Reflow method

Abstract

To provide a reflow method with which reflow can be performed in a short time at low cost.SOLUTION: A reflow method to be disclosed is a reflow method for solder connecting lands of a substrate and a plurality of electronic components to each other. The reflow method includes: a step (i) introducing, into a reflow device, a substrate 10x in which a plurality of electronic components 30 is arranged on lands 10b through a layer including solder (for example, layer of solder paste 11); and (ii) heating the substrate 10x introduced into the reflow device in the reflow device to fuse the solder, thereby soldering the plurality of electronic components 30 to the lands 10b. In the step (ii), while the surface temperature of a surface of the surfaces of the substrate 10x where the lands 10b are formed rises from 50°C to the melting point of the solder, the substrate 10x is heated so that the surface temperature rises at a rate of temperature rise of 4°C / sec or more.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a reflow method.

Background Art

[0002] As a method for mounting a plurality of electronic components on a substrate, a reflow method of mounting electronic components on a substrate using solder paste is known.

[0003] Patent Document 1 (Japanese Patent Laid-Open No. 61-289697) discloses "a soldering method characterized in that, in an apparatus for heating and reflowing a printed circuit board or the like coated with a solder material and having electronic components or the like mounted thereon while continuously conveying it by a conveyor, after preliminary heating, heating is performed by flowing hot air substantially in parallel with the moving direction of the conveyor simultaneously with heating by an infrared heater."

[0004] Patent Document 2 (Japanese Patent Laid-Open No. 4-250691) discloses "a reflow method for cream solder, characterized in that, in a reflow method of printing and applying cream solder on a printed board and then performing preliminary heating and main heating in a reflow furnace, the temperature rising rate during preliminary heating is set to 1 to 5 °C / sec."

[0005] Patent Document 3 (Japanese Patent Application Laid-Open No. 2004-255426) discloses, "In the reflow preheating and drying step of the reflow temperature profile when heating a printed circuit board with electronic components mounted thereon by cream solder to perform reflow soldering of the electronic components, after starting heating of the printed circuit board at a preheating temperature increase rate equal to or higher than the maximum value of the sag allowable temperature increase rate range, which is a range of preheating temperature increase rates capable of suppressing the amount of sag generated in the cream solder due to heating to a level where there is no risk of printing defects, heating is continued until the sag generation temperature at which sag occurs in the cream solder, a first preheating step; a second preheating step of heating the printed circuit board that has undergone the first preheating step at a preheating temperature increase rate within the sag allowable temperature increase rate range and continuing heating until the sag end temperature at which the progress of sag in the cream solder stops; and a third preheating step of heating the printed circuit board that has undergone the second preheating step at a preheating temperature increase rate equal to or higher than the maximum value of the sag allowable temperature increase rate range and continuing heating until the target temperature is reached. A reflow preheating and drying method characterized by having the above steps."

[0006] Patent Document 4 (Japanese Patent Application Laid-Open No. 2004-304098) discloses, "In a reflow method of applying solder paste to a soldering portion of a printed board, mounting an electronic component on the applied portion, and then heating the printed board in a reflow furnace to solder the printed board and the electronic component, the printed board is heated from a temperature of 100°C or lower to the peak temperature at a substantially constant heating rate (Claim 1). Further, Patent Document 4 describes, "In the present invention, a substantially constant heating rate of 0.5 to 2.0°C / second is appropriate. If the heating rate is slower than 0.5°C / second, the solder paste will sag and generate micro balls, and not only will the time taken to pass through the reflow furnace be prolonged, deteriorating productivity, but there is also a risk of thermal impact on the electronic components, causing functional degradation. However, if the heating rate is faster than 2.0°C / second, the evaporation of the solvent in the flux will occur rapidly, causing the solder paste to scatter or the position of the electronic components to shift."

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] As shown in the above patent documents, in the conventional reflow process, it was the common technical knowledge of those skilled in the art to perform a preheating process. Further, even if the preheating process was not performed, it was common to set the heating rate to 2°C / second or less.

[0009] On the other hand, in the mounting of electronic components by reflow, the solder surface in the solder paste oxidizes during reflow, and soldering defects are likely to occur. In order to prevent such oxidation, reflow is performed in a nitrogen gas atmosphere inside the reflow apparatus. However, in that case, the cost of the reflow process increases. In addition, the reflow apparatus becomes more complex and expensive.

[0010] In such a situation, one of the objects of the present invention is to provide a reflow method capable of performing reflow in a short time and at low cost.

Means for Solving the Problems

[0011] One aspect of the present invention relates to a reflow method. The reflow method is a reflow method for soldering a land of a substrate and a plurality of electronic components, and includes a step (i) of introducing the substrate on which the plurality of electronic components are arranged on the land through a layer containing solder into a reflow apparatus, and a step (ii) of melting the solder by heating the substrate introduced into the reflow apparatus in the reflow apparatus to solder the plurality of electronic components to the land. In the step (ii), while the surface temperature of the surface of the substrate on the side where the land is formed rises from 50°C to the melting point of the solder, the substrate is heated so that the surface temperature rises at a rate of 4°C / second or more.

Advantages of the Invention

[0012] According to the reflow method of the present invention, reflow can be performed in a short time and at low cost.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with examples, but the present invention is not limited to the examples described below. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less".

[0015] (Reflow method) The reflow method according to this embodiment is a reflow method for soldering the lands of a substrate and a plurality of electronic components. This reflow method includes steps (i) and (ii). These steps will be described below.

[0016] (Step (i)) Step (i) is a step of introducing a substrate on which a plurality of electronic components are arranged on lands through a layer containing solder into a reflow apparatus. The layer containing solder may hereinafter be referred to as "layer (L) containing solder" or "layer (L)". The substrate (for example, a printed circuit board) includes a substrate portion and lands formed on the surface of the substrate portion. The lands are part of the wiring, and electronic components are soldered to the lands. The substrate is not particularly limited, and a known printed circuit board may be used.

[0017] The layer (L) containing solder may be a layer formed of a solder paste containing solder particles. The solder paste contains solder particles and other components (for example, flux). The layer (L) containing solder may include a solder precoat layer. The layer containing solder may include a solder precoat layer and a flux layer disposed so as to cover the solder precoat layer.

[0018] The melting point of the solder (solder particles or solder precoat layer) contained in the layer (L) containing solder is, for example, 200°C or higher (for example, 210°C or higher and 215°C or higher). The melting point of the solder is preferably 240°C or lower (for example, 230°C or lower and 225°C or lower). The melting point of the solder may be in the range of 200°C to 240°C (for example, in the range of 200°C to 230°C or in the range of 215°C to 230°C). Lead-free solder is preferably used for the solder.

[0019] (Step (ii)) Step (ii) is a step of melting the solder in layer (L) by heating the substrate introduced into the reflow apparatus in the reflow apparatus, thereby soldering a plurality of electronic components to the lands. In step (ii), while the surface temperature of the surface of the substrate on the side where the lands are formed rises from 50°C to the melting point of the solder, the substrate is heated so that the surface temperature rises at a rate of 4°C / second or more. Hereinafter, the surface of the substrate on the side where the lands are formed may be referred to as the "upper surface", and the surface on the opposite side of the upper surface may be referred to as the "lower surface". From another perspective, the upper surface is the surface on the side where the electronic components to be soldered are arranged.

[0020] The reflow apparatus includes a heater. The reflow apparatus may include a fan for facilitating the transfer of the heat of the heater to the substrate. The reflow apparatus may have the same configuration as a known reflow apparatus, or a known reflow apparatus may be used. However, in the reflow method of the present invention, it is possible to shorten the length of the apparatus (the length in the substrate conveyance direction).

[0021] Normally, in step (ii), the lower surface of the substrate also heats up in the same manner as the upper surface of the substrate. Normally, in step (ii), until the surface temperature of the lower surface of the substrate rises from 50°C to the melting point of the solder in layer (L), the substrate is heated so that the surface temperature of the lower surface rises at a rate of 4°C / second or more. That is, until the surface temperature of the lower surface of the substrate rises from 50°C to the melting point of the solder in layer (L), the heating rate of the lower surface does not become less than 4°C / second. As long as the heating rate is 4°C / second or more, the heating rate may or may not be constant.

[0022] The above-mentioned heating rate with respect to the lower surface and the upper surface of the substrate is 4 °C / second or more, and may be 4.5 °C / second or more, or 5 °C / second or more. There is no particular limitation on the upper limit of the above-mentioned heating rate with respect to the lower surface and the upper surface of the substrate. The heating rate may be, for example, 10 °C / second or less, 8 °C / second or less, 6 °C / second or less, or 5 °C / second or less. By setting the heating rate to 10 °C / second or less, it is possible to suppress the peeling of the wiring and the land from the substrate portion. These lower limits and upper limits can be arbitrarily combined as long as the lower limit is not higher than the upper limit. For example, the above-mentioned heating rate may be in the range of 4 °C / second or more to 10 °C / second or less, or the lower limit and / or the upper limit of this range may be replaced with the lower limit and / or the upper limit exemplified above.

[0023] In step (ii), heating may be performed such that the surface temperature of the lower surface of the substrate is higher than the surface temperature of the upper surface of the substrate. The higher the temperature on the side where the electronic component is disposed, the more likely the solder is to oxidize. Therefore, it is preferable to raise the temperature of the lower surface side to melt the solder. In order to make the temperature of the lower surface side of the substrate higher, the number and / or output of the heaters for heating from the lower surface side of the substrate may be made larger than the number and / or output of the heaters for heating from the upper surface side of the substrate. Regarding the above-mentioned heating rate, the heating rate of the lower surface of the substrate may be made higher than the heating rate of the upper surface of the substrate.

[0024] Even after the surface temperature of the substrate exceeds the melting point of the solder, heating of the substrate may be continued as necessary. In step (ii), the maximum temperature of the surface temperature of the upper surface and the lower surface of the substrate is preferably lower than the temperature obtained by adding 40 ° C (for example, 20 ° C or 30 ° C) to the melting point of the solder. In step (ii), the maximum temperature of the surface temperature of the upper surface and the lower surface of the substrate may be 250 ° C or lower (for example, 240 ° C or lower or 230 ° C or lower) in consideration of the influence on the electronic components. After the surface temperature of the upper surface of the substrate reaches the melting point of the solder, there is no particular limitation on the surface temperature of the upper surface of the substrate as long as all the solder contained in the layer (L) melts. For example, after the surface temperature of the upper surface of the substrate reaches the melting point of the solder, the heating rate may be less than 4 ° C / second, the surface temperature may be kept constant, or the surface temperature may be decreased. Heating of the substrate is performed at least until all of the solder contained in the layer (L) melts.

[0025] After melting the solder by raising the temperature of the substrate at the above heating rate, the substrate may be cooled to solidify the melted solder. Step (ii) may include a step (ii-a) of melting the solder in the layer (L) by heating the substrate introduced into the reflow apparatus in the reflow apparatus, and a step (ii-b) of cooling and solidifying the melted solder.

[0026] Conventionally, in the reflow method, the temperature of the entire substrate is raised to a predetermined temperature (a temperature lower than the melting point of the solder paste) by a preheating process of holding the substrate at a temperature of about 100 to 150°C, and then the substrate is further heated to melt the solder (for example, the solder particles in the solder paste). It has been considered necessary to do so. This is because it has been considered that if the preheating process is not performed, the temperature of the substrate will vary and soldering defects will easily occur. Patent Document 4 discloses a method of heating a printed circuit board from a temperature of 100°C or lower to a peak temperature at a substantially constant heating rate without providing a preheating process. However, even in that case, Patent Document 4 discloses that "when the heating rate becomes faster than 2.0°C / second, the solvent in the flux evaporates rapidly, causing the solder paste to scatter or the electronic components to be displaced." That is, also in the method of Patent Document 4, it has been considered necessary to set the heating rate to 2.0°C / second or lower.

[0027] However, as a result of investigations, the inventors of the present application have found that reflow can be performed by a method that completely contradicts the above-mentioned common technical knowledge. Specifically, the inventors of the present application have newly found that reflow can be performed by raising the temperature of the substrate to a temperature equal to or higher than the melting point of the solder at a heating rate that is not considered in the conventional common technical knowledge. The present invention is based on this new finding.

