JP2003174254A - Connecting method of electronic component to circuit substrate employing lead-free solder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a connecting method through soldering capable of obtaining connecting reliability same as or much more than conventional Sn-Pb base solder by employing lead-free solder containing Sn and Zn. <P>SOLUTION: Soldering connection is effected employing a solder ball, molten at a temperature lower than the lead-free solder alloy containing Sn and Zn, and a Cu ball as well as an Ni-Fe alloy ball, jointed to the lead-free solder without being molten. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to S, for connecting parts and the like to a circuit board of an electronic device such as a television, a personal computer, an audio, a video, a rice cooker and a refrigerator.
Conventional Sn-P using lead-free solder containing n and Zn
The present invention relates to a soldering method having reliability equal to or higher than that of eutectic solder.

[0002]

2. Description of the Related Art As a solder used for soldering a circuit board of an electronic device, a solder composed of Sn and Pb (hereinafter Sn-Pb type solder) is generally used and has been used for a long time since ancient times. . The Sn-Pb solder has a low eutectic composition (63Sn-Pb) melting point of 183 degrees Celsius (hereinafter simply referred to as "degree"), and its soldering temperature is 220 to 230 degrees, which is a weak electronic component. Is a temperature that does not cause heat damage.

Moreover, the Sn-Pb type solder has an excellent feature that it solidifies immediately during soldering and does not crack or peel even if vibration or impact is applied to the soldered joint.

In general, electronic devices such as televisions, personal computers, audio, video, rice cookers, and refrigerators are discarded when they are broken down or become old and unusable. In these electronic devices, the exterior and printed circuit boards are synthetic resins such as plastic,
In addition, since the conductor and frame are made of metal, they cannot be incinerated and are almost buried underground.

By the way, in recent years, due to heavy use of petrochemical fuels such as gasoline and heavy oil, a large amount of sulfur oxides is released into the atmosphere, and as a result, the rain on the ground is acid rain. Acid rain elutes the solder on the circuit boards of electronic devices buried in the ground, soaks underground and pollutes groundwater. When drinking groundwater containing lead in this way for many years, lead may accumulate in the human body and lead poisoning may occur.

Under such circumstances, it has been desired in the electronic equipment industry to develop lead-free solder, so-called “lead-free solder”, and it is urgently necessary to develop a substitute material for Sn—Pb solder. .

Sn-Pb type solder has been widely used for joining electronic parts such as QFP (quad flat package) and BGA (ball grid array). However, since it contains lead and adversely affects the environment when it is discarded, it is considered to use lead-free solder for joining electronic components.

Conventionally, as a lead-free solder, a solder composed of Sn and Ag having Sn as a main component (hereinafter Sn-Ag solder) or a solder composed of Sn and Sb (hereinafter Sn-).
There were Sb-based solders), solders composed of Sn and Zn (hereinafter Sn-Zn-based solders), and the like.

The Sn-Ag solder has a eutectic composition of Sn-3.5Ag having the lowest melting temperature, and the melting temperature is 221 degrees. The soldering temperature of the solder having this composition is a considerably high temperature of 260 to 270 degrees.

The Sn-Sb type solder has a composition with the lowest melting temperature of Sn-5Sb. The melting temperature of this composition is as high as 235 degrees in the solidus temperature and 240 degrees in the liquidus temperature. Since it is temperature, the soldering temperature is S
280-300, which is higher than n-3.5 Ag solder
It becomes degree.

The Sn-Zn solder has a eutectic composition of Sn-.
With 9Zn, the eutectic temperature is 199 degrees, and the melting temperature is close to the eutectic temperature of 183 degrees of the conventional 63Sn-Pb eutectic solder, which is a temperature superiority. In addition, Sn-Zn
The system solder is superior in mechanical strength to the Sn-Pb system solder.

Using Sn-Zn type solder, QFP and B
When soldering components such as GA, it is necessary to solder at a slightly higher temperature than Sn-Pb solder. Therefore, a solder in which Bi is added to the Sn-Zn-based solder to lower the melting temperature (hereinafter, Sn-Zn-Bi-based solder) is being developed.

