Method of mounting an electric part on a circuit board

Abstract: A paste-like composition containing a lead salt of an organic acid and tin powder is applied on a pad array portion of a circuit board. Then, the paste-like composition is heated so as to cause precipitation, thereby forming a solder layer formed of an Sn-Pb alloy substantially only on pads. This precipitation is performed in a state that a liquid pool is formed on the pad array portion when the paste-like composition is liquefied by heating, and the tin powder is settled in the liquid pool. When an electronic part is to be mounted on the pads, first, preparatory solder layers are formed on the pads by the above-mentioned precipitation process. After a paste-like composition is applied on the preparatory solder layers, the electronic part is put on the paste-like composition. Then, the paste-like composition is heated, thereby soldering leads of the electronic part with the pads.

Description: This invention relates to a method of mounting an electronic part on a circuit board. In general, before mounting electronic parts on a circuit board, a thin solder layer is formed on pads of the circuit board to prevent the pads from being oxidized and to improve the wettability of solder used in mounting elements. (The term "pads" used here is a general term intended to designate a part-mounting portion including, for example, a through hole to which parts are mounted, and the term will be used hereinafter in this general meaning). The thin solder layer is formed by a hot gas leveling method, an electroplating method, or the like. In the hot gas leveling method, first, a circuit board is dipped in melted solder to apply the solder to a pad array formed on the board. After drawing up the board from the melted solder, a hot gas is sprayed on the board before the solder is hardened, thereby blowing off superfluous solder from the pads and the gap between adjacent pads. As a result, thin solder layers are formed on the pads only. This method is disadvantageous in that it is difficult to form a solder layer having a uniform thickness. The thickness of a solder layer tends to vary widely. Particularly, if the pitch of the pads is fine, bridging (conduction between the pads through the solder) occurs easily. To avoid bridging, it is necessary to form a thin solder layer. However, if a portion having a thickness of 1 µm or less is generated due to a variety of the thickness of a solder layer, an intermetallic compound layer such as Cu₃Sn formed between copper pads and the solder layer may be exposed, and the surface of the solder layer may be oxidized. Hence, the wettability of solder is greatly reduced at the time of mounting parts in a next step. On the other hand, the electroplating method is advantageous in that a solder layer can be formed on a fine pattern. However, it is necessary to heat and melt the solder layer after the electroplating, since an intermetallic compound layer for adhering copper foil of the pads with the solder layer is not formed. In the heating step, a blister of the resist of the circuit board and the above-mentioned bridging tend to occur. Moreover, the manufacturing cost is twice that required in the hot gas leveling method. When electronic parts are to be mounted on both sides of a circuit board, first, solder layers are formed on pads of both sides, and then parts are mounted on one side thereafter on another side (it is impossible to mount parts on both sides simultaneously). When parts are mounted to the solder layer on a first side, the solder layer on a second side on which parts are to be mounted later is heated to a high temperature in a reflow furnace process. At this time, if the solder layer on the second side has a portion where the thickness is too small, intermetallic compound layer may be exposed, then, oxidation occurs at the portion, and the wettability of the solder layer tends to be reduced. Therefore, a method of forming a preparatory solder layer having a relatively large thickness uniformly is demanded. In recent years, the lead pitches of electronic parts have been reduced to 0.8mm, 0.65mm, or 0.5mm in order to make the electronic device lighter, thinner, and more compact. Electronic parts having lead pitches of 0.36mm, 0.3mm, or 0.15mm have been devised. The conventional methods such as printing for applying a soldering paste on each pad can be applied to a part of a lead pitch of about 0.5mm or larger. However, it is very difficult to supply a soldering paste in proper quantity on a individual pad. If the quantity of a soldering paste is too much, bridging tends to be formed between adjacent pads. On the other hand, if the quantity of a soldering paste is too little, it is impossible to mount a part on a circuit board. Hence, it is demanded that a part mounting method which can be applied to a part having leads of a fine pitch is developed. EP-A-0 307 888 discloses a method of forming a solder layer on pads of a circuit board, comprising the steps of: applying a paste-like composition containing a lead salt of an organic acid and tin powder on a pad array portion on the circuit board; and heating the composition to cause a solder precipitation, thereby