JP4259431B2 - Solder paste and solder joining method - Google Patents

Description

  The present invention relates to a solder paste used for soldering an electronic component to a substrate and a solder bonding method using the solder paste.

  As a method for mounting electronic components on a substrate, a solder bonding method is widely used. If the electronic component to be mounted is a fine pitch component and the solder amount of the solder joint is small, or if the strength of the solder joint material used is low and it is difficult to ensure sufficient joint strength, the solder joint should be A solder joining method in which the reinforcing resin is reinforced is adopted.

A thermosetting resin such as an epoxy resin is often used as the reinforcing resin, but the process can be simplified if the curing process for curing the reinforcing resin can be performed in the same process as the reflow process for solder bonding. This is convenient. For this reason, when mounting electronic components on a substrate, so-called “resin pre-coating” in which a bonding material containing a thermosetting resin that forms a reinforcing resin prior to electronic component mounting is applied in advance to the mounting position may be employed. As a solder bonding material used in such a construction method, a thermosetting flux containing a thermosetting resin or a solder paste in which solder powder is premixed in a thermosetting flux has been proposed (for example, Patent Document 1). reference). By using such a solder paste, there is an excellent advantage that solder bonding between electrodes and formation of a reinforcing resin can be performed simultaneously in the same process without supplying a separate solder material.
JP 2001-219294 A

  However, the solder paste in which the solder is mixed with the above-mentioned thermosetting flux has a normal solder joint that gives sufficient joint strength due to the difficulty in flowing the molten solder in a desired form during the reflow process. There was a problem that it was difficult to form. That is, in the reflow process, the solder in the solder paste is melted by heating for solder joining, and the thermosetting reaction of the thermosetting resin proceeds simultaneously.

  At this time, when the curing of the thermosetting flux is delayed, the flux component flow causes the molten solder to solidify from the joint site where it was initially supplied and solidifies to form solder balls, or when the distance between adjacent electrodes is narrow However, there is a problem that such a solder ball may cause a short circuit between the electrodes, and a normal solder joint portion is not formed, and it is difficult to ensure a sufficient solder joint strength.

  Therefore, an object of the present invention is to provide a solder paste capable of forming a normal solder joint portion and ensuring a sufficient solder joint strength, and a solder joint method using the solder paste.

  The solder paste of the present invention is a solder paste used for mounting an electronic component on a substrate by solder bonding, and is a metal component containing solder particles, a thermosetting resin and a solid at room temperature, and becomes liquid by heating. And a thermosetting flux containing an active action of removing the solder oxide film, and the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin.

  The solder bonding method of the present invention is a solder bonding method for soldering a connection electrode of an electronic component to a circuit electrode of a substrate, and is a metal component containing solder particles, a thermosetting resin, and solid at room temperature and heated A thermosetting flux containing a solid resin having a property of changing to a liquid state and having an action of removing a solder oxide film, and the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin. A step of interposing a solder paste between the circuit electrode and the connecting electrode; heating the substrate to melt the solder; and promoting the curing reaction of the thermosetting resin It includes a heating step for changing to a liquid state and a solidification step for solidifying the solid resin and solder by returning the substrate to room temperature.

  According to the present invention, a particulate metal component containing solder, a thermosetting resin, and a solid resin that has a property of being solid at normal temperature and changing to a liquid state upon heating, and having an active action of removing the solder oxide film are provided. By adopting a solder paste with a solid resin softening temperature that is equal to or higher than the curing start temperature of the thermosetting resin, it is possible to prevent the resin curing delay in the reflow process. In addition, it is possible to eliminate defects such as the generation of solder balls and form a normal solder joint portion to ensure a sufficient solder joint strength.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a process explanatory diagram of an electronic component mounting method using a solder bonding method according to an embodiment of the present invention, FIG. 2 is a sectional view of a solder bonding structure according to an embodiment of the present invention, and FIG. It is explanatory drawing of the malfunction example in the used soldering method.

