JPH11121921A - Method and device for soldering electronic components - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a method and a device for soldering, by which a proper solder fillet can be formed in a step for mounting chip elements to a circuit board through soldering by preventing an undercut such as soldering failures in a solder layer under the chip. SOLUTION: In a soldering furnace, in which a heating block 14 is provided in a first half as a temperature rising zone along a substrate transfer route within a tunnel-like chamber 4, and a cooling block 15 is provided in a second half as a temperature dropping zone therealong, a circuit board 1 and a chip element 2 applied with a spare solder are fed into the furnace and they are overlapped with each other. Then a solder is melted and solidified for soldering, riding midway between the blocks. In the transfer zone of a temperature dropping zone within the furnace, a needle bundle 18 consisting of many needle-like columns 18a is provided on the substrate-supporting surface of a cooling block 15a close to a positions, where a solder is solidified into a solid phase from a liquid phase, and a circuit board passing there is supported by a point contact. Thus, the unevenness of temperature on the substrate can be suppressed and the entire solder layer be solidified at the same time, thereby forming a solder fillet having no undercut.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic component for mounting various electronic components (surface mounted chip elements) including a semiconductor chip (bare chip) on a circuit board by a reflow soldering method, for example, for a power transistor module. And a soldering apparatus.

[0002]

2. Description of the Related Art A chip element (bare chip) having an electrode surface formed on a lower surface is solder-mounted on a circuit board (for example, a direct bonded copper substrate in which copper foil is directly bonded to both surfaces of a ceramic substrate). A reflow soldering method using a soldering furnace using a hot plate as a reflow heat source, in which a chip element subjected to preliminary soldering is superimposed on a circuit board, is generally adopted.

Next, taking the power transistor module described above as an example, a basic configuration of a module assembly thereof and a conventional soldering apparatus employed as a die bonder will be described. First, FIG. 3A shows a power transistor assembly. In the figure, reference numeral 1 denotes a circuit board (copper foil 1
b, 1c, etc.), 2 is a power transistor chip element (bare chip) solder-mounted on a circuit pattern formed on the upper surface side copper foil 1b of the circuit board 1, and the chip element 2 is An electrode serving as a soldering surface is formed on the lower surface, and a solder (for example, Pb 98% -Sn 2% (melting point 3
22 ° C), Pb95% -Sn5% (melting point 314 ° C), Pb9
3.5% -Sn5% -Ag1.5% (melting point 296 [deg.] C.)
3 are joined.

Next, the construction of the soldering apparatus is shown in FIGS.
This will be described in (a). In each figure, the soldering equipment
Tunnel-shaped main chamber 4 of the soldering furnace, substrate loading / unloading conveyors 5 and 6 laid on the entrance and exit sides of main chamber 4, and one circuit board 1 described above at the entrance side of main chamber 4. Loader 7 to supply to carry-in conveyor 5
An unloader 8 for taking out the circuit board soldered to the chip element 2 at the outlet side from the unloading conveyor 6; and a branch for carrying in the chip element drawn laterally from the soldering section 9 set at the center of the main chamber 4. A chamber 10 and a pre-soldering portion 11 installed inside the branch chamber 10
The chip element 2 is sucked and held on a vacuum chuck with the soldering surface facing down, and is transferred from the entrance side of the branch chamber 10 to the soldering section 9 of the main chamber 4 via the preliminary soldering section 11. A chip transfer mechanism 12 with a unit, a chip loader 13 for transferring chip elements 2 one by one to the chip transfer mechanism 12 at the entrance side of the branch chamber 10, and a furnace bottom side along a transfer path in the main chamber 4. And a hot plate (reflow heat source) laid on the floor. Here, the hot plate includes a heating block 14 with a built-in electric heater arranged in a heating zone between the entrance of the main chamber 4 and the intermediate soldering section 9, and a temperature decrease between the soldering section 9 and the chamber outlet. A cooling block 15 of a water-cooling type arranged in a zone, and a walking beam type substrate transport mechanism for intermittently pitching the circuit board 1 carried in from the entrance of the main chamber 4 toward the exit while transiting between these blocks ( The mechanism will be described with reference to FIG. 6).

