the circuit formed a daisy chain pattern at the time of connection, so that the connection resistance could be measured. For the printed circuit board 3, a glass epoxy material was used as a substrate material, and copper was used as an electrode and wiring material. The surface of the substrate was treated with a gold (Au) plating solution (ASH Gold: manufactured by Okuno Pharmaceutical Co., Ltd.) to produce a printed substrate 3 on which the bonding method according to the present invention was performed.
[0037]
FIG. 2 is an enlarged cross-sectional view of the vicinity of the joint (the part surrounded by the circle indicated by A) of the semiconductor device mounting board shown in FIG. 1 manufactured by the above-described method. According to the bonding method of the present invention, the gold plating layer 5 is formed directly on the electrode pads 4 of the printed circuit board 3. Then, the solder bumps 2 (conductive bonding members) are bonded to the gold plating layer 5 by reflow.
[0038]
After the printed circuit board 3 was manufactured, the gold plating layer on the surface was removed and analyzed by an energy dispersive X-ray analyzer (EDX) to measure the film thickness of gold on the surface. 0.06 μm.
[0039]
Here, the above-mentioned gold plating method is reduction-type electroless gold plating, not replacement-type gold plating. When gold (Au) is plated on copper (Cu) by conventional substitutional gold plating, a gold plating layer is formed while partially dissolving copper. However, in the above-described bonding method according to the present invention, a reduction type gold plating method is used, and a gold plating layer can be formed without dissolving copper. This is presumed to be the reason why the reliability of joining with the solder is not reduced.
[0040]
The LSI package 1 having the Sn-3Ag-0.5Cu solder ball mounted thereon was bonded to the electrode pad 4 of the printed circuit board 3 manufactured as described above using a solder paste having the same composition by a reflow heating device. A free fall test was performed on the semiconductor device mounting substrate manufactured in this manner, and the drop life was examined. That is, when the resistance value between the solder bump 2 and the electrode pad 4 increases by 10% or more from the initial resistance value, it is determined that the joint is broken, and the number of drops at this time (hereinafter, referred to as the number of breaks and drops) is determined. Examined.
[0041]
As a result, in the case where the joining method according to the present invention was used, when the reflow was performed twice, the number of breaks and drops was 7 times. When reflow was performed four times under the same conditions, the number of times of breaking and falling was 24 times. Thus, it was found that even if the number of reflows was increased from two to four, the drop life was greatly extended. It is considered that the reason for this is that the mounting temperature (reflow peak temperature 231 ° C.) has an effect. That is, when gold plating is directly applied on copper, it is considered that more heat is required for bonding copper and solder via gold.
[0042]
In the above experiment, as a comparative example, the substrate surface was subjected to electroless nickel plating using a medium phosphorus concentration electroless nickel plating solution (ICP Nicolon GSR-M: manufactured by Okuno Pharmaceutical Industries) as in the conventional case, The printed circuit board 4 which was processed by electroless gold plating (flash gold 2000-M: manufactured by Okuno Pharmaceutical) was produced.
[0043]
FIG. 3 is an enlarged cross-sectional view of the vicinity of the bonding portion (portion surrounded by a circle indicated by A) of the semiconductor device mounting board shown in FIG. 1 manufactured as a comparative example by the above-described method. In the comparative example, a nickel coating 6 is formed on the electrode pads 4 of the printed circuit board 3, and a gold plating layer 5 is formed thereon. Then, the solder bumps 2 (conductive bonding members) are bonded to the gold plating layer 5 by reflow.
[0044]
Thus, in the comparative example, the nickel coating 6 is interposed between the electrode pad 4 made of copper and the gold plating layer 5. The phosphorus concentration of the produced printed board 4 was 5.7%, the thickness of the nickel film 6 was 5.2 μm, and the thickness of the gold plating layer 5 was 0.06 μm.
[0045]
When a drop test was performed using the comparative example manufactured as described above, when the reflow was performed twice, the number of times of breaking and dropping was 5 times. When the reflow was performed four times under the same conditions, the number of times of breaking and falling was reduced to one. This is the opposite phenomenon to the case where the above-described bonding method according to the present invention is used. When a gold plating layer is formed on a copper film via a nickel film, the strength of the bonding portion is increased by applying heat. It shows that it decreases.