  First, an electronic component mounting method will be described with reference to FIG. In this electronic component mounting method, an electronic component is mounted on a substrate by soldering the connection electrode of the electronic component to a circuit electrode formed on the substrate.

  In FIG. 1A, a circuit electrode 2 (hereinafter simply referred to as “electrode 2”) is formed on a substrate 1. Connection electrodes for electronic components mounted on the substrate 1 are connected to the electrodes 2 by solder bonding. Prior to mounting the electronic component, first, as shown in FIG. 1B, a solder paste 3 is applied to the upper surface of the electrode 2. For the application of the solder paste 3, a method of applying by screen printing or a dispenser is used.

  Here, the solder paste 3 used for mounting the electronic component on the substrate by solder bonding will be described. The solder paste 3 contains a metal component containing solder particles, a thermosetting resin, and a solid resin that is solid at room temperature and has a property of changing to a liquid state upon heating, and has an active action of removing the solder oxide film. The composition contains a curable flux. Here, a so-called lead-free solder that does not contain a lead component is employed as the solder, and two types of solder are properly used according to the characteristics of the substrate to be mounted and the electronic component.

  That is, Sn (tin) -Ag (silver) -Cu (copper) -based solder (liquidus temperature 220 ° C.) is selected and heated for an object that can be heated in a relatively high temperature range. Sn (tin) -Bi (bismuth) -based solder (liquidus temperature 139 ° C.) is selected for an object for which it is desired to set the temperature as low as possible. Note that the strength of the solder can be improved by adding Ag (silver) to the Sn—Bi solder at a blending ratio of 1 wt% to 3 wt%. And these solders are contained in the solder paste in a blending ratio in the range of 70 wt% to 92 wt% in the form of particles.

In addition to solder particles, metal powder in the form of foil such as Ag (silver), palladium (Pd), Au (gold) is mixed as a metal component at a mixing ratio of 0.5 wt% to 10 wt%. As a result, the solderability can be improved. That is, the above-mentioned metal has a melting point higher than the melting point of the solder to be used, does not generate an oxide film in the atmosphere, and the solder in a fluid state in which solder particles are melted easily wets along the surface. Therefore, in the solder joining process by reflow, these metal powders have the effect of aggregating the molten solder and improving the wettability of the solder.

  Further, as the thermosetting resin and the solid resin, a combination is selected such that the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin. By selecting such a combination, as will be described later, it is possible to prevent problems caused by delaying thermosetting of the thermosetting resin in the reflow process, for example, joint shape defects such as generation of solder balls.

  Next, a chip-type electronic component 4 is mounted on the substrate 1. That is, as shown in FIG. 1C, the terminals 4 a that are connection electrodes provided at both ends of the electronic component 4 are aligned with the electrodes 2, and the terminals 4 a are landed on the solder paste 3. Thereby, the electronic component 4 is temporarily fixed by the adhesive force of the solder paste 3. Thereafter, the substrate 1 on which the electronic component 4 is mounted is sent to a reflow apparatus, where it is heated to a temperature higher than the liquidus temperature of the solder in the solder paste 3. This heating melts the solder in the solder paste 3 and promotes the curing reaction of the thermosetting resin in the solder paste 3 as shown in FIG. Change.

  After that, the substrate 1 is taken out from the reflow apparatus and the substrate 1 is returned to room temperature, thereby solidifying the solid resin and the molten solder in the solder paste 3. Thereby, in the electronic component 4, a fillet-shaped solder joint portion 5a (see FIG. 2) in which the molten solder connects the electrode 2 and the terminal 4a is formed. As a result, using the solder paste 3, a solder joint structure 5 is formed by soldering the terminal 4a, which is the connection electrode of the electronic component 4, and the electrode 2 of the substrate 1.

