Connection structure, and method for manufacturing the connection structure

Abstract

To provide a connection structure capable of suppressing an increase in connection resistance, and a method for manufacturing the connection structure.SOLUTION: A connection structure includes a surface mount component having a first terminal row, a wiring board having a second terminal row corresponding to the first terminal row, and an adhesive layer having air bubbles between the surface mount component and the wiring board and bonding the first terminal row and the second terminal row with solder particles, and the area ratio of air bubbles to the bonding area of the adhesive layer is 17 to 72%. As a result, the stress caused by the difference in linear expansion between the surface mount component and the wiring board due to thermal shock can be alleviated, and an increase in the connection resistance value can be suppressed.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present technology relates to a connection structure in which surface mount components are connected by solder, and a method for manufacturing the connection structure.

Background Art

[0002] The mounting of surface mount components (SMD: Surface Mount Device) on a rigid substrate or a flexible substrate is generally performed by printing solder paste on the substrate, mounting the surface mount components thereon by a mounter, and then performing the mounting using a reflow process.

[0003] In a general solder paste, the absolute amount of solder itself in the paste is large, and there is a limit because solder bridges occur in fine pitch connections. Therefore, in order to reduce the solder remaining between wirings in response to fine pitch, a fine pitch compatible anisotropic conductive adhesive with a reduced solder content and utilizing the self-aggregation performance of solder has been proposed (see, for example, Patent Documents 1 and 2).

[0004] FIG. 4 is a cross-sectional view schematically showing component mounting on a substrate using a conventional anisotropic conductive adhesive containing solder. In the reflow mounting using the conventional anisotropic conductive adhesive 103 containing solder, a large stress is applied to the connection portion due to the linear expansion difference between the surface mount component 101 and the substrate 102, which may lead to lifting between the surface mount component 101 and the adhesive and between the substrate 102 and the adhesive. In addition, there are concerns such as crack generation in the metal bonding portion of the solder.

[0005] Particularly, in a reliability test with mechanical expansion and contraction due to thermal expansion and thermal contraction such as a thermal shock test, the stress during the test cannot be relaxed, which may cause an increase in resistance value.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007] This technology was proposed in light of the conventional situation described above, and provides a connection structure that can suppress the increase in connection resistance, and a method for manufacturing the connection structure. [Means for solving the problem]

[0008] The connection structure relating to this technology comprises a surface mount component having two opposing rows of terminals, a wiring board having two opposing rows of terminals corresponding to the surface mount component, and an air bubble between the surface mount component and the wiring board. Two opposing surface mount components The terminal array and Two opposing parts of the aforementioned wiring board The surface mount component comprises an adhesive layer that joins the terminal rows with solder particles, wherein the area ratio of the air bubbles to the adhesive area of ​​the adhesive layer in the central part between two opposing terminal rows of the surface mount component is 17-72%.

[0009] The manufacturing method of the connecting structure related to this technology involves using an anisotropic conductive adhesive containing solder particles and a foaming component that generates bubbles, It has two opposing rows of terminals. On the wiring board It has two opposing rows of terminals. A placement step for arranging surface-mount components, and a reflow oven set to a temperature above the melting point of the solder particles to form an adhesive layer between the surface-mount components and the wiring board, and the surface-mount components Two opposing terminal rows and the aforementioned wiring board Two opposing terminal rows The bonding process involves joining the surface mount component with the solder particles, and the surface mount component Between two opposing rows of terminals The area ratio of the air bubbles to the adhesive area of ​​the adhesive layer in the central part is 17-72%. [Effects of the Invention]

[0010] According to this technology, the presence of a predetermined amount of air bubbles in the adhesive layer mitigates the stress caused by the difference in linear expansion between surface-mount components and the wiring board due to thermal shock, thereby suppressing the increase in connection resistance. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a cross-sectional view showing an example of the short-side orientation of a plug and receptacle in a vertical mating type connector. [Figure 2] Figure 2 is a schematic cross-sectional view showing the longitudinal direction of a connection structure in which a connector plug and a wiring board are connected. [Figure 3] Figure 3 is a schematic cross-sectional view showing the short-side direction of a connection structure in which a connector plug and a wiring board are connected. [Figure 4] This is a schematic cross-sectional view illustrating the mounting of components onto a substrate using a conventional solder-containing anisotropic conductive adhesive. [Modes for carrying out the invention]

[0012] The embodiments of this technology will be described in detail below with reference to the drawings, in the following order. 1. Connection Structure 2. Manufacturing method of the connecting structure 3. Anisotropic conductive adhesive 4. Examples

[0013] <1. Connection Structure> The connection structure according to this embodiment comprises a surface mount component having a first row of terminals, a wiring board having a second row of terminals corresponding to the first row of terminals, and an adhesive layer having air bubbles between the surface mount component and the wiring board, which joins the first row of terminals and the second row of terminals with solder particles. By keeping the area ratio of the air bubbles to the adhesive area of ​​the adhesive layer within a predetermined range, the stress caused by the difference in linear expansion between the surface mount component and the wiring board due to thermal shock can be mitigated, and the increase in connection resistance can be suppressed.

[0014] The area ratio of the bubbles to the adhesion area of the subsequent layer can be calculated, for example, by cutting the adhesive layer perpendicular to the connection direction, that is, parallel to the terminal surface, and calculating from the observation image obtained by using an electron microscope such as a metal microscope, an optical microscope, or a SEM (Scanning Electron Microscope). Also, the area ratio of the bubbles may be measured using known image analysis software for the observation image. Further, the measurement location is preferably the central part of the surface-mounted component, that is, the so-called belly part. For example, in the case of a connector, it is preferably the insulator part (resin part) where the lead terminals are incorporated.

[0015] The linear expansion coefficient of the wiring board is preferably 1.5 times or more, more preferably 3.0 times or more, and even more preferably 5.0 times or more the linear expansion coefficient of the surface-mounted component. The linear expansion coefficient of the surface-mounted component is, for example, 0.1 to 1 ppm / °C, and the linear expansion coefficient of the wiring board is, for example, 10 to 30 ppm / °C. When the linear expansion coefficient of the wiring board and the linear expansion coefficient of the surface-mounted component are in the above relationship, although mechanical stress is likely to be applied by thermal shock, by applying this technology, the stress can be relaxed and the increase in the connection resistance value can be suppressed.

[0016] The surface-mounted component preferably has a plurality of terminals in the longitudinal direction, and the terminals may face each other in the short-side direction. Also, the ratio of the length in the short-side direction to the length in the longitudinal direction of the surface-mounted component is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. When the surface-mounted component has an aspect ratio within the above range, although a large stress due to the linear expansion difference is likely to be applied in the longitudinal direction by thermal shock, by applying this technology, the stress can be relaxed and the increase in the connection resistance value can be suppressed.

[0017] Surface mount components are not particularly limited as long as they can be joined by solder reflow, and examples include connectors, IC (Integrated Circuit) and LSI (Large Scale Integration) packages, LEDs (Light Emitting Diodes), and switches. Among these, resin-molded connectors with a large aspect ratio and multiple pins in the longitudinal direction are prone to significant stress at the connection point, but by applying this technology, stress can be alleviated and the increase in connection resistance can be suppressed.

[0018] In the following, we will describe the connection structure in detail using a vertical mating type connector as a specific example of a surface mount component. The connector is mainly a resin molded product having a mating portion, and may be, for example, a resin molded product with multiple electrodes protruding longitudinally (a so-called centipede-type connector) or one without protruding electrodes (a flip-chip type connector).

[0019] Figure 1 is a cross-sectional view showing an example of the short-side configuration of a plug and receptacle in a vertical mating type connector. The example connector shown in Figure 1 consists of a receptacle 1 with receptacle terminals 11A and 11B fixed by insulating resin, and a plug 2 with plug terminals 21A and 21B fixed by insulating resin. In the longitudinal direction of the receptacle 1, the receptacle terminals 11A and 11B are formed at a predetermined pitch, and in the longitudinal direction of the plug 2, the plug terminals 21A and 21B are formed at a predetermined pitch.

[0020] Figure 2 is a schematic cross-sectional view showing the longitudinal direction of a connection structure connecting a connector plug and a wiring board, and Figure 3 is a schematic cross-sectional view showing the short direction of the connection structure connecting a connector plug and a wiring board. That is, the longitudinal cross-section shown in Figure 2 is a Y-section perpendicular to the short-direction cross-section shown in Figure 3, and the short-direction cross-section shown in Figure 4 is an X-section perpendicular to the longitudinal cross-section shown in Figure 2.

[0021] As shown in Figures 2 and 3, the connection structure comprises a plug 2 having plug terminals 21A and 21B as a first terminal row, a wiring board 3 having substrate terminals 31A and 31B as a second terminal row corresponding to the plug terminals 21A and 21B, and an adhesive layer 4 that joins the plug terminals 21A and 21B and the substrate terminals 31A and 31B with solder 41.

[0022] Plug 2 comprises plug terminals 21A and 21B, connecting pins 22A and 22B made of bent conductive metal continuous with the plug terminals 21A and 21B, and insulating resin 23 for fixing the plug terminals 21A and 21B and the connecting pins 22A and 22B.

[0023] The insulating resin 23 is made of, for example, polyamide or LCP (Liquid Crystal Polymer), and fixes the plug terminals 21A and 21B and the connecting pins 22A and 22B, for example, by resin molding. Leads are formed at the ends of the plug terminals 21A and 21B, and the plug terminals 21A and 21B constitute a surface-mount type lead array.

[0024] The upper limit of the minimum distance (space distance) between adjacent terminals at plug terminals 21A, 21B and board terminals 31A, 31B is preferably 0.8 mm or less, more preferably 0.3 mm or less, and even more preferably 0.2 mm or less. The lower limit of the minimum distance between adjacent terminals at plug terminals 21A, 21B and board terminals 31A, 31B is preferably 50 μm or more, more preferably 60 μm or more, and even more preferably 70 μm or more. Furthermore, the surfaces of plug terminals 21A and 21B are preferably gold-plated.

