JP6511768B2 - Method of forming solder bumps - Google Patents

Description

The present invention relates to a method of forming a solder bump by reflow processing in a formic acid gas atmosphere using a solder paste containing an activator.

  Conventionally, as a method of forming a solder bump using a solder paste composed of a solder powder and a flux, there is disclosed a method of including an activator in the flux and reflowing in a nitrogen atmosphere or a low oxygen atmosphere (for example, Patent Document 1) reference). The solder paste used in this method is a mixture of solder powder and flux at a predetermined ratio, and the solder powder is a Sn—Ag—Cu based, Pb—Sn based alloy solder powder with a particle size of 5 μm to 32 μm. The flux contains a resin component such as rosin, an activator, a thixo agent, and a solvent, and is halogen free, active (RA), weak active (RMA), water soluble, etc. I use the one.

  On the other hand, when two terminals such as a resistance element and a capacitance element are soldered to a substrate, the non-uniform difference in the amount of solder between the two terminals is eliminated to improve the reliability when soldering portions with small spacing. In order to reduce the oxide film such as solder powder by spraying a formic acid gas when melting the solder in the reflow process using a paste that does not contain an activator that has a reducing power to the oxide film such as solder powder. A mounting method is disclosed (see, for example, Patent Document 2). The solder paste used in this component mounting method consists of solder powder and an organic material such as polybden, and does not use flux.

Unexamined-Japanese-Patent No. 2014-107373 (Paragraph [0017], [0021]) JP, 2008-246563, A (claim 3, paragraph [0010], [0011], [0015])

  In the method for forming a solder bump represented by Patent Document 1, an oxide film such as solder powder is reduced by an activator at the time of solder melting in the reflow process. When the method of printing this using the solder paste shown by patent document 2 instead of reducing with this activator, and spraying a formic acid gas at the time of solder melting in a reflow process is applied, the solder paste of patent document 2 In this case, there was a defect that the bump precursor at the time of mask peeling after paste printing was chipped and the shape of the bump precursor was not good.

  On the other hand, this is printed using a solder paste containing a resin component such as rosin shown in Patent Document 1, an activator, a thixo agent, and a solvent containing a solvent, and formic acid gas is sprayed when the solder is melted in the reflow process. When using the method described above, there was a defect that extremely many voids occurred in the bumps.

  An object of the present invention is a method of forming a solder bump in which there is no chip in a bump precursor at the time of mask peeling after paste printing, the shape and size of a bump after reflow are uniform, and generation of voids in the bump is small. It is to provide.

According to a first aspect of the present invention, a mask having an opening is disposed on a substrate, and the solder paste is printed so as to fill the opening with a solder paste containing flux and solder powder, and the mask is It is improvement of the method of carrying out the reflow process of the bump precursor on the said base material, and forming a solder bump, after peeling. Its distinctive point, the flux, rosin, a solvent, containing a switch shit agents and active agents, acid value of the flux is not more than 100 mg KOH / g, halogen content of the flux is 0.03 wt% The following is to perform the reflow process in a formic acid gas atmosphere and / or an atmosphere of a gas in which formic acid is thermally decomposed. However, halogen flux contains refers to halogen ionizing.

  A second aspect of the present invention is the invention based on the first aspect, wherein the rosin is a polymerized rosin ester, rosin ester, terpene resin, petroleum resin, hydrogenated rosin ester, maleic acid resin, rosin modification It is a formation method of the solder bump which is a phenol resin, rosin modified xylene resin, a ketone resin, or an alkylphenol resin.

  A third aspect of the present invention is the invention based on the first or second aspect, wherein the solder powder is Sn—Ag—Cu alloy solder powder, Sn—Cu alloy solder powder, Sn—Ag based It is a formation method of the solder bump which is alloy solder powder or Pb-Sn type alloy solder powder.

A fourth aspect of the present invention is the invention based on any of the first to third aspects, wherein the flux comprises 20 to 60% by mass of the solvent and 1.0 to 10% by mass of the thixotropic agent. The method is a method for forming a solder bump in which the activator is 0.1% by mass or less and the rosin is the balance.