[0013]

[Problems to be Solved by the Invention] In the electronic parts such as BGA and QFP, the heat resistance guaranteed temperature of the resin part is generally 220-
It is as low as about 230 degrees, and the solder joint becomes lower than the resin section by about 10 to 20 degrees due to temperature variation in the reflow furnace caused by the shape, specific heat and size of the parts.

However, even if the Sn-Zn-Bi solder is used, it must be soldered at a higher temperature than the Sn-Pb solder. Therefore, the melting time required for soldering is shortened, and stable joining reliability cannot be obtained within the heat-resistant guaranteed temperature of electronic components.

Further, the solder balls used in the BGA are Sn-Pb type solders, and when a solder containing Bi such as Sn-Zn-Bi type solder and Pb are mixed, a compound is formed and the reliability of the joint is impaired. It has been known. Although Sn-Ag-based lead-free balls have been proposed, their melting points are higher than that of Sn-Zn-based solder, so that stable bonding reliability cannot be obtained within the heat-resistant guaranteed temperature of electronic components in consideration of temperature variations.

The present invention provides a soldering method using lead-free solder containing Sn and Zn and capable of obtaining connection reliability equal to or higher than that of a conventional Sn-Pb-based solder.

[0017]

In order to solve the above problems, the present invention is characterized in that the melting temperature of a lead-free solder ball used for BGA is lower than the melting temperature of a lead-free solder containing Sn and Zn. Can be achieved in any way. As a result, the lead-free solder balls used in the BGA are melted first, so that the lead-free solder in contact with the lead-free solder balls can be melt-bonded and stable even within the heat-resistant guaranteed temperature of the electronic component. It is possible to obtain the joined reliability.

Further, in the above soldering method, the surface of the lead-free solder ball is covered with an organic substance to suppress the oxidation of the lead-free solder ball in the reflow furnace, and further, the meltability is improved and stable joint reliability is obtained. You can get sex.

In order to solve the above-mentioned problems, the present invention is B
Lead-free balls used for GA are Cu balls, Ni-Fe
This can be achieved by a soldering method, which is a ball capable of forming an intermetallic compound with the lead-free solder such as an alloy ball. This allows BGA
Even if the ball used for melting is not melted, if the lead-free solder is melted, an intermetallic compound layer is formed, so soldering can be performed, and stable joining reliability can be obtained even within the heat-resistant guaranteed temperature of the component. it can.

In the soldering method, the lead-free solder is partially melted by covering the surface of the lead-free ball with an alloy film that melts at a temperature lower than that of the lead-free solder. It can be melted for a longer period of time than that of reflow soldering at the same temperature, and more stable joining reliability can be obtained.

Further, in the above soldering method, the surface of the lead-free ball is covered with an organic substance, a metal film composed of an Ni film, a Pd film and an Au film in this order, a metal film composed of an Ni film and an Au film in this order. Thereby, the reaction product of the lead-free ball and Zn is suppressed before the lead-free solder is melted in the reflow furnace, so that more stable joining reliability can be obtained.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a sectional view of a main part of a soldering method according to this embodiment. A Ni film 14 is formed on the Cu film 12 in order to connect the solder balls 20 to the circuit board 10 on which the semiconductor element is mounted, and Au is further formed thereon.
The electrode 18 is formed by forming the film 16. In order to bond the electrode 18 and the solder ball 20, it is desirable to use a low-activity flux and to bond them in a nitrogen atmosphere.

In order to solder the semiconductor package 22 on which the solder balls 20 are formed and the glass epoxy substrate 30 as described above, a lead-free solder containing Sn and Zn is formed on the Cu film 32 formed on the glass epoxy substrate 30. A Cu paste 32 is formed by screen-printing a solder paste 34 containing a mixture of solder, solvent, rosin, activator, viscosity stabilizer, and the like.
Is formed on the pre-flux film 33 formed above.

The solder paste 34 formed on the glass epoxy substrate 30 by screen printing and the solder balls 20 formed on the semiconductor package 22 are soldered by heating them in a reflow oven. In the reflow furnace, pre-heating 150 degrees about 60 seconds, main heating 200 degrees about 3
The temperature was 0 second and the maximum temperature was 220 degrees.