forming solder layers formed of an Sn-Pb alloy substantially only on pads; said solder precipitation for forming a solder alloy being performed in a state that a liquid pool is formed on the pad array portion when the paste-like composition is liquefied by heating, and the tin powder is settled in the liquid pool. In this prior art method, a preparatory tin layer may be formed. The prior art method may be used for surface mounting of electronic components having very small lead pitches on printed circuit boards. The object of the present invention is to improve the method of mounting a part on a circuit board which can be applied in mounting an electronic part having leads of a fine pitch. According to the present invention, there is provided a method having the features indicated in claim 1. Preferred embodiments of the invention are indicated in the dependent claims. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a plane view showing a part of a circuit board used in the method according to the present invention; Fig. 2 is a sectional view showing a pad with a solder layer having a half circle-like section; and Fig. 3 is a diagram showing a relationship between the width of a pad and the thickness of a solder layer precipitated thereon. The present inventors have proposed a method of precipitating solder formed of an Sn-Pb alloy on pads only (Published Unexamined Japanese Patent Application No. 1-157796). In the method, a paste-like composition containing a lead salt of an organic acid and tin powder is applied on a pad array on a circuit board, and the composition is heated. As a result, an Sn-Pb solder is precipitated substantially only on pads. The method of forming a solder layer used here is based on the above-mentioned method of precipitating solder, and characterized in that the state of a paste-like composition for use in precipitating solder is defined. More specifically, when the paste-like composition is heated and liquefied due to the heat, a liquid pool in which tin powder is settled is formed on a pad array portion, and the precipitation of solder is performed in a state that the tin powder is settled in the liquid, that is, the powder is covered by the liquid. According to the method used here, a relatively thick solder layer is formed uniformly on a fine pattern of a pad pitch of 0.5mm or smaller. This method was first devised by the present inventors as a result of their research. In general, when a solder layer is formed by applying melted solder on pads of a circuit board, a layer of a compound formed of Cu and Sn (an intermetallic compound such as Cu₃Sn, or Cu₆Sn₅) is formed to a thickness of 1 to 2 µm on the surface of the pads made of copper foil. Hence, to prevent the compound layer from being exposed, it is desirable that the solder layer be 5 µm thick or more. In contrast to the conventional method in which a solder layer is formed by remelting an Sn-Pb alloy, the method used here is based on the following principles: in a high temperature solution containing tin powder and a lead salt of an organic acid, Sn and Pb are replaced by each other due to an ionization level difference between Pb and Sn, thereby precipitating Pb; and the precipitated Pb and tin powder are melted in atomic level, thereby constituting an Sn-Pb alloy. Remelting of a conventional cream solder used in general occurs instantaneously as the temperature rises, and the melted solder swells due to surface tension. According to the method used here, since the precipitation reaction requires a relatively long period of time, and therefore solder is precipitated gradually, a solder layer having a uniform thickness can be formed. The solder precipitating composition mainly comprises a lead salt of an organic acid and tin powder for forming a solder alloy, at least one kind of rosins and amines, and a viscosity modifier for keeping the blend in a paste state. These materials are kneaded thereby forming a paste in room temperature. The paste is applied on the pad array portion of a circuit board in solid form, and heated, thereby precipitating solder substantially only on pads. It is important for this process that a liquid pool in which tin powder is settled is caused to be formed on the pad array portion when the paste is liquefied by heating, and precipitation is performed in a state where the tin powder is settled in the liquid, that is, the tin powder is covered by the liquid. The reason for this is as follows: When the paste is heated, the viscosity thereof is considerably reduced. When the viscosity is too low, the liquid extends in a wide range at the pad array portion and its neighborhood. As a result, the amount of the lead salt of the organic acid on the pads does not suffice to react with the tin powder, and a sufficient amount of solder cannot be obtained. A little extension is allowed. However, to form a relatively thick solder layer (e.g., 5 µm) of a uniform thickness, it is necessary that a large amount of tin powder is settled on the pad array portion and kept covered with a melted liquid. In the above-described process, Pb atoms separated from a lead salt of an organic acid by above-mentioned replacing reaction collide against the tin powder, thereby