[0025] The wiring board 3 is provided with substrate terminals 31A and 31B on the substrate that correspond to plug terminals 21A and 21B. The wiring board 3 can be any type that can be broadly defined as a so-called printed wiring board (PWB), and may be a rigid substrate or a flexible printed circuit board (FPC). Examples of substrates based on substrate type include glass substrates, ceramic substrates, and plastic substrates.

[0026] From an economic standpoint, it is preferable that no short-circuit prevention processing (such as walls or grooves) used in solder resist is formed between adjacent terminals of substrate terminals 31A and 31B. For example, the upper limit of the height of the space between adjacent terminals of substrate terminals 31A and 31B is preferably 100 μm or less of the height of substrate terminals 31A and 31B, more preferably 35 μm or less, and even more preferably 12 μm or less. The lower limit of the height from the terminals between adjacent terminals of substrate terminals 31A and 31B may be the same as the height of substrate terminals 31A and 31B (the space between terminals is horizontal and flush with the substrate surface, there are no protrusions on the terminals, and the connection surface of the substrate is flat), and the substrate terminals 31A and 31B provided on the substrate may protrude from the substrate plane. Furthermore, it is preferable that the surfaces of substrate terminals 31A and 31B are gold-plated.

[0027] The adhesive layer 4 comprises solder 41 that joins the plug terminals 21A, 21B to the substrate terminals 31A, 31B, and air bubbles 42. The adhesive layer 4 is formed by the curing of an anisotropic conductive adhesive, which will be described later. The plug terminals 21A, 21B and the substrate terminals 31A, 31B are joined by solder 41, and an anisotropic conductive adhesive binder is filled between the plug 2 and the wiring board 3. Multiple solder joints may exist within a single substrate terminal 31A, 31B, and bonding points by the binder may exist in areas other than the multiple solder joints. The electrodes of the plug terminals 21A, 21B and the substrate terminals 31A, 31B may be in direct contact with each other, and the binder may hold them in place. In addition, solder particles in the vicinity of the connection between the electrodes may act to assist in this. Solder particles may be sandwiched between electrodes, and after the electrodes are directly connected, solder particles in the surrounding area may melt and contribute to the joining.

[0028] The bubbles 42 are generated by the foaming component in the anisotropic conductive adhesive described later and are formed within the adhesive layer 4. The area ratio of bubbles 42 to the adhesive area of ​​the adhesive layer 4 is 17-72%, preferably 20-70%, and more preferably 30-60%. If the area ratio of bubbles 42 is too small, the stress relief ability due to the difference in linear expansion caused by thermal shock will be low, and if the area ratio of bubbles 42 is too large, the cohesive force of the binder itself will decrease, making it difficult to suppress the increase in connection resistance.

[0029] According to the connection structure of this embodiment, the ratio of the area of ​​air bubbles to the adhesive area of ​​the adhesive layer is within a predetermined range, thereby mitigating the stress caused by the difference in linear expansion between the surface-mount component and the wiring board due to thermal shock, and suppressing the increase in connection resistance. In particular, when a resin-molded connector with a large aspect ratio and multiple pins in the longitudinal direction is mounted, the stress on the connection part can be mitigated, and the increase in connection resistance can be suppressed. Furthermore, the connection body according to this embodiment is connected using solder particles, which are widely used in BGA (Ball Grid Array) and other applications, and has high connection reliability, so it can be applied to many applications such as sensor equipment, in-vehicle equipment, and IoT (Internet of Things) equipment.

[0030] <2. Manufacturing method of connecting structure> The manufacturing method for the connection structure according to this embodiment includes a placement step of placing surface-mount components on a wiring board via an anisotropic conductive adhesive containing solder particles and foaming agent that generates bubbles, and a joining step of curing the anisotropic conductive adhesive using a reflow oven set to a temperature above the melting point of the solder particles to form an adhesive layer between the surface-mount components and the wiring board, and joining the first terminal row of the surface-mount components and the second terminal row of the wiring board with the solder particles. By keeping the area ratio of bubbles to the adhesive area of ​​the adhesive layer within a predetermined range, the stress caused by the difference in linear expansion between the surface-mount components and the wiring board due to thermal shock can be mitigated, and the increase in connection resistance can be suppressed. Here, a connection structure refers to a structure in which two materials or components are electrically connected. Joining refers to connecting two materials or components together.

[0031] [Placement process] In the placement process, surface-mount components are placed on a wiring board via an anisotropic conductive adhesive containing solder particles and a foaming component that generates bubbles. For example, a tool is used to align the first terminal row of the surface-mount component with the second terminal row of the wiring board, and the surface-mount component is mounted on the anisotropic conductive adhesive. The tool is preferably a heated pickup tool and preferably has a suction mechanism for adsorbing the surface-mount component. The anisotropic conductive adhesive may be in paste form or film form. A paste-like adhesive may be made into a form close to a film when mounting the surface-mount component.

[0032] Furthermore, in the placement process, temporary crimping may be performed by pressing from the surface mount component side with a tool. The temporary crimping conditions are preferably 2.0 MPa or less, more preferably 1.5 MPa or less, and even more preferably 1.0 MPa or less.

[0033] When surface-mount components are pressed, the upper limit of the ratio of the average particle size of solder particles to the thickness of the anisotropic conductive adhesive is preferably 1.4 or less, more preferably 1.2 or less, and even more preferably 1.0 or less. By pressing the anisotropic conductive adhesive to a distance close to the solder particle diameter between the first terminal row of the surface-mount component and the second terminal row of the wiring board, the contribution of solder particles to bonding in the reflow process is relatively reduced, and stable conductivity can be obtained.

[0034] Furthermore, during temporary crimping, the anisotropic conductive adhesive may have already begun to cure, may be in a semi-cured state where it has not yet fully cured, or may be fully cured. Additionally, during temporary crimping, a buffer material may be used between the tool and the surface-mount component. Polytetrafluoroethylene (PTFE) or silicone rubber can be used as the buffer material. This further reduces damage to the surface-mount component.

[0035] [Joining process] In the bonding process, a reflow oven set to a temperature above the melting point of the solder particles is used to cure the anisotropic conductive adhesive, forming an adhesive layer containing air bubbles between the surface mount component and the wiring board, and bonding the first terminal row of the surface mount component and the second terminal row of the wiring board with solder particles.

[0036] Reflow ovens allow for heat bonding without mechanical pressure, thus minimizing damage to surface-mount components and circuit boards. Furthermore, compared to anisotropic conductive connections using conventional heating and pressing tools, the absence of unwanted resin flow reduces the likelihood of air bubbles being trapped. Additionally, the lack of pressure reduces solder particle movement, resulting in higher solder particle capture efficiency. Here, "no pressure" refers to a state without mechanical pressure.

[0037] Examples of reflow ovens include atmospheric pressure reflow, vacuum reflow, atmospheric pressure oven, and autoclave (pressure oven). Among these, it is preferable to use a vacuum reflow oven or autoclave, which can eliminate air bubbles contained within the joint.

[0038] The lower limit of the peak temperature (maximum temperature achievable) in the reflow oven is above the temperature at which solder particles melt and above the temperature at which the anisotropic conductive adhesive begins to harden, preferably 150°C or higher, more preferably 180°C or higher, and even more preferably 200°C or higher. The upper limit of the peak temperature in the reflow oven is 300°C or lower, more preferably 290°C or lower, and even more preferably 280°C or lower.

[0039] In a reflow oven, heating melts the anisotropic conductive adhesive, and the main heating, which is above the solder melting point, melts the solder particles sandwiched between the electrodes, wetting and spreading the solder onto the electrodes. Cooling then joins the first terminal row of the surface mount component to the second terminal row of the wiring board. Additionally, foaming components in the anisotropic conductive adhesive generate bubbles, forming bubbles within the adhesive layer. Reflow may include a heating step, a cooling step, and a step to maintain a constant temperature (a keep step). There may also be a peak step with the highest temperature, or the peak step may be included during the heating or cooling process. The heating step may consist of two stages: a step to melt the binder (e.g., up to 120°C) and a step to melt and spread the solder particles (e.g., 120-175°C). Therefore, the heating rate may be, for example, 10-120°C / min or 20-100°C / min. The holding time during the holding process (e.g., 175-180°C) also serves as the process for hardening the binder. This temperature can be anywhere from 160-230°C, with a difference of 5-10°C, or it can be the same as the peak temperature. This time can be selected as appropriate, for example, 0.5 min or more or 0.75 min or more. If it is too long, the manufacturing efficiency will deteriorate, so it should be, for example, 5 min or less or 3 min or less.

[0040] For example, if the binder of an anisotropic conductive adhesive is thermosetting, the resin melting, solder particle placement between terminals, solder melting, and resin curing during the reflow process can be optimized by matching the temperature rise, maintenance, and cooling during the reflow process with the thermosetting behavior of the binder. The thermosetting behavior of the binder can be determined by DSC measurement or viscosity measurement using a rheometer.

[0041] According to the manufacturing method of the connection structure of this embodiment, bubbles are generated by the foaming component in the anisotropic conductive adhesive, and the formation of bubbles in the adhesive layer alleviates the stress caused by the difference in linear expansion between the surface mount component and the wiring board due to thermal shock, thereby suppressing the increase in connection resistance.

[0042] <3. Anisotropic conductive adhesives> The anisotropic conductive adhesive according to this embodiment contains solder particles and a foaming component that generates bubbles, with the foaming component content relative to the binder component of the anisotropic conductive adhesive being 0.3 to 1.5 wt%. This makes it possible to form an adhesive layer in which the area ratio of bubbles to the adhesive area is within a predetermined range, thereby mitigating the stress caused by the difference in linear expansion between surface-mount components and the wiring board due to thermal shock, and suppressing the increase in connection resistance.

[0043] Anisotropic conductive adhesives may be in the form of a film or a paste. Alternatively, a paste can be formed into a film during connection, or a form similar to a film can be achieved by mounting components. In the case of a paste, it is sufficient to uniformly apply a predetermined amount onto the substrate; for example, application methods such as dispensing, stamping, or screen printing can be used, and drying may be permitted as needed. In the case of a film, the amount of bonding material (e.g., anisotropic conductive bonding material) can be made uniform by adjusting the film thickness, and its ease of handling improves work efficiency.