The following effects can be obtained by using a solder ball whose melting temperature is lower than that of the solder paste 34. That is, since the melting temperature of the solder ball 20 is low, the lead-free solder containing Sn and Zn in contact with the solder ball starts melting at a temperature lower than the melting point of the alloy. Therefore, even if there is a temperature variation that occurs during reflow soldering, the melting time required for solder joining can be secured, and stable joining reliability can be obtained.

Further, since the solder balls do not contain lead, the joint reliability is not impaired. Furthermore, the oxidation of the lead-free solder containing Sn and Zn used in the solder paste in the reflow furnace rapidly increases when the melting temperature exceeds the solidus temperature. Therefore, by starting to melt the solder balls 20 before the lead-free solder containing Sn and Zn is melted, the time during which the solder in the solder paste 34 comes into contact with the atmosphere is reduced, and Sn and Zn oxidized by the main heating are removed. By reducing the amount of oxidation of the lead-free solder containing it, there is also an effect of suppressing the generation of solder balls.

(Embodiment 2) The organic material used in the present invention is an organic material that is easily bonded to the lead-free solder ball component, preferably insoluble in the solvent of the solder paste, and particularly preferably anionic. Organic substances contained in the flux are preferable from the viewpoint of the reliability of the flux residue of the soldered portion after the reflow, and by way of example, copper phthalate, copper citrate, and particularly copper stearate are preferable.

The solvent for dissolving the above-mentioned organic acid copper is not particularly limited, such as soluble or insoluble, but it is safer as a solvent, easier to dry the solder powder at the time of forming the organic acid copper, and cost. Alcohol-based solvents are preferred.
When performing soldering as in the first embodiment, B
By coating the surface of the solder ball mounted on the GA with an organic substance or an organic substance melted in an organic solvent in advance before mounting it on the BGA, it is possible to suppress the oxidation of the solder ball in the reflow furnace and it is stable. It is possible to obtain the joined reliability.

(Embodiment 3) FIG. 2 is a sectional view of soldering in a soldering method according to this embodiment. An electrode 18 is formed by forming a Ni film 14 on the Cu film 12 and further forming an Au film 16 on the Ni film 14 in order to connect the circuit board 10 on which the semiconductor element is mounted and the Cu ball 21. Cu ball 2 covered with electrode 18 and alloy film 23
In order to bond No. 1, it is preferable to use a solder paste containing lead-free solder containing Sn and Zn and to bond them in a nitrogen atmosphere.

Since the semiconductor package 22 on which the Cu balls 21 are formed and the glass epoxy substrate 30 are soldered in this way, a lead-free film containing Sn and Zn is formed on the Cu film 32 formed on the glass epoxy substrate 30. A solder paste 34 in which solder, solvent, rosin, activator, viscosity stabilizer, etc. are mixed is formed on the pre-flux film 33 formed on the Cu film 32 by screen printing.

The solder paste 34 formed on the glass epoxy substrate 30 by screen printing and the Cu balls 21 formed on the semiconductor package 22 are soldered by heating in a reflow oven. In the reflow furnace, preheat 150 degrees for about 60 seconds, main heating 200 degrees for about 30
Second, the maximum temperature was 220 degrees.

The use of the Cu balls 21 has the following effects. That is, in order to bond the Cu-ball and the lead-free solder containing Sn and Zn used for the solder paste in the reflow furnace, it is necessary to form the intermetallic compound layer. Although the melting point of the intermetallic compound is much higher than the maximum temperature in the reflow furnace, the intermetallic compound layer is formed even by soldering in the reflow furnace under the above conditions.
By soldering in this way, it is possible to bond at a lower temperature than when using Sn-Ag based balls, which have been proposed as lead-free solder balls, and to achieve stable bonding reliability within the heat-resistant guaranteed temperature of electronic components. Obtainable.

By covering the surface of the Cu ball with the alloy film 23 that melts at a temperature lower than that of the solder contained in the solder paste 34, the same effect as that of the first embodiment can be obtained. Therefore, the amount of oxidation of the lead-free solder containing Sn and Zn that is oxidized by the main heating can be reduced, and the generation of solder balls can be suppressed.