forming a Pb-Sn alloy. This is a mechanism of the solder precipitating. Hence, the occasion of the collision determines the amount of precipitated alloy. For this reason, it is desirable that the tin powder deposited on the pad array portion be entirely covered by the liquid containing a lead salt of an organic acid and rosins. Next, methods of mounting an electronic part on pads will be described below. The method described below is also based on the method disclosed in Published Unexamined Japanese Patent Application No. 1-157796. In the soldering step for mounting an electronic part on pads, the solder precipitating reaction disclosed in this reference is generated. An embodiment of the method comprises the steps of forming a preparatory solder layer made of an Sn-Pb alloy substantially only on pads, applying a paste-like composition containing a lead salt of an organic acid and tin powder on the preparatory solder layer, putting an electronic part on the preparatory solder layer applied the paste-like composition, and heating the paste-like composition to precipitate solder, thereby soldering leads of an electronic part on the pads. As described before, in the conventional soldering method, a cream solder is remelted instantaneously as the temperature rises, and the solder is melted entirely in a shorter period of time than that required for the solder to extend over the pads. Hence, the melted solder is swelled due to surface tension. When the solder is swelled beyond the limit of the surface tension, it spreads out of the pads. Thus, bridging may occur on a fine pattern. Additionally, flux components of the soldering paste liquefy by heating and flow to between pads. In this time, solder particles flowing together with the flux component are melted in a moment, and this is a cause of bridging. In contrast, the method used here utilizes a solder precipitating reaction based on the following principles: in a high temperature solution containing tin powder and a lead salt of an organic acid, Sn and Pb replace each other due to an ionization level difference between Pb and Sn, thereby precipitating Pb; and the precipitated Pb and Sn are melted in atomic level, thereby constituting an Sn-Pb alloy. In this method, since it requires a long period of time to replace Sn by Pb, and to alloy every grain of the tin powder with precipitated Pb, the precipitation proceeds more gradually than above-mentioned conventional technique. Further, since the precipitation reaction is performed at high temperature, the Sn-Pb alloy solder precipitated on pads immediately reacts with Cu of pad material in the surface of the pads, thereby forming an intermetallic compound layer such as Cu₃Sn, Cu₆Sn₅, or the like. Thereafter, a solder layer is formed on the intermetallic compound layer by the Sn-Pb alloy solder precipitated on the intermetallic compound layer, while the chemical bonding power acts between the alloy solder and the intermetallic compound layer. The primary difference between this method and the conventional method is as follows: In the conventional soldering paste, solder particles are melted in a moment and at uniform time, thereby forming a solder layer. In contrast, in this method, fine solder particles having a various period of time until precipitating as a solder on a pad are precipitated on a pad. Thereafter, the solder particles which are adjacent to each other diffuse into each other and are united, thereby forming a solder layer on a pad. That is, the solder layer is formed in a longer period of time than that in the conventional method. Additionally, in the case of the method used here since the precipitated solder particles have various compositions, the particles are hardly melted and united, unlike in the conventional method, even if solder particles precipitate between pads. For the above reason, according to the method used here, bridging between pads hardly occurs. In addition, when the precipitated Pb collides against Sn powder existing near the pads or between adjacent pads, an Sn-Pb alloy is formed at that portion. The Sn-Pb alloy thus formed is drawn to and absorbed by the solder precipitated portion on the pads. In this way, the solder layer can be formed substantially only on pads of a fine pitch, e.g., 0.5mm or less, without forming bridges. It is desirable that the formation of the solder layer in the step of soldering an electronic part on pads is performed in the same manner as in the above-described method of forming a solder layer. According to the method, since a relatively thick solder layer is formed uniformly, a pitch between leads of an electronic part can be finer. Also, it is desirable that a preparatory solder layer be formed on pads of a circuit board by applying a paste-like composition containing a lead salt of an organic acid and tin powder on a pad array portion of the circuit board and thereafter heating the paste-like composition. In other words, the above-described solder precipitation reaction is utilized also in the step of forming a preparatory solder layer. As a result, a preparatory solder layer can be formed in accordance with a fine wiring pattern. In this embodiment, for example, when forming a relatively thick