[0044] When the anisotropic conductive adhesive is in film form, the lower limit of the ratio of the average particle size of solder particles to the thickness of the anisotropic conductive adhesive is preferably 0.6 or higher, more preferably 0.8 or higher, and even more preferably 0.9 or higher. If the ratio of the average particle size of solder particles to the thickness of the anisotropic conductive adhesive is large, it becomes easier to sandwich the solder particles between electrodes in the aforementioned arrangement step, but there is a risk that the handling difficulty will increase when it is made into a film.

[0045] The film thickness can be measured using a known micrometer or digital thickness gauge (e.g., Mitutoyo Corporation: MDE-25M, minimum display unit 0.0001 mm) capable of measuring 1 μm or less, preferably 0.1 μm or less. The film thickness can be determined by measuring at 10 or more locations and averaging the results. However, if the film thickness is thinner than the particle diameter, a contact-type thickness gauge is not suitable, and it is preferable to use a laser displacement meter (e.g., Keyence Corporation, spectral interference displacement type SI-T series). Here, film thickness refers to the thickness of the binder resin layer only and does not include the particle diameter.

[0046] The binder for anisotropic conductive adhesives may be thermosetting or thermoplastic, but it is preferable that it be thermosetting, which can be melted and cured by temperature control during the reflow process. The following describes thermosetting binders (insulating binders).

[0047] [Thermosetting binder] The thermosetting binder preferably has an exothermic peak temperature higher than the melting point of the solder particles, and preferably a melting temperature lower than the melting point of the solder particles. Here, the exothermic peak temperature can be measured using a rotary rheometer (manufactured by Thermo Fisher) under the following conditions: measurement pressure 1N, temperature range 30~200℃, heating rate 10℃ / min, measurement frequency 1Hz, and measurement plate diameter 8mm. As a result, the thermosetting binder melts upon heating, and the solder melts with the solder particles sandwiched between the terminals, making it possible to join electronic components equipped with fine-pitch electrodes.

[0048] Examples of thermosetting binders include thermo-radical polymerized resin compositions containing a (meth)acrylate compound and a thermo-radical polymerization initiator, thermo-cationic polymerized resin compositions containing an epoxy compound and a thermo-cationic polymerization initiator, and thermo-anionic polymerized resin compositions containing an epoxy compound and a thermo-anionic polymerization initiator. Known adhesive compositions may also be used. Note that (meth)acrylic monomer includes both acrylic monomers and methacrylic monomers.

[0049] In the following section, we will explain using a thermo-anionic polymerization type resin composition containing a solid epoxy resin, a liquid epoxy resin, and an epoxy resin curing agent as a specific example.

[0050] The solid epoxy resin is not particularly limited as long as it is solid at room temperature and has one or more epoxy groups in its molecule; for example, it may be a bisphenol A type epoxy resin, a biphenyl type epoxy resin, etc. This allows the film shape to be maintained. Room temperature refers to the range of 20°C ± 15°C (5°C to 35°C) as defined in JIS Z 8703.

[0051] The liquid epoxy resin is not particularly limited as long as it is liquid at room temperature; for example, it may be bisphenol A type epoxy resin, bisphenol F type epoxy resin, or urethane-modified epoxy resin.

[0052] The amount of liquid epoxy resin added is preferably 160 parts by mass or less, more preferably 100 parts by mass or less, and even more preferably 70 parts by mass or less, per 100 parts by mass of solid epoxy resin. If the amount of liquid epoxy resin added is too high, it becomes difficult to maintain the film shape. Also, if the amount of liquid epoxy resin added is too high, the cured material after heat curing generally becomes highly elastic due to the high crosslink density, resulting in a reduced stress relief capacity.

[0053] The epoxy resin curing agent is not particularly limited as long as it is a thermosetting agent that begins to cure with heat. Examples include anionic curing agents such as amines and imidazoles, and cationic curing agents such as sulfonium salts. The curing agent may also be microencapsulated to provide resistance to the solvent used during film formation.

[0054] [Solder particles] Solder particles can be appropriately selected from, for example, those specified in JIS Z 3282-1999, such as Sn-Pb, Pb-Sn-Sb, Sn-Sb, Sn-Pb-Bi, Bi-Sn, Sn-Cu, Sn-Pb-Cu, Sn-In, Sn-Ag, Sn-Pb-Ag, and Pb-Ag, depending on the electrode material and connection conditions. Bi-based solder is a highly tough alloy, and although there are concerns about crack formation in the metal bonding area of ​​the solder, applying this technology can alleviate stress, suppress crack formation in the metal bonding area of ​​the solder, and reduce the increase in connection resistance.

[0055] The lower limit of the melting point of the solder particles is preferably 110°C or higher, more preferably 120°C or higher, and even more preferably 130°C or higher. The upper limit of the melting point of the solder particles may be 200°C or lower, preferably 180°C or lower, more preferably 160°C or lower, and even more preferably 150°C or lower. Furthermore, the flux compound may be directly bonded to the surface of the solder particles for the purpose of activating the surface. Activating the surface can promote metallic bonding with the electrode portion.

