JP2015082630A - Soldering method by powder solder, and fluxless continuous reflow furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soldering method by powder solder which enables the optimization of soldering by solder powder.SOLUTION: A fluxless continuous reflow furnace for soldering an electronic component to a substrate through a solder paste including solder powder and a binder comprises: first and second preliminary heating zones 2A and 2B and a regularly heating zone 3. The substrate 7 is passed through the first and second preliminary heating zones 2A and 2B, which are put almost in an atmospheric pressure condition and then, sent to the regular heating zone 3. The regular heating zone 3 has a closed chamber 5. In the closed chamber 5, the substrate is subjected to a heat treatment. The closed chamber 5 communicates with evacuation means and therefore, the inside of the closed chamber 5 can be decompressed until reaching a substantial vacuum condition. On receipt of the substrate 7, the closed chamber 5 is decompressed until reaching the substantial vacuum condition. Subsequently, a formic acid (HCOOH) is filled in the closed chamber 5 while using an inactive atmospheric gas (Ngas) as a carrier gas. As a result, the pressure inside the closed chamber 5 rises to 70 KPa at one stroke.

Description

  The present invention relates to a soldering method using powder solder and a fluxless continuous reflow furnace.

  In order to cope with recent high-density mounting boards, development of solder paste that does not leave a flux residue after reflow soldering is in progress. Patent Document 1 discloses a solder paste (also called “cream solder”) in which powder solder and a binder made of a polyhydric alcohol having a boiling point between the solid phase line and the liquid phase line of the powder solder are mixed. Disclosure. This solder paste is reflowed in a reducing atmosphere.

  Patent Document 2 discloses a solder paste in which powder solder and an organic polyvalent hydroxy compound (evaporation temperature is 170 ° C. or higher) are mixed.

  Patent Document 3 is a flux that does not contain rosin and evaporates at a reflow temperature, and at least one high-viscosity solvent that is a high-viscosity fluid at room temperature and evaporates at a reflow temperature. A solder paste is disclosed in which at least one liquid solvent that is liquid at normal temperature and evaporates at a reflow temperature and powder solder are mixed.

  A reflow furnace suitable for soldering using this type of solder paste has been proposed (for example, Patent Document 4). This reflow furnace employs a configuration in which the oxide film on the metal surface is removed using the reducing power of formic acid instead of the flux, and is referred to as a “fluxless reflow furnace” in the industry. In this fluxless reflow furnace, there is also a type that uses hydrogen gas as a reducing agent instead of formic acid (Patent Document 5).

JP-A-2-25291 JP-A-9-94691 JP 2004-25305 A JP 2007-125578 A JP 2004-111607 A

  FIG. 6 shows a typical control example of a conventional fluxless reflow furnace. The illustrated fluxless reflow furnace is continuous, and has an inlet at one end in the longitudinal direction and an outlet at the other end. The substrate entering from the entrance passes through the preheating zone while moving intermittently in the reflow furnace, and then enters the main heating zone, where soldering is performed. Then exit from the outlet after being cooled in the cooling zone.

  A conventional fluxless continuous reflow furnace is depressurized before entering the first preheating zone. Further, the conventional fluxless continuous reflow furnace has a gas supply port in the vicinity of the entrance of the first preheating zone, and formic acid is injected into the first preheating zone from the gas supply port. Typically, the first and second preheating zones, the main heating zone, and the cooling zone of the fluxless continuous reflow furnace are filled with nitrogen gas to maintain a substantially atmospheric pressure state.

  Since formic acid is known to exhibit reducing power from 150 ° C., formic acid exhibits reducing power in the first and second preheating zones, and the solvent (binder) contained in the solder paste is vaporized. Since the heating temperature in the main heating zone is set to a temperature at which the powder solder melts, soldering is performed in the main heating zone.

  An object of the present invention is to provide a soldering method and a fluxless continuous reflow furnace that can optimize soldering with solder powder.

The above technical problem is, according to the first aspect of the present invention,
A method of soldering an electronic component to a substrate via a solder paste containing solder powder and a binder,
A preheating step of heating the substrate on which the electronic component is mounted at a temperature lower than a temperature at which the solder powder melts and higher than a temperature at which the binder vaporizes;
Subsequent to the preheating step, a degassing step of evacuating the sealed chamber to remove gas between the electronic component and the substrate;
Next to the degassing step, a reducing agent and an inert atmosphere gas are introduced into the sealed chamber to increase the pressure in the sealed chamber so that the reducing agent enters between the electronic component and the substrate. A reducing gas intrusion process;
There is provided a soldering method using powder solder, comprising: a soldering step of heating the solder powder to a temperature at which the solder powder melts in the sealed chamber following the reducing gas intrusion step. Is achieved.

The above technical problem is, according to the second aspect of the present invention,
A fluxless continuous reflow furnace for soldering an electronic component to a substrate via a solder paste containing solder powder and a binder, the preheating zone for preheating the substrate, and a main heating zone following the preheating zone In a fluxless continuous reflow furnace with
The preheating zone is in an atmospheric pressure state, the preheating zone is set to a temperature at which the solder powder does not melt and the solder paste binder is vaporized,
The main heating zone has a sealed chamber capable of taking an open state and a sealed state,
The sealed chamber is connected to a decompression means for evacuating the sealed chamber;
The closed chamber can be supplied with a reducing agent and an inert atmosphere gas through a supply port leading to the closed chamber,
After the substrate is received in the sealed chamber, the sealed chamber is evacuated, and then the reducing agent is supplied to the sealed chamber and together with the reducing agent or before or after the reducing agent. Supplying the inert gas to the sealed chamber to increase the pressure in the sealed chamber;
This is achieved by providing a fluxless continuous reflow furnace characterized in that the solder powder is heated in the sealed chamber.

  Other objects and operational effects of the present invention will become apparent from the following detailed description of the preferred embodiments of the present invention.

It is a schematic block diagram of the continuous-type fluxless reflow furnace of an Example. It is a figure for demonstrating the pressure state and temperature state of each zone and sealed chamber in the reflow furnace shown in FIG. It is a figure for demonstrating the modification of the pressure state and temperature state of each zone and sealed chamber in the reflow furnace shown in FIG. It is operation | movement explanatory drawing of the process in a preheating process. It is operation | movement explanatory drawing of the process in a sealed chamber. It is a figure for demonstrating the pressure state and temperature state of each zone and sealed room in the conventional fluxless continuous reflow furnace.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 shows a fluxless reflow furnace 1 of the embodiment. The illustrated fluxless reflow furnace 1 is a continuous reflow furnace. As shown in FIG. 1, the fluxless reflow furnace 1 has a plurality of zones arranged in series that are partitioned in the workpiece conveyance direction. In the following description, the fluxless reflow furnace 1 is referred to as “reflow furnace” or “furnace”. Specifically, the reflow furnace 1 includes, in order in the workpiece transfer direction, first and second preheating zones 2A and 2B located on the right side of the drawing, a main heating zone 3 located in the center of the drawing, and a left side of the drawing. One cooling zone 4. The main heating zone 3 has a sealed chamber 5 therein.

  Reference numeral 6 is a partition wall for partitioning each chamber. In each of the zones 2A, 2B, 3, and 4 in the reflow furnace 1, an inert gas (nitrogen gas in the present embodiment) is circulated as an atmospheric gas to prevent solder oxidation.

  The printed wiring board 7 (hereinafter simply referred to as “substrate”) is intermittently conveyed in the reflow furnace 1. Electronic components are mounted on the substrate 7 via a solder paste containing solder powder and a solvent (binder). The substrate 7 sequentially fed into the first and second preheating zones 2A and 2B, the main heating zone 3 and the cooling zone 4 stays in each zone for a predetermined time.

  The sealed chamber 5 of the main heating zone 3 has an upper housing 5A and a lower housing 5B which are divided into upper and lower portions. The lower housing 5B is fixed to the main heating zone 3. On the other hand, the upper housing 5A can be moved up and down by an actuator, specifically, a cylinder device (not shown).

  The sealed chamber 5 can take two states: a sealed state in which the upper housing 5A is lowered and in close contact with the lower housing 5B, and an open state in which the upper housing 5A is lifted. In the sealed chamber 5, the substrate 7 is taken in and out of the sealed chamber 5 with the upper housing 5 </ b> A raised. When the substrate 7 is fed into the sealed chamber 5 from the second preheating zone 2B, the upper housing 5A is lowered and the sealed chamber 5 is sealed. When the processing in the sealed chamber 5 is completed, the upper housing 5A is raised and the sealed chamber 5 is opened, and the substrate 7 processed in the sealed chamber 5 is taken out from the sealed chamber 5. The substrate 7 taken out from the sealed chamber 5 is transferred to the next cooling zone 4.

  In the cooling zone 4, the substrate 7 is cooled for a certain time by the cooling device 8, thereby solidifying the solder. Thereafter, the substrate 7 is unloaded from the cooling zone 4 through the outlet 12 of the furnace 1.

