JP4367630B2 - Bump formation method - Google Patents

Description

  The present invention relates to a bump forming method. More specifically, the present invention relates to a method for forming a bump on an electrode provided on a printed wiring board, a wafer, a ceramic substrate, or the like using a resin film as a mask.

  In recent years, regarding the mounting of electronic components on a printed wiring board or a ceramic substrate, the demand for higher density has been increasing year by year, and a bare chip mounting method has attracted attention as a method that satisfies such a requirement. In the bare chip mounting method, instead of the conventional face-up mounting in which electrical connection between the chip and the substrate wiring is achieved through wire bonding, face-down mounting in which metal chip bumps are used tends to be widely adopted.

  According to the so-called metal bump method in which face-down mounting is performed through metal bumps, it can be expected to form a low-resistance connection between electronic components. However, many technical matters are required in the metal bump method. For example, when the electrodes of the electronic component are provided with a fine pitch, it is required to accurately form metal bumps with a fine pitch on the electrode. This requirement is particularly strong when metal bumps are formed on electrodes of semiconductor elements. In addition, in order to obtain stable connection reliability between electronic components, it is also required to ensure the height of the metal bumps with a certain degree of accuracy and to reduce the manufacturing cost.

  As a method of forming metal bumps for performing face-down mounting, plating methods and vapor deposition methods have been conventionally used. However, according to these methods, a large amount of capital investment is required, and bump height and metal composition It also had problems such as difficulty in control. Therefore, recently, a metal mask printing method or a resin mask filling method, which can form metal bumps at low cost and has a high degree of freedom in metal composition, has been adopted.

  A series of steps for forming metal bumps by a metal mask printing method is shown in FIGS. In the metal mask printing method, first, (a) a metal mask 22 having an opening 22a provided in advance at a position corresponding to the electrode portion 21 on the substrate 20 is prepared. (B) The opening portion 22 a of the metal mask 22 and the electrode portion 21 on the substrate 20 are aligned, and the metal mask 22 is placed on the substrate 20. (C) A solder paste 23 containing a predetermined solder powder is supplied to the opening 22a of the metal mask 22 by a printing method. (D) After removing the metal mask 22 from the surface of the substrate 20, (e) by performing a heat treatment, the solder powder in the solder paste 23 is melted, whereby a substantially spherical shape is formed on the electrode portion 21 of the substrate 20. Metal bumps 24 are formed. For example, Patent Document 1 below discloses a metal bump forming method using such a metal mask printing method.

JP-A-7-302972

  A series of steps for forming metal bumps by a resin mask filling method is shown in FIGS. In the resin mask filling method, first, (a) a resin film 32 is formed on a substrate 30 provided with an electrode portion 31. (B) An opening 32 a that exposes the electrode portion 31 of the substrate 30 is provided to the resin film 32 by etching. (C) The opening 32a of the resin mask 32 is filled with a solder paste 33 containing a predetermined solder powder. (D) By performing the heat treatment, the solder powder in the solder pace 33 is melted, whereby a substantially spherical metal bump 34 is formed on the electrode portion 31 of the substrate 30. (E) Finally, the resin mask 32 is removed from the surface of the substrate 30.

  Among these, the metal mask printing method has a problem that it becomes difficult to control the height of the bump when the bump is formed at a finer pitch. Specifically, when the openings 22a of the metal mask 22 are formed with a fine pitch, the solder paste 23 is removed when the metal mask 22 is removed after supplying the solder paste to the openings 22a (step of FIG. 2D). A part of the solder paste 23 is removed together with the metal mask 22 by being caught in the opening 22a. As a result, the variation in the height of the formed metal bumps 24 becomes remarkable, making it difficult to mount good electronic components.

  On the other hand, in the resin mask filling method, after the solder paste 33 is heated and melted, as shown in FIG. 3E, the resin film 32 as a printing mask is removed, so that electrodes provided at a fine pitch are used. The metal bumps 35 can be reliably formed on the solder paste 31 with a necessary amount of solder paste. As described above, it can be understood that the resin mask filling method can more appropriately meet the demand for fine pitch metal bumps accompanying the recent increase in the density of electronic component mounting than the metal mask printing method.