[0056] The average particle size of the solder particles is preferably 0.2 times or less, more preferably 0.15 times or less, and even more preferably 0.1 times or less, the minimum distance between terminals (space distance) between the first terminal row of the surface mount component and the second terminal row of the wiring board. Based on this relationship between the space distance and the average particle size of the solder particles, the first terminal row of the surface mount component and the second terminal row of the wiring board can be joined using a reflow oven. If the average particle size of the solder particles becomes larger than 0.2 times the minimum distance between terminals between the first terminal row of the surface mount component and the second terminal row of the wiring board, the likelihood of a short circuit increases.

[0057] The lower limit of the average particle size of the solder particles is preferably 0.5 μm or more, more preferably 3 μm or more, and more preferably 5 μm or more. This allows for a consistent film coating thickness. If the average particle size of the solder particles is smaller than 0.5 μm, a good solder joint state cannot be obtained with the electrode portion, and reliability tends to deteriorate. The upper limit of the average particle size of the solder particles may be 50 μm or less, preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0058] The average particle size is the average value of the long axis diameter of particles measured in observation images using a metallurgical microscope, optical microscope, or electron microscope such as a Scanning Electron Microscope (SEM), for example, with N=20 or more, preferably N=50 or more, and more preferably N=200 or more. If the particles are spherical, it is the average value of the particle diameter. Alternatively, the measurement may be obtained by measuring the observation image using known image analysis software ("WinROOF": Mitani Corporation, "A-zo-kun (registered trademark)": Asahi Kasei Engineering Corporation, etc.) or by measuring the measurement using an image-type particle size distribution analyzer (for example, FPIA-3000 (Malvern)) (N=1000 or more). The average particle size obtained from observation images or an image-type particle size distribution analyzer can be the average value of the maximum length of the particles. When manufacturing anisotropic conductive adhesives, manufacturer values ​​such as the particle size at which the cumulative frequency in the particle size distribution obtained by laser diffraction / scattering method reaches 50% (D50) or the arithmetic mean diameter (preferably volume-based) can be used.

[0059] Furthermore, in the case of aggregates formed by the aggregation of multiple solder particles, it is preferable to make the size of the aggregates smaller than the thickness of the anisotropic conductive adhesive. The size of individual solder particles can be determined by the image observation method described above.

[0060] The solder particles are preferably dispersed in the binder, and may be arranged randomly or according to a certain rule. Furthermore, the solder particles may be aggregates formed by the aggregation of multiple particles.

[0061] The lower limit of the mass ratio range for the amount of solder particles is preferably 20 wt% or more, more preferably 30 wt% or more, and even more preferably 35 wt% or more. The lower limit of the volume ratio range for the amount of solder particles is preferably 5 vol% or more, more preferably 10 vol% or more, and even more preferably 15 vol% or more. By satisfying the above-mentioned mass ratio range or volume ratio range for the amount of solder particles, excellent conductivity, heat dissipation, and adhesion can be obtained. When the solder particles are present in the binder, the volume ratio may be used, and when manufacturing anisotropic conductive bonding materials (before the solder particles are present in the binder), the mass ratio may be used. The mass ratio can be converted to a volume ratio from the specific gravity or mixing ratio of the compounds. If the amount of solder particles is too low, excellent conductivity, heat dissipation, and adhesion cannot be obtained, and if the amount is too high, anisotropy is easily impaired, and it becomes difficult to obtain excellent conductivity reliability.

[0062] [Foaming ingredients] The foaming component is preferably one or more selected from the group consisting of toluene, methyl ethyl ketone (MEK), acetone, ethyl acetate, propylene glycol monomethyl ether acetate methoxypropyl acetate (PMA), and isopropyl alcohol (IPA). Alternatively, liquid reactive monomers such as epoxy monomers and (meth)acrylic monomers can be volatilized before the reaction to form the foaming component. Because the foaming component is a volatile component as described above, bubbles can be easily generated during the reflow process.

[0063] The content of the foaming component relative to the binder component is preferably 0.3 to 1.5 wt%. This makes it possible to obtain an adhesive layer in which the area ratio of air bubbles to the adhesive area is within a predetermined range, thereby mitigating the stress caused by the difference in linear expansion between the surface mount component and the wiring board due to thermal shock, and suppressing the increase in connection resistance.

[0064] [Other additives] In addition to the binder, solder particles, and foaming components described above, various additives conventionally used in adhesives can be incorporated into the anisotropic conductive adhesive, as long as they do not impair the effects of the present invention. While it is desirable that the particle size of the additives be smaller than the average particle size of the solder particles, there are no particular limitations as long as they do not hinder inter-electrode bonding.