A fifth aspect of the present invention, rosin, a solvent, containing a switch shit agents and active agents, acid value is not more than 100 mg KOH / g, the content of the active agent is not more than 0.1 mass%, It is a solder paste containing flux and solder powder whose halogen content is 0.03 mass% or less.

In a first aspect of the solder bump forming method of the present invention, rosin, a solvent, for using a flux containing a Chi shit agents and active agents, shape retention of the paste filling property and bump precursor to the mask opening Since it is high, there is no chip in the bump precursor at the time of mask peeling after paste printing, the shape and size of the bump after reflow become uniform, and the printability is excellent. Since the acid value of the flux is 100 mg KOH / g or less and the halogen content of the flux is 0.03 mass% or less, the generation of reduced water caused by the reduction reaction between the flux and the oxide of the solder powder is small, and the bumps are Less occurrence of voids inside. Further, by performing the reflow process in a formic acid gas atmosphere and / or in an atmosphere of a gas in which formic acid is thermally decomposed, the oxide film such as the solder powder is reduced to make the solder melting smooth.

  In the method for forming a solder bump according to the second aspect of the present invention, as rosin, polymerized rosin ester, rosin ester, terpene resin, petroleum resin, hydrogenated rosin ester, maleic acid resin, rosin modified phenolic resin, rosin modified xylene By using a resin, a ketone resin or an alkylphenol resin, the acid value of the flux can be reduced to 100 mg KOH / g or less.

  In the method of forming a solder bump according to the third aspect of the present invention, a Sn—Ag—Cu alloy solder powder, a Sn—Cu alloy solder powder, a Sn—Ag alloy solder powder, or a Pb—Sn alloy solder powder is used as a solder powder. Solder powder can be used.

  In the method of forming a solder bump according to the fourth aspect of the present invention, the printability of the solder paste and the shape retention of the bump precursor are improved by setting the composition ratio of the solvent in the flux to 20 to 60 mass%. . In addition, by setting the composition ratio of the thixotropic agent to 1.0 to 10% by mass, the viscosity of the solder paste can be adjusted to improve the shape retention of the bump precursor.

A fifth aspect of the solder paste of the present invention, rosin, a solvent, containing a switch shit agents and active agents, acid value is not more than 100 mg KOH / g, the content of the active agent 0.1 wt% Since it contains flux and solder powder having a halogen content of 0.03% by mass or less, the printability and bumpability of the solder paste can be obtained by using the method for forming a solder bump in which reflow processing is performed in a formic acid gas atmosphere. The shape retention of the precursor is improved, and the occurrence of voids in the bumps is reduced.

  Next, an embodiment of the present invention will be described.

[Content of solder paste]
The solder paste of the present invention is a mixture containing 5 to 20% by mass of a flux with respect to 100% by mass of the solder paste, with the balance being solder powder. Examples of solder powder include Sn-Ag-Cu alloy solder powder, Sn-Cu alloy solder powder, Sn-Ag alloy solder powder, and Pb-Sn alloy solder powder. The average particle size of the solder powder is, for example, in the range of 0.1 to 30 μm. Thereby, the paste filling property to the mask opening and the shape retention of the bump precursor can be enhanced. In order to narrow the bump formation, the average particle size of the solder powder is preferably in the range of 0.1 to 10 μm.

Flux of the present invention have a rosin, a solvent, containing a switch shit agents and active agents, acid value is not more than 100 mg KOH / g, the characteristics of the silver content is 0.03 mass% or less. If the acid value is less than 100 mg KOH / g, when the acid value exceeds this value, the generation of reduced water generated by the reduction reaction between the flux and the oxide of the solder powder increases, and the voids in the formed bumps become It is to increase the number. Also, if the halogen content exceeds 0.03% by mass, the generation of reduced water generated by the reduction reaction between the flux and the oxide of the solder powder increases, and the voids in the similarly formed bumps also increase. . The acid value is preferably 50 mg KOH / g or less, and the halogen content is preferably 0.01 mass% or less. Halogen includes chlorine and / or bromine.