  However, the conventional resin mask filling method has a problem in removing the resin film. Specifically, in the step shown in FIG. 3D, when the solder powder in the solder paste 33 is melted, it is usually heated at a temperature 30 to 50 ° C. higher than the melting point of the metal constituting the solder powder. However, this heating promotes the curing reaction of the resin film, and when the resin film is removed in the step of FIG. 3E, a part of the resin film may remain on the substrate surface. When the resin film remains on the substrate surface, good electronic component mounting is hindered.

  Therefore, the present invention aims to solve or reduce such conventional problems, and in the formation of metal bumps using a resin film, the resin film can be favorably removed or peeled off. It is an object of the present invention to provide a bump forming method that enables good electronic component mounting.

  The bump forming method provided by the first aspect of the present invention includes a step of forming a resin film on a substrate surface provided with an electrode portion, and an opening portion so that the electrode portion is exposed to the resin film. A step of filling the opening with a bump forming material containing a plurality of metals having different melting points, and the melting point of the plurality of metals is equal to or higher than the lowest melting point and the highest of the melting points of the plurality of metals. A step of heating to below the melting point, a step of cooling to below the lowest melting point, and a step of heating the substrate to the highest melting point or higher without removing the resin film and mounting the substrate on an object to be joined. And

  In the first aspect of the present invention, the bump forming material filled in the opening formed in the resin film contains a plurality of metals having different melting points. Here, the melting point means a melting point in a normal meaning in the case of a single metal, and a liquidus temperature under a certain pressure in the case of an alloy. At the melting point, single metals and alloys can be completely melted. Therefore, in the first aspect of the present invention, the bump forming material filled in the opening is heated to a temperature not lower than the lowest melting point of the plurality of metals and less than the highest melting point of the plurality of metals. Then (hereinafter, this heating step is also referred to as “primary heating”), a part of the metal contained in the bump forming material is melted. Once this is cooled, the entire bump forming material is temporarily fixed to the electrode portion, and in this state, the resin film formed on the substrate surface as a mask can be removed. After removing the resin film, all of the plurality of metals contained in the bump forming material are heated to a temperature at which the metal becomes a liquid (hereinafter, this heating process is also referred to as “secondary heating”), and then the cooling is performed. It will be completed. That is, in the bump forming method according to the first aspect, the bump forming material is temporarily fixed to the electrode portion by primary heating performed at a lower temperature than the secondary heating, and then the resin film is removed before the secondary heating. Therefore, the resin film removal process can be easily performed as compared with the conventional case, and as a result, the electronic component (joining object) can be satisfactorily passed through the formed bumps without the resin film remaining on the substrate surface. It can be implemented.

  Moreover, since the bump forming method according to the first aspect can independently select a metal that becomes liquid phase by primary heating and a metal that becomes liquid phase only by secondary heating, it does not hinder the removal of the resin film. There is also an advantage that the composition of the bumps finally formed can be easily controlled.

  The bump forming method provided by the second aspect of the present invention includes a step of forming a resin film on a substrate surface provided with an electrode portion, and an opening portion so that the electrode portion is exposed to the resin film. A step of filling the opening with a bump forming material containing one or more types of metal, and heating to a temperature above the temperature at which only a part of the metal melts and below the temperature at which all the metal melts And a step of cooling below the temperature at which all of the metal solidifies, and a step of heating to a temperature at which all of the metal melts without mounting the substrate on the object to be joined after removing the resin film. It is characterized by including.