[0046]
FIG. 4 is a graph showing a change in the drop life (the number of times of break-down) by increasing the number of reflows. According to the joining method of the present invention, when the number of reflows is increased from 2 to 4 times, the number of break and drop is greatly increased from 7 to 24, whereas in the comparative example, the number is decreased from 5 to 1 . That is, when the number of times of reflow is two, the difference between the number of times of break and drop between the present invention and the comparative example is two. However, when the number of times of reflow is increased to three or more, an example using the joining method according to the present invention. The difference between the comparative example and the comparative example was greatly increased, and it was found that the drop life when the joining method according to the present invention was used was dramatically increased.
[0047]
The tensile strength of the joint is also one of the measures for evaluating the joint reliability. Then, the present inventor examined the relationship between the thickness of the gold plating layer 5 and the tensile strength of the joint. FIG. 5 is a graph showing the result. It was found that the smaller the thickness of the gold plating layer 5 was, the higher the tensile strength of the joint portion was, and the tensile strength was sharply reduced from around 0.1 μm in thickness.
[0048]
Thus, it was found that the thickness of the gold plating layer 5 on the printed circuit board 4 before the reflow mounting was preferably 0.1 μm or less. The thickness of the gold plating layer formed by flash gold plating was about 0.1 μm, and it was found that the gold plating layer 5 was preferably formed by flash gold plating.
[0049]
Further, when the thickness of the gold plating layer 5 exceeds 0.1 μm, not only the tensile strength decreases, but also the reliability of bonding with the solder decreases, and it is evident from the results of experiments that fracture due to a drop impact easily occurs. became.
[0050]
As described above, the present specification discloses the following inventions.
[0051]
(Supplementary Note 1) A joining method for joining an electrode of an electronic component to an electrode pad of a substrate or another electronic component,
Forming a gold plating layer on the electrode pad made of copper by a reduction type gold plating method,
A bonding method, wherein the electrode and the electrode pad are bonded via the conductive bonding member by bringing a conductive bonding member made of an alloy containing silver into contact with the gold plating layer and performing reflow.
(Supplementary Note 2) The joining method according to Supplementary Note 1, wherein
A bonding method, wherein the gold plating layer is formed so that the thickness of the gold plating layer is 0.1 μm or less.
(Supplementary Note 3) The joining method according to Supplementary Note 1 or 2, wherein
A bonding method comprising reflowing the conductive bonding member three times or more.
(Supplementary Note 4) The joining method according to any one of Supplementary Notes 1 to 3, wherein
A bonding method using an alloy containing copper as the conductive bonding member.
[0052]
(Supplementary Note 5) The joining method according to any one of Supplementary Notes 1 to 4, wherein
A bonding method, wherein an alloy having a melting point of 200 ° C. or more is used as the conductive bonding member.
[0053]
(Supplementary Note 6) The joining method according to any one of Supplementary Notes 1 to 5, wherein
A bonding method, wherein an alloy having a Young's modulus of 25 GPa or more is used as the conductive bonding member.
[0054]
(Supplementary Note 7) The joining method according to any one of Supplementary Notes 1 to 6, wherein
A joining method, wherein a lead-free solder is used as the conductive joining member.
[0055]
(Supplementary Note 8) A connection structure for electronic components,
An electrode pad made of copper,
A gold plating layer formed on the electrode pad,
A connection structure comprising a silver-containing alloy and a conductive bonding member bonded to the gold plating layer.
(Supplementary Note 9) The connection structure according to Supplementary Note 8, wherein
The connection structure, wherein the thickness of the gold plating layer is 0.1 μm or less.
[0056]
(Supplementary Note 10) The joint structure according to Supplementary Note 8 or 9, wherein
The joining structure, wherein the conductive joining member is made of an alloy containing copper.
[0057]
(Supplementary Note 11) The joint structure according to any one of Supplementary Notes 8 to 10, wherein
The bonding structure, wherein the conductive bonding member is made of an alloy having a melting point of 200 ° C. or more.