[0028] Step (ii) may be performed in an air atmosphere inside the reflow apparatus. In the conventional reflow method, the solder in layer (L) is exposed to a high temperature for a long time. Therefore, the surface of the solder in layer (L) is oxidized, and soldering defects are likely to occur. When soldering an electronic component using a solder paste, when the size of the electronic component to be soldered becomes smaller, the amount of solder paste disposed on one land becomes smaller. When the amount of solder paste disposed on one land becomes smaller, (the exposed surface area S (mm 2 )) / (the volume V (mm of the solder paste 3)) value increases. Therefore, the smaller the size of the electronic component, the easier it is for oxidation to occur on the surface of the solder, and the more likely it is for soldering defects to occur. Therefore, in the conventional reflow method, when soldering electronic components including minute electronic components, in order to prevent oxidation of the solder, the reflow process may be performed in a nitrogen gas atmosphere. However, when performing reflow in a nitrogen gas atmosphere, the cost of the reflow process itself and the cost of the reflow apparatus increase. On the other hand, according to the method of the present invention, even when the inside of the reflow apparatus is in an air atmosphere, minute electronic components can be soldered with good yield. However, it is also possible to perform the reflow method of the present invention in a nitrogen gas atmosphere.

[0029] The plurality of electronic components to be soldered to the substrate may include electronic components having a side length of the planar shape of 0.4 mm or less. As described above, when soldering minute electronic components, the method of the present invention is particularly preferably used. When the plurality of electronic components include electronic components having a side length of the planar shape of 0.4 mm or less and electronic components having a larger size, the method of the present invention is particularly preferably used. Examples of electronic components having a side length of the planar shape of 0.4 mm or less include electronic components having a size of 0402 size or less according to the JIS standard. In this specification, the "planar shape" is the shape when the electronic component is viewed from above the substrate in a state where the electronic component is mounted on the substrate.

[0030] In the reflow apparatus, the distance by which the substrate is conveyed may be 2 m or less. In the reflow method of the present invention, a preheating process for slowly raising the temperature of the substrate is not performed. Therefore, the region where the substrate is heated can be shortened, and as a result, the distance by which the substrate is conveyed in the reflow apparatus can be significantly shortened compared to the distance (for example, about 5 m) in the conventional reflow apparatus. Therefore, miniaturization of the apparatus is possible. Of course, the distance by which the substrate is conveyed in the reflow apparatus may be longer than 2 m.

[0031] In the reflow method of the present invention, the preheating process is not performed, and the substrate is heated at a very high heating rate compared to the conventional method. Therefore, in the reflow method of the present invention, it is possible to significantly shorten the processing time in the reflow apparatus.

[0032] (Reflow apparatus) The present disclosure further relates to a reflow apparatus. The reflow apparatus includes a substrate transfer device for transferring a substrate, a heater (heating device) for heating the substrate, and a control device. The reflow apparatus may further include a measuring device for measuring the surface temperature of the substrate. Note that the surface temperature of the substrate may be measured only when determining the heating conditions of the substrate. Once the heating conditions of the substrate are determined, the substrate can be heated according to those conditions in the reflow process. When using different substrates, different electronic components, and different solder-containing layers (e.g., solder paste), the heating conditions may be determined for each of their types. Alternatively, the reflow apparatus may heat the substrate while monitoring the surface temperature of the substrate during the reflow process. The control device includes an arithmetic processing device and a storage device. The control device performs the above-described steps (i) and (ii).

[0033] The storage device stores a program for executing the reflow method of the present invention. Specifically, the storage device stores a program for executing the above-described steps (i) and (ii). The arithmetic processing device executes the program to perform steps (i) and (ii). At this time, the control device may control the heater and / or the substrate transfer device based on the output from the measuring device. For each device constituting the reflow apparatus, a device used in a known reflow apparatus may be applied, or a device used in a known reflow apparatus may be modified to be adapted to the apparatus of the present invention and then applied.

[0034] Hereinafter, examples of embodiments according to the present invention will be specifically described with reference to the drawings. The embodiments described below can be modified based on the above description. Also, the matters described below may be applied to the above embodiments. In addition, matters that are not essential to the embodiments according to the present invention can be omitted. Note that, for ease of understanding, the following figures may show members with their scales changed. Also, in the following figures, the hatching of some members may be omitted.

[0035] (Embodiment 1) In Embodiment 1, an example of the case where a layer formed of solder paste is used as the layer (L) will be described. An example of the reflow apparatus 100 used in Embodiment 1 is schematically shown in FIG. 1. The reflow apparatus 100 includes a housing 101, a heating zone 110, a cooling zone 120, and a conveyor (substrate transfer mechanism) 131. The reflow apparatus 100 includes a measuring instrument for monitoring the surface temperature of the substrate 10x and a control device for controlling the equipment (both not shown). The inside of the housing 101 is partitioned into a heating zone 110 and a cooling zone. The conveyor 131 is arranged so as to be able to move the substrate 10x (finally the substrate 10y) from the substrate loading section 102 to the substrate unloading section 103.