The composition of the flux of the present invention is 20 to 60% by mass of the solvent, 1.0 to 10% by mass of the thixotropic agent, 0.1 % by mass or less of the activator and the balance of the rosin. If the solvent content is less than 20% by mass, the solder paste is unlikely to be in the form of a paste. When the amount of the solvent exceeds 60% by mass, the shape retention of the bump precursor becomes poor. If the thixotropic agent is less than 1.0% by mass, the viscosity of the solder paste decreases, and if it exceeds 10% by mass, the solder paste becomes too hard. The preferred composition of the flux is 30 to 50% by mass of the solvent, 2 to 8% by mass of the thixotropic agent, 0.1 % by mass of the activator and the balance of the rosin.

  As rosin contained in the flux of the present invention, polymerized rosin ester (acid number: 10 to 25), rosin ester (acid number: 50 or less), terpene resin in order to make the acid value of the flux 100 mg KOH / g or less (Acid number: 45 to 90), petroleum resin (1 or less), hydrogenated rosin ester (acid number: 20 or less), maleic acid resin (acid number: 50 or less), rosin modified phenolic resin (acid number: 30 or less) And rosin-modified xylene resin (acid number: 75 or less), ketone resin (acid number: 1 or less) or alkylphenol resin (acid number: 85 to 100) are preferably used.

  As the solvent contained in the flux of the present invention, solvents such as alcohols, ketones, esters, ethers, aromatics and hydrocarbons are used. Specifically, benzyl alcohol, ethanol, ethyl alcohol, isopropyl alcohol, butanol, diethylene glycol, ethylene glycol, ethyl cellosolve, butyl cellosolve, butyl carbitol, isopropyl alcohol, ethyl acetate, butyl acetate, butyl benzoate, diethyl adipate, dodecane To tetradecene, alpha terpineol, 2-methyl 2,4-pentanediol, 2-ethylhexanediol, toluene, xylene, propylene glycol monophenyl ether, diethylene glycol monohexyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, diisobutyl adipate, Xylene glycol, cyclohexane dimethanol, 2-terpinyloxy ether Lumpur, 2-dihydro-terpineol oxyethanol is used alone or as a mixture thereof.

  Thixotropic agents contained in the flux of the present invention include hydrogenated castor oil, hydrogenated castor oil, carnauba wax, amides, hydroxy fatty acids, dibenzylidene sorbitol, bis (p-methylbenzylidene) sorbitol, beeswax, stearic acid amide, hydroxy Stearic acid ethylene bisamide or the like is used. Furthermore, if necessary, fatty acids such as caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid, hydroxy fatty acids such as 1,2-hydroxystearic acid, antioxidants, surfactants, if necessary. Amines and the like are added and used.

[Step of forming solder bump]
The step of forming the solder bumps comprises a printing step, a reflow heating step in a formic acid gas atmosphere, and a cleaning step of cleaning flux residue and the like. In the printing step, a mask having an opening is disposed on a base material such as a silicon wafer or a glass epoxy resin substrate, and a solder paste is printed so as to fill the opening with the solder paste. After printing, the mask is peeled from the substrate (plate missing) to form a bump precursor on the substrate. The solder paste of the present invention uses a flux consisting of rosin, a solvent and a thixo agent, so that the paste filling property to the mask opening and the shape retention property of the bump precursor are high. The precursor is chipped, the shape and size of the bump after reflow become uniform, and the printability is excellent.

  In the reflow heating step, the bump precursor formed on the substrate is heated at a temperature lower than the melting point of the solder powder, for example, at a temperature of 160 to 200 ° C. in a nitrogen atmosphere or a low oxygen atmosphere as preliminary heating. And volatilize the solvent which is a void source in the flux. Thereafter, the solder powder is melted by heating at a temperature higher than the melting point of the solder powder, for example, at a temperature of 217 to 230 ° C. Formic acid may be introduced into the reflow oven prior to preheating, or may be introduced into the reflow oven just prior to the melting point of the powder. The reducing power of formic acid gas reduces the oxide film such as solder powder. When the molten solder is cooled in the next cooling step, surface tension forms a substantially hemispherical solder bump. Subsequently, the substrate on which the solder bumps are formed is transferred to a cleaning step, and the flux residue and the like are removed by cleaning. In the solder bumps thus formed, the acid value of the flux in the used solder paste was 100 mg KOH / g or less, and the halogen content was 0.03 mass% or less, so the oxides of the flux and the solder powder were used. The generation of reduced water resulting from the reduction reaction with is reduced, and the generation of voids is reduced.