  When the solder is melted, the solid resin in the thermosetting flux contained in the solder paste 3 changes to a liquid state, so that the thermosetting flux does not lose its fluidity even when heated to the solder melting temperature. It does not hinder the solder self-alignment phenomenon. And after this soldering process is completed, the thermosetting flux is solidified once the solid resin once liquefied by heating is cooled to room temperature along with the curing by the thermosetting resin completing the thermosetting. Therefore, it becomes a completely solid state and functions as a resin reinforcing portion 5b that covers and reinforces the solder joint portion 5a on the upper surface of the electrode 2.

  As shown in FIG. 2, the solder joint structure 5 is formed by heating the substrate 1 with the solder paste 3 interposed between the electrode 2 and the terminal 4a to melt the solder and then cooling it. A solder joint portion 5a formed between the terminal 4a, a thermosetting resin formed so as to cover the surface of the solder joint portion 5a and cured by heating, and a resin reinforcing portion 5b made of a solid resin solidified by cooling. It becomes the composition.

  The above-described solder joining method includes the step of interposing the solder paste 3 having the above-described composition between the electrode 2 and the terminal 4a, heating the substrate 1 to melt the solder, and curing the thermosetting resin. It has a form including a heating step for changing the solid resin to liquid while promoting, and a solidification step for solidifying the solid resin and solder by returning the substrate 1 to room temperature.

  By using this solder bonding method, it is possible to prevent a bonding failure that is likely to occur when a similar electronic component 14 is soldered using a conventional solder paste. For example, FIG. 3A shows a solder paste in which Sn—Bi-based lead-free solder particles are mixed with a thermosetting flux that tends to delay the curing of the thermosetting flux during the reflow process. A solder joint structure when 14a is soldered to the electrode 12 is shown.

  In the solder joint example shown here, it is difficult to form a solder fillet having a sound shape due to the correlation between the melting timing of the solder and the curing characteristics of the thermosetting flux, and sufficient bonding strength is often not ensured. That is, most of the solder supplied between the electrode 12 and the terminal 14a tends to be dispersed together with the flux component 15c in a fluid state without being agglomerated in the reflow process, becoming a solder ball 15c, and the solder joint 15a is soldered. It tends to be in an irregular shape with a small amount. When such a problem occurs when the distance between adjacent electrodes is narrow as shown in FIG. 3B, the generated solder balls 15c may cause a problem of short-circuiting the electrodes 12. .

  Even in the case of targeting such solder joint examples, as shown in the present invention, the thermosetting flux contains a solid resin as a plasticizer, and the softening temperature of the solid resin is the curing of the thermosetting resin. By using a solder paste having a temperature higher than the starting temperature, the following effects are obtained. In other words, the solder paste whose softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin starts to harden the thermosetting resin prior to the softening of the plasticizer in the reflow process, so that the flux component is excessive. The state that flows is not generated. Therefore, the situation in which the molten solder is dispersed in a form that accompanies the flow of the flux component does not occur, and the molten solder can be aggregated at the site to be joined to form a solder joint having a healthy shape.

  Further, after the reflow, the resin reinforced portion, which is solid in a state of being compatible with the thermosetting resin that has been cured by cooling and the plasticizer that has been solidified by cooling, is formed so as to cover the solder joint portion, so that it is brittle. Even when a low melting point type lead-free solder having inferior bonding strength is used, the solder bonding portion is reinforced by the resin reinforcing portion, and bonding reliability can be ensured.

  Here, a detailed example of the component composition of the solder paste 3 will be described. As described above, the solder paste 3 has a configuration in which solder particles are mixed in the thermosetting flux. In the present embodiment, the thermosetting flux includes, as a basic composition, a main agent containing epoxy as a thermosetting resin as a component, a curing agent and a curing accelerator for thermosetting the main agent, and an activator for removing the oxide film of the solder. In addition, a plasticizer made of a thermoplastic solid resin and a solvent are included.

  Next, the kind and compounding ratio of each component in the basic composition will be described. First, as a main agent, hydrogenated bisphenol A type epoxy resin (30 wt% to 40 wt%), as a curing agent, methyltetrahydrophthalic anhydride (30 wt% to 40 wt%), and as a curing accelerator, 2-phenyl 4-methyl 5- Hydroxymethylimidazole (5 wt% to 10 wt%), m-hydroxybenzoic acid (3 wt% to 10 wt%) as an activator, highly polymerized rosin (3 wt% to 20 wt%) as a plasticizer, and butyl carbitol as a solvent (0 wt% to 5 wt%).