The heating in the reflow furnace 1 will be described again. The substrate 7 is first transferred from the inlet 11 of the furnace 1 to the first first preheating zone 2A. In the first preheating zone 2A, an inert atmosphere gas (N ₂ gas) heated by the heater 14 is circulated, and the substrate 7 is heated by the inert atmosphere gas (N ₂ gas) for a certain period of time. The

The substrate 7 is then sent to the second preheating zone 2B, and the inert gas (N ₂ gas) heated by the heater 15 is also circulated in the second preheating zone 2B. It is heated for a certain period of time by atmospheric gas (N ₂ gas). Of course, the heating method in the first and second preheating zones 2A and 2B is not limited to the heating method using hot air (inert atmosphere gas), or the heating method using far infrared heating (radiant heat) or heat plate (conduction) or You may employ | adopt the heating method by these combination.

The substrate 7 is then sent to the main heating zone 3. In the main heating zone 3, an inert atmosphere gas (N ₂ gas) heated by the heater 16 is circulated. A heater 17 is also disposed in the sealed chamber 5, and an inert atmosphere gas (N ₂ gas) heated by the heater 17 is circulated in the sealed chamber 5. The solder powder of the substrate 7 accommodated in the sealed chamber 5 is in a molten state in the sealed chamber 5. Of course, the heating method in the sealed chamber 5 is not limited to a heating method using hot air (inert atmosphere gas), but a heating method using far infrared heating (radiant heat), a heating plate (conduction), or a combination thereof is adopted. May be.

The sealed chamber 5 communicates with a decompression means (a vacuum source in this embodiment) in order to lower the pressure in the sealed chamber 5, and the inside of the sealed chamber 5 can be decompressed to a substantially vacuum state. The sealed chamber 5 has a gas supply port 18, and formic acid as a reducing agent can be supplied into the sealed chamber 5 through the gas supply port 18 using an inert atmosphere gas (N ₂ gas) as a carrier gas. .

  FIG. 2 is a diagram for explaining the temperature control of the reflow furnace 1 and the pressure control of the first and second preheating zones 2A and 2B, the sealed chamber 5, and the cooling zone 4 in the furnace 1. A substantially atmospheric pressure state is maintained from the inlet 11 of the reflow furnace 1 to the second preheating zone 2B.

When the sealed chamber 5 receives the substrate 7 and is in a sealed state, the sealed chamber 5 is depressurized and almost in a vacuum state. Further, the heater 17 in the sealed chamber 5 is turned on. Immediately after the vacuum state is generated or after the vacuum state is continued for several seconds to several minutes, formic acid (HCOOH) is filled in the sealed chamber 5 using an inert atmosphere gas (N ₂ gas) as a carrier gas. Thereby, the pressure in the sealed chamber 5 rises at once to 70 KPa in this embodiment. This inert atmosphere gas functions as a heat medium for heating the solder powder 24 interposed between the substrate 7 and the electronic component 22. Accordingly, the amount of the inert atmosphere gas filled in the sealed chamber 5 is set to an amount sufficient for the inert atmosphere gas to function as a heat medium in consideration of economy. If this is described in the pressure state of the sealed chamber 5, it is preferably 25 to 110 KPa. Setting the pressure state of the sealed chamber 5 to a pressure (negative pressure) lower than the atmospheric pressure (100 KPa) can easily suppress leakage of the inert atmosphere gas and formic acid (reducing gas) to the outside. Specific examples of the “negative pressure” are 60 KPa to 90 KPa, preferably 65 to 85 KPa. Of course, when the pressure state of the sealed chamber 5 is set to atmospheric pressure (100 KPa) or higher than atmospheric pressure (positive pressure), the reducing gas easily enters between the solder powders.

By the above process, first, the sealed chamber 5 is depressurized to a substantially vacuum state, so that the solvent (binder) gasified in the preheating zones 2A and 2B in the atmospheric pressure state is forced from between the electronic component 22 and the substrate 7. The electronic component 22 and the substrate 7 are in a negative pressure state. Next, formic acid (HCOOH) using an inert atmosphere gas (N ₂ gas) as a carrier gas is introduced into the sealed chamber 5, whereby the pressure in the sealed chamber 5 rapidly rises to a pressure state close to atmospheric pressure, Formic acid (HCOOH) is sucked between the electronic component 22 and the substrate 7. That is, the pressure in the sealed chamber 5 is first reduced to a vacuum state, and then the formic acid is introduced into the sealed chamber 5 and the pressure in the sealed chamber 5 is rapidly increased with an inert atmosphere gas (N ₂ gas). The solvent (binder) gasified in 2B is forcibly replaced with formic acid in the sealed chamber 5.

As a modification of the operation after decompressing the sealed chamber 5, first, an inert atmosphere gas (N ₂ gas) may be introduced into the sealed chamber 5, and then vaporized formic acid may be introduced into the sealed chamber 5. . As another modification, first, vaporized formic acid may be introduced into the sealed chamber 5, and then an inert atmosphere gas (N ₂ gas) may be introduced into the sealed chamber 5.

  An inert atmosphere gas circulates in the sealed chamber 5, and the temperature of the inert atmosphere gas is set to a temperature at which the solder melts. Since this temperature is 150 ° C. or higher, formic acid filled in the sealed chamber 5 immediately exhibits a reducing power.

Instead of the wet chemical reduction method using formic acid (HCOOH) as the reducing agent, a gas reduction method using hydrogen gas (H ₂ ) as a reducing gas may be employed.

By supplying an inert atmosphere gas (N ₂ gas) into the sealed chamber 5 and rapidly increasing the pressure in the sealed chamber 5 to 70 KPa close to atmospheric pressure, the gasified solvent (binder) in the solder paste is converted into formic acid. Forced replacement. Of course, the solder powder is also heated in the sealed chamber 5 by the heated inert atmosphere gas, and the solder powder is melted.

With reference to FIG. 3, a modified example of the above-described control of the sealed chamber 5 will be described. After the solder powder is melted by the heating in the sealed chamber 5, the second decompression is performed to evacuate the sealed chamber 5 again. It may be. This second decompression can suppress the generation of solder voids. After the second decompression, the inert atmosphere gas (N ₂ gas) is preferably supplied again into the sealed chamber 5. This inert atmosphere gas is circulated in the sealed chamber 5.

  FIG. 4 is a view for explaining a preparation process for soldering with solder powder. FIG. 4I shows the substrate 7 on which the electronic component 22 is mounted via the solder paste 20. The solder paste 20 includes a solder powder 24 and a solvent (binder) 26. FIG. 4 (II) shows a step of vaporizing the solvent (binder) 26. The vaporized solvent (binder) is indicated by dots and indicated by reference numeral 26G. The state of (II) in FIG. 4, that is, the vaporization of the solvent 26 occurs in the first and second preheating zones 2A and 2B.

  In order to forcibly remove the solvent 26G gasified by the preheating in the preheating zones 2A and 2B from between the electronic component 22 and the substrate 7, as described above, the gas that first depressurizes the sealed chamber 5 to a vacuum state. A punching process is performed. Next, the pressure in the sealed chamber 5 is increased at once, for example, up to 70 KPa in this embodiment, by filling the sealed chamber 5 with an inert atmosphere gas together with formic acid or before or after formic acid. Thus, a reducing gas intrusion step in which a reducing agent (formic acid in this embodiment) flows between the electronic component 22 and the substrate 7 is executed. This reducing gas intrusion step is performed at a temperature equal to or higher than the temperature at which formic acid performs the reducing action. Next, in the sealed chamber 5, a soldering process is performed in which the solder powder 24 is heated at a temperature at which the solder powder 24 existing between the electronic component 22 and the substrate 7 is melted.

  That is, the solvent (binder) 26 is vaporized in the preheating zones 2A and 2B, and the sealed chamber 5 heated at a temperature at which the solder powder 24 is melted is evacuated, so that the solder powder 24 is dissolved before 7 and the electronic component 22 are forcibly removed from the solvent (binder) 26G. As a result, the gasified solvent (binder) 26G between the solder powder 24 existing between the substrate 7 and the electronic component 22 is removed, and a void 28 ((II) in FIG. 5) is formed between the solder powders 24. Is generated. Next, the formic acid 30 is filled in the sealed chamber 5 and an inert atmosphere gas is introduced to rapidly increase the pressure in the sealed chamber 5 to a pressure state close to the atmospheric pressure. Invades at once. That is, the solvent (binder) 26G gasified in the preheating zones 2A and 2B is forcibly replaced by formic acid. Then, the solder powder 24 is melted.

  FIG. 5 is a diagram for explaining the processing in the sealed chamber 5. As described above, when the closed chamber 5 is received, the sealed chamber 5 is immediately decompressed to a substantially vacuum state. The state at this time is shown in (I) and (II) of FIG. The solvent gas 26G shown in FIG. 4 (II) described above is extracted from between the substrate 7 and the electronic component 22 by reducing the pressure in the sealed chamber 5 to a vacuum state ((I) in FIG. 5). As a result, a space 28 (vacuum state) is formed between the solder powder 24 remaining between the substrate 7 and the electronic component 22 ((II) in FIG. 5).

