JP2008110380A - Flux for cream solder, and cream solder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide flux composition and cream solder, by which flux composition and cream solder, the release property in screen printing can be remarkably improved, and further the generation of voids in a solder joint portion can be suppressed. <P>SOLUTION: The flux for the cream solder, which flux contains 0.05 to 10 vol.% of at least one of spheroidal polymer fine particles to be selected from the group consisting of polyimide fine particles, polyurea fine particles, benzoguanimine fine particles, acrylic cross linking fine particles, and styrenic crosslinking fine particles, and the cream solder, which contains solder particles and the flux for the cream solder, are used. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a flux for cream solder and cream solder.

  In surface mounting of electronic components, so-called cream solder kneaded with flux and solder powder is printed on a printed board using a stencil mask, etc., and after mounting the components, it is heated and melted using a reflow oven, The method of connecting parts is mainly adopted.

  In such soldering, a technique based on screen printing technology is used. In this method, as shown in FIG. 1, a stencil mask is placed at a predetermined position on a substrate to be mounted, and paste-like solder is applied and filled into a window-like opening formed in the mask with a squeegee, and thereafter By removing the mask, the cream solder is transferred onto the mounting substrate in accordance with the pattern of the opening previously made in the metal mask.

In recent years, electronic components mounted on a substrate have been miniaturized. For example, in an IC package, the lead pitch has been reduced from 0.5 mm to 0.4 mm, and further to 0.3 mm. In addition, 1 mm × 0.5 mm size and 0.6 mm × 0.3 mm size micro components have been used. With such a narrow pitch, there is a need for a more reliable mounting technique that can accurately transfer the mask pattern without causing a shortage of supply of cream solder. That is, in mounting such a narrow-pitch component, the printability of cream solder, in particular, the plate release property is very important. In particular, when the mask opening diameter is reduced, problems such as the occurrence of defects due to the paste-like cream solder adhering to the opening of the metal mask and the decrease in pattern transfer accuracy become obvious. Factors that affect the plate-release property generally include cream solder viscosity, cream solder application accuracy during the printing process, and the like.

In order to solve such problems, by adjusting the viscosity of the cream solder, it is possible to suppress dripping and bleeding during printing, and to improve the plate release and printability. A method of adding a thixotropic agent (Patent Document 1), a method of adding a specific amide compound to cause gelation (Patent Document 2), and the like have been proposed.

However, in the process operation of screen printing, it is indispensable that the cream solder has a certain amount of viscosity and fluidity. For this reason, in adjusting the viscosity of the cream solder as shown in Patent Documents 1 and 2, It is very difficult to maintain the fluidity that can be filled in the narrow pitch openings without any shortage, and to maintain the form as a solder joint without being attached to the mask and without sagging or bleeding. Furthermore, if the viscosity is adjusted as described above, that is, when the viscosity is increased so that the form at the time of printing can be maintained, bubbles that are inevitably generated at the solder joints are not released to the outside of the solder during solder reflow. The problem of remaining as bubbles (hereinafter referred to as voids) becomes more prominent. The generation of such voids causes the mechanical strength of the joint to deteriorate, resulting in a problem that the reliability of the electronic component is greatly reduced.

JP 2001-347395 A JP-A-60-170594

It is an object of the present invention to provide a flux composition and a cream solder that can remarkably improve the plate slippage during screen printing even when mounting electronic components with a narrow pitch, and can suppress the generation of voids at the solder joints. Objective.

In order to solve the above problems, the present inventor focused on the paste shape and void generation mechanism during cream solder printing, and by incorporating spherical fine particles of an appropriate size in the flux, the mask opening was narrowed. Even in narrow pitch mounting, it has been found that the plate slippage at the time of printing can be remarkably improved, and furthermore, the occurrence of voids at the time of reflow can be remarkably improved, and the present invention has been completed.

That is, it is characterized by containing 0.05 vol% to 10 vol% of at least one spherical polymer fine particle selected from the group consisting of polyimide fine particles, polyurea fine particles, benzoguanamine fine particles, acrylic crosslinked fine particles, and styrene crosslinked fine particles. The present invention relates to a cream solder containing a solder powder and the cream solder flux.