  Here, the curing start temperature of the thermosetting resin is 140 ° C. or less, and the softening temperature of the highly polymerized rosin used as a plasticizer (solid resin) is 140 ° C. A combination in which the softening temperature is equal to or higher than the curing start temperature of the thermosetting resin is realized.

In addition, as the above-mentioned components, the following substances can be selected as alternative substances. First, instead of hydrogenated bisphenol A type epoxy resin, 3,4 epoxycyclohexenylmethyl-3, '4' epoxycyclohexene carboxylate, bisphenol F type epoxy resin or bisphenol A type epoxy resin can be selected as the main agent. . In addition, methyltetrahydrophthalic anhydride is used as a curing agent, methylhexahydrophthalic anhydride is used as an activator, and m-hydroxybenzoic acid is used as an activator, mesaconic acid is used as a solvent, and butyl carbitol is used as a solvent. It is possible to select carbitol. The blending ratio of the above components is the same as the numerical values shown in the basic blending example. Further, since the acid anhydride used as the curing agent has an active action of removing the oxide film by itself, the blending of the activator may be omitted.

  In addition, as a thermosetting resin, a material containing any one of acrylic, urethane, phenol, urea, melamine, unsaturated polyester, amine, and silicon as the main agent in addition to epoxy. Can be selected. And as solid resin used as a plasticizer, terpene resin, phenol resin, xylene resin, urea resin, melanin resin, amorphous rosin, imide resin, olefin resin, acrylic resin, amide resin, polyester resin, styrene, polyimide, At least one selected from fatty acid derivatives is mixed in the thermosetting resin.

  When selecting these solid resins, by selecting a solid resin that is compatible with the main agent in relation to the components of the main agent, when mixing the solid resin into the main agent, the vaporizable gas content is reduced. It is possible to realize a liquid resin having fluidity without using a solvent containing it. As a result, it is possible to reduce the environmental burden due to the use of the solvent, such as adhesion of gas components into the reflow apparatus by gas vaporized from the solvent and contamination of the work environment in the factory.

  Further, by using Sn-Bi solder which is a low melting point type lead-free solder, excellent effects as described in detail below are obtained. In recent years, the use of lead-free solder has become the mainstream in the electronic equipment manufacturing industry due to the demand for environmental protection. However, a commonly used SN-Ag-Cu solder has a liquidus temperature of 220 ° C. Since the temperature is higher than the liquidus temperature of the SnPb eutectic solder used mainly, it is difficult to apply depending on the target substrate or component.

  On the other hand, since Sn-Bi solder has a liquidus temperature of 139 ° C., it is desired to be applied to components having a low heat-resistant temperature (for example, a CCD element or an aluminum electrolytic capacitor). However, Sn-Bi solder has mechanically fragile strength characteristics, and, as described above, it is difficult to form a solder joint with a healthy shape in the reflow process, and thus there is a problem in joint reliability. Conventionally, the applicable range is limited.

  In this embodiment, the applicable range can be greatly expanded by using Sn-Bi solder having such characteristics as a solder paste mixed in a thermosetting flux containing a plasticizer. It is possible. That is, by adopting the solder paste 3 having such a configuration and further appropriately setting the relative relationship between the curing start temperature of the thermosetting resin and the softening start temperature of the solid resin as the plasticizer, the reflow is performed as described above. The flow characteristics of the flux component in the process can be made desirable for good solder joint formation.

  Thereby, the degree to which the agglomeration of molten solder is inhibited by the flux component is small, and it is possible to form a solder joint having a more healthy shape. Further, since the formed solder joint portion is covered and reinforced by the resin reinforcing portion made of the cured thermosetting resin and the solidified plasticizer, the resin lacks the strength due to the strength characteristics of the Sn-Bi solder. It can be compensated by the reinforcing portion, and the joining reliability is improved.