[0036] The heating zone 110 includes a plurality of heaters 111 and a plurality of fans 112. The cooling zone 120 includes a plurality of fans 122. In the example shown in the figure, the heaters 111, the fans 112, and the fans 122 are arranged on both the lower side and the upper side of the conveyor 131. By adjusting the number and output of the heaters 111 and the fans 112 arranged under the conveyor 131, the number and output of the heaters 111 and the fans 112 arranged above the conveyor 131, the conveyance speed of the conveyor 131, etc., the temperature rising speed of the upper surface of the substrate 10x, the temperature rising speed of the lower surface, and the ratio of those temperature rising speeds can be controlled.

[0037] The processing by the reflow method of Embodiment 1 may be performed with nitrogen gas flowing in the reflow apparatus. However, from the viewpoints of cost and simplification of the apparatus, it is preferable to perform the processing with the inside of the reflow apparatus in an air atmosphere.

[0038] In the reflow method of Embodiment 1, first, the substrate 10x is introduced into the reflow apparatus 100 from the substrate loading section 102 by the conveyor 131 (step (i)). The substrate 10x includes a plurality of electronic components arranged on the lands. An example of the steps of the manufacturing method of the substrate 10x on which a plurality of electronic components are arranged is shown in FIG. 2.

[0039] First, a printed circuit board (substrate) 10 shown in FIG. 2(a) is prepared. The printed circuit board 10 includes a plate-shaped substrate portion 10a and lands 10b formed on the surface of the substrate portion 10a. The lands 10b are connected to wiring (not shown) as necessary.

[0040] Next, as shown in FIG. 2(b), a mask 20 is placed on the printed circuit board (substrate) 10. The mask 20 has an opening 20h at a position corresponding to the land 10b where the solder paste is to be applied.

[0041] Next, as shown in FIG. 2(c), the solder paste 11 is placed on the land 10b by applying the solder paste 11. The solder paste 11 includes solder particles 11a and a flux 11f. The solder paste 11 forms a layer (L) containing solder.

[0042] Next, the mask 20 is moved from above the printed circuit board 10. At this time, as shown in FIG. 2(d), solder particles 11a may be arranged between two adjacent lands 10b. Next, as shown in FIG. 2(e), an electronic component 30 is placed on the solder paste 11. An example of the electronic component 30 shown in FIG. 2(e) includes a terminal portion 30a connected to the land 10b and an element portion 30b. In this way, the substrate 10x on which the electronic component 30 is arranged is obtained. Although only one electronic component 30 is shown in FIG. 2(e), a plurality of electronic components 30 are arranged on the printed circuit board 10. Through the above steps, the substrate 10x on which a plurality of electronic components 30 are arranged is obtained. The method of arranging the solder paste 11 and the method of arranging the electronic component 30 are not limited, and known methods may be used.

[0043] The substrate 10x introduced into the reflow apparatus 100 is conveyed by the conveyor 131 and passes through the heating zone 110 and the cooling zone 120. Usually, the substrate 10x is conveyed through the reflow apparatus 100 at a constant speed by the conveyor 131. In the heating zone 110, the substrate 10x is heated, and the solder particles 11a in the solder paste 11 melt. In the cooling zone 120, the substrate 10x is cooled, and the melted solder solidifies. In this way, a plurality of electronic components 30 are soldered to the lands 10b.

[0044] In step (ii), the substrate 10x is heated from room temperature to a temperature exceeding 50°C and further heated to a temperature equal to or higher than the melting point of the solder. An example of the change in the substrate 10x in step (ii) is shown in the cross-sectional views of FIGS. 3(a) to 3(c). FIG. 3(a) shows the initial stage of heating of the substrate 10x in the heating zone 110. At the stage of FIG. 3(a), a part of the solder particles 11a has melted to form the molten nuclei 11b. When the substrate 10x is heated, heat is easily transferred from the land 10b to the solder particles 11a. Therefore, the molten nuclei 11b are formed on the land 10b.