According to the present invention, it is possible to provide a flux composition and a cream solder which can remarkably improve the plate slippage property at the time of screen printing even when mounting electronic components with a narrow pitch and can further suppress the generation of voids at the solder joints. be able to.

The cream solder in the present invention is a composition in which a liquid so-called cream solder flux and solder powder are kneaded in the same manner as that usually used for mounting electronic components. The liquid flux is generally composed of a flux base, a solvent, an activator, a thixotropic agent, and the like.

In the present invention, spherical polymer fine particles are contained in the flux. The polymer fine particles are at least one selected from the group consisting of polyimide fine particles, polyurea fine particles, benzoguanamine fine particles, acrylic crosslinked fine particles, and styrene crosslinked fine particles. Here, the acrylic crosslinked fine particles are obtained by, for example, radical copolymerizing a monomer component mainly composed of a known (meth) acrylic monomer and a crosslinked monomer containing at least two vinyl groups. The resulting fine particles are styrenic crosslinked fine particles, for example, fine particles obtained by radical copolymerization of a monomer component mainly composed of a styrene monomer and a crosslinked monomer containing at least two vinyl groups. That is. Specifically, for example, the acrylic crosslinked fine particles include divinylbenzene-acrylic acid ester copolymer, and the styrene crosslinked fine particles include divinylbenzene-styrene copolymer. These polymer fine particles are prepared by a known method described in, for example, JP-A-1-213303, JP-A-11-140181, JP-A-2006-183018, JP-A-5-178912, and the like. Alternatively, commercially available products such as benzoguanamine resin (for example, trade name: Eposter, Nippon Shokubai Co., Ltd.) may be used as they are.

By adding the above polymer fine particles to the flux, even in a paste-like cream solder that maintains a relatively low viscosity suitable for printing, the fine particles function as a thixotropic agent, and the form of the printed cream solder By physically holding the film, the plate-release property is remarkably improved even in a narrow pitch mounting in which the mask openings are narrowed. Furthermore, the melt viscosity of the flux base can be made relatively low, so that bubbles generated by foaming during reflow can be easily discharged out of the system, or the diffusion path of voids generated in the solder joints in the molten solder And promotes the coalescence growth of bubbles, making it easier to be discharged out of the system, or if very fine particles are added, a defoaming effect that adsorbs gas components on the surface of the particles appears By doing so, void generation is remarkably improved.

As the polymer fine particles, those having a diameter of 5 μm or less, more preferably 1 μm or less are preferably added. A diameter of 5 μm or less is preferable because the fluidity during reflow can be improved and the coalescence of solder metal particles can be promoted. The lower limit of the particle size is not particularly limited, but a diameter of 0.01 μm or more is preferable in consideration of the minimum particle size by a normal polymer fine particle production method.

The polymer fine particles added to the flux preferably have a melting point of more than 260 ° C. measured by a differential thermal analysis method, a differential scanning calorimetry method or the like. Usually, since soldering by reflow is performed at 230 to 260 ° C., polymer fine particles having a melting point exceeding 260 ° C. are preferable because the action of promoting the coalescence of bubbles and the defoaming action are enhanced during reflow.

The flux base used in the production of the flux of the present invention is not particularly limited, and a known one can be used. Specifically, rosin resins such as gum rosin, polymerized rosin, hydrogenated rosin, disproportionated rosin, modified rosin (for example, acrylic acid modified rosin), rosin esters, and other various rosin derivatives, polyester resins, poly Examples thereof include synthetic resins such as ether ester amide resins, polyamide resins, polyimide resins, phenoxy resins, and terpene resins. These can be used individually by 1 type or in mixture of 2 or more types.

It does not specifically limit as a solvent used for manufacture of the flux of this invention, A well-known thing can be used. Specifically, alcohols such as ethanol, n-propanol, isopropanol and isobutanol, glycol ethers such as butyl carbitol and hexyl carbitol, esters such as isopropyl acetate, ethyl propionate, butyl benzoate and diethyl adipate And hydrocarbons such as n-hexane, dodecane, and tetradecene. These can be used individually by 1 type or in mixture of 2 or more types.

