Patent Application
20150115020
The present invention relates to a solder bump, a method for forming the solder bump, a substrate provided with the solder bump, and a method for manufacturing the substrate. More particularly, the invention relates to a solder bump having a constant thickness capable of preventing corrosion of copper forming a copper electrode on the copper electrode of a substrate such as a printed circuit board, a wafer, or a flexible substrate, a method for forming the solder bump, a substrate provided with the solder bump, and a method for manufacturing the substrate.
Recent years have seen increasing improvement in wiring density and mounting density on substrates such as printed circuit boards, wafers, and flexible substrates (which may be hereinafter referred to as “mounting substrates”). Solder bumps used for soldering electronic components on a mounting substrate are required to be minute and uniform in shape, size, and the like. As a solder bump forming method satisfying such requirements, Patent Document 1 has proposed a technique and the like for easily forming bumps dense and constant in shape by using a screen plate provided with openings for forming paste bumps with a paste and characterized in that the plate includes a rigid first metal layer, a resin-based adhesive layer, and a second metal layer and the openings of the adhesive layer and the second layer have smaller diameters than that of the openings of the first metal layer.
Meanwhile, in electronic components such as a connector, a QFP (Quad Flat Package), a SOP (Small Outline Package), and a BGA (Ball Grid Array), there may be dimensional variations in connection terminals such as lead terminals. In order to solder electronic components with such connection terminals having varied dimensions without soldering failure, it is necessary to reduce influence of the dimensional variations on the electronic components by increasing thickness of solder bumps formed on a mounting substrate. Accordingly, a metal mask such as the screen plate used in Patent Document 1 needs to be thick enough to allow a considerable amount of solder to be provided on the mounting substrate.
On the other hand, when small electronic components such as a CSP (Chip Size Package) are among electronic components mounted on a mounting substrate, the sizes of solder bumps for such small electronic components are very minute. Thus, a metal mask used to form the solder bumps on the mounting substrate is provided with small openings for small electronic components. Meanwhile, the metal mask for providing the considerable amount of solder has a thickness enough to absorb the dimensional variations of connection terminals. Openings for small electronic components provided in the metal mask having such a thickness have a large aspect ratio (thickness/opening width). When a solder paste is screen printed using the metal mask, the solder paste remains on the metal mask and thus the thickness of the solder paste becomes unstable. In order to solve the problem, it is necessary to appropriately form a plurality of solder bumps having different height directions (thicknesses) so that electronic components having connection terminals having large dimensional variations and small electronic components can be simultaneously mounted on the same mounting substrate. However, manufacturing of a plurality of bumps having different thicknesses is a complicated task and therefore not preferable in terms of manufacturing cost.
Patent Document 2 has proposed a method for forming solder bumps suitable for both of large electronic components such as a connector, a QFP, a SOP, and a BGA and small electronic components such as a CSP even when the large and small electronic components are mounted together. In the method, a solder print mask including a first metal layer with openings and a second layer with other openings is arranged on a printed circuit board with electrodes (mounting substrate) such that the second layer closely contacts with the printed circuit board, then a solder paste is supplied from the first layer side and squeegeed by a squeegee jig to form solder bumps having different heights.
In addition, there is another known solder bump forming method, in which a mounting substrate provided with a copper electrode is directly dipped (immersed) in molten solder. However, when the mounting substrate is directly dipped in molten solder, tin included in the solder corrodes copper of the copper electrode to cause a so-called “copper corrosion”, leading to disappearance of the copper pattern. Accordingly, a method has been considered in which, in order to shorten a time for dipping a mounting substrate in molten solder to suppress copper corrosion, a preliminary solder layer is formed on a copper electrode of the mounting substrate and then the mounting substrate is dipped in the molten solder (a dipping method).
Patent Document 1: Japanese Laid-Open Patent Application No. H10-286936
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-66811
However, the method of Patent Document 2 has a problem in that it is a method for forming solder bumps having different heights by squeegeeing the supplied solder paste with a squeegee jig, thus leading to low productivity. Additionally, the dipping method described above is problematic in that the problem of copper corrosion still remains unsolved.
The present invention has been accomplished to solve the above problems. It is an object of the invention to provide a solder bump and a method for forming the solder bump in which solder bumps having a desired constant thickness can be formed without problems such as copper corrosion on a substrate with a minute electrode, such as a printed circuit board, a wafer, or a flexible substrate. It is another object of the invention to provide a substrate provided with such a solder bump and a method for manufacturing the substrate.
In order to solve the problems described above, the method for forming the solder bump includes: preparing a substrate that has a copper electrode; preparing a mask that has an opening portion formed at a necessary place on the copper electrode to form the solder bumps; piling up a molten solder by spraying a jet stream of the molten solder from a surface side of the mask to the opening portion of the mask until a thickness of the molten solder is greater than a thickness of the mask, after overlapping the mask and the substrate; forming the solder bumps having the predetermined thickness by removing an excess of the molten solder that is plied up beyond the thickness of the mask; and removing the mask from the overlapped substrate, wherein the molten solder is molten lead-free solder that includes tin as a main component and at least nickel as an accessory component, and further includes one or more components selected from among silver, copper, zinc, bismuth, antimony and germanium as an optional accessory component, and removing the excess of the molten solder plied up beyond the thickness of the mask is performed by a removal unit, such as a blade and an air cutter, and a removal unit spraying organic fatty acid-containing solution having 12 to 20 carbon atoms.