  Even with such a configuration, the same effects as described above with respect to the first aspect of the present invention can be achieved. Specifically, only a part of the metal contained in the bump forming material is melted by the primary heating, and the bump forming material is temporarily fixed to the electrode portion of the substrate by cooling this. Then, all of the metal contained in the bump forming material is melted by the secondary heating, and the bump fixed to the electrode portion is completed by cooling the metal. The metal component contained in the bump forming material includes two or more single metals having different melting points, two or more single metals and alloys having different melting points and liquidus temperatures, and solidus and / or liquidus temperatures. Two or more types of alloys having different values can be used. When two or more kinds of alloys are used, the liquid phase of the metal having the highest liquidus temperature that is higher than the solidus temperature of the metal that exhibits the lowest solidus temperature among the solder-forming materials. Primary heating is performed at a temperature lower than the linear temperature, and secondary heating is performed at a temperature equal to or higher than the highest liquidus temperature.

  As the substrate in the present invention, for example, a silicon wafer or a circuit substrate made of glass fiber reinforced epoxy resin can be used. On the substrate, a plurality of electrode portions made of, for example, copper, nickel, gold or the like are provided at predetermined locations.

  For the resin film as a mask formed on the substrate, for example, acrylic resin, epoxy resin, imide resin or a combination thereof is used, and etching is performed by photolithography (exposure / development method). Is preferably used. However, as the resin film of the present invention, a non-photosensitive resin can also be used. In that case, a laser or the like is used for etching. The form of the resin film may be a film or a liquid. Further, from the viewpoint of forming high bumps on the electrode portions provided at a fine pitch, the resin film preferably has a thickness of 30 to 150 μm.

  For peeling of the resin film, a strong alkali stripping solution such as an aqueous sodium hydroxide solution, an organic alkali stripping solution such as a monoethanolamine aqueous solution or an aqueous tetramethylammonium hydroxide solution, and a predetermined additive were added thereto. Things can be used. It is preferable that the additive has an action of breaking the resin film to be peeled into pieces and preventing the peeling residue.

  The bump forming material for forming metal bumps is preferably a paste obtained by mixing powdered metal with a flux. As the flux, a kneaded rosin resin, thixotropic agent, activator, solvent and the like can be used.

  In the first and second aspects, the alloy included in the bump forming material has a composition in which one or two or more kinds selected from Sn, Pb, Ag, Sb, Bi, Cu, In, Zn and the like are combined. Can be used. Specifically, for example, Sb—Sn alloy, Sn—Bi alloy, Sn—In alloy, Sn—Pb alloy, Sn—Ag alloy, Sn—Cu alloy, Sn—Zn alloy, Sn—Pb—Sb alloy can be cited. It is done. More specifically, a 5% Sn-95% Pb alloy, a 43% Sn-57% Bi alloy, and a 48% Sn-52% In alloy may be mentioned. Examples of the single metal contained in the bump forming material on the first side surface include Sn, Pb, and In.

  The rosin is mainly for enhancing the adhesiveness of the solder paste, and a polymerized rosin, a hydrogenated rosin, an esterified rosin or the like can be used.

  The thixotropic agent mainly imparts shape retention to the solder paste, and hardened castor oil, stearamide, and the like can be used.

  The activator removes the oxide film formed on the surface of the solder powder and the electrode part during the heat treatment, thereby improving the solder wettability with respect to the electrode part by cleaning the solder powder surface and the electrode part surface, This contributes to the formation of good metal bumps. As the activator, an organic acid and / or an organic amine can be used. For example, one or more organic acids and / or organic amines selected from sebacic acid, succinic acid, adipic acid, glutaric acid, triethanolamine, monoethanolamine and the like can be used.

  The solvent is for dissolving soluble components and making the flux vehicle into a paste. As the solvent, one or two or more solvents having a boiling point near the melting point or above the temperature are used in combination depending on the melting point of the solder which varies depending on the solder composition. For example, one or more selected from higher alcohols such as diethylene glycol dimethyl ether, n-butyl phenyl ether, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, and glycol ether solvents can be used.

  The opening formed in the resin film in order to fill the solder paste described above is preferably formed with an opening area of 25 times or less the area of the corresponding electrode part. This is because, when the solder paste is melted, a situation in which solder components do not collect on the electrode is avoided, and a good spherical bump is formed.

  Hereinafter, a preferred embodiment of the present invention will be specifically described with reference to FIGS.