Next, when the closed chamber 5 is filled with formic acid 30, and at the same time, before or after that, the closed chamber 5 is filled with an inert atmosphere gas (N ₂ gas), the pressure in the closed chamber 5 rapidly rises to 70 KPa. The formic acid 30 filled in the sealed chamber 5 enters the vacuum space 28 between the solder powders 24 at once ((III), (IV) in FIG. 5). The formic acid 30 that has entered the space 28 immediately exhibits a reducing power. Next, the solder powder 24 starts to melt.

  From the above description with reference to FIG. 5, it will be understood that the essential points requiring formic acid reduction are forcibly replaced with formic acid. Thereby, soldering with solder powder can be optimized.

  The present invention is not limited to the continuous reflow furnace described in the above embodiment, but can be applied to a batch reflow furnace.

DESCRIPTION OF SYMBOLS 1 Fluxless reflow furnace 2A 1st preheating zone 2B 2nd preheating zone 3 Main heating zone 5 Sealed chamber 7 Printed wiring board 17 Heater arrange | positioned in sealed chamber 18 Gas supply port (nitrogen gas and formic acid to sealed chamber Supply)
20 Solder paste 22 Electronic component 24 Solder powder 26 Solvent (binder)
26G Solvent gasified in the preheating process (binder)
30 formic acid

Claims (9)

A method of soldering an electronic component to a substrate via a solder paste containing solder powder and a binder,
A preheating step of heating the substrate on which the electronic component is mounted at a temperature lower than a temperature at which the solder powder melts and higher than a temperature at which the binder vaporizes;
Subsequent to the preheating step, a degassing step of evacuating the sealed chamber to remove gas between the electronic component and the substrate;
Next to the degassing step, a reducing agent and an inert atmosphere gas are introduced into the sealed chamber to increase the pressure in the sealed chamber so that the reducing agent enters between the electronic component and the substrate. A reducing gas intrusion process;
A soldering method using powder solder, comprising: a soldering step of heating the solder powder to a temperature at which the solder powder melts in the sealed chamber following the reducing gas intrusion step. The soldering method is performed in a fluxless continuous reflow furnace,
The fluxless continuous reflow furnace has a preheating zone in an atmospheric pressure state and a main heating zone including the sealed chamber,
The soldering method according to claim 1, wherein the preheating step is performed in the preheating zone. The reducing agent is formic acid;
The soldering method according to claim 2, wherein the formic acid is introduced into the sealed chamber together with the inert atmosphere gas.   The pressure of the sealed chamber when the reducing agent and the inert atmosphere gas are introduced into the sealed chamber in the reducing gas intrusion step is 25 KPa to 110 KPa. Soldering method. A fluxless continuous reflow furnace for soldering an electronic component to a substrate via a solder paste containing solder powder and a binder, the preheating zone for preheating the substrate, and a main heating zone following the preheating zone In a fluxless continuous reflow furnace with
The preheating zone is in an atmospheric pressure state, the preheating zone is set to a temperature at which the solder powder does not melt and the solder paste binder is vaporized,
The main heating zone has a sealed chamber capable of taking an open state and a sealed state,
The sealed chamber is connected to a decompression means for evacuating the sealed chamber;
The closed chamber can be supplied with a reducing agent and an inert atmosphere gas through a supply port leading to the closed chamber,
After the substrate is received in the sealed chamber, the sealed chamber is evacuated, and then the reducing agent is supplied to the sealed chamber and together with the reducing agent or before or after the reducing agent. Supplying the inert gas to the sealed chamber to increase the pressure in the sealed chamber;
A fluxless continuous reflow furnace, wherein the solder powder is heated in the sealed chamber.   The fluxless continuous reflow furnace according to claim 5, wherein the reducing agent is supplied to the sealed chamber together with the inert atmosphere gas.   The fluxless continuous reflow furnace according to claim 5 or 6, wherein the reducing agent is formic acid. A heater is installed in the sealed chamber,
The fluxless continuous reflow furnace according to any one of claims 5 to 7, wherein the inert atmosphere gas is circulated in the sealed chamber.   9. The pressure in the sealed chamber when the reducing agent and the inert atmosphere gas are introduced into the sealed chamber in the reducing gas intrusion step is 25 KPa to 110 KPa. 9. Fluxless continuous reflow furnace.

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