According to the present invention, the jet stream of the molten solder including tin as the main ingredient and at least nickel as the accessory ingredient is sprayed to the opening portion, whereby the Cu—Ni—Sn intermetallic compound layer obtained by alloying of the copper of the copper electrode and the nickel included in the molten solder is formed on a surface of the copper electrode. The intermetallic compound layer formed on the surface of the copper electrode serves as a barrier layer against copper corrosion. Since the barrier layer prevents copper corrosion of the copper electrode, defects and disappearance of the copper electrode caused by copper corrosion can be prevented. As a result, reliability can be ensured in the copper electrodes such as a copper land of the mounting substrate. Thus, when electronic components are soldered to a mounting substrate, copper corrosion occurring in conventional soldering can be suppressed. In addition, according to the invention, the removal of molten solder exceeding the thickness of the mask is performed by a simple means, that is, a removing means such as a blade or an air cutter or a removing means that sprays an organic fatty acid-containing solution having from 12 to 20 carbon atoms. Thus, solder bumps having a desired constant thickness can be easily formed with high dimensional precision on the mounting substrate with the minute copper electrode. As a result, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joints at which electronic components are solder-bonded to the solder bumps. This is advantageous in terms of manufacturing a low-cost mounting substrate.
In the forming method of solder bumps, a coating film having organic fatty acid in the organic fatty acid-containing solution is formed on the solder bumps.
In the forming method of solder bumps, contacting the organic fatty acid-containing solution to the copper electrode before by spraying a jet stream of the molten solder to the copper electrode.
In the forming method of solder bumps, the organic fatty acid-containing solution is solution including palmitic acid having 16 atoms.
In order to solve the problems described above, the method for manufacturing a mounting substrate includes: forming solder bumps on a surface of the copper electrode by the forming method of solder bumps according to the present invention; and mounting an electronic component by soldering on the formed solder bumps.
According to the present invention, since the solder bump is formed by the solder bump forming method described above, reliability can be ensured in the copper electrode such as a copper land of the mounting substrate, and when electronic components are soldered to the mounting substrate, copper corrosion occurring in conventional soldering can be suppressed. In addition, the solder bumps having a desired constant thickness can be easily formed with high dimensional precision on the mounting substrate with the minute copper electrode. In the invention described above, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joint portions at which electronic components are solder-bonded to the solder bumps, so that a low-cost mounting substrate can be manufactured.
The solder bump forming method according to the present invention allows for the formation of a Cu—Ni—Sn intermetallic compound layer obtained by alloying of copper of the copper electrode and nickel included in the molten solder on a surface of the copper electrode. Thus, the intermetallic compound layer serves as a barrier layer against copper corrosion, and the barrier layer can prevent copper corrosion of the copper electrode, thereby allowing for the prevention of defects and disappearance of the copper electrode due to copper corrosion. As a result, reliability can be ensured in the copper electrode such as a copper land of the mounting substrate. Thus, when electronic components are soldered to a mounting substrate, copper corrosion occurring in conventional soldering can be suppressed. In addition, the present invention allows solder bumps having a desired constant thickness to be easily formed with high dimensional precision on a mounting substrate with a minute copper electrode. Thus, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joint portions at which electronic components are solder-bonded to the solder bumps. This is advantageous in terms of manufacturing a low-cost mounting substrate.
The mounting substrate manufacturing method according to the present invention can ensure reliability of a copper electrode such as a copper land of a mounting substrate and can suppress copper corrosion occurring in conventional soldering when electronic component are soldered to a mounting substrate. In addition, the manufacturing method allows solder bumps having a desired constant thickness to be easily formed with high dimensional precision on a mounting substrate with a minute copper electrode thereon. As a result, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joint portions at which electronic components are solder-bonded to the solder bumps, so that a low-cost mounting substrate can be manufactured.
Hereinafter, a description will be given of a solder bump forming method and a method for manufacturing a mounting substrate according to the present invention, with reference to the drawings. In the present application, the term “the present invention” can be rephrased as “embodiment of the present application”.
A method for forming solder bump 11 according to the present invention includes a step of preparing substrate 1 on which copper electrode 2 is formed, a step of preparing mask 5 on which opening 6 for forming solder bump 11 at a necessary position on copper electrode 2 is formed, a step of superimposing substrate 1 and mask 5 and then spraying jet stream 11′ of molten solder from a surface S1 side of mask 5 to deposit molten solder 11a on opening portion 6 of mask 5 until a thickness of molten solder 11a becomes larger than a thickness of mask 5, a step of removing molten solder 11a exceeding the thickness of mask 5 to form solder bump 11 having a predetermined thickness, and a step of removing mask 5.
Each of the steps will be described in detail below.
Substrate 1 prepared is a substrate such as a printed circuit board, a wafer, or flexible substrate, as depicted in
On the copper electrode 2 formed on substrate 1, only portions for mounting electronic components (solder joint portions) are exposed, whereas a region thereof other than the solder joint portions is covered with an insulation layer or an insulation film. Solder bump 11 is formed at the exposed solder joint portions by a means that will be described later, and solder bump 11 provided at the solder joint portions is used for solder bonding an electronic component and bonding a connection terminal (connector).
In mask 5 to be prepared, there is formed opening portion 6 for forming solder bump 11 at necessary positions on copper electrode 2, as depicted in
The thickness of mask 5 is arbitrarily selected according to the thickness of solder bump 11 to be formed. Accordingly, a peripheral thickness of opening portion 6 of mask 5 restricts a height (thickness) of solder bump 11. The peripheral thickness of mask 5 is arbitrarily selected in a range of from 20 to 500 μm. In the present invention, the use of mask 5 having a thickness within such a range allows for the formation of solder bumps 11 having the same or substantially the same thickness as the peripheral thickness in a uniform thickness without variations.
Regarding a thickness of mask 4, usually, as depicted in
Preferably, a boundary between the thin portion and the thick portion of each of masks 5A and 5B is gently inclined surface S2, as depicted in
The solder deposition step is a step in which after superimposing substrate 1 and mask 5 as depicted in
Molten solder 11a to be used is prepared by thermally melting solder and then fluidizing to an extent allowing the solder to be sprayed as jet stream 11′. Heating temperature for solder is arbitrarily selected according to a solder composition. Usually, a favorable temperature is determined in a range of from 150 to 300° C. The present invention uses a molten lead-free solder including tin as a main ingredient, at least nickel as an accessory ingredient, and furthermore, arbitrarily, one or two or more selected from silver, copper, zinc, bismuth, antimony, and germanium, as one or more accessory ingredients.
A preferable solder composition is a Sn—Ni—Ag—Cu—Ge alloy. Specifically, it is preferable to use a solder alloy including from 0.01 to 0.5% by mass of nickel, from 2 to 4% by mass of silver, from 0.1 to 1% by mass of copper, from 0.001 to 0.02% by mass of germanium, and tin for the rest in order to form Cu—Ni—Sn intermetallic compound 13a (see
In addition, when the solder includes bismuth, the heating temperature for molten solder 11a can be further lowered, and adjustment of an ingredient composition thereof can lower the heating temperature, for example, to near 150° C. A solder composition including bismuth also, as in the above, includes preferably from 0.01 to 0.5% by mass of nickel, and more preferably from 0.01 to 0.07% by mass of nickel. This composition can provide a low-temperature type molten solder 11a that can easily form Cu—Sn intermetallic compound layer 13a.
Besides, zinc and/or antimony may be added as needed. In any case, the solder composition includes at least nickel in an amount ranging preferably from 0.01 to 0.5% by mass, and more preferably from 0.01 to 0.07% by mass.
Molten solder 11a having the above composition is a lead-free solder that does not include lead and essentially includes nickel in the amount mentioned above. Thus, as depicted in
The amount of nickel included in molten solder 11a affects a thickness of the Cu—Ni—Sn intermetallic compound layer 13a, as depicted in Examples described later. Specifically, when the nickel content is in a range of from 0.01 to 0.5% by mass (preferably to 0.07% by mass), there can be formed Cu—Ni—Sn intermetallic compound layer 13a having a substantially uniform thickness of around from 1 to 3 μm. Cu—Ni—Sn intermetallic compound layer 13a having a thickness in the above range can prevent the copper in copper electrode 2 from being dissolved into molten solder 11a or solder bump 11 to be corroded.
When the nickel content is 0.01% by mass, the thickness of Cu—Ni—Sn intermetallic compound layer 13a is from around about 1 to about 1.5 μm; when the nickel content is, for example, 0.07% by mass, the thickness of Cu—Ni—Sn intermetallic compound layer 13a is about 2 μm; and when the nickel content is 0.5% by mass, the thickness of Cu—Ni—Sn intermetallic compound layer 13a is around about 3 μm.
When the nickel content is less than 0.01% by mass, the thickness of Cu—Ni—Sn intermetallic compound layer 13a is less than 1 μm, which leads to the occurrence of a region of copper electrode 2 that cannot be covered with Cu—Ni—Sn intermetallic compound layer 13a. Then, copper corrosion can easily begin from the region. When the nickel content exceeds 0.5% by mass, the thickness of hard Cu—Ni—Sn intermetallic compound layer 13a exceeds the thickness of 3 μm to be thicker and a crack occurs in the thicker Cu—Ni—Sn intermetallic compound layer 13a. As a result, copper corrosion tends to begin from the cracked portion. The nickel content is preferably from 0.01 to 0.07% by mass. In molten solder 11a having a nickel content in the above range does not cause a crack in Cu—Ni—Sn intermetallic compound layer 13a and can form a smooth uniform layer, as compared to those having a nickel content more than 0.07% by mass and not more than 0.5% by mass.
Solder used as molten solder 11a is preferably purified solder. Specifically, a solution including from 5 to 25% by mass of an organic fatty acid having from 12 to 20 carbon atoms is heated to a temperature of from 180 to 350° C.; then, the heated solution is contacted with molten solder 11a to be mixed by intensely stirring. In this way, molten solder 11a before purification, which has been contaminated with a copper oxide, a flux component, and the like, can be purified, whereby there can be obtained molten solder 11a from which the copper oxide, the flux component, and the like have been removed. Then, the mixed solution including the molten solder 11a but not including any copper oxide, any flux component, and the like is introduced in an organic fatty acid-containing solution tank. In the organic fatty acid-containing solution tank, the molten solder 11a after purification, which has been separated by a specific gravity difference, is returned to a lead-free solder solution tank from a bottom of the organic fatty acid-containing solution tank by a pump. Performing such purification processing can suppress time-dependent increases in copper concentration and impurity concentration in molten solder 11a used as a jet stream, as well as can prevent entry of impurities such as copper oxide, flux residue, and the like in the lead-free solder solution tank. As a result, a time-dependent composition change of the molten solder 11a in the lead-free solder solution tank can be suppressed, thereby allowing for the continuous formation of solder bump 11 using molten solder 11a stable and highly reliable in bonding. Additionally, a mounting substrate provided with such solder bumps 11 can be continuously manufactured.
Molten solder 11a purified does not include any impurities such as copper oxide and flux residue that affect bonding quality of solder bump 11. As a result, there is no interlot variation in the bonding quality between solder bump 11 and an electronic component, which can be a contribution to time-dependent quality stability.
In addition, results have shown that molten solder 11a purified with the organic fatty acid-containing solution has less wettability than molten solder not purified therewith. Specifically, as seen from the results of wettability (meniscograph) tests of
The above characteristic differences between the purified molten solder 11a and the unpurified molten solder means that jet stream 11′ of molten solder 11a uniformly spreads with favorable solder wettability throughout the surface of copper electrode 2. Particularly, before spraying molten solder 11a onto copper electrode 2, the organic fatty acid-containing solution used for purification is contacted (sraying or immersion) with the copper electrode 2 to perform cleaning so that the organic fatty acid-containing solution removes oxides, impurities, and the like present on the copper surface. Then, molten solder 11a similarly purified with the organic fatty acid-containing solution is sprayed onto the copper surface thus cleaned to allow the solder to adhere thereto, whereby molten solder 11a can wet-spread with good wettability on copper surface 2. On the other hand, when unpurified molten solder was sprayed onto a cleaned copper surface to allow the solder to adhere thereto, the solder did not wet spread with good wettability, unlike the above, and there were cases in which the molten solder did not wet-spread uniformly on the copper surface. These results show that it is significantly effective to spray molten solder 11a purified with the organic fatty acid-containing solution onto a copper surface cleaned therewith to allow the solder to adhere to the surface.
The organic fatty acid contained in the organic fatty acid-containing solution used for purification may be one having 11 or less carbon atoms. However, such an organic fatty acid is water-absorbent and not very preferable for use in the higher temperature range of from 180 to 350° C. mentioned above. Additionally, organic fatty acids having 21 or more carbon atoms are problematic in terms of high melting points, poor permeability, poor handleability, and the like, and also the surface of molten solder 11a after purification has insufficient anti-rust effect. Typically, palmitic acid having 16 carbon atoms is preferable. It is particularly preferable to only use the palmitic acid having 16 carbon atoms, and, as needed, it is possible to contain an organic fatty acid having from 12 to 20 carbon atoms, such as stearic acid having 18 carbon atoms.
The organic fatty acid-containing solution used for purification preferably includes from 5 to 25% by mass of palmitic acid and an ester synthetic oil for the rest. The use of such an organic fatty acid-containing solution heated to a temperature of from 180 to 350° C. allows the solution to selectively capture impurities such as oxides and flux ingredients present in molten solder 11a to purify molten solder 11a. Particularly preferred is an organic fatty acid-containing solution that contains around 10% by mass (for example, from 5 to 15% by mass) of palmitic acid having 16 carbon atoms. The organic fatty acid-containing solution includes neither metal salts such as nickel salt and cobalt salt nor additives such as an antioxidant.
The use of organic fatty acids having concentrations of below 5% by mass reduces the effect of selectively capturing impurities such as oxides and flux ingredients present in molten solder 11a to purify, and furthermore makes control at low temperature complicated. On the other hand, when the concentration of the organic fatty acid exceeds 25% by mass, there arise problems, such as a significant increase in the viscosity of the organic fatty acid-containing solution, the occurrence of fumes and odors in a high temperature range of 300° C. or more, and insufficient stirring mixability with molten solder 11a. Therefore, the content of the organic fatty acid is preferably from 5 to 25% by mass, and particularly when using only palmitic acid having 16 carbon atoms, the content of the organic fatty acid is preferably around 10% by mass (for example, from 5 to 15% by mass).
