JP2006320943A - Solder paste and solder printing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To conduct precoating by soldering on a fine-pitched pattern without generating a solder bridge or a non-soldered section. <P>SOLUTION: A solder paste contains a solder powder, a filler having a melting point higher than that of the solder powder and being secondarily aggregated into the form of sea urchins, and a flux. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a solder paste precoated on a pad formed on the surface of an insulating substrate such as a printed wiring board, and a solder printing method using the solder paste.

  Conventionally, in order to mount an electronic component on a substrate, a solder paste is individually supplied to each of the soldering electrodes of the substrate by printing using a screen plate opened for each electrode, and an electrode portion of the electronic component is mounted on the reflow furnace. Generally, the solder paste is heated and melted by passing it through and soldered together. By the way, the miniaturization of electronic parts is not known, and in recent years, flip chip mounting in which a semiconductor bare chip obtained by applying Au stud bumps or plating bumps to a semiconductor bare chip is directly mounted on a substrate has been actively performed. NCP, ACP All the methods using bonding materials such as ACF are mechanical pressure welding methods. Especially when the thickness of the semiconductor bare chip is less than 200μm, the characteristic shift and reliability after flip chip mounting due to excessive pressure and heat during pressure welding Therefore, it is an obstacle to miniaturization and thinning.

  On the other hand, when flip-chip mounting is performed on an electrode pre-soldered on a substrate, almost no pressure is required and mounting is possible only by heating. Therefore, the electrode pitch is an ultrafine pitch of 100 μm or less and from several hundreds. Thousands of pin-class flip chip mounting is considered to be an effective method that can ensure miniaturization, thinning, and stability of characteristics.

  However, it is very difficult to stably supply solder to individual mounting electrodes having a pitch of 100 μm or less by a printing method. In particular, when the pitch is 70 μm or less, the dimensional tolerance of the substrate, the dimensional tolerance of the screen plate, and the printing machine It is almost impossible to manage all of the positioning tolerances.

  As a technique for solving the above problems, there is a plating method. However, the plating method cannot supply a sufficient amount of solder necessary for soldering electronic components. In addition, when a material composed of several kinds of metal composition such as solder is deposited by plating, the metal composition ratio varies, so that the melting point is not stable. Therefore, the solder deposited by this method is not suitable as a bonding material.

  Japanese Patent Application Laid-Open No. 63-114979 describes a method of depositing solder on a wide area including a portion other than an electrode and selectively depositing the solder on the electrode. The method described in the publication does not require a special printing machine or technology, so it is excellent in mass productivity, but because of the chemical precipitation method, the ease of precipitation differs depending on the metal involved in the reaction. Then, it can be put into practical use only with some metal composition solders. In addition, since solder cannot be easily deposited on preflux other than Au plating or chemically rust-prevented electrodes, it is necessary to expose the surface of the clean metal by acid cleaning. If these processes and equipment are not provided, equipment for acid treatment equipment, washing equipment and washing water treatment is necessary, which increases the manufacturing process of printed circuit boards and also increases costs. Connected. Also, since the oxygen concentration in the reflow is required to be around 100 ppm, an expensive and high-performance nitrogen atmosphere compatible reflow furnace and nitrogen supply device are required, and the solder material itself is very expensive and the cost of the product is increased. Leads to.

  Japanese Patent Laid-Open No. 6-326453 discloses that after applying a chemical that reacts only with the electrode metal to produce adhesiveness, it is washed with water and dried, and then the solder powder is sprinkled onto the electrode to obtain the adhesiveness. After fixing the solder, a method is described in which excessive solder powder is removed, a strong active flux is sprayed, and the solder is precoated by reflow heating. In such a method, in addition to expensive dedicated equipment and a special water-soluble organic compound, an acid treatment process is required. Therefore, management man-hours, waste water treatment equipment and waste liquid treatment are necessary, and the manufacturing process is multi- In addition to incurring complexity, if you do not have such equipment, including new equipment investment and environmental conservation costs, manufacturing costs will be incurred and it will not be possible to supply products at low cost.