Further, a mixed gas of nitrogen and hydrogen is blown into the main chamber 4 from the inlet side, and a reducing gas atmosphere is flowed in the first half of the temperature rising zone, and a nitrogen gas is flowed in the second half of the temperature drop zone to flow the solder of the substrate 1. The attachment surface is prevented from being oxidized, and at the same time, the oxide film formed on the soldering surface of the circuit board 1 is reduced and removed, thereby ensuring the “wetability” of the solder.
Further, nitrogen gas is blown into the inside of the branch chamber 10 to maintain the atmosphere in a non-oxidizing atmosphere, thereby preventing the preliminary solder layer applied to the chip element 2 from being oxidized.

Further, the heating block 14 and the cooling block 15 perform temperature control for each block, and the circuit board 1 transferred in the furnace while being connected on the block is shown in FIG.
As shown by the temperature distribution in (b), the process proceeds from a rise in temperature to a fall in temperature. Here, t _H is a soldering temperature set slightly higher than the solder melting point, and t _L is a temperature at which the soldered substrate is taken out from the outlet of the main chamber 4 and is set to, for example, about 50 ° C. In the soldering apparatus having the above-described configuration, a tact system is assembled to automatically control the operation of each step according to a command from an operation control unit. In particular, the circuit board is transferred between the heating and cooling blocks while being transferred. In this case, preheating and cooling are performed by stopping the substrate for about several tens of seconds in a state where the substrate is transferred onto each block.

When the circuit boards 1 are supplied one by one to the carry-in conveyor 5 with the soldering surface facing upward by the loader 7, the circuit boards 1 enter the main chamber 4 and are transferred to the heating block 14. After that, the heating block 14 is intermittently transferred and transferred to the soldering section 9 where it is soldered to the chip element 2 as described later. Conveyed in the furnace. Here, in the first half of the heating zone in the furnace, the circuit board 1 is heated by the heat transfer from the heating block 14 and rises to the soldering temperature t _H , and in the second half of the cooling zone between the cooling block 15 Cooled by heat transfer, the temperature at the outlet drops to t _L.

On the other hand, with the soldering surface facing down, the loader 1
3 is supplied to the chip transport mechanism 1
The chip element 2 is sucked and held by a vacuum chuck with a preheater attached to the lifting unit 2 and then transferred to the pre-soldering section 11 arranged on the way of the transport path, where the chip element 2 Form a preliminary solder layer. Subsequently, the chip element 2 is carried into the soldering section 9 of the main chamber 4 through the branch chamber 10, where the chip element 2 waits in a preheated state, and is placed on the circuit board 1.
2 lowers the chip element 2 from above to the circuit board 1
And the surface of the preliminary solder layer is pressed against the substrate 1. As a result, the preliminary solder is melted and the chip element 2 and the circuit board 1 are soldered. After that, after separating the chip transport mechanism 12 from the chip element 2, the circuit board 1 is transferred from the soldering section 9 to the cooling block 15 laid in the lower temperature zone in the latter half.
The transfer between the cooling blocks is carried to the outlet of the main chamber 4. In this transport process, the molten solder solidifies from a liquid phase to a solid phase, and when it reaches the outlet end, it is cooled down to near normal temperature, and is taken out from the outlet side of the main chamber 4 via the unloading conveyor 6 by the unloader 8.

Next, as described above, a walking beam type substrate transfer mechanism for transferring the substrate 1 in the furnace while transferring between the heating block 14 and the cooling block 15 in the main chamber 4 is shown in FIGS. This will be described with reference to the principle diagram (b). That is, the cooling and heating blocks 14 and 15 arranged in a straight line have a pair of left and right concave grooves 16 formed on the upper surface side. Further, a pair of long left and right substrate support beams 17 (the beam is smaller than the depth of the concave groove 19) are laid so as to pass through the concave groove 16 between the blocks, and the beam 17 is driven by a driving unit ( (Not shown) to form a walking beam type substrate transfer mechanism.

In such a configuration, the substrate support beam 17 is operated by operating the drive unit, as shown in FIG.
Circuit board 1 placed between right and left beams 17
Is intermittently conveyed in the direction of the arrow P while being transferred between blocks. When the beam 17 is lowered, the circuit board 1 is placed on the upper surfaces of the heating block 14 and the cooling block 15, and the board is heated or cooled at this position.

[0011]

By the way, in the case where the chip element 2 is solder-mounted on the circuit board 1 by using the above-mentioned soldering furnace, many defects such as the following defective soldering occur. That is, the heating block 14 with a built-in electric heater laid along the substrate transfer path in the furnace,
In the cooling block 15 having a built-in water cooling tube, it is actually difficult to uniformly heat and cool the entire block, and the temperature distribution on the upper surface of the block is not uniform due to local temperