Paste for soldering and soldering method using the same

The present invention relates to a solder bonding paste used for solder bonding and a solder bonding method using the solder bonding paste.

Solder bonding has been widely used as a bonding method for mounting electronic components on a substrate. As a form of soldering, various methods such as a method of forming a metal bump as a bonding electrode provided on an electronic component by soldering and a solder precoating of forming a solder layer on the electrode surface of a substrate are used. In recent years, from the viewpoint of environmental protection, so-called lead-free solder containing almost no harmful lead has been adopted in the above-mentioned soldering.

Since lead-free solder has a composition that is significantly different from that of lead-based solder that has been conventionally used, it is not possible to use the flux that is generally used in the past as it is as the flux used in the solder bonding process. That is, the conventional flux has insufficient activating action, insufficient removal of the oxide film on the solder surface, and it is difficult to secure good solder wettability. For such solders having poor solder wettability, a flux having a composition in which a metal powder made of a metal having excellent solder wettability such as silver is mixed in a flux component has been proposed (see, for example, Patent Document 1). By using such a flux, the solder melted in the reflow process can be wetted and spread along the surface of the metal powder in the flux, and the melted solder can be guided to the electrode to be bonded.<patcit num="1"><text>Japanese Unexamined Patent Publication No. 2000-31210</text></patcit>

<p> However, the flux shown in the above patent document example may have the following problems depending on the content ratio of the metal powder. In recent years, a non-cleaning method that omits cleaning to remove the flux component after solder bonding has become the mainstream, so after reflow, the flux remains as a residue around the solder joint and remains in the flux. The contained metal powder also remains around the solder joint.</p><p> At this time, if the residual amount of metal powder is large, insulation failure due to migration may occur. If the content of the metal powder is reduced in order to prevent this insulation defect, the effect of guiding the molten solder by the metal powder during reflow is reduced, resulting in a decrease in solder bondability. As described above, the conventional solder bonding paste such as flux containing metal powder has a problem that it is difficult to maintain both solder bonding property and secure insulating property.</p><p> Therefore, an object of the present invention is to provide a solder bonding paste and a solder bonding method capable of ensuring both solder bondability and insulating property.</p>

<p> The solder bonding paste of the present invention is a solder bonding paste that is interposed between the solder portion and the second electrode when the first electrode on which the solder portion is formed is soldered to the second electrode. It has a liquid base composed of a resin component, an active component having an action of removing an oxide film formed on the surface of the solder portion, a core metal, and a surface metal covering the surface of the core metal.<u style="single">Scale-like</u>The surface metal contains metal powder and has good wettability with respect to the solder forming the solder portion.<u style="single">Includes either gold or silver</u>It is made of metal, and the core metal can be taken into the inside by solid-solving the surface metal by heating by reflow.<u style="single">Contains tin, zinc, lead or indium</u>It is made of metal.</p><p> The solder bonding method of the present invention is a solder bonding method in which a first electrode on which a solder portion is formed is solder-bonded to a second electrode, and is formed on a liquid base composed of a resin component and the surface of the solder portion. An active ingredient that has the effect of removing the formed oxide film,<u style="single">Contains tin, zinc, lead or indium</u>Cover the core metal and the surface of this core metal<u style="single">Includes either gold or silver</u>Has a surface metal<u style="single">Scale-like</u>The solder bonding is performed by aligning the first electrode and the second electrode with the first step of applying the solder bonding paste containing the metal powder to the solder portion or at least one of the second electrodes. In the second step of interposing the paste between the solder portion and the second electrode, the solder melted by melting the solder by heating and spreading it wet along the surface of the metal powder is spread over the surface of the metal powder. A third step of bringing the surface metal into contact with the first electrode and the second electrode and diffusing and taking in the surface metal inside the core metal, and a fourth step of solidifying the molten solder after the third step. Includes steps.</p>

<p> According to the present invention, the metal powder mixed for the purpose of inducing molten solder during reflow is composed of a core metal and a surface metal covering the surface of the core metal, and the surface metal has wettability with the solder. By forming the core metal with a metal that can take in the surface metal inside by heating by reflow, the metal powder can remain as a residue in a state where migration is likely to occur after the reflow. It is possible to secure both solder bondability and insulation.</p>

Next, an embodiment of the present invention will be described with reference to the drawings. 1 and 2 are explanatory views of the process of mounting electronic components according to the embodiment of the present invention, FIG. 3 is an explanatory view of the solder bonding process using the solder bonding paste of the embodiment of the present invention, and FIG. 4 is an explanatory diagram. A cross-sectional view of the metal powder mixed in the solder bonding paste according to the embodiment of the present invention, FIG. 5 is an explanatory view of a method of supplying the solder bonding paste in the mounting of electronic components according to the embodiment of the present invention.

First, with reference to FIGS. 1 and 2, the mounting of electronic components by solder bonding using the solder bonding paste of the present invention will be described. In this electronic component mounting, the electronic component 4 is mounted by solder bonding on the substrate 1 on which the circuit electrode 2 (second electrode) is formed on the upper surface. In FIG. 1A, the electronic component 4 is provided with an external connection electrode 4b (first electrode) on the lower surface of the resin substrate 4a having the component mounting portion 5 on the upper surface thereof, and further, a solder portion is provided on the external connection electrode 4b. It has a structure in which bump 6 is formed as. The bump 6 is formed by soldering fine granular solder balls to the external connection electrode 4b. The solder refers to a metal having a low melting point (for example, tin) or an alloy of a plurality of types of metals (for example, a silver-tin alloy). Here, a lead-free solder containing almost no lead in these metals or alloys is used as a solder material. It is used as.

The component mounting portion 5 is formed by sealing a semiconductor element (not shown) mounted on the upper surface of the resin substrate 4a with a resin. In this resin sealing step, a molten high-temperature resin is injected into a mold cavity, the resin is thermoset to form a resin mold, and then the resin mold is removed from the mold cavity and cooled in the air. In this cooling process, the component mounting portion 5 on the upper surface side of the resin substrate 4a shrinks more than the resin substrate 4a due to the difference in the coefficient of thermal expansion between the substrate 2 and the resin mold, so that the entire electronic component 4 is the end of the resin substrate 4a. The part is deformed so as to warp on the component mounting part 5 side.

Therefore, among the plurality of bumps 6 formed on the lower surface side of the electronic component 4, the lower end portion of the bump 6 * located at the outer edge portion is only the displacement d1 due to warpage deformation than the lower end portion of the bump 6 located inside. It is located above. Therefore, the heights of the lower ends of the bumps 6 are not on the same plane, and as will be described later, when the electronic component 4 is mounted on the substrate, a gap tends to occur between the bumps 6 * and the circuit electrodes. is there.

Flux 3, which is a solder bonding paste described below, is applied to the bump 6 by transfer. That is, by moving the electronic component 4 up and down on the transfer table 7 on which the coating film of the flux 3 is formed, the flux 3 is transferred and applied to the lower end portion of the bump 6 as shown in FIG. 1 (b). The flux 3 is used by interposing it between the bump 6 and the circuit electrode 2 in order to improve the solder bondability in the solder bonding for mounting the electronic component 4 on the substrate 1 described below.

Here, the composition of flux 3 will be described. Flux 3 is a mixture of a highly viscous liquid base in which a resin component such as rosin is dissolved in a solvent, an activator and a metal powder 8 as additive components. The activator is added for the purpose of removing the oxide film of the solder formed on the surface of the bump 6, and an organic acid having such an oxide film removing ability is used. Here, as the activator, a low-activity agent that does not require cleaning after solder bonding is used.

As the metal powder 8, as shown in FIG. 4A, a metal powder 8 having a core metal 8a as a core body and a surface metal 8b covering the surface of the core metal 8a is used. In this configuration, a metal species used as the core metal 7a is selected from tin (Sn), zinc (Zn), lead (Pb), and indium (In), and a flaky metal foil is formed by this metal species. Then, a film of the surface metal 8b is formed on the surface of the metal foil by a method such as electroplating.

Here, the metal type of the surface metal 8b has a melting point higher than the melting point of the solder used for the bump 6, and does not form an oxide film on the surface of the metal powder 8 in the atmosphere, and further, the bump 6 Select a material (for example, precious metal such as gold (Au) or silver (Ag) with a purity of 90% or more) that has good wettability to the solder that forms the solder and the solder in a fluid state in which the bump 6 is melted easily wets and spreads along the surface. Will be done. The addition to the flux 3 is carried out by mixing these metal powders 8 in the base at a ratio in the range of 1 to 20 vol%.

Here, in the combination of the metal species used for the core metal 8a and the surface metal 8b, diffusion from the surface metal 8b to the inner core metal 8a (see FIG. 4 (b)) is easily generated by heating in the reflow process, and the reflow occurs. At the end, a combination is selected in which the diffusion of the surface metal 8b into the core metal 8a is completed and the diffusion characteristics are realized so that the surface metal 8b is almost incorporated into the core metal 8a. That is, in this configuration, the surface metal 8b is formed of a metal having good wettability with solder, and the core metal 8a is formed of a metal capable of solid-solving the surface metal 8b by heating by reflow and taking it inside. Has been done. By adopting such a structure as the metal powder mixed in the solder paste 3, an excellent effect as described later can be obtained in the solder bonding by the non-cleaning method.

Next, as shown in FIG. 1 (c), the electronic component 4 after the flux transfer coating is mounted on the substrate 1. The mounting of the electronic component 4 on the substrate 1 is performed by melting the bump 6 by heating and solder-bonding it to the upper surface of the circuit electrode 2, whereby the external connection electrode 4b is attached to the corresponding circuit electrode 2. While being electrically connected, the electronic component 4 is fixed to the substrate 1 by a solder joint formed by solidifying the molten solder.

In this mounting process, the electronic component 4 is positioned on the substrate 1, the bump 6 is aligned with the circuit electrode 2, and is lowered with respect to the substrate 1. Then, the bump 6 coated with the flux 3 is landed on the circuit electrode 2 and pressed by a predetermined pressing load. As a result, in the bump 6 whose lower end is at the average height position, the higher bump 6 is somewhat crushed in the height direction by the pressing force even if there is some variation in the bump height. As a result, the lower end of the circuit electrode 2 comes into contact with the upper surface of the circuit electrode 2. On the other hand, in the bump 6 * located at the outer edge, even if the other bumps 6 are crushed to some extent and the entire electronic component 4 is lowered by that amount, the lower end is still the table of the circuit electrode 2. It does not come into contact with the surface, and a gap is formed between the lower surface of the bump and the circuit electrode 2.

Next, a solder joining process in which the bump 6 is melted and soldered to the circuit electrode 2 will be described. The substrate 1 after mounting the components shown in FIG. 1 (c) is sent to the reflow furnace and heated. At this time, as shown in FIG. 2A, the bump 6 near the center where the height of the lower end is at an average position is located at the outer edge of the bump 6 with the lower end in contact with the circuit electrode 2. The bump 6 * is heated with the flux 3 interposed between the lower end and the circuit electrode 2.

By this heating, both bumps 6 and 6 * are solder-bonded to the circuit electrode 2, but the behavior of the solder at this time differs depending on whether or not the lower end of the bump is in contact with the circuit electrode 2. .. That is, as shown in FIG. 2 (b), in the bump 6 whose lower end is in contact with the circuit electrode 2, when the bump 6 is melted by heating, the molten solder 6a is immediately a circuit electrode made of a material having good solder wettability. It spreads well along the surface of 2, and the external connection electrode 4b is connected to the circuit electrode 2 by the solder 6a. At this time, the oxide film on the surface of the bump 6 is removed by the activator contained in the flux 3.

On the other hand, in the bump 6 *, since there is a gap between the bump 6 * and the circuit electrode 2, the connection between the external connection electrode 4b and the circuit electrode 2 by the solder 6a is performed through the process shown in FIG. .. FIG. 3 (a) shows the state at the start of heating in the reflow process. Here, since the metal powder 8 in the flux 3 interposed between the lower end of the bump 6 * and the surface 2a of the circuit electrode 2 contains scaly ones, a large number of metal powders are present in a random posture. With 8, a bridge of metal powder 8 connecting the lower end of the bump 6 * and the surface 2a of the circuit electrode 2 is formed with a high probability (see the part indicated by the arrow a in FIG. 3A).

Here, the bridge means a state in which the metal powders 8 are continuously connected in a state of being close to each other. And the close state means that when the fluid solder that wets and covers the surface of one metal powder 8 forms a certain thickness due to surface tension, the surface of the solder thickness becomes the other metal powder 8 adjacent to it. A state in which a plurality of metal powders 8 are present at intervals such that they come into contact with each other.

That is, since a large number of metal powders 8 are continuously present in such a close state, the solder in contact with the metal powder 8 on one side of the connection is the surface of the metal powder 8 containing a metal having good solder wettability. By wrapping and spreading wet, it comes into contact with the adjacent metal powder 8 in sequence. Then, the flow of solder due to this wet spread continuously occurs to the other side of the connection, so that the metal powder 8 of these connections is connected to the lower end of the bump 6 * and the circuit as shown in FIG. 3 (b). It functions as a bridge that connects the surface 2a of the electrode 2 and allows the solder to flow.

At this time, since a noble metal such as gold or silver having a melting point higher than the melting point of solder, which is usually used as a material of the surface metal 8b constituting the metal powder 8, was used, the surface metal 8b was heated to a higher temperature than the melting point of solder. Even in the case, the surface metal 8b is surely present in the solid state. That is, in the solder bonding method using cream solder containing solder particles in flux 3, the solder particles in the cream solder are also melted at the same time by heating during reflow, and the bridge function that bridges the molten solder in the gap is provided. On the other hand, the flux 3 of the present embodiment can surely fulfill the above-mentioned bridge function.

The metal powder 8 used in the flux 3 is expensive in the conventional flux containing metal powder because the precious metal such as gold or silver is used as the surface metal 8b covering the surface of the inexpensive core metal 8a. Compared with the method of using a precious metal as it is as a powder, it is possible to significantly reduce the cost. Although there is already a solder (for example, Sn-Ag-based solder) made of an alloy of a metal type and silver that can be selected as the core metal 8a, such solder and the metal powder 8 in the present embodiment are metals. Clearly distinguished in terms of the action and effect produced by the powder 8. Should be.

Here, as the shape of the metal powder 8, by using the above-mentioned metal processed into a scale shape, a bridge is formed by the metal powder 8 existing in a posture in which the longitudinal direction of the scale shape is directed to the bridging direction of the gap. This facilitates the formation of bridges efficiently with a relatively low content. Once the solder 6a reaches the electrode surface 2a through such a bridge, the flowing solder 6a wets and spreads along the electrode surface 2a having good solder wettability. Due to the wet spread of the solder 6a, the flux 3 near the electrode surface 2a is pushed outward, and even in the bump 6 * where a gap is initially formed between the solder 6a and the circuit electrode 2, the external connection electrode 4b is circuited by the solder 6a. It is fully connected to the electrode 2.

Even in this case, the bondability is improved by the activator contained in the flux 3, but the solder bondability is good even when the oxide film on the bump surface is only partially removed due to the bridge forming effect described above. Is ensured, the activator contained in Flux 3 is not required to have a strong active action. In other words, the addition of the metal powder 8 makes it possible to use a low-activity flux with a weak active action, and the degree to which the circuit electrode 2 is corroded by the active component is low even when the flux 3 remains after solder bonding. .. Therefore, in combination with the effect of improving the insulating property due to the characteristics of the metal powder 8 described later, sufficient reliability can be ensured even in the non-cleaning method in which cleaning for flux removal is not performed after solder bonding.

FIG. 3 (c) shows a cooled state after completing a predetermined heating cycle in the reflow process. That is, when the solder 6a in which the bumps 6 are melted is solidified by cooling, a solder joint portion 16 for connecting the external connection electrode 4b and the circuit electrode 2 by solder joint is formed. In the vicinity of the electrode surface 2a of the solder joint portion 16, the metal powder 8 taken into the solder during the soldering process exists in an alloy state or a solid solution state. Then, around the electrode surface 2a and the circuit electrode 2, the residue (resin component and activator) 3a after the solvent component evaporates from the flux 3 remains together with the metal powder 8 that has not been incorporated into the solder joint portion 16. ..

In FIG. 2 (c), the solder joint portion 16 connecting the external connection electrode 4b and the circuit electrode 2 is formed for all the external connection electrodes 4b and the circuit electrode 2 in this way, and is described above around the circuit electrode 2. The residue 3a containing the metal powder 8 of the above is shown in a state of residual adhesion. In this way, in the combination of the external connection electrode 4b and the circuit electrode 2 to be soldered, when the bump 6 * located at the outer edge has a gap between the lower end and the circuit electrode 2. Even in this case, good solder bondability can be ensured by applying the flux 3 as the solder bonding paste and the solder bonding method shown in the present embodiment.

Here, the change of the metal powder 8 in the above-mentioned reflow step will be described. In each metal powder 8, as shown in FIG. 4 (b), the surface metal 8b is gradually incorporated into the core metal 8a by diffusion as the heating is continued. Depending on the metal type of the core metal 8a and the heating temperature, the surface metal 8b may diffuse to the liquid phase core metal 8a or to the solid phase core metal 7a. In either case, the surface metal 8b diffuses to the solid phase core metal 7a. The metal 8b is gradually incorporated into the core metal 8a. Then, as the surface metal 8b is completely taken in and the surface of the core metal 8a is exposed, as shown in FIG. 4C, an oxide film 8c obtained by oxidizing the core metal 8a by heating is formed on the surface of the metal powder 8. It is formed. The oxide film 8c has the effect of improving the insulating property after solder bonding, as described below.

In the non-cleaning method in which cleaning for flux removal is not performed after the solder joining step, the residue 3a and the metal powder 8 shown in FIG. 2 (c) remain around the circuit electrode 2 as they are. If a metal such as gold or silver is used as it is as a metal powder mixed in the solder paste, migration may occur that electrically corrodes between the circuit electrodes and reduces the insulating property depending on the residual amount. There is. For this reason, conventionally, it has been necessary to keep the mixing ratio of the metal powder low in consideration of ensuring the insulating property, and as a result, the effect of improving the solder wettability for guiding the molten solder has not been sufficiently realized in the reflow process.

On the other hand, by using the metal powder 8 having the above configuration, the surface of the metal powder 8 is electrically stable even when a considerable amount of the metal powder 8 remains around the circuit electrode 2 after the solder joining step. Since it is covered with the oxide film 8c, migration does not occur and good insulation is ensured. Therefore, by using the metal powder 8 having the above configuration, the solder bondability is improved by mixing a sufficient amount of metal powder in the solder paste, and the insulation property after the solder bond is ensured to improve the mounting reliability. It is possible to make it.

In other words, by using the metal powder 8 having the above-mentioned structure, a non-cleaning type flux 3 having excellent solder bondability and insulating properties can be realized. That is, when targeting electronic components in which bumps are formed by lead-free solder, which has high hardness and the bumps are not easily crushed, there is a gap between the bumps and the circuit electrodes of the substrate due to warpage deformation of the electronic components and variations in bump size. Even in the state where the bumps are not soldered normally to the circuit electrodes, it is possible to effectively prevent the occurrence of mounting defects, and a non-cleaning method that omits cleaning to remove flux after soldering is adopted. Even in such a case, good insulation can be ensured.

The solder bonding method in the above-mentioned electronic component mounting is a solder bonding method in which the external connection electrode 4b on which the bump 6 as the solder portion is formed is solder-bonded to the circuit electrode 2, and the flux 3 having the above configuration is bonded to the bump 6 or By heating, the first step of applying to at least one of the circuit electrodes 2 and the second step of interposing the flux 3 between the bump 6 and the circuit electrode 2 by aligning the bump 6 and the circuit electrode 2. By melting the bump 6 and spreading it wet along the surface of the metal powder 8, the molten solder is brought into contact with the circuit electrode 2, and the surface metal 8b of the metal powder 8 is diffused and taken into the core metal 8a. The form includes a third step and a fourth step of solidifying the molten solder after the third step.

In the above example, in the step of applying the flux 3, the flux 3 is transferred to the bump 6 and applied, but various methods can be used in addition to this. For example, as shown in FIG. 5A, the flux 3 may be discharged by the dispenser 9 to supply the flux 3 to the circuit electrode 2. Further, as shown in FIG. 5B, the flux 3 may be supplied on the circuit electrode 2 by transfer by the transfer pin 10.

Further, as shown in FIG. 5 (c), the flux 3 may be printed on the circuit electrode 2 by screen printing. That is, a mask plate 11 provided with a pattern hole 11a corresponding to the circuit electrode 2 is mounted on the substrate 1, and the flux 3 is filled in the pattern hole 11a by the squeegee 12 and printed on the surface of the circuit electrode 2.

In the above embodiment, the first electrode is the external connection electrode 4b formed on the electronic component 4, the solder portion is the bump 6 formed on the external connection electrode 4b, and the bump 6 is the second. Although an example of solder bonding to the circuit electrode 2 which is an electrode of the above is described, the present invention is not limited to the above example. For example, the first electrode is a circuit electrode formed on a substrate, and the solder portion is a circuit electrode. The present invention can also be applied to the case where the solder precoat is formed.

Further, even when solder particles are mixed with the flux 3 shown in the present embodiment and used as a solder paste, the same effect can be obtained by mixing the metal powder 8 having the above-mentioned structure into the flux 3. .. In this case, in the reflow process, the metal powder 8 is used to agglomerate the molten solder when the solder particles existing in the dispersed state in the solder paste are melted. Functions as a core.

That is, since the surface of the metal powder 8 has good wettability of the solder, the molten solder in contact with the metal powder 8 moves along the surface of the metal powder due to the surface tension while wetting the surface of the metal powder 8, and eventually the molten solder. Wraps the metal powder 8 in one unit. Due to the cohesive effect of the molten solder, good solder bondability can be ensured even when solder having inferior wettability such as lead-free solder is used. In addition to this effect of improving solder wettability, an electrically stable oxide film in which the core metal is oxidized is formed on the surface of the metal powder 8 after solder bonding as in the above example, so that migration after solder bonding can be performed. It prevents the occurrence and ensures good insulation.

The solder bonding paste of the present invention has the effect of obtaining a high-quality solder joint portion without causing poor bonding or deterioration of insulating properties, and solders electronic components to a substrate by lead-free soldering. It is useful for solder joining methods.

<figref num="1">Diagram of process of mounting electronic components according to an embodiment of the present invention</figref><figref num="2">Diagram of process of mounting electronic components according to an embodiment of the present invention</figref><figref num="3">Explanatory drawing of solder bonding process using solder bonding paste of one Embodiment of this invention</figref><figref num="4">Cross-sectional view of the metal powder mixed in the solder bonding paste according to the embodiment of the present invention.</figref><figref num="5">Explanatory drawing of method of supplying solder bonding paste in electronic component mounting of one Embodiment of this invention</figref>

Code description

1 board 2 circuit electrodes 3 flux 4 Electronic components 4b External connection electrode 6, 6 * bump 8 metal powder 8a core metal 8b Surface metal