The temperature of the organic fatty acid-containing solution used for purification is determined by a melting point of molten solder 11a to be purified, and the organic fatty acid-containing solution and molten solder 11a are contacted with each other by intensely stirring in a high temperature region (one example is from 240 to 260° C.) of not less than at least a melting point of molten solder 11a. In addition, an upper limit temperature of the organic fatty acid-containing solution is around 350° C. from the viewpoint of a fuming problem and energy saving, and preferably in a range of from a temperature of not less than the melting point of molten solder 11a to be purified to 300° C. For example, the solder alloy including from 0.01 to 0.07% by mass of nickel, from 0.1 to 4% by mass of silver, from 0.1 to 1% by mass of copper, from 0.001 to 0.01% by mass of germanium, and tin for the rest is used as molten solder 11a at a temperature of from 240 to 260° C. Thus, preferably, the temperature of the organic fatty acid-containing solution is also in the same temperature range of from 240 to 260° C. as the solder alloy.
Reasons for mixing an ester synthetic oil in the organic fatty acid-containing solution are to facilitate uniform stirring mixing between the organic fatty acid-containing solution and molten solder 11a by lowering the viscosity of the organic fatty acid-containing solution and to suppress high-temperature fuming properties of the organic fatty acid contained in the solution.
Molten solder 11a purified with such an organic fatty acid-containing solution is sprayed as jet stream 11′ to opening portion 6 of mask 5 from a spray nozzle, as depicted in
An atmosphere for spraying is not particularly limited. However, since excess molten solder 11a is removed after that by a squeegee, liquid shower, or gaseous shower, an atmosphere temperature during processing in both steps of “solder deposition step” and “excess solder removal step” is needed to be maintained at a temperature that can keep the molten state of molten solder 11a as it is (a temperature of from 240 to 260° C. in the above example). Specifically, the atmosphere temperature is preferably a temperature equal or close to a temperature of molten solder 11a to be used for soldering. The temperature is preferably set to be slightly higher than the temperature of molten solder 11a, although can be equal thereto. For example, the atmosphere temperature is set to be preferably from 2 to 10° C. higher, and more preferably from 2 to 5° C. higher than a jet stream temperature of molten solder 11a. Setting the atmosphere temperature within the above temperature range allows jet stream 11′ of molten solder 11a sprayed on the surface of copper electrode 2 to be flown uniformly on the surface thereof, and particularly allows molten solder 11a to spread throughout the inside of mask 5. Atmosphere temperatures lower than the temperature of the jet stream of molten solder 11a can reduce the fluidity of molten solder 11a. On the other hand, atmosphere temperatures higher than 10° C. may thermally damage the substrate due to the excessively high temperatures.
The excess solder removal step is a step of removing molten solder 11a protruding from a surface of mask 5 in a thickness direction thereof (which can also be referred to as exceeding the thickness of mask 5) to form solder bump 11 having a predetermined thickness, as depicted in
Blade 15 usable may be a plate-shaped blade generally used for scraping off, as depicted in
In addition, in the present invention, preferred is a removing means that sprays organic fatty acid-containing solution 18 having from 18 to 20 carbon atoms from nozzle 17 to blow off molten solder 11a protruding from the surface of mask 5 in the thickness direction thereof. Furthermore, such a means that removes molten solder 11a by spraying can form coating film 19 of organic fatty acid contained in organic fatty acid-containing solution 18 on solder bump 11.
A flow rate of jet stream 11′ of molten solder sprayed from nozzle 17 and a time for spraying processing are arbitrarily determined in consideration of the kind of molten solder 11a, a thickness of coating film 19, and the like. Additionally, conditions such as a shape of nozzle 17 and a spraying angle thereof are also arbitrarily applied or determined in consideration of the kind of molten solder 11a, the thickness of coating film 19, and the like. Although the spraying angle is not particularly limited, a virtual angle between a center axis of nozzle 17 and the mask surface is set to be preferably within a range of from 30 to 45 degrees. Depending on the flow rate of jet stream 11′ of molten solder and the time for spraying processing, molten solder 11a under the surface of mask 5 may also be removed along with the removal of molten solder 11a protruding from the surface of mask 5 in the thickness direction thereof. As a result, the surface of molten solder 11a after the spraying may be slightly concaved. A configuration after the removal of the excess also molten solder 11a includes a configuration after such a removal.
Organic fatty acid-containing solution 18 to be sprayed may include some amount of an inert gas such as nitrogen gas. On the other hand, oxygen-containing air, water, and the like are not allowed to be included from the viewpoint of oxidation of molten solder 11a and compatibility with the organic fatty acid-containing solution.
The atmosphere temperature during processing in the removal step is maintained at a temperature that can keep the molten state of molten solder 11a, as it is, deposited the opening portion 6. Such a temperature varies depending on the kind of molten solder 11a to be used. In the above example, the temperature needs to be maintained at a temperature of from 240 to 260° C.
In the removal of an excess of molten solder 11a by spraying organic fatty acid-containing solution 18 using a spray, the organic fatty acid-containing solution 18 to be used is preferably the same organic fatty acid-containing solution as that used in the purification processing described above. In other words, it is preferable to use an organic fatty acid-containing solution containing from 5 to 25% by mass of organic fatty acid having from 12 to 20 carbon atoms. Particularly preferred is an organic fatty acid-solution containing from 5 to 15% by mass of palmitic acid having 16 carbon atoms. As needed, an organic fatty acid having from 12 to 20 carbon atoms, such as stearic acid having 18 carbon atoms may be contained. The organic fatty acid-containing solution includes neither metal salts such as nickel salt and cobalt salt nor additives such as an antioxidant.
The organic fatty acid-containing solution is preferably heated in the same temperature range as the temperature of molten solder 11a, and for example, an organic fatty acid-containing solution heated to from 180 to 350° C. is used. When molten solder 11a is heated to from 240 to 260° C. and sprayed as jet stream 11′ as in the above, it is preferable to spray an organic fatty acid-containing solution having the same temperature of from 240 to 260° C. as the temperature of the sprayed molten solder 11a to remove an excess of molten solder 11a. The organic fatty acid-containing solution sprayed by a spray and the molten solder 11a removed together with the sprayed organic fatty acid-containing solution are separated by a specific gravity difference, and molten solder 11a that has sunk at a bottom of the organic fatty acid-containing solution is taken out and can be separated from the organic fatty acid-containing solution. The molten solder 11a and the organic fatty acid-containing solution separated from each other can be reused.
As described above, the organic fatty acid contained in the organic fatty acid-containing solution may be one having 11 or less carbon atoms. However, such an organic fatty acid is water-absorbent and thus not very preferable for use in the high temperature range of from 180 to 350° C. mentioned above. In addition, organic fatty acids having 21 or more carbon atoms are problematic in terms of high melting points, poor permeability, poor handleability, and the like, and also the anti-rust effect of coating film 19 formed on the surface of molten solder 11a is insufficient. Typically, palmitic acid having 16 carbon atoms is preferable, and the use of the above palmitic acid alone is particularly preferable. When needed, it is possible to contain an organic fatty acid having from 12 to 20 carbon atoms, such as stearic acid having 18 carbon atoms.
Preferably, the temperature of the organic fatty acid-containing solution is the same or substantially the same as the temperature of molten solder 11a to be removed, thereby allowing molten solder 11a to be blown off without being cooled down.
After removing the excess of molten solder 11a by spraying the organic fatty acid-containing solution to molten solder 11a, coating film 19 of the organic fatty acid forming the organic fatty acid-containing solution is formed on the surface of molten solder 11a. The formed coating film 19 cleans the surface of molten solder 11a and additionally suppresses oxidation of molten solder 11a so that the formation of an oxide film can be prevented.
Finally, mask 5 is removed, as depicted in
The thickness of solder bump 11 is the same or substantially the same as the thickness of mask 5, and thicknesses of nearly all of a plurality of solder bumps 11 formed were the same. When molten solder 11a has been removed by a removing means using a member such as blade 15 or an air cutter, coating film 19 of the organic fatty acid as depicted in
Particularly, palmitic acid is preferable since the acid contacts with molten solder 11a or solder bump 11 and cleans the surface of molten solder 11a or solder bump 11 more effectively. Furthermore, coating film 19 with the palmitic acid adsorbed thereto is formed on the surface of molten solder 11a or solder bump 11. Accordingly, even after that, the coating film 19 can maintain the surface of molten solder 11a or solder bump 11 in such a clean state as to suppress the occurrence of an oxide. As a result, even when the substrate passes through a plurality of heating furnaces during mounting of electronic components thereon, it can be suppressed that the heat oxidizes the surface of solder bump 11 to bring the surface into a state that hinders soldering. Then, the electronic components can be soldered to solder bumps 11 without soldering failure.
Moreover, solder bump 11 formed by the method for forming solder bump 11 according to the present invention can minimize the occurrence of micro voids, as depicted in
As described hereinabove, the solder bump forming method according to the present invention sprays jet stream 11′ of molten solder including tin as the main ingredient and at least nickel as the accessory ingredient to opening portion 6. Accordingly, Cu—Ni—Sn intermetallic compound layer 13a produced by alloying of the copper of copper electrode 2 and the nickel of molten solder 11a is formed on the surface of copper electrode 2. The Cu—Ni—Sn intermetallic compound layer 13a formed on the surface of copper electrode 2 serves as a barrier layer against copper corrosion. The barrier layer prevents copper corrosion of copper electrode 2, so that defects or disappearance of copper electrode 2 due to copper corrosion can be prevented. As a result, reliability can be ensured for copper electrode 2 such as a copper land of a mounting substrate, and when soldering an electronic component on a mounting substrate, copper corrosion occurring in conventional soldering can be suppressed.