  Japanese Patent Laid-Open No. 3-50853 proposes a method of applying a solder powder having a solder powder surface coated with a flux to a substrate, but cannot cope with a fine pattern whose pitch is extremely narrowed.

  Japanese Patent Application Laid-Open No. 4-10694 proposes a method in which, after supplying flux to a substrate, solder powder is sprinkled, the solder powder is heated to a melting point or higher and melted, and then excess molten solder is blown off with an air knife. However, there is almost no solder by blowing away, leaving only the amount of solder that is close to plating without thickness, and there is a risk of short-circuiting on a substrate with a fine pitch pattern, and it is a very dangerous operation Therefore, practical use is difficult.

  Furthermore, in Patent No. 3169719, an irregularly shaped filler is used. However, as a result of confirmation at each pitch of 120 μ, 100 μ, 70 μ, and 60 μ in a verification experiment, a pitch interval of about 120 μ is the limit for simple amorphous and spherical shapes. However, at a pitch less than this, an effect of suppressing the bridge was recognized, but an unsoldered area was observed, and it was impossible to achieve compatibility with the wettability of the solder.

Japanese Unexamined Patent Publication No. 63-114979 JP-A-6-326453 Japanese Patent Laid-Open No. 3-50853 Japanese Patent Laid-Open No. 4-10694 Japanese Patent No. 3169719

  Accordingly, an object of the present invention is to provide a solder paste and a solder printing method capable of performing solder pre-coating without generating solder bridges and unsoldered areas on a fine pitch pattern.

  In order to solve the above-described problems, a solder paste according to the present invention contains solder powder, a filler having a melting point higher than the melting point of the solder powder and secondarily aggregated in a sea chestnut shape, and a flux. Is.

  Moreover, the solder printing method according to the present invention includes a solder powder, a filler having a melting point higher than the melting point of the solder powder and a secondary agglomeration in a sea chestnut shape on the electrode formed on the substrate, a flux, The substrate is exposed to a temperature not lower than the melting point of the flux and lower than the melting point of the solder powder for a predetermined time, and then the substrate is not lower than the melting point of the solder powder and higher than the melting point of the filler. Is exposed to a low temperature for a predetermined time, and then the flux residue is removed.

  According to the solder paste and the solder printing method according to the present invention, by heating the substrate on which the solder paste is printed, the filler that is secondarily aggregated in a sea chestnut shape is formed into a three-dimensional network without overlapping densely. Therefore, the flux can convect between the fillers, and the powdered solder metal having a high specific gravity can settle between the electrodes and advance the pre-coating. Therefore, it is possible to ensure solder wettability and prevent the occurrence of unsoldered areas while suppressing solder bridges.

  Hereinafter, a solder paste and a solder printing method to which the present invention is applied will be described in detail with reference to the drawings. This solder paste uses QFP (quad flat package), SOP (small outline package), and other electronic components and flip chips with L- and gull-wing lead terminals protruding around the package on insulating substrates such as interposers and motherboards. When the surface mounting is performed, printing is performed to pre-coat the electrodes such as pads and lands formed on the insulating substrate with solder. The pads and lands have a fine pitch of 100 μm or less in response to the recent demand for smaller and thinner electronic components. A solder paste is printed on the pad or land by, for example, screen printing.

  The solder paste to which the present invention is applied is formed by blending powdered solder metal, a filler formed by agglomerating organic or inorganic materials, and flux, and kneading these. The composition of the powdered solder metal is not particularly limited, but those having a particle size of 10 μm or less are used.