(Embodiment 4) When soldering is carried out in the same manner as in Embodiment 3, the surface of the Cu ball 21 in the reflow furnace is covered by coating the surface of the ball mounted on the BGA with an organic substance. Oxidation of the covered alloy film 23 can be suppressed, soldering with less generation of solder balls can be performed, and stable joining reliability can be obtained.

(Embodiment 5) When soldering is carried out in the same manner as in Embodiment 3, by coating the surface of the ball mounted on the BGA with a Ni film, an Au film and a Pd film in this order, a reflow furnace is formed. Zn contained in the solder paste 34 in the inside suppresses the generation of a reaction product generated by contact with the Cu balls 21 and heating, and further, soldering with less generation of solder balls becomes possible, and stable joint reliability is achieved. Can be obtained.

[0037]

EXAMPLES Examples will be described below. The compositions of the solder and BGA solder balls used in Examples and Comparative Examples and the state of the solder ball coating film were as follows.

Example 1 Solder (composition 1 in Table 1) Sn 91% by weight Zn 9% by weight BGA solder ball (composition 2 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight ○ Solder ball coating Example 2 ○ Solder (composition 2 in Table 1) Sn 89 wt% Zn 8 wt% Bi 3 wt% BGA solder ball (composition 3 in Table 1) Sn 87 wt% Zn 7 wt% Bi 6 wt% ○ Solder ball coating film None Example 3 ○ Solder (composition 1 in Table 1) Sn 91 wt% Zn 9 wt% ○ BGA solder ball (composition 2 in Table 1) Sn 89 wt% Zn 8 wt% Bi 3 wt% ○ Solder ball coating film Organic substance Example 4 ○ Solder (composition 2 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight ○ BGA solder ball (composition 3 in Table 1) Sn 87% by weight Zn 7% by weight B 6 wt% ○ Solder ball coating film Organic substance Example 5 ○ Solder (composition 1 in Table 1) Sn 91 wt% Zn 9 wt% ○ BGA ball Cu 100 wt% ○ BGA ball alloy film (Composition 2 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight ○ BGA ball coating film None Example 6 ○ Solder (composition 3 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight ○ BGA ball Cu 100% by weight ○ BGA ball alloy film (composition 3 in Table 1) Sn 87% by weight Zn 7% by weight Bi 6% by weight ○ No ball coating film Example 7 ○ Solder (composition 1 in Table 1) Sn 91% by weight Zn 9% by weight ○ BGA ball Cu 100% by weight BGA ball alloy film (composition 2 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight BGA ball coating film Organic Example 8 ○ Solder (composition 2 in Table 1) ) Sn 89 wt% Zn 8 wt% Bi 3 wt% ○ BGA ball Cu 100 wt% ○ BGA ball alloy film (composition 3 in Table 1) Sn 87 wt% Zn 7 wt% Bi 6 wt% ○ Ball coating film organic substance Comparative Example 1 ○ Solder (composition 1 in Table 1) Sn 91 wt% Zn 9 wt% ○ BGA solder ball (composition 1 in Table 1) Sn 91 wt% Zn 9 wt% ○ Solder ball coating film None Comparative Example 2 ○ Solder (composition 2 in Table 1) Sn 89% by weight Zn 8% by weight Bi 3% by weight ○ BGA solder ball (composition 4 in Table 1) Sn 96.5% by weight Ag 3.0% by weight Cu 0.5% by weight ○ Solder ball coating film None Comparative example 3 ○ Solder (composition 2 in Table 1) Sn 89 wt% Zn 8 wt% Bi 3 wt% ○ BGA solder ball (composition 5 in Table 1) Sn 63 wt% Pb 37 wt% ○ Han The solder composition and a melting point which was used without the Examples and Comparative Examples ball coated film shown in Table 1. Solder pastes are produced using the solders of the above-mentioned examples and comparative examples, trial test boards are manufactured, and reliability evaluation is performed.