In addition, in the method for forming solder bump 11 according to the invention, the removal of molten solder 11a exceeding the thickness of mask 5 is performed by a simple means, that is, a removing means such as blade 15 or an air cutter or a removing means that sprays organic fatty acid-containing solution 18 having from 12 to 20 carbon atoms. Thus, solder bumps 11 having a desired constant thickness can be formed easily with high dimensional precision on a mounting substrate with minute copper electrode 2 thereon. Consequently, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joint portions where electronic components are solder-bonded to solder bumps 11. This is advantageous in terms of manufacturing a low-cost mounting substrate.
Particularly, recent years have seen improvement in the wiring density of copper electrode 2 and the mounting density of electronic components. Accordingly, in order to achieve high-yield solder-bonding of electronic components onto substrate 1, solder bump 11 is becoming more minute and it is moreover expected that there is no thickness variation. The present invention is a method for forming solder bump 11 that can meet the expectation. The invention can form solder bumps 11 having a constant thickness on substrate 1 even when there are dimensional variations between connection terminals such as a connector, a QFP (Quad Flat Package), a SOP (Small Outline Package), a BGA (Ball Grid Array), and the like, so that there is an advantage that can achieve stable and highly reliable solder bonding. As a result, even when electronic components having such variations or large and small electronic components are mounted on a substrate, stable solder bonding can be achieved.
A method for manufacturing a mounting substrate according to the present invention is characterized in that solder bump 11 is formed at a joint portion of copper electrode 2 for mounting an electronic component by the above-described solder bump forming method according to the invention and the electronic component is soldered and mounted to the formed solder bump 11. This can ensure the reliability of copper electrode 2 such as a copper land of a mounting substrate and can suppress copper corrosion occurring in conventional soldering when an electronic component is soldered to a mounting substrate. In addition, solder bump 11 having a desired constant thickness can be formed easily with high dimensional precision on the mounting substrate provided with minute copper electrode 2. In the present invention described above, such a high-cost step as in the conventional art is unnecessary, thus allowing for increase in reliability and yield of joint portions where electronic components are solder-bonded to the solder bumps, so that a low-cost mounting substrate can be manufactured.
Examples of the substrate include various kinds of substrates such as a printed circuit board, a wafer, and a flexible substrate. Particularly, it is preferable for a wafer to apply the method according to the present invention, since the width and pitch of copper electrodes on the wafer are narrow, and solder bump 11 can be provided with high precision on the narrow-pitch micro electrodes. In addition, even on a printed circuit board or a flexible substrate where large electronic components are to be provided, solder bump 11 can be formed with a uniform thickness, and moreover the surface of solder bump 11 can be kept in a cleaned state. Accordingly, the invention can have a significantly great advantageous effect in that even electronic components having dimensional variations can be mounted without any soldering failure and without any a concern about the occurrence of micro voids as depicted in
Examples of the electronic components include a semiconductor chip, a semiconductor module, an IC chip, an IC module, a dielectric chip, a dielectric module, a resistor chip, and a resistor module.
The present invention will be described in more detail below with reference to Examples and Comparative Examples.
As one example, there was prepared substrate 1 having a copper wiring pattern having a width of, for example, 200 μm and a thickness of, for example, 10 μm formed thereon. On the substrate 1, there were exposed only joint portions of the copper wiring pattern that were to be portions for mounting electronic components and had a width of, for example, 200 μm and a length of, for example, 50 μm, whereas the other portions of copper electrode 2 were covered with an insulation layer. There was prepared mask 5 provided with opening portions 6 used for forming solder bumps 11 at such joint portions. Positions of the joint portions where copper electrode 2 was exposed on substrate 1 were matched with positions of the opening portions 6 of mask 5 to superimpose with each other. Then, using a quinary lead-free solder consisting of Ni: 0.05% by mass, Ge: 0.005% by mass, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest, the solder was heated to 250° C. to obtain molten solder 11a, and jet stream 11′ of the molten solder was jetted to opening portions 6. Jet stream 11′ of the molten solder was sprayed using a jetting apparatus in a state where nozzle 12 was inclined, for example, by 45 degrees at an atmosphere temperature of around 250° C. so that the temperature of molten solder 11a heated to 250° C. was not lowered. In the configuration of opening portions 6 after the jetting of jet stream 11′ of the molten solder, molten solder 11a was in a swollen state due to surface tension. The molten solder 11a was allowed to keep the molten state by a vapor atmosphere temperature.
Next, palmitic acid was added up to 10% by mass in an ester synthetic oil including neither metal salts such as nickel salt and cobalt salt nor additives such as an antioxidant to prepare an organic fatty acid-containing solution. The organic fatty acid-containing solution was heated to 250° C. to obtain heated organic fatty acid-containing solution 18, which was then jetted to opening portions 6 with the molten solder 11a swollen therein. Molten solder 11a at this time kept the molten state. The jetting of organic fatty acid-containing solution 18 was performed in a state where nozzle 17 was inclined, for example, by 30 degrees using a spraying apparatus heated so that the temperature of the organic fatty acid-containing solution heated to 250° C. was not lowered. In opening portions 6 after the jetting of organic fatty acid-containing solution 18, it was found that molten solder 11a was provided at the same height as that of a surface of mask 5.
After that, molten solder 11a was allowed to cool to obtain solder bumps 11, and then mask 5 was removed to form solder bumps 11 according to Example 1 of the present invention. Scanning electron micrographic views of a section of obtained solder bump 11 were shown in
Solder bumps 11 according to Example 2 were formed in the same manner as in Example 1 except for using a quinary lead-free solder consisting of, as solder materials, Ni: 0.03% by mass, Ge: 0.005% by mass, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed with a thickness of 1 μm, although unevenness was slightly observed.
Solder bumps 11 according to Example 3 were formed in the same manner as in Example 1 except for using a quinary lead-free solder consisting of, as solder materials, Ni: 0.07% by mass, Ge: 0.005% by mass, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed uniformly with a thickness of 2 μm.
Solder bumps 11 according to Example 4 were formed in the same manner as in Example 1 except for changing the content of palmitic acid in the organic fatty acid-containing solution to 7% by mass. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed uniformly with a thickness of 1.5 μm.
Solder bumps 11 according to Example 5 were formed in the same manner as in Example 1 except for changing the content of palmitic acid in the organic fatty acid-containing solution to 12% by mass. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed uniformly with a thickness of 1.5 μm.
Solder bumps 11 according to Comparative Example 1 were formed in the same manner as in Example 1 except for using a ternary lead-free solder consisting of, as solder materials, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that no Cu—Ni—Sn intermetallic compound layer was present (see
Solder bumps 11 according to Comparative Example 2 were formed in the same manner as in Example 1 except for using a quinary lead-free solder consisting of, as solder materials, Ni: 0.005% by mass, Ge: 0.005% by mass, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was extremely uneven and not continuous as a film.
Solder bumps 11 according to Comparative Example 3 were formed in the same manner as in Example 1 except for using a quinary lead-free solder consisting of, as solder materials, Ni: 1% by mass, Ge: 0.005% by mass, Ag: 3% by mass, Cu: 0.5% by mass, and Sn for the rest. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed with a thickness of 5 μm and cracking was observed.
Solder bumps 11 according to Comparative Example 4 were formed in the same manner as in Example 1 except for changing the content of palmitic acid in the organic fatty acid-containing solution to 1% by mass. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed uniformly with a thickness of 1.5 μm. but the surface of the layer had discoloration that seemed to be due to oxidation.
Solder bumps 11 according to Comparative Example 5 were formed in the same manner as in Example 1 except for changing the content of palmitic acid in the organic fatty acid-containing solution to 30% by mass. Similarly to Example 1, scanning electron micrographs of a section of obtained solder bump 11 were taken and indicated that Cu—Ni—Sn intermetallic compound layer 13a was formed uniformly with a thickness of 1.5 μm.
The thickness of Cu—Ni—Sn intermetallic compound layer 13a was measured from the sectional scanning electron micrograph of solder bump 11. In addition, the presence or absence of a void near a bonding interface between copper electrode 2 and solder bump 11 was evaluated from the sectional scanning electron micrograph thereof taken after aging at 150° C. for 240 hours.
Viscosity was measured on molten solder 11a purified with the organic fatty acid-containing solution and molten solder 11a unpurified with the organic fatty acid-containing solution by a vibrating reed viscometer. Molten solder 11a purified with the organic fatty acid-containing solution had a viscosity of from 0.003 to 0.004 Pa·s, more particularly 0.0038 Pa·s at 220° C., 0.0036 Pa·s at 240° C., 0.0035 Pa·s at 260° C., and 0.0034 Pa·s at 280° C. On the other hand, molten solder 11a unpurified therewith had a viscosity of from 0.005 to 0.006 Pa·s, more particularly 0.0060 Pa·s at 220° C., 0.0058 Pa·s at 240° C., 0.0056 Pa·s at 260° C., and 0.0054 Pa·s at 280° C. Thus, both solders had a significant difference.
In addition,
The characteristic differences between the purified molten solder and the unpurified molten solder as described above indicate the reason that jet stream 11′ of molten solder spreads uniformly with favorable solder wettability throughout the surface of copper electrode 2. In other words, the purified molten solder wet-spread favorably on copper surface 2 due to the small viscosity and good wettability thereof, whereas the unpurified molten solder did not wet-spread uniformly on the copper surface. The same results were also shown on copper surfaces cleaned with the organic fatty acid-containing solution.
The amounts of oxygen in the solder layers depicted in
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/060301 | 4/17/2012 | WO | 00 | 10/15/2014 |