  The filler is a material having a melting point higher than the melting point of the solder metal. For example, resin powder, glass powder, mica powder, alumina powder or the like having heat resistance that does not melt or decompose even at 250 ° C. or higher is used. Further, the filler is used by secondary agglomeration, that is, irreversibly agglomeration of these powdery materials into a sea chestnut shape having needle-like protrusions around the filler. The filler has an average particle size of 10 μm or less and a composition that does not fuse or react with the flux or the molten solder metal. In addition, as such a sea chestnut-shaped filler, Kao: Lunamos SP etc. can be used, for example.

  As the flux, a general flux in which a thixotropic agent or a thickener is blended with an active rosin or organic acid, a halogen-based or amine-based active agent is used. In addition, according to the flux in which the rosin and the thixotropic agent which are hard to be heated are blended, the solder bridge can be effectively suppressed in the reflow process.

  The solder paste to which the present invention is applied has a solder metal content of 10 to 30% by weight, an agglomerated filler content of 40% by weight or less, and the remainder contains a flux. The solder metal content is adjusted with a flux containing an activator and a thickener. The composition of the solder metal is not required and can be selected according to the purpose of the user. The content of the agglomerated filler varies depending on the particle size, specific gravity, and particle size distribution of the powder constituting the filler. However, if the content is more than 40% by weight, the convection of the flux in the softening temperature region of the flux is almost eliminated, and the solder metal is reduced. This is because the electrode (pad, land) formed on the insulating substrate cannot be contacted, and an unsoldered area is formed on the electrode.

  Such solder paste is printed with a certain thickness on the electrode portion of the substrate by screen printing or a dispenser, and is heat-treated in a reflow furnace. By heating the solder paste, it is possible to prevent the filler in the solder paste from dispersing in the flux and aggregating and enlarging the melted solder metals. Therefore, pre-coating can be performed by allowing molten solder to settle through the gap between the filler or flux having a low specific gravity and contact the electrode.

  Hereinafter, a printing method to which the present invention is applied will be described by taking as an example a process of printing a solder paste on a component mounting portion specifically formed on an interposer substrate. As shown in FIG. 1, a mounting portion 2 on which QFP is mounted is provided on the surface of an interposer substrate 1. The mounting portion 2 is formed by etching a copper foil or the like to form a plurality of pads 3 serving as terminal electrodes on the side of the interposer substrate 1 according to terminals protruding around the QFP. The pad 3 has a pad pitch of 100 μm or less in accordance with electronic components such as QFP and downsizing of the substrate. The interposer substrate 1 is coated with a solder resist 4 for preventing the solder paste from adhering to the region excluding the mounting portion 2.

When this interposer substrate 1 is printed with a solder paste 7 to which the present invention is applied, as shown in FIGS. 2 and 3, a metal screen 5 in which an opening 6 corresponding to the mounting portion 2 is formed is attached. Is done. Next, as shown in FIG. 4, the interposer substrate 1 is supplied with the solder paste 7 on the metal screen 5 and is stretched by the squeegee 8 to fill the opening 6 with the solder paste 7. Thereafter, the interposer substrate 1 is subjected to a normal reflow process, heated in a reflow furnace, and solder pre-coating on the pads 3 is performed.
FIG. 5 shows a heating profile when using a solder paste of Sn / 3Ag / 0.5Cu (3.0% Ag: 0.5% Cu: Sn residue) as an example of the composition of the solder metal.

  Specifically, the solder paste 7 becomes liquid and causes rapid convection when the flux reaches around 80 ° C. which is the softening temperature during reflow heating. At this time, powdery solder metal having a specific gravity heavier than that of the filler settles on and between the pads 3. Next, when moving to a preheating step of holding 100 ° C. to 150 ° C. for a certain time (90 seconds to 150 seconds here), volatile components contained in the flux are lost, and the filler and solder metal are included in the flux with increased viscosity, Solder metal and filler are fixed. That is, the filler and the solder metal cannot be moved by the flux having increased viscosity, and the solder metal powder is prevented from agglomerating to become a solder lump.