[0065] The aforementioned anisotropic conductive adhesive can be obtained, for example, by mixing an insulating binder, solder particles, and a foaming component in a solvent, applying this mixture to a release film to a predetermined thickness using a bar coater, and then drying it to evaporate the solvent. Alternatively, the mixture may be applied to the release film using a bar coater and then pressurized to a predetermined thickness. Furthermore, to improve the dispersibility of the solder particles, it is preferable to apply high shear while the solvent is still present. For example, a known batch-type planetary stirring device can be used. The amount of residual solvent in the anisotropic conductive adhesive is preferably 2% or less, more preferably 1% or less.

[0066] According to the anisotropic conductive adhesive of this embodiment, by having a predetermined content of foaming components that generate bubbles, it is possible to form an adhesive layer in which the area ratio of bubbles to the adhesive area is within a predetermined range. This reduces stress caused by the difference in linear expansion between surface mount components and wiring boards due to thermal shock, and suppresses the increase in connection resistance. [Examples]

[0067] <4. Examples> In this embodiment, a connector was mounted on a substrate using an anisotropic conductive film containing solder particles. The resistance value of the mounted assembly was then evaluated both initially and after thermal shock testing. However, this embodiment is not limited to these examples.

[0068] [Fabrication of anisotropic conductive films] An anisotropic conductive film with a thickness of 35 μm was prepared by blending 80 parts by mass of solid epoxy resin (bisphenol F type epoxy resin, Mitsubishi Chemical Corporation, JER4007P), 20 parts by mass of liquid epoxy resin (dicyclopentadiene skeleton epoxy resin, ADEKA Corporation, EP4088L), 5 parts by mass of epoxy resin curing agent (imidazole-based curing agent, Shikoku Chemicals Co., Ltd., Cureazole 2P4MHZ-PW), 3 parts by mass of flux compound (glutaric acid (1,3-propanedicarboxylic acid), Tokyo Chemical Co., Ltd.), and 50 parts by mass of solder particles with an average particle size of 30 μm (MCP-137, 5N Plus inc, Sn-58Bi alloy, solidus temperature 138℃). A predetermined amount (wt%) of highly volatile toluene was added to the binder component of the above blend as a bubble-generating component to adjust the amount of bubbles.

[0069] [Connector implementation] The connector components and circuit board described below were prepared. An anisotropic conductive film was placed on the circuit board, the plug connector was mounted according to the wiring on the circuit board, and the plug connector was mounted through a reflow process under the reflow conditions described below. Connector component: 10 pins on one side (20 pins on both sides), 0.35mm pitch plug connector, Hirose Electric Co., Ltd., BM23FR0.6-20DP-0.35V(895) Circuit board: Rigid circuit board compatible with the above plug connector (Dexerials evaluation glass epoxy circuit board, Ni-Au plated) Reflow conditions: 150°C to 260°C for 100 seconds, peak top set to 260°C.

[0070] [Percentage of bubble area after curing of anisotropic conductive film] As a method for evaluating the amount of air bubbles after implementation, the material was cut in the Z direction as shown in Figure 3, the cross-section was observed, the area of ​​air bubbles in the hardened binder was measured, and the percentage was calculated.

[0071] [Evaluation of resistance values ​​of the implemented object] The continuity resistance between the connector terminals and the PCB terminals in the reflow-processed assembly was measured both initially and after thermal shock testing. The thermal shock test conditions were -55°C for 30 minutes ⇔ 125°C for 30 minutes, repeated 100 times. The continuity resistance between the plug connector terminals (upper terminals) and the PCB terminals after assembly was measured and evaluated based on the maximum increase in resistance before and after the thermal shock test. The evaluation criteria for A to C were as follows. A: The maximum resistance increase is 30 mΩ or less. B: The maximum resistance increase is greater than 30mΩ but less than or equal to 50mΩ. C: Maximum resistance increase exceeds 50 mΩ

[0072] [Evaluation of the handling properties of anisotropic conductive films] In accordance with JIS Z 0237 "Test Methods for Adhesive Tapes and Sheets," a tack tester (TACII, Resca Co., Ltd.) was used to measure tack force in an atmosphere of 22°C. The probe was pressed against an anisotropic conductive film under the following measurement conditions: probe diameter 5 mm (stainless steel, mirror finish, cylindrical), pressing load 196 kgf, pressing speed 30 mm / min, and peeling speed 5 mm / min. The peak intensity on the measurement chart was defined as the tack force (kPa). A tack force of less than 10 kPa was judged as "OK," and a tack force of 10 kPa or less was judged as "NG."

[0073] Table 1 shows the evaluation results of the implementations for the examples and comparative examples.

[0074] [Table 1]

[0075] As in Comparative Examples 1 and 2, when the proportion of air bubble area in the binder after curing is 8% or less, that is, when the proportion of volatile components in the binder before curing is 0.2 wt% or less, The stress relief performance due to the difference in linear expansion caused by thermal shock was low, and the increase in resistance value after the thermal shock test was large.

[0076] Furthermore, as in Comparative Examples 3 and 4, when the proportion of air bubble area in the binder after curing was 82% or more, that is, when the proportion of volatile components in the binder before curing was 2.0 wt% or less, the handling properties of the anisotropic conductive film deteriorated, and the increase in resistance value after the thermal shock test was significant. This is presumed to be because the cohesive force of the binder itself decreased due to the large number of air bubbles, thus reducing its ability to suppress the increase in resistance value.

[0077] On the other hand, as in Examples 1-5, the proportion of the air bubble area in the binder after curing is 17-72 wt% In that case, that is, the proportion of volatile components in the binder before hardening is 0.3~1.5% In this case, the anisotropic conductive film exhibited good handling properties, and the increase in resistance after thermal shock testing was suppressed. This is presumed to be because the appropriate amount of air bubbles enhances the stress-relieving properties of the binder, thereby mitigating the stress caused by the difference in linear expansion due to thermal shock.

[0078] As described above, by incorporating volatile components into the binder to generate air bubbles during the process of mounting the anisotropic conductive adhesive, and by positioning these air bubbles in the cured binder after mounting, the stress relief performance was enhanced, and the increase in resistance value after reliability testing was suppressed. [Explanation of symbols]

[0079] 1 Receptacle, 2 Plug, 3 Wiring board, 4 Adhesive layer, 11A, 11B Receptacle terminals, 21A, 21B Plug terminals, 22A, 22B Connection pins, 23 Insulating resin, 31A, 31B Board terminals, 41 Solder, 42 Air bubbles, 101 Surface mount component, 102 Circuit board, 103 Anisotropic conductive adhesive

Claims

1. A surface mount component having two opposing rows of terminals, A wiring board having two opposing terminal rows corresponding to the surface-mount component, The surface mount component and the wiring board have air bubbles between them, and the adhesive layer joins two opposing terminal rows of the surface mount component and two opposing terminal rows of the wiring board with solder particles, A connection structure in which the area ratio of the air bubbles to the adhesive area of ​​the adhesive layer in the central part between two opposing terminal rows of the surface mount component is 17 to 72%.

2. The connection structure according to claim 1, wherein the coefficient of thermal expansion of the wiring board is 1.5 times or more the coefficient of thermal expansion of the surface mount component.

3. The connection structure according to claim 1 or 2, wherein the surface mount component has a plurality of terminals in the longitudinal direction.

4. The connection structure according to any one of claims 1 to 3, wherein the surface mount component is a resin-molded connector having a ratio of its length in the short direction to its length in the longitudinal direction of 2 or more, and having a plurality of pins in the longitudinal direction.

5. A placement step of placing a surface mount component having two opposing terminal rows on a wiring board having two opposing terminal rows via an anisotropic conductive adhesive containing solder particles and a foaming component that generates bubbles, The process includes forming an adhesive layer between the surface mount component and the wiring board using a reflow oven set to a melting point above that of the solder particles, and joining two opposing terminal rows of the surface mount component and two opposing terminal rows of the wiring board with the solder particles. A method for manufacturing a connection structure, wherein the area ratio of the air bubbles to the adhesive area of ​​the adhesive layer in the central part between two opposing terminal rows of the surface mount component is 17 to 72%.

6. The method for manufacturing a connection structure according to claim 5, wherein the coefficient of thermal expansion of the wiring board is 1.5 times or more the coefficient of thermal expansion of the surface mount component.

7. The method for manufacturing a connection structure according to claim 5 or 6, wherein the surface mount component has a plurality of terminals in the longitudinal direction.

8. The method for manufacturing a connection structure according to any one of claims 5 to 7, wherein the surface mount component is a resin-molded connector having a ratio of its length in the short direction to its length in the longitudinal direction of 2 or more, and having a plurality of pins in the longitudinal direction.

9. A method for producing a connecting structure according to any one of claims 5 to 8, wherein the foaming component is one or more selected from the group consisting of toluene, methyl ethyl ketone (MEK), acetone, ethyl acetate, propylene glycol monomethyl ether acetate methoxypropyl acetate (PMA), and isopropyl alcohol (IPA).

10. A method for producing a connecting structure according to any one of claims 5 to 8, wherein the foaming component is a volatile reactive monomer.

11. A method for manufacturing a connecting structure according to any one of claims 5 to 10, wherein the content of the foaming component relative to the binder component of the anisotropic conductive adhesive is 0.3 to 1.5 wt%.

12. A method for manufacturing a connecting structure according to any one of claims 5 to 11, wherein the solder particles are an alloy containing Bi.

13. A method for manufacturing a connecting structure according to any one of claims 5 to 12, wherein the anisotropic conductive adhesive is in the form of a film.

14. It contains solder particles, a foaming component that generates bubbles, and a binder. The content of the foaming component in the binder is 0.3 to 1.5 wt%, A reflow connecting film, wherein the area ratio of the air bubbles to the adhesive area of ​​the hardened adhesive layer of the binder in the central portion between two opposing terminal rows of a surface mount component is 17 to 72%.

15. The connecting film according to claim 14, wherein the solder particles are an alloy containing Bi.

16. The connecting film according to claim 14 or 15, wherein the foaming component is one or more selected from the group consisting of toluene, methyl ethyl ketone (MEK), acetone, ethyl acetate, propylene glycol monomethyl ether acetate methoxypropyl acetate (PMA), and isopropyl alcohol (IPA).

17. The connecting film according to claim 14 or 15, wherein the foaming component is a volatile reactive monomer.

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