preparatory solder layer on pads in uniformly thickness, a paste-like composition containing a lead salt of an organic acid and tin powder is applied thereon. Further, an electronic part is set on the paste-like composition, and then the composition is heated, thereby soldering the pads with leads of an electronic part. Alternatively, a preparatory solder layer can be formed by electroless plating, since electroless plating also enables a preparatory solder layer to be formed on pads in accordance with a fine wiring pattern, without forming bridges between adjacent pads. Further, although a paste containing a lead salt of an organic acid and tin powder is applied on pads after forming a preparatory solder layer on pads, the paste can be applied on leads of an electronic part elements which are to be mounted on the pads. Furthermore, it is possible that, when forming preparatory solder layers on pads of a circuit board, the above-mentioned paste is applied to leads of an electronic part and heated, to form preparatory solder layers also on surfaces of the leads, and thereafter a flux is applied to at least one of the pads and the leads, and subsequently the pads and the leads are heated and soldered. In the case where the preparatory solder layers are formed on leads of an electronic part, it is also desirable that the above-described method of precipitating solder or an electroless plating method be employed. The method according to the present invention will be described hereinafter. The method comprises, among other steps the steps of applying a paste-like composition containing a lead salt of an organic acid and tin powder on a pad array portion of a circuit board, heating the paste-like composition, thereby forming a solder layer substantially only on pads, the solder layer being formed of an Sn-Pb alloy by an amount necessary for mounting an electronic part on pads, applying a flux on the solder layer, putting an electronic part on the flux, and heating and melting the solder layer, thereby soldering leads of the electronic part on the pads. The flux can be applied also on the surfaces of leads of an electronic part. In other words, the flux may be applied to at least one of the solder layers on pads and the surfaces of leads. In the method according to the present invention, the solder layer is formed by an amount necessary for mounting electronic parts on the pads. Since no preparatory solder layer is formed unlike mentioned above, an electronic part can be mounted on the pads in a smaller number of steps than that in the above-indicated method. If a non-halogen type flux is used, it is unnecessary to clean the circuit board after soldering, or at least, it is possible to clean it with cleaner other than freon. Thus, the present invention contributes to the prevention of atmospheric pollution which is much talked about. If a circuit board has pad array portions including a small-pitch portion of a pad pitch of 0.5mm or smaller and a large-pitch portion of a larger pitch, the following method can be applied. First, a solder layer having a thickness of, for example, several ten microns (by the amount necessary for soldering) is formed on the small pitch portion, whereas a preparatory solder layer of about 5 to 10 µm thickness is formed on the large pitch portion. Then, a soldering paste is printed on the preparatory solder layer on the large pitch portion, and an electronic part is provisionally fixed to the pad array portion by adhesion of the cream solder. Thereafter, the circuit board is heated in a reflow furnace or the like, thereby soldering the electronic part with the pads. On the other hand, a solder flux is applied on the pads on which the solder layers having thickness of several ten µm require soldering by the above-mentioned method, and an electronic part is provisionally fixed to the pads by adhesion of the flux. Thereafter, the circuit board is heated in a reflow furnace or the like, thereby soldering the electronic part with the pads. Further, individual pad array portions may require solder layers of largely different thicknesses because of different areas of pads or pad pitches. In this case, the paste-like composition is applied on each pad array portion in different amounts corresponding to the required thicknesses, and thereafter heated. Thus, solder layers of different thicknesses can be formed respectively on the pad array portions. On the other hand, a solder layer forming a thickness of several ten µm sometimes has a half circle-like section, as shown in Fig. 2. In this case, an electronic part can not be provisionally fixed exactly to the pad only by adhesion of a solder flux, if lead of the electronic part is put on the solder layer. That is, sometimes a displacement between pads and leads can not be entirely prevented. In Fig. 2, reference numeral 1 designates an electric insulating substrate, 2 a pad, and 4 a solder layer, H a thickness of the solder layer and W a width of the pad. This problem can be overcome by the following procedure. A solder layer having a large thickness, for example several ten µm thick, is precipitated on a pad having a small area or a small pitch, and thereafter, to the solder layer pressure is applied from the upper surface, thereby making the surface of the solder layer flat. In this flattening step, it is effective that pressure is applied to the solder layer while the layer is heated, when the solder layer has specified thickness and width. The temperature of this procedure depends on the solder composition, having a small area or a small pitch, but it is desirable that the solder layer by heated to a temperature which is 20 to 40°C lower than that at which the precipitated solder is deformed. The pressure and the period of time required for flattening the projections are determined in accordance with the thickness and area of the solder layer. Thus, displacement of parts can be prevented with a simple process. When an electronic part is mounted as mentioned above, electronic parts can be soldered with pads at a lower temperature than in a case where a conventional soldering paste is used. This advantage is very prominent particularly in a case where a solder alloy having a Pb rich composition with a high melting point is used. For example, to manufacture an IC device such as a PGA (pin grid array) packaged by plastic or a multi-chip module, if an IC chip is to be soldered to a circuit in a package by a bump formed of a solder alloy having a Pb rich composition, it is desirable that the bump be formed on the circuit board rather than on the IC chip. The reason for this is that, if a bump is formed on the circuit board, the IC device can be manufactured at a low cost, not adversely affecting the IC chip. However, to form a bump on the circuit board, the circuit board must be resistant to heat which is applied thereto for forming a solder layer having a Pb rich composition with a high melting point. For this reason, an expensive substrate material resistant to a high temperature is required, resulting in a high cost in total. In the conventional soldering method using a soldering paste, it is necessary to heat the solder to a temperature which is 30 to 60°C higher than the melting point of the solder. This is because diffusion of Cu atoms from the pad to the solder layer and diffusion of Pb and Sn atoms from the solder to the pad must be performed within a short period of time while the solder is being melted. According to the present invention, soldering can be performed at substantially the same solder precipitating temperature no matter what the Sn/Pb ratio of a solder layer formed would be, by appropriately selecting the composition ratio between a lead salt of an organic acid and tin powder in the above paste-like composite material. For example, in the case of a Pb rich solder having a high melting point, a solder layer can be formed at a temperature much lower than the melting point of the solder formed by precipitation. Further, since an intermetallic compound layer is formed by diffusion between the solder and the copper which is a material of the pad at formation of the solder layer, additional energy, which is needed in the conventional electrical plating method, is no longer required for forming an intermetallic compound by a post-heating method, for example, fusion, when soldering is performed later using the solder layer. Thus, the number of the method steps can be decreased, reducing the cost of the overall process. The method of forming a solder layer on pads of a circuit board and the method of mounting an electronic part on a circuit board according to the present invention are applied to, for example, a circuit board as is shown in Fig. 1. In Fig. 1 reference numeral 1 designates an electric insulating substrate, and pads 2 for mounting electronic parts are formed thereon. In pad array portions 3 each of the pads 2 are arranged corresponding to a part to be mounted. Examples Examples of details of the present invention will be described below. Example 1 The following is an example of the method of forming a solder layer. 46% (by weight) of a lead salt of an organic acid, 18% of an activator, 25% of tin powder, and 11% of a viscosity modifier were mixed, thereby forming a paste-like composition of viscosity of about 280,000 to 290,000cps. The paste-like composition was applied in a thickness of 0.5mm on a pad array portion on a circuit board for incorporating a 284-pins QFP of a pad size of 0.26mm × 2mm and a pad pitch of 0.36mm. Then, the circuit board was heated for two minutes at the temperature of 215°C. Observing the paste-like composition, it was confirmed that the paste-like composition was liquefied, but spread little and accumulated stratifiedly on and near the pads array portion, and the tin powder was settled in the accumulated liquid, and solder layers were gradually formed on the pads. The solder layers ultimately formed on the pads have an average thickness of 9 to 10 µm with a dispersion σ of about 1 to 2 µm. The solder layers contain 64 to 69% by weight of Sn. No bridges were formed in the layer. In the same manner, solder layer were formed on pads on a pad array portion with a pad pitch of 0.15mm, with no bridges. As a comparative example, solder layers were formed on pads under the same condition as mentioned above by the conventional hot-gas leveling method, in a manner such that no bridges were formed. The solder