  At this time, the filler projections aggregated in a sea chestnut shape are entangled with each other to form a three-dimensional network structure, so that the flux can flow without densely overlapping the fillers. Accordingly, the pre-coating proceeds by the solder metal existing on the pad 3 and the solder metal settled between the pad electrodes. Next, the process proceeds to a heating process in which 250 ° C. is maintained for a certain time (here, 25 seconds to 40 seconds). At 250 ° C., the solder metal melts in the solder paste, but the filler does not melt. Then, only the solder metal precoated on the pad 3 reacts with the pad and melts to selectively form a molten solder layer on the pad 3 as shown in FIG. On the other hand, the solder metal that cannot contact the pad 3 is included in the flux having increased viscosity as described above, and the occurrence of solder bridges is suppressed because the metal powder is prevented from agglomerating and becoming a lump of solder. The

  Furthermore, since the filler aggregated in a sea chestnut shape has a large surface area, it has a large ability to adsorb substances. Therefore, in the heat process, the flux is adsorbed and dried to generate innumerable micropores, from which nitrogen enters and the flux's active action can be maximized. Sex can be secured.

  After the heating step, the interposer substrate 1 is washed with an alcohol or glycol ether solvent, and as shown in FIG. 7, the residue such as solder metal that is not absorbed on the pad 3 and remains with the filler is removed. Solder pre-coating is performed on electrodes such as pads and lands formed on the substrate as described above. Thereafter, terminals of electronic components such as QFP and flip chip are mounted on the solder precoated electrodes, and the solder coated on the electrodes is melted and solidified by being subjected to a reflow process, and the electronic components are mounted. .

  As described above, according to the solder paste to which the present invention is applied and the solder printing method using the solder paste, the substrate on which the solder paste is printed is heated in a reflow furnace, thereby secondary aggregation into a sea chestnut shape. Since the filler being deposited is deposited in a three-dimensional network without densely overlapping, the flux can convect between the fillers, and the powdered solder metal having a high specific gravity settles between the pad electrode and the pad electrode, The precoat can be advanced. Therefore, it is possible to ensure solder wettability while preventing bridges and prevent the occurrence of unsoldered areas.

Next, an embodiment of a solder paste and a solder printing method to which the present invention is applied will be described. In this example, the following were used as the composition of the solder metal constituting the solder paste, the substrate, the screen plate, the heating profile, the atmosphere in the reflow furnace, and the cleaning conditions.
Solder powder: Sn / 3Ag / 0.5Cu (with a particle size of 10 μm or less, average particle size of 7 μm)
Filler: Silica (Agglomerated in chestnut shape, average particle size 2.5 μm, specific gravity 2.6)
Flux: RA type substrate:
Pad pitch 60μm (L / S = 30/30)
FR4 copper electrode (no preflux treatment, no rust treatment)
192-pin × 8-sided metal screen: thickness t = 50 μm
Heating profile:
Preheat 150 ° C./90 seconds Heat 245 ° C./35 seconds Reflow oven atmosphere: nitrogen atmosphere with oxygen concentration of 1000 ppm Cleaning solution: ultrasonic cleaning with ethanol Under the above conditions, the composition of solder powder, filler and flux constituting the solder paste The ratio is fixed at a filler content of 15%, and the solder bridging rate and the unsoldered region rate of solder pre-coated on the pad in 5 wt% increments from 10 wt% to 40 wt% of the solder powder content It was measured. The measurement results are shown in Table 1 below. In this measurement, the flux content was the remainder excluding the solder powder and filler contents in Table 1. Further, as the filler content, a value in which the aggregation of the solder powder was suppressed and the bridge was reduced in the hit test was adopted.

  As shown in Table 1, it was recognized that the content of the solder powder has an appropriate value when the solder bridge generation rate is less than 30% by weight where the generation rate is rapidly decreased.