  Next, an example of the present invention will be described in detail along with a comparative example.

Example 1
Flux consisting of 55% by weight of solvent of alpha terpineol, 3.5% by weight of thixotropic agent of hydrogenated castor oil, 0.1% by weight of activator of diphenyl guanidine hydrobromide and the balance being rosin of alkylphenol resin, 96.5 Sn-Ag-Cu-based alloy solder powder (hereinafter referred to as SAC 305) having an average particle diameter of 3.5 μm consisting of mass% Sn-3 mass% Ag-0.5 mass% Cu, flux content 12.0 mass The solder paste was prepared by mixing in a percentage ratio. The acid value of the flux was 100 mg KOH / g, and the halogen content was 0.03 mass%. The solder paste is printed so as to be filled in the opening of a metal mask disposed on a glass epoxy resin substrate which is a base material, and after peeling off the mask, the substrate on which a bump precursor is formed is nitrogen It preheat-treated by hold | maintaining for 1 minute at the temperature of 150-200 degreeC under atmosphere. Also, formic acid was introduced into the reflow furnace before preheating. Subsequently, after reflow heat treatment at a temperature of 230 to 250 ° C., it was cooled to form 1,600 solder bumps. The dimensions of the bumps formed on the substrate were 95 μm in diameter and 43 μm in height, and the bump pitch was 150 μm.

Example 2
A solder paste was prepared in the same manner as in Example 1 using the same flux as in Example 1 and the same solder powder as in Example 1 except that a rosin ester was used as the rosin and no activator was added. The acid value of this flux was 5 mg KOH / g, and its halogen content was below the detection limit (N.D.). Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Example 3
As a solder powder, a Sn-Ag-Cu based alloy solder powder (hereinafter referred to as SAC 405) having an average particle diameter of 15.6 μm consisting of 95.5 mass% Sn-4 mass% Ag-0.5 mass% Cu was used. A solder paste was prepared using the same flux as in Example 2 except that the flux content was 9.0% by mass. The acid value of this flux was 5 mg KOH / g, and its halogen content was below the detection limit. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Example 4
Example 2 except using Sn-Cu-based alloy solder powder (hereinafter referred to as Sn0.7Cu) having an average particle diameter of 6.8 μm consisting of 99.3 mass% Sn-0.7 mass% Cu as the solder powder The solder paste was prepared by mixing using the same flux as in the above and at a ratio of flux content of 10.4% by mass. The acid value of this flux was 5 mg KOH / g, and its halogen content was below the detection limit. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Example 5
Example 2 except using Sn-Ag-based alloy solder powder (hereinafter referred to as Sn2.3Ag) having an average particle diameter of 5.2 μm consisting of 97.7 mass% Sn-2.3 mass% Ag as the solder powder A solder paste was prepared by mixing using the same flux as in the above and at a ratio of flux content of 11.3% by mass. The acid value of this flux was 5 mg KOH / g, and its halogen content was below the detection limit. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Example 6
The same flux as in Example 2 was used except that a Pb-Sn alloy solder powder (hereinafter referred to as Pb63Sn) having an average particle diameter of 9.8 μm consisting of 37 mass% Pb-63 mass% Sn was used as the solder powder. The solder paste was prepared by mixing in a proportion of 9.8% by mass of flux content. The acid value of this flux was 5 mg KOH / g, and its halogen content was below the detection limit. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Comparative Example 1
An activator comprising 55% by mass of a solvent of alpha terpineol, 3.5% by mass of thixo agent of hydrogenated castor oil, 0.4% by mass of diphenylguanidine hydrobromide activator, and the balance being hydrogenated rosin A solder paste was prepared in the same manner as in Example 1 using the same flux as in Example 1 and using the same solder powder as in Example 1. The acid value of this flux was 150 mg KOH / g, and its halogen content was 0.1% by mass. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