layer ultimately formed has a thickness of 1 to 5 µm. Thus, the thickness has a large dispersion, and the intermetallic compound layers are exposed from thinner portions of the solder layers. When a large amount of solder was applied on the pads to form a thicker layer, a large number of bridges were formed. Next, on the assumption that parts are mounted on both sides of a circuit board, several samples were prepared to check the wettability of solder, before and after heating, before heating (on the assumption of a first mounting) and after heating (on the assumption of a second mounting which was performed on the rear-surface of the board after first mounting). The samples are as follows: A: A board on which solder layers were formed by the method of present invention. The board consisted of a 0.3mm thick copper plate and a solder layer. The solder layer was formed by applying a paste-like composition (containing 37% of a lead salt of an organic acid, 10% of an activator, 40% of tin powder, 13% of a viscosity modifier) on the copper plate to a thickness of 300 µm, and thereafter heating the plate on a hot plate for two minutes at a temperature of 220°C. B: The same board as A. The solder layer was formed by the hot gas leveling method. C: A 0.3mm thick copper plate with no solder layer. Five pieces of each of samples A to C were prepared, and a non-halogen type soldering paste was applied thereon. Then, all pieces were heated, and thereafter the solder spreading ratio was measured on the basis of JIS (Japanese Industrial Standard) Z3197. Next, the samples A to C (five pieces for each) were heated in a hot air dryer at the temperature of 150°C for an hour. Then, the non-halogen type soldering paste was applied thereon, and thereafter the solder spreading ratio was measured. The results of the tests are indicated Table 1 below. As is obvious from Table 1, the solder layer according to the present invention keeps a high solder spreading ratio (wettability), even after being subjected to heating on the assumption of a second mounting of an electronic part. Thus, the paste-like composition of the present invention is suitable for a preparatory solder for use in a case where electronic parts are soldered to both sides of a circuit board. Example 2 The following is an example of the method of soldering an electronic part on a circuit board. 46% (by weight) of a lead salt of an organic acid Pb, 18% of an activator, 25% of tin powder, and 11% of a viscosity modifier were mixed, thereby forming a paste-like composition of a viscosity of about 280,000 to 290,000cps. The paste-like composition was applied in a thickness of 0.5mm all over a pad array portion of a circuit board, for mounting 284-pins QFP, having a pad size of 0.26mm × 2mm and a pad array pitch of 0.36mm. Then, the circuit board was heated for two minutes at the temperature of 215°C, thereby forming a preparatory solder layer on the pad array portion. The preparatory solder layer has an average thickness of 9 to 10 µm with a dispersion σ of about 1 to 2 µm, and contains 64 to 69% by weight of Sn. No bridges were formed. Thereafter, a paste-like composition containing 40% of a lead salt of an organic acid, 33% of an activator, 18.4% of tin powder, and 8.6% of a viscosity modifier was applied on all over the preparatory solder layer on the pad array portion. A 284-pins QFP of the lead pitch of 0.36mm was provisionally fixed to the paste-like composition. Then, the circuit board was heated in a reflow furnace at the temperature of 215°C for 30 seconds, and subsequently at the temperature of 220°C for 2 minutes, thereby precipitating solder. By using the solder, the 284-pins QFP was soldered to the pad array. As a result, no bridges were formed, and leads of the 284-pins QFP were satisfactorily soldered to the pad array. Example 3 The following is a example of the method of soldering an electronic part on a circuit board according to details of the present invention. Parts which were to be soldered on a circuit board are as follows. 1) A QFP having 100 pins. The lead pitch thereof is 0.65mm, and the entire size thereof is 20mm × 13mm. 2) A chip of the entire size of 4.2mm × 7mm. 3) A SOP having 20 pins. The lead pitch thereof is 0.8mm, and the entire size thereof is 9mm × 8mm. First, a glass epoxy board sized 200mm × 250mm was prepared, on which pads corresponding to the above three elements were formed. Pads for the QFP were sized 0.5mm × 2mm, pads for the chip sized 1.5mm × 1.5mm, and pads for the SOP sized 1.2mm × 0.8mm. A paste-like composition used in this example contains 40% of a lead salt of an organic acid, 33% of an activator, 18.4 % of tin powder, and 8.6% of a viscosity modifier. The paste-like composition was applied on the pad array portion in a ratio of 0.5 to 0.9g per one pad array portion. Then, the circuit board was heated in a reflow furnace having a far infrared radiation heater at the temperature of 215°C for 30 seconds, and subsequently at the temperature of 220°C for 2 minutes, thereby precipitating solder. As a result, a solder layer of the thickness of 30 µm containing 74% by weight of Sn was formed. Then, a rosin flux (Microsolder F-40 produced by HARIMA CHEMICALS, INC.) having an aqueous