[0045] When heating the substrate 10x, as described above, the substrate 10x is heated so that the surface temperature of the upper surface 10sa of the printed circuit board 10 rises at a rate of 4°C / second or more until the surface temperature rises from 50°C to the melting point of the solder particles 11a. The upper surface 10sa is the surface on which the lands 10b where the electronic components 30 are arranged are formed.

[0046] As described above, it is preferable that the surface temperature of the lower surface 10sb on the opposite side of the upper surface 10sa is higher than the surface temperature of the upper surface 10sa. That is, it is preferable that the rate of increase in the surface temperature of the lower surface 10sb is higher than the rate of increase in the surface temperature of the upper surface 10sa.

[0047] As the melting of the solder particles 11a progresses, as shown in FIG. 3(b), all of the solder particles 11a melt to become the molten solder 11c. At this time, the solder particles 11a melt while being attracted to the melting nucleus 11b that they first contacted. Therefore, the solder particles 11a existing between the lands 10b are also attracted to the melting nucleus 11b on the lands 10b. As a result, short - circuiting of two adjacent lands 10b by solder is suppressed.

[0048] Next, the substrate 10x is cooled in the cooling zone 120. As a result, as shown in FIG. 3(c), the molten solder 11c solidifies to become the solder 11d. Also, the flux 11f becomes the flux residue 11g. In this way, a substrate 10y on which the electronic component 30 is soldered to the lands 10b is obtained. The substrate 10y is an electronic component mounting substrate on which the electronic component 30 is mounted.

[0049] An example of the temperature profile of the surface temperatures of the upper surface 10sa and the lower surface 10sb in step (ii) is schematically shown in FIG. 4. In the example shown in FIG. 4, from room temperature (25°C in this example) to the maximum temperature (230°C in this example), the upper surface 10sa and the lower surface 10sb of the substrate 10x are heated at a substantially constant heating rate. The heating rate of the lower surface 10sb is higher than the heating rate of the upper surface 10sa. The maximum temperature reached on the upper surface is higher than the melting point of the solder particles (for example, 221°C) and lower than 230°C. The heating rate of the lower surface 10sb is 4°C / second or more from 25°C (room temperature) to the melting point of the solder particles. Similarly, the heating rate of the upper surface 10sa is also 4°C / second or more from 25°C to the melting point of the solder particles. That is, the heating rates of the surface temperatures of the upper surface 10sa and the lower surface 10sb are both 4°C / second or more from 50°C to the melting point of the solder and do not become less than 4°C / second. As described above, the heating rate may or may not be constant.

[0050] Regarding step (ii) of Embodiment 1, the results of actual experiments will be described below. In this experiment, first, a test substrate was prepared assuming mounting a 0402-sized chip and a 0201-sized chip. Then, solder paste was applied onto the lands of the substrate. The substrate thus obtained was heated to a temperature equal to or higher than the melting point of the solder particles to melt the solder particles. At this time, five types of experiments (Examples 1 to 5) were conducted while changing the heating rate. Thereafter, the melted solder was solidified by cooling the substrate. The meltability of the solder was evaluated by visually observing the state of the solidified solder. Specifically, if the solder formed after the solder paste melted and solidified aggregated on the land, it was determined as "good"; otherwise, it was determined as "bad".

[0051] Table 1 shows the heating conditions and the evaluation results of the meltability of the solder. In Table 1, the "set temperature of the heater" is the set temperature of the heaters arranged above and below the substrate. T(50) indicates the time taken for the surface temperature of the upper surface of the substrate to reach 50°C from room temperature. T(melting point) indicates the time taken for the surface temperature of the upper surface of the substrate to reach the melting point of the solder particles (221°C) from room temperature. The "time required for temperature rise" indicates the time taken for the surface temperature of the upper surface of the substrate to reach the melting point of the solder particles from 50°C. The "heating rate" indicates the heating rate when the surface temperature of the upper surface of the substrate rises from 50°C to the melting point of the solder particles.

[0052]

Table 1

[0053] As shown in Table 1, when the heating rate was 4.0°C / second or higher, the solder particles melted appropriately, contrary to the common technical knowledge of the prior art. On the other hand, in Examples 1 and 2, the solder particles did not melt appropriately. This is presumably because the time required for temperature rise became long and the surface of the solder particles was oxidized.

[0054] (Embodiment 2) In Embodiment 2, an example in the case where the layer (L) containing solder includes a solder precoat layer will be described. As for the apparatus used for reflow, for example, the reflow apparatus 100 described in Embodiment 1 can be used.

[0055] In Embodiment 2, steps (i) and (ii) can be performed in the same manner as in Embodiment 1, except that the substrate 10X shown in FIG. 5(a) is used instead of the substrate 10x. The change in the substrate 10X due to step (ii) is shown in FIG. 5.

[0056] FIG. 5(a) shows the substrate 10X introduced into the reflow apparatus 100. The substrate 10X includes a printed circuit board 10 including lands 10b, a solder precoat layer 11p formed on the lands 10b, and electronic components 30 disposed on the solder precoat layer 11p. The printed circuit board 10 includes a board portion 10a and lands 10b. The solder precoat layer 11p is a layer (L) containing solder.

[0057] Residue 11pg exists on the surface of the solder precoat layer 11p. The residue 11pg is a residue of the flux generated when forming the solder precoat layer 11p. Flux (solder flux) 12 is disposed around the solder precoat layer 11p. The printed circuit board 10 is the board described in Embodiment 1.

[0058] There is no limitation on the method for preparing the substrate 10X, and it may be prepared by a known method. For example, the substrate 10X may be prepared by the following method. First, a solder precoat layer 11p is formed on the lands 10b of the printed circuit board 10, and then the flux 12 is disposed so as to cover the solder precoat layer 11p. Next, the electronic components 30 are disposed on the lands 10b via the solder precoat layer 11p. In this way, the substrate 10X is obtained. The formation of the solder precoat layer 11p, the disposition of the flux 12, and the disposition of the electronic components 30 can be performed by known methods.

[0059] Next, the solder precoat layer 11p is melted by heating the substrate 10X in the heating zone 110. FIG. 5(b) shows an initial stage of heating the substrate 10X in the heating zone 110. At the stage of FIG. 5(b), a part of the solder precoat layer 11p is melted to form a molten core 11b. The heating can be performed as described in Embodiment 1. As described above, the rate of temperature increase of the surface temperature of the upper surface 10sa and the rate of temperature increase of the surface temperature of the lower surface 10sb are both 4°C / second or more from 50°C to the melting point of the solder, and do not become less than 4°C / second.

[0060] As the melting of the solder precoat layer 11p progresses, as shown in FIG. 5(c), all of the solder precoat layer 11p is melted to become molten solder 11c. Next, the substrate 10X is cooled in the cooling zone 120. As a result, as shown in FIG. 5(d), the molten solder 11c solidifies to become solder 11d. The flux 12 becomes flux residue 12g. In this way, a substrate 10y with the electronic component 30 soldered to the land 10b is obtained. The substrate 10y is an electronic component mounting substrate on which the electronic component 30 is mounted.

Industrial Applicability

[0061] The present invention can be used in a reflow method.

Explanation of Signs

[0062] 10: Printed circuit board (substrate) 10b: Land 10sa: Upper surface 10sb: Lower surface 10x, 10X, 10y: Substrate 11: Solder paste (layer containing solder) 11a: Solder particles 11c, 11d: Solder 11f, 12: Flux 11g, 12g: Flux residue 11p: Solder precoat layer (layer containing solder) 30: Electronic component 100: Reflow apparatus 110: Heating zone 111: Heater 120: Cooling zone 122: Fan 131: Conveyor

Claims

Claim 1 A reflow method for soldering a land of a substrate and a plurality of electronic components, comprising: Step (i) of introducing the substrate having the plurality of electronic components disposed thereon via a layer containing solder into a reflow apparatus; Step (ii) of melting the solder by heating the substrate introduced into the reflow apparatus within the reflow apparatus to solder the plurality of electronic components to the land; In step (ii), while the surface temperature of the surface of the substrate on the side where the land is formed rises from 50°C to the melting point of the solder, the substrate is heated such that the surface temperature rises at a rate of 4°C / second or more; Step (ii) is performed in an air atmosphere within the reflow apparatus. A reflow method. Claim 2 The reflow method according to claim 1, wherein the layer containing solder is a layer formed of a solder paste containing solder particles. Claim 3 The reflow method according to claim 1 or 2, wherein the plurality of electronic components include electronic components having a side of a planar shape of 0.4 mm or less. Claim 4 The reflow method according to any one of claims 1 to 3, wherein the distance traveled by the substrate within the reflow apparatus is 2 m or less.

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