[0039]

[Table 1]

As the test board, a printed wiring board having a land portion Cu-plated to be soldered with glass epoxy is used, and a solder paste is printed with a metal squeegee using a metal mask having an opening by etching with a thickness of 150 μm. The electronic component has a ball diameter of 760 μm, 1.
It is a BGA with a pitch of 27 mm, and is mounted on a land portion printed with a solder paste, and the maximum temperature of the soldering portion is set to 210 ° C. using an atmospheric hot air reflow furnace to melt and join the solder.

The soldering bondability was evaluated as follows: -40 ° C for 30 minutes, + 125 ° C for 30 minutes for a BGA mounting board, and 1 for this.
A temperature cycle test as a cycle was performed, and the continuity between the solder ball and the joint portion of the circuit board was evaluated. In addition, the number of balls generated around the BGA balls during reflow was counted. The evaluation results are shown in Table 2.

[0042]

[Table 2]

As shown in Examples 1 to 8, in the soldered joints of BGA, no continuity was lost in the temperature cycle test, and there was no problem even after 1500 cycles had passed.

On the other hand, when the melting temperature of the BGA solder ball is higher than the temperature of the solder as in Comparative Example 2, it can be seen that there is no continuity immediately after reflow and soldering cannot be achieved. Further, when the solder balls for BGA are used in the conventionally used Sn-Pb system as in Comparative Example 3, the conduction is lost after 500 temperature cycles.

When the surface of the BGA ball was coated with an organic substance as in Examples 3, 4 and 5, 6 the performance was equivalent to that without the organic substance in the temperature cycle test, but the ball generated during reflow jumped dramatically. Decrease to. From this, it is understood that the generation of balls is suppressed by coating the organic substance.

[0046]

As described above, by performing the soldering joining according to the present invention, it becomes possible to set the soldering temperature within the heat-resistant guaranteed temperature of the electronic component, and the soldering with high reliability becomes possible. .

Further, if the soldering method of the present invention is used, since Pb is not contained in the solder and alloy film, it is not harmful to people and the environment and safe.

[Brief description of drawings]

FIG. 1 is a sectional view of an essential part of a soldering method according to a first embodiment of the present invention.

FIG. 2 is a sectional view of an essential part of a soldering method according to a third embodiment of the present invention.

[Explanation of symbols]

10 circuit board 12 Cu film 14 Ni film 16 Au film 18 electrodes 20 solder balls 21 Cu ball 22 Semiconductor package 23 Alloy film 30 glass epoxy substrate 32 Cu film 33 Preflux 34 Solder paste

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hisahiko Yoshida             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F term (reference) 5E319 AA03 AB05 AC01 BB01 CC36

Claims (11)

[Claims] 1. A BGA having lead-free solder balls.
(Ball grid array) and lead-free solder containing Sn and Zn are used to bond to the circuit board, and the liquidus temperature of the lead-free solder balls is Sn and Zn.
A soldering method characterized in that it is lower than the liquidus temperature of lead-free solder containing. 2. The solidus temperature of a lead-free solder ball is S
The soldering method according to claim 1, wherein the temperature is lower than the solidus temperature of the lead-free solder containing n and Zn. 3. The liquidus temperature of a lead-free solder ball is S
The soldering method according to claim 1, wherein the temperature is lower than the solidus temperature of the lead-free solder containing n and Zn. 4. The soldering method according to claim 1, wherein the surface of the lead-free solder ball is covered with an organic substance. 5. A method of joining a BGA having a lead-free ball and a lead-free solder containing Sn and Zn, wherein the lead-free ball is Cu. 6. The soldering method according to claim 5, wherein the surface of the Cu ball is covered with an alloy film that melts at a temperature lower than that of lead-free solder. 7. The surface of the Cu ball is a Ni film, a Pd film, A
The soldering method according to claim 5, wherein the u film is covered in this order. 8. The soldering method according to claim 5, wherein the surface of the Cu ball is covered with a Ni film and an Au film in this order. 9. A method for joining a BGA having a lead-free ball and a lead-free solder containing Sn and Zn, wherein the lead-free ball is a Ni—Fe alloy. 10. The soldering method according to claim 9, wherein the surface of the Ni—Fe alloy ball is covered with an alloy film that melts at a temperature lower than that of the lead-free solder. 11. The soldering method according to claim 5, wherein the surface of the lead-free ball is covered with an organic substance.