solution resistance of 100,000 Ω · cm or more was painted on the layer. The above-mentioned parts were put on the flux, and the board was heated in a reflow furnace to melt the solder, thereby soldering leads of each part to the pad array portion. As a result, the leads were soldered to the pad array with no bridges and solder balls. Moreover, after the soldering, an ionic residue was measured by means of an omegameter (Model 600 produced by KENKO Corporation, U. S. A.) without cleaning the solder layer. The measured value was satisfactorily small, i.e., 0.1 to 0.3 µg NaCℓ/sq.in. In this example, the solder precipitation amount depends on the amount of the Sn powder settled on and around the pads, unlike the case of the conventional soldering paste. In more detail, the thickness H of the solder precipitated on the pads and the width W of the pads have a relationship shown in Fig. 3. When the relationship is represented as H = aW, the range of the value a must be 0.05 > a > 0.5, more preferably 0.1 > a > 0.3. If 0.05 > a, the precipitated solder layer will have, regionally, one or several recesses (the sections where extremely less solder is settled), affecting the formation of a fillet which is required for soldering. If a > 0.5, the solder layer will have regionally one or several raised sections which might interfere with the lead of the electronic part to be displaced. Example 4 The following is an example of the method of forming a solder layer having Pb rich composition, before mounting electronic elements on a circuit board. (1) A paste-like composition containing 67% of a lead salt of an organic acid, 10% of an activator, 13% of tin powder, and 10% of a viscosity modifier was applied on a copper plate to a thickness of 400 to 500 µm, and heated at the temperature of 220°C for two minutes, thereby forming a solder layer. The obtained solder layer was 8 to 10 µm thick and contained Sn and Pb in the ratio 2:8, and its melting point was 293°C. When the above-mentioned solder layer is formed by using the conventional soldering paste, it is required that the circuit board is dipped in a melted solder (Sn/Pb = 2/8) kept at 293°C or higher, which is far higher than 220°C in the present invention. As a result, the reliability of the circuit board significantly drops. (2) A paste-like composition containing 55% of a lead salt of an organic acid, 15% of an activator, 13% of tin powder, and 17% of a viscosity modifier was applied on a copper plate to a thickness of 300 to 400 µm, and heated at the temperature of 220°C for two minutes, thereby forming a solder layer. The obtained solder layer was 4 to 8 µm thick and contained Sn and Pb in the ratio 3:7, and its melting point was 278°C. In the conventional method using a soldering paste containing the above-mentioned components in the same ratios, it is necessary to heat a circuit board at a temperature of about 320°C. Note that, lead naphthenate, lead rosinate, lead octylate, lead oleate, lead stearate, or the like can be used as a lead salt of an organic acid. Rosin, an organic acid, alkanolamine, amine acid, or the like can be used as an activator. Custer wax, cellulose powder, butyl carbitol, hexyl carbitol, squalene, or the like can be used as a viscosity modifier. It is desirable that tin powder is constituted by spherical grains of a diameter of 80 µm or less, preferably 25 µm or less, containing oxygen in a ratio 1500ppm or less, preferably 1000ppm or less, formed by an atomizing method.

Claims: A method of mounting an electronic part on a circuit board (1), comprising the steps of: applying a paste-like composition containing a lead salt of an organic acid and tin powder on a pad array portion (3) on the circuit board (1); heating the paste-like composition to cause solder precipitation, thereby forming solder layers (4) formed of an Sn-Pb alloy by an amount required for mounting an electronic part on pads (2) of the pad array portion (3); applying pressure to the upper surfaces of the solder layers (4), while heating the solder layers, after the step of forming the solder layers (4), and before the step of putting an electronic part on the pads; applying a flux to at least one of the solder layer (4) and leads of an electronic part which is to be mounted on the pads (2); putting the electronic part on the pads (2); heating the solder layers (4); and soldering the pads (2) with leads of the electronic part. A method according to claim 1, characterized in that said solder precipitation is performed in a state that a liquid pool is formed on the pad array portion (3), when the paste-like composition is liquefied by heating, and the tin powder is settled in the liquid pool. A method according to claim 1, characterized in that said flux is of a non-halogen type. A method according to claim 1, characterized in that said solder alloy contains an Sn-Pb alloy having a composition rich in Pb.

Inventors: FUKUNAGA TAKAO, FUSE KENICHI, IRIE HISAO, KOHNO MASANAO, FUKUNAGA, TAKAO, FUSE, KENICHI, IRIE, HISAO, KOHNO, MASANAO
Applicants: FURUKAWA ELECTRIC CO LTD, HARIMA CHEMICALS INC, HARIMA CHEMICALS, INC., THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1990
Main classification: H05K3
IPC codes: H05K3, B23K35