Comparative Example 2
Polybden having an average molecular weight of 300 or less and polybden having an average molecular weight of about 500 were mixed at a mass ratio of 5: 1. A solder paste was prepared in the same manner as in Example 1 by mixing this organic material and the same solder powder as in Example 1 at a ratio of the organic material content of 10% by mass. The acid value of this organic material was less than 0.01 mg KOH / g, and its halogen content was below the detection limit. Solder bumps were formed in the same manner as in Example 1 using this solder paste.

<Evaluation test>
The acid value of the flux or the organic material used in Examples 1 to 6 and Comparative Examples 1 and 2 and the halogen content thereof were measured by the following methods. In addition, the shape retention of the bump precursor after printing the solder pastes obtained in Examples 1 to 6 and Comparative Examples 1 and 2 and in the solder bumps obtained in Examples 1 to 6 and Comparative Examples 1 and 2 The maximum void fraction was measured and evaluated by the following method.
(a) Acid value of flux The acid value of flux was measured in accordance with JIS Z 3197.
(b) Halogen content of flux The halogen content of flux was measured in accordance with JIS Z 3197.
(c) Shape Retaining Property of Bump Precursor The substrate after printing the paste was observed visually or with an optical microscope, and it was judged as good if missing, printing sag, chipping of the bump precursor and the like did not occur. It was judged that the occurrence of missing etc. was a defect.
(d) Maximum Void Ratio in Bumps Voids generated inside the formed solder bumps were observed by transmission X-ray to evaluate how large a void is generated in each bump. The void ratio is the total void area ratio generated with respect to the bump area, and the highest void ratio among the 1600 bumps measured is taken as the maximum void ratio.

<Result of evaluation>
As is clear from Table 1, in Comparative Example 1 in which the activator was used for the flux as in the conventional solder paste, the shape retention of the bump precursor was good, but the maximum void fraction in the bump was 9.%. It was as high as 5%. This is considered to be due to the large amount of reduced water generated by the reduction reaction between the flux and the oxide of the solder powder. Further, the solder paste according to Patent Document 2 of Comparative Example 2 had poor fluidity of the paste and poor filling property to the mask opening, so that printing could not be performed and bumps could not be formed. Therefore, the void fraction could not be measured. In Examples 1 to 6 as compared with Comparative Examples 1 and 2 described above, the shape retention of the bump precursor was all good, and the maximum void ratio in the bump was as small as 5.8% or less.

Claims (5)

A mask having an opening is disposed on a substrate, the solder paste is printed so that a solder paste containing flux and solder powder is filled in the opening, and the mask is peeled off, and then the mask is removed. In a method of reflowing a bump precursor to form a solder bump,
The flux contains rosin, solvent, Ji shit agents and active agents, acid value of the flux is not more than 100 mg KOH / g, halogen content of the flux is not more than 0.03 wt%, the reflow A method of forming a solder bump, wherein the treatment is performed in a formic acid gas atmosphere and / or in an atmosphere of a gas in which formic acid is thermally decomposed.   The rosin is a polymerized rosin ester, rosin ester, terpene resin, petroleum resin, hydrogenated rosin ester, maleic acid resin, rosin modified phenolic resin, rosin modified xylene resin, ketone resin or alkylphenol resin. Method.   The method according to claim 1 or 2, wherein the solder powder is Sn-Ag-Cu alloy solder powder, Sn-Cu alloy solder powder, Sn-Ag alloy solder powder, or Pb-Sn alloy solder powder. The flux according to any one of claims 1 to 3, wherein 20 to 60% by mass of the solvent, 1.0 to 10% by mass of the thixotropic agent, 0.1 % by mass or less of the activator, and the balance rosin. mETHODS. Rosin, a solvent, containing a switch shit agents and active agents, acid value is not more than 100 mg KOH / g, the content of the active agent is not more than 0.1 mass%, the halogen content of 0.03 wt% Solder paste containing the following flux and solder powder.