  Thus, by securing a bonding method that enables the practical use of Sn-Bi solder, which is a low melting point lead-free solder, the application to low heat-resistant substrates and components as described above is expanded. At the same time, a secondary effect of being able to set the heating temperature low, that is, the reflow device can be reduced in size and power consumption can be reduced by reducing the number of preheating stages.

In addition, due to restrictions on the upper limit temperature of heating, a high-cost joining method employed when solder joining at a lower temperature than before has been essential, such as a method using an Ag paste containing silver powder in a resin adhesive, or a substrate There is no need to adopt an individual joining method in which soldering is performed by locally heating with a laser, a soft beam or the like without heating the whole, and the use of expensive materials and devices is not required, and the manufacturing cost can be reduced.

  Furthermore, since the liquidus temperature is significantly lower than the liquidus temperature (183 ° C.) of the conventional SnPb eutectic solder, a low heat-resistant material that has been impossible to adopt conventionally, such as paper phenol, is used. An inexpensive material such as BT resin can be used for the substrate, and it is not necessary to use an expensive substrate material such as BT resin, so that the material cost can be reduced.

  As described above, in the soldering method shown in the present embodiment, in a joining method employing a so-called `` resin pre-coating '' in which a joining material including a thermosetting resin that forms a reinforcing resin is applied in advance to a mounting position. A solder paste having a metal component including solder particles and a thermosetting flux having the above-described composition, in which the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin is employed. This prevents the thermosetting delay of the thermosetting resin during the solder melting process in the reflow process, eliminates defects such as the generation of solder balls, forms a normal solder joint, and provides sufficient solder joint strength. Can be secured.

  The solder paste, the solder bonding method, and the solder bonding structure of the present invention have an effect that a normal solder bonding portion can be formed and sufficient solder bonding strength can be secured, and the electronic component is mounted on the substrate by solder bonding. It can be used for the purpose.

Process explanatory drawing of the electronic component mounting method by the soldering method of one embodiment of the present invention Sectional drawing of the solder joint structure of one embodiment of this invention Explanatory drawing of a failure example in a conventional soldering method using solder paste

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Electrode 3 Solder paste 4 Electronic component 4a Terminal 5 Solder joint structure 5a Solder joint part 5b Resin reinforcement part

Claims (5)

  A solder paste used for mounting electronic components on a substrate by solder bonding, a metal component containing solder particles, a thermosetting resin, and a solid resin that is solid at room temperature and changes to a liquid state upon heating And a thermosetting flux having an active action of removing the solder oxide film, wherein the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin.   The solder paste according to claim 1, wherein the solder contains tin and bismuth.   The thermosetting resin includes an epoxy resin, an acrylic resin, a urethane resin, a phenol resin, a urea resin, a melamine resin, an unsaturated polyester resin, an amine resin, a silicon resin, and a thermosetting resin. The solid resin is terpene resin, phenol resin, xylene resin, urea resin, melanin resin, amorphous rosin, imide resin, olefin resin, acrylic resin, amide resin, polyester resin, styrene, polyimide The solder paste according to claim 1, wherein the solder paste is at least one selected from fatty acid derivatives.   4. The solder paste according to claim 3, wherein a solid resin having compatibility with the main agent is selected. A soldering method for soldering an electrode for connecting an electronic component to a circuit electrode on a substrate, having a property of being a solid at normal temperature and a metal component containing solder particles, a thermosetting resin, and changing to a liquid state upon heating. A solder paste containing a solid resin and having an active action of removing a solder oxide film, wherein the softening temperature of the solid resin is equal to or higher than the curing start temperature of the thermosetting resin. And a step of interposing between the connecting electrode, a heating step of heating the substrate to melt the solder and changing the solid resin into a liquid state while promoting a curing reaction of the thermosetting resin, And a solidification step of solidifying the solid resin and solder by returning the substrate to room temperature.

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