  First, as shown in FIG. 1A, a substrate 10 to be a target for forming bumps is prepared. A plurality of electrode portions 11 are provided on the surface of the substrate 10 at a predetermined pitch in advance. A wiring part (not shown) that is electrically connected to the electrode part 11 is formed on the surface of the substrate 10.

  As shown in FIG. 1B, a film-like photosensitive resin film 12 is placed and pressure-bonded on such a substrate 10 so as to cover each electrode portion 11. The resin film 12 may be formed by applying a liquid resin to the surface of the substrate 10 by spin coating and thermosetting it.

  Next, as shown in FIG. 1C, the exposure process through a predetermined photomask (not shown) and the subsequent development process are performed on the portions corresponding to the electrode portions 11 of the resin film 12. To form the opening 12a so that each electrode 11 is exposed.

  Next, as shown in FIG. 1 (d), the opening 12 a is filled with a metal paste 13. When filling the metal paste 13, it is desirable that a large amount of excess metal paste does not remain on the upper surface of the resin film 12. For this purpose, for example, it is applied to the upper surface of the resin film 12 using a squeegee. What is necessary is just to scrape off the excess metal paste. The metal contained in this metal paste is in powder form.

  In the present invention, two or more kinds of metals having different melting points are employed as the metal component contained in the metal paste 13. The metal here includes a single metal and an alloy. The melting point means a melting point in a normal sense for a single metal, and a liquidus temperature under a certain pressure for an alloy.

  Next, in the primary heating step shown in FIG. 1 (e), heating is performed to a primary heating temperature that is equal to or higher than the lowest melting point of the plurality of metal components contained in the metal paste 13 and less than the highest melting point. Then, the heating state is maintained for a predetermined time. At this primary heating temperature, a metal component having a melting point equal to or lower than that temperature melts. Further, most of the components such as rosin resin other than the alloy volatilize and disappear from the metal paste 13. Then, as shown in FIG. 1 (e), the metal paste material remaining in the opening 12a gathers in a substantially spherical shape due to the surface tension of the dissolved metal component or the slightly remaining rosin resin. Then, once cooled to below the lowest melting point among the melting points of the plurality of metal components contained in the metal paste 13, some of the dissolved metal components change to a solid phase in an integrated state, and the metal paste The entire material is temporarily fixed to the electrode part. That is, an incomplete bump 14 containing a metal component having a thermal history that is not dissolved by primary heating is formed on the electrode portion 11.

  After the incomplete bumps 14 are formed, the resin film 12 is removed or peeled off from the surface of the substrate 10 as shown in FIG. As the stripper, an appropriate solvent for removing the used resin film 12 is selected. At this time, since the resin film 12 has not undergone the secondary heating step for completely melting all the metal components, it can be easily removed under mild conditions.

  Next, in the secondary heating step shown in FIG. 1G, the secondary heating temperature is higher than the highest melting point among the melting points of the plurality of metal components contained in the metal paste 13, and the heating state is maintained for a predetermined time. maintain. As a result, both the metal component once melted in the primary heating step and the metal component not melted in the primary heating step are melted. When this is cooled, a complete bump 14 ′ in which all metal components are melted is formed on the electrode portion 11 of the substrate 10.

  Further, by adding two or more kinds of alloys to the solder forming material, the highest liquidus temperature is higher than the solidus temperature of the metal showing the lowest solidus temperature among the solder forming materials. A series of steps shown in FIGS. 1A to 1G can also be performed by primary heating at a temperature lower than the liquidus temperature of the metal and secondary heating at a temperature equal to or higher than the highest liquidus temperature. Thus, it is possible to form a good metal bump 14 ′ while easily removing the resin film 12.

  Next, examples of the present invention will be described.

<Preparation of solder paste>
Solder of 63% Sn-37% Pb as metal I showing a relatively low liquidus temperature (liquidus temperature: 183 ° C.), and 2 as metal II showing a relatively high liquidus temperature Solder of% Sn-98% Pb (liquidus temperature: 320 ° C.) was pulverized to an average particle diameter of 20 μm, and these were mixed at a weight ratio of 1: 9. This mixed solder was mixed with flux to prepare a solder paste. The flux is 50% polypeel as rosin resin, 20% diethylene glycol monobutyl ether and 20% 2-methyl-2,4-pentanediol as solvent, 8% sebacic acid as activator, 2% hardened castor oil as thixotropic agent What kneaded beforehand (both volume percentage) was used.

<Bump formation>
Thermo-compression of a film-like acrylic photosensitive resin film (trade name: NIT250, manufactured by Nichigo Morton) with a thickness of 50 μm on a wafer provided with 300,000 electrode parts (electrode diameter: 70 μm, pitch: 150 μm) 100 ° C., pressure 3.5 kg / mm 2). Next, using a glass mask, a portion corresponding to the electrode portion was exposed, and then etched and developed with a 1.0% aqueous sodium carbonate solution to form an opening having a diameter of 130 μm at the portion corresponding to the electrode portion. Next, the above-described solder paste was applied onto the film, and this was filled into the opening by a printing method using a urethane squeegee. Next, the solder was generally integrated by primary heating at 213 ° C., which is higher than the liquidus temperature of 63% Sn-37% Pb solder, for 1 minute. Next, this was cooled and temporarily fixed to the electrode portion as an incomplete bump. Then, it was immersed in a 10% monoethanolamine aqueous solution to remove the resin film. Thereafter, flux (trade name: R5003, manufactured by Alpha Metals) is applied to the solder temporarily fixed to the electrode portion, and 350 ° C. which is higher than the liquidus temperature of 2% Sn-98% Pb solder. The solder was completely melted and integrated by second heating for 2 minutes. This was cooled and a complete bump was formed on the electrode part.

<Result>
In this example, the resin film after the primary heating could be peeled off satisfactorily. Further, the height of the formed bump was 80 μm ± 3 μm, and it was possible to form a highly accurate bump with little variation in height. In addition, the composition of the finally formed bump could be controlled within a range of ± 0.2% with respect to the target composition of 8% Sn-92% Pb. Table 1 lists the metal composition, the metal mixing ratio, the primary heating temperature, the secondary heating temperature, and the final bump composition for Example 1. The same applies to Examples 2 to 4 below.

  Solder of 35% Sn-65% Pb as metal I (liquidus temperature: 246 ° C.) and 2% Sn-98% Pb as metal II (liquidus temperature: 320 ° C.) A solder paste was prepared in the same manner as in Example 1 using a mixture of 1: 9. Then, bumps were formed by the same method as in Example 1 using this solder paste. However, the primary heating temperature was 265 ° C. As a result, the resin film after the primary heating was successfully peeled off. Further, the height of the formed bump was 80 μm ± 3 μm, and it was possible to form a highly accurate bump with little variation in height. Further, the composition of the finally formed bump could be controlled within a range of ± 0.2% with respect to the target composition of 5% Sn-95% Pb.

  Example using 100% Sn as metal I (melting point: 232 ° C.) and 100% Pb solder as metal II (melting point: 327 ° C.) mixed at a weight ratio of 1: 9 A metal paste was prepared in the same manner as in 1. Then, using this metal paste, bumps were formed by the same method as in Example 1. However, the primary heating temperature was 262 ° C., and the secondary heating temperature was 357 ° C. As a result, the resin film after the primary heating was successfully peeled off. Further, the height of the formed bump was 80 μm ± 3 μm, and it was possible to form a highly accurate bump with little variation in height. In addition, the composition of the finally formed bump could be controlled within a range of ± 0.2% with respect to the target composition of 10% Sn-90% Pb.

  Example using 100% Sn as metal I (melting point: 232 ° C.) and 100% Pb solder as metal II (melting point: 327 ° C.) mixed at a weight ratio of 1:19 A metal paste was prepared in the same manner as in 1. Then, using this metal paste, bumps were formed by the same method as in Example 1. However, the primary heating temperature was 262 ° C., and the secondary heating temperature was 357 ° C. As a result, the resin film after the primary heating was successfully peeled off. Further, the height of the formed bump was 80 μm ± 3 μm, and it was possible to form a highly accurate bump with little variation in height. Further, the composition of the finally formed bump could be controlled within a range of ± 0.2% with respect to the target composition of 5% Sn-95% Pb.

As a summary of the above, the configurations of the present invention and variations thereof are listed below as additional notes.
(Additional remark 1) The process of forming a resin film with respect to the substrate surface in which the electrode part was provided,
Forming an opening so that the electrode portion is exposed to the resin film;
Filling the opening with a bump forming material containing a plurality of metals having different melting points;
A step of heating the melting point of the plurality of metals to the lowest melting point or higher and less than the highest melting point of the plurality of metals;
Cooling to below the lowest melting point;
And a step of heating to the highest melting point or higher after removing the resin film.
(Additional remark 2) The process of forming a resin film with respect to the substrate surface in which the electrode part was provided,
Forming an opening so that the electrode portion is exposed to the resin film;
Filling the opening with a bump forming material containing one or more kinds of metals;
Heating above a temperature at which only a portion of the metal melts and less than a temperature at which all of the metal melts;
Cooling to below the temperature at which all of the metal solidifies;
And a step of heating to a temperature at which all of the metal is melted after removing the resin film.
(Additional remark 3) The bump formation method of Additional remark 1 or 3 whose said resin film is photosensitive resin.
(Additional remark 4) The said metal contained in the said bump forming material is a powder form, and the said bump forming material is a solder paste made by mixing the powdered metal and a vehicle component containing a resin and a solvent into a paste form. The bump forming method according to any one of appendices 1 to 3, wherein
(Additional remark 5) After heating the said liquidus temperature or more, or the said metal is integrated by heating until all the metals contained in the said solder formation material become a liquid, the alloy of a desired composition The method of forming a bump according to any one of appendices 1 to 4, wherein the composition of the metal contained in the bump forming material is controlled so as to be formed.

  According to the present invention, in a bump forming method using a resin film as a mask, curing of the resin film can be suppressed, and the resin film can be easily removed or peeled off from the substrate surface. As a result, good electronic component mounting is possible through the formed bumps.

It is sectional drawing showing a series of processes of the bump formation method which concerns on this invention. It is sectional drawing showing a series of processes of the conventional metal mask printing method. It is sectional drawing showing a series of processes of the conventional resin film filling method.

Explanation of symbols

10, 20, 30 Substrate 11, 21, 31 Electrode part 12, 32 Resin film 22 Metal mask 12a, 22a, 32a Opening part 13, 23, 33 Solder paste 14 ', 24, 34 Bump

Claims (3)

Forming a resin film on the surface of the substrate provided with the electrode part;
Forming an opening so that the electrode portion is exposed to the resin film;
Filling the opening with a bump forming material containing a plurality of metals having different melting points;
A first heating step for forming a bump that is not lower than the lowest melting point among the melting points of the plurality of metals and is heated below the highest melting point among the melting points of the plurality of metals;
Cooling to below the lowest melting point;
A bump forming method comprising: a second heating step for forming a bump that heats the substrate and the object to be bonded to the highest melting point or higher without removing the resin film and then bonding the substrate and the object to be bonded . Forming a resin film on the surface of the substrate provided with the electrode part;
Forming an opening so that the electrode portion is exposed to the resin film;
Filling the opening with a bump forming material containing two or more kinds of metals;
A first heating step for forming a bump that is not less than a temperature at which only a part of the metal melts and is less than a temperature at which all of the metal melts;
Cooling to below the temperature at which all of the metal solidifies;
After removing the resin film, without bonding the substrate and the object to be bonded, a second heating step for forming a bump that heats the metal to a temperature at which all of the metal melts or more, Bump formation method. The metal contained in the bump forming material is in a powder form, and the bump forming material is a solder paste made into a paste form by mixing the powdered metal and a vehicle component containing a resin and a solvent. Item 3. A bump forming method according to Item 1 or 2.

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