The activator used in the production of the flux of the present invention is not particularly limited and known ones can be used. For example, amine hydrohalides, organic acids and the like can be used. Specific examples of amine hydrohalides include diethylamine hydrobromide and cyclohexylamine hydrobromide. Specific examples of organic acids include adipic acid, stearic acid, and benzoic acid. Etc. In addition, these activators can be used individually by 1 type or in mixture of 2 or more types.

The thixotropic agent used in the production of the flux of the present invention is not particularly limited as long as it is a thixotropic agent used in the production of a flux, and known ones can be used. Specifically, for example, hardened castor oil, beeswax, carnauba wax, stearamide, hydroxystearic acid ethylene bisamide and the like can be used. These can be used individually by 1 type or in mixture of 2 or more types.

The amount of each of these components may be appropriately adjusted according to the application, but is usually about 30 to 75 parts by weight of flux base, about 20 to 60 parts by weight of solvent, about 1 to 10 parts by weight of thixotropic agent, and 0. About 1 to 20 parts. In addition, in the flux for cream solder of this invention, additives, such as antioxidant, an antifungal agent, and a matting agent, can be contained as needed.

The polymer fine particles are added so as to be about 0.05 to 10% by volume of the flux, preferably 0.05 to 5% by volume. By making the addition amount 0.05% by volume or more, sufficient thixotropy can be imparted to the paste-like cream solder, and preferably by 10% by volume or less, it promotes melting and coalescence of solder metal particles during reflow. This is preferable.

  The cream solder of the present invention contains solder powder and the cream solder flux.

  The alloy composition of the solder powder of the present invention is not particularly limited, and various known ones can be used. For example, as a solder alloy, a conventionally known tin-lead alloy, a solder alloy composition such as a tin-silver alloy and a tin-zinc-based alloy that have been developed as a lead-free solder; , Indium, antimony and the like can be used. Further, the shape of the solder powder is not particularly limited, and any shape such as a true sphere, an indefinite shape, or a mixture of both can be used.

Although the usage-amount of each component should just be determined suitably according to a use etc., a solder powder is about 80-95 weight part normally, and the flux for cream solder is about 5-20 weight part. Moreover, you may add various well-known additives as needed.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to the following Example.

Examples 1-6 and Comparative Examples 1-3
Using the components shown in Table 1, polyimide fine particles, a flux for cream solder, and cream solder were prepared by the following method and evaluated as follows. The results are shown in Table 1.

(1) Preparation of polyimide fine particles 150 ml of an acetone solution (0.06 mol / l) of 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4′-diaminodiphenyl ether (DPE) ) After mixing 150 ml of acetone solution (0.06 mol / l), ultrasonic waves of 38 kHz (sonic cleaning machine SONO manufactured by Kaijo Corporation)
The mixture was reacted at 25 ° C. for 30 minutes while irradiating CLEANER 100Z) to obtain 300 ml of an acetone dispersion of polyamic acid fine particles. Using a micromixer (YM-1 type, manufactured by Yamatake Corporation), a reaction apparatus as shown in FIG. 2 was assembled to synthesize polyamic acid fine particles. That is, 30 ml of the above-mentioned acetone dispersion of polyamic acid fine particles was added to a 5000 ml fine particle preparation tank equipped with a stirrer, and 0.06 mol / l BTDA, DPE-acetone solution was added to the non-pulsating pump while stirring. Then, each 15 ml / min was introduced into the micromixer at room temperature and mixed. The initial flow of 30 ml was discarded without being placed in the receiver, and the subsequent mixed liquid was collected in the receiver. Stirring while keeping the temperature constant, when the liquid mixture reached 4800 ml, the liquid feeding was stopped, and then left to stand for 30 minutes, suction filtered, and washed with 400 ml of acetone three times. This was heat-dried at 60 ° C. for 4 hours. While dispersing 60 g of the polyamic acid fine particles obtained here as shown in FIG. 3 in 540 ml of tetralin and stirring with a stirrer so as not to cause bumping, the water produced using a water separator was removed while Imidization was carried out at 4 ° C. for 4 hours to obtain polyimide fine particles having a diameter of 1 μm. In addition, the polyimide fine particle of another particle size was manufactured by adjusting the amount of acetone dispersion liquid and the total liquid feeding amount thrown into the initial particle preparation tank. By reducing the amount of the acetone dispersion liquid charged into the initial fine particle preparation tank and increasing the total liquid feeding amount, polyimide fine particles having a large particle diameter can be obtained. The melting point of these polyimide fine particles was determined by using a differential calorimeter EXSTAR 6200 (manufactured by SEIKO Instruments), sealing 5 to 10 mg of polyimide fine particles in an Al container, raising and lowering the temperature to 300 ° C. at 4 ° C./min, It was determined by evaluating the endothermic peak. As a result, it was confirmed that the melting point exceeded 260 ° C. in any sample.