  Next, in order to fix the content of the solder powder having a solder bridge occurrence rate of 0% and fix the content of the filler, the filler content is reduced from 6 wt% to 14 wt% in increments of 1 wt%. The solder bridge occurrence rate of the solder precoated on the pad and the occurrence rate of the unsoldered area were measured. The measurement results are shown in Table 2 below. In this measurement, the flux content is the remainder excluding the solder powder and filler contents in Table 2, and other solder metal composition, substrate, screen plate, heating profile, reflow atmosphere and cleaning conditions. Was the same as above.

  As shown in Table 2, it was recognized that the filler content had an appropriate value in the range of 7% by weight to 9% by weight where the occurrence rate of the unsoldered region was low.

  Based on the above results, based on the combination of the content of the solder powder and filler, in which appropriate values were observed in the occurrence rate of solder bridges and the occurrence rate of unsoldered areas, the occurrence of solder bridges at the mixing ratios shown in Table 3 below The rate and the rate of occurrence of unsoldered areas were measured. In this measurement, the flux content is the remainder excluding the solder powder and filler contents in Table 3, and other solder metal composition, substrate, screen plate, heating profile, reflow atmosphere and cleaning conditions. Was the same as above.

  As shown in Table 3, optimum values were obtained for the solder powder content: 26.5 ± 5 wt%, the filler content: 7.5 ± 5 wt%, and the flux: the remainder.

  In this embodiment, a metal screen with a thickness of 50 μm was used with a substrate with a pad pitch of 60 μm. However, by changing the mixing ratio of solder powder, filler and flux, substrates with different pad pitches and metals with different thicknesses were used. Even when a screen is used, it is possible to optimize the occurrence rate of solder bridges and unsoldered areas.

  In addition to screen printing, a solder paste may be supplied onto the substrate by a dispensing method.

It is a top view which shows the board | substrate with which the electrode on which the solder paste concerning this invention was printed was formed. It is a top view which shows the board | substrate with which the screen plate was stuck on the surface. It is sectional drawing which shows the board | substrate with which the screen plate was stuck on the surface. It is sectional drawing which shows the board | substrate with which the mounting part was filled with the solder paste. It is a graph which shows the heating profile of a board | substrate. It is sectional drawing which shows the board | substrate with which the solder paste was pre-coated on the pad. It is sectional drawing which shows the board | substrate by which the residue which remains in a mounting part was wash | cleaned.

Explanation of symbols

1 Interposer substrate, 2 mounting part, 3 pad, 4 solder resist, 5 metal screen, 6 opening, 7 solder paste, 8 squeegee

Claims (8)

Solder powder,
A filler having a melting point higher than the melting point of the solder powder and secondary agglomerated in a sea chestnut shape;
With flux,
Containing solder paste.   The solder paste according to claim 1, wherein the solder powder has an average particle size of 10 μm or less, and the filler has an average particle size of 10 μm or less.   3. The solder paste according to claim 2, wherein the blending ratio of the solder powder and the filler is solder powder: filler = 26.0-27.0% by weight: 7.0-8.0% by weight. .   The solder paste according to claim 1, wherein the filler has a composition that does not fuse or react with the flux and the molten solder powder. On the electrode formed on the substrate, a solder paste containing a solder powder, a filler having a melting point higher than the melting point of the solder powder and secondarily aggregated in a sea chestnut shape, and a flux is printed.
Exposing the substrate to a temperature above the melting point of the flux and lower than the melting point of the solder powder for a predetermined time;
Next, the substrate is exposed for a predetermined time at a temperature higher than the melting point of the solder powder and lower than the melting point of the filler,
Next, the flux residue is removed.
Solder printing method.   6. The solder printing method according to claim 5, wherein the average particle size of the solder powder is 10 [mu] m or less, and the average particle size of the filler is 10 [mu] m or less.   7. The solder printing according to claim 6, wherein a mixing ratio of the solder powder and the filler is solder powder: filler = 26.0-27.0 wt%: 7.0-8.0 wt%. Method. 6. The solder printing method according to claim 5, wherein the filler has a composition that does not fuse or react with the flux and the molten solder powder.

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