[0058]
(Supplementary Note 12) The joint structure according to any one of Supplementary Notes 8 to 11, wherein
The bonding structure, wherein the conductive bonding member is made of an alloy having a Young's modulus of 25 GPa or more.
[0059]
(Supplementary Note 13) The joint structure according to any one of Supplementary Notes 8 to 12, wherein
The bonding structure, wherein the conductive bonding member is a lead-free solder.
[0060]
(Supplementary Note 14) A board on which electronic components are mounted,
An electrode pad made of copper,
Having a gold plating layer formed on the electrode pad,
The substrate, wherein the gold plating layer is a gold plating layer having a thickness of 0.1 μm or less formed by reduction gold plating, and is a layer that comes into contact with the conductive bonding member.
【The invention's effect】
As described above, according to the present invention, the following various effects can be realized.
According to the first aspect of the present invention, it is possible to improve the bonding strength between the conductive bonding member and the electrode pad by directly performing gold plating on the electrode pad made of copper without using nickel plating. The reliability of the joint can be increased. Thereby, sufficient joining reliability can be ensured even if lead-free solder is used as the conductive joining member.
[0061]
According to the second aspect of the present invention, the tensile strength of the joining portion can be maintained at a high value, and the joining reliability can be further improved.
[0062]
According to the third aspect of the invention, by increasing the number of times of reflow, the joining reliability of the joining portion can be further improved.
[0063]
According to the fourth aspect of the present invention, the bonding strength between the conductive bonding member and the electrode pad can be improved by directly plating the gold on the electrode pad made of copper without using nickel plating. The reliability of the joint can be increased.
[0064]
According to the invention as set forth in claim 5, by bonding the conductive bonding member and the electrode pad by directly applying a gold plating having a thickness of 0.1 μm or less on the electrode pad made of copper without using nickel plating. The strength can be improved, and the reliability of the joint can be increased.
[Brief description of the drawings]
FIG. 1 is a diagram showing a semiconductor device mounted on a printed circuit board by a bonding method according to the present invention.
FIG. 2 is an enlarged cross-sectional view of the vicinity of a bonding portion (a portion surrounded by a circle indicated by A) of the semiconductor device mounting substrate shown in FIG. 1 manufactured by a bonding method according to the present invention.
3 is an enlarged cross-sectional view of the vicinity of a joint (a portion surrounded by a circle indicated by A) of the semiconductor device mounting board shown in FIG. 1 manufactured as a comparative example.
FIG. 4 is a graph showing a change in a drop life (the number of times of break-down) by increasing the number of reflows.
FIG. 5 is a graph showing the relationship between the thickness of a gold plating layer and the tensile strength of a joint.
[Explanation of symbols]
1 LSI package 2 Solder bump 3 Printed circuit board 4 Electrode pad 5 Gold plating layer 6 Nickel coating

Claims (5)

A joining method for joining an electrode of an electronic component to an electrode pad of a substrate or another electronic component,
Forming a gold plating layer on the electrode pad made of copper by a reduction type gold plating method,
A bonding method, wherein the electrode and the electrode pad are bonded via the conductive bonding member by bringing a conductive bonding member made of an alloy containing silver into contact with the gold plating layer and performing reflow. The joining method according to claim 1, wherein
A bonding method, wherein the gold plating layer is formed so that the thickness of the gold plating layer is 0.1 μm or less. The joining method according to claim 1 or 2,
A bonding method comprising reflowing the conductive bonding member three times or more. A connection structure for electronic components,
An electrode pad made of copper,
A gold plating layer formed on the electrode pad,
A connection structure comprising a silver-containing alloy and a conductive bonding member bonded to the gold plating layer. A substrate on which electronic components are mounted,
An electrode pad made of copper,
Having a gold plating layer formed on the electrode pad,
The substrate, wherein the gold plating layer is a gold plating layer having a thickness of 0.1 μm or less formed by reduction gold plating, and is a layer that comes into contact with the conductive bonding member.

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