(2) Preparation of flux Each component of the flux other than the polyimide fine particles shown in Table 1 (in the table, the amount used of each component is expressed in parts by weight) is charged into a container, heated to 180 ° C and dissolved. I let you. After cooling, the polyimide fine particles were mixed to obtain a cream solder flux. The addition amount and particle diameter of each polyimide fine particle are as shown in Table 1 (in the table, each numerical value is volume% in the flux composition). A flux not added with polyimide fine particles was used as Comparative Example 4. In addition, the particle diameter in a table | surface observed the synthesized sample with the scanning electron microscope (SEM), and calculated | required the average diameter in the microparticles | fine-particles of a total of 500 or more by image-processing the SEM photograph.

(3) Preparation of cream solder Solder powder (Sn—Ag—Cu alloy having a particle size of 20 to 40 μm, Sn / Ag / Cu content is 96.5 wt% / 3 wt% / 0.5 wt% .) 89 parts by weight and 11 parts by weight of each flux composition prepared by the above method were placed in a container and stirred to prepare a cream solder composition.

(4) Evaluation of cream solder (4-1) Printability Cream solder stored at 25 ° C. for 15 days was continuously printed 20 times on a copper-clad laminate using a stencil mask having a screen thickness of 150 μm. The pattern shape was a hole width of 0.25 mm, a length of 2.0 mm, and a pitch of 0.5, 0.4, and 0.3 mm. Using an optical microscope, the adhesion state of the solder at the mask opening after printing and sagging / bleeding of the printed solder paste were evaluated, and clear solder adhesion to the mask and sagging / bleeding of the transfer pattern were observed. In this case, the printability was poor. A: Defect rate 0%, O: Defect rate exceeding 0% and less than 5%, Δ: Defect rate 5% to less than 15%, X: Defect rate 15% or more (4-2) Void evaluation
As with 4-1, cream solder is printed on a copper-clad laminate on a land pattern with a width of 0.25 mm, a length of 2.0 mm, and a pitch of 0.5, 0.4, 0.3 mm. After mounting the chip parts, it was reflowed in nitrogen. The peak temperature during reflow was 250 ° C. The inside of this sample was observed for the occurrence of voids with an X-ray microscope (three-dimensional X-ray inspection apparatus XVA-160 manufactured by Uniheight System Co., Ltd.). The observed void diameter and the area ratio of the void to the land area were determined by calculating an average value of 20 connections. In addition, NG in the table indicates that the measurement could not be performed because the soldering was not successful.

FIG. 1 is a schematic diagram showing a cream solder printing process. FIG. 2 is a schematic view showing an example of a polymer fine particle synthesis apparatus. FIG. 3 is a scanning electron micrograph of polyamic acid fine particles obtained by the polymer fine particle preparation method shown in the examples.

Claims (2)

A cream containing 0.05 vol% to 10 vol% of at least one spherical polymer fine particle selected from the group consisting of polyimide fine particles, polyurea fine particles, benzoguanamine fine particles, acrylic crosslinked fine particles, and styrene crosslinked fine particles. Solder flux. A cream solder comprising the solder powder and the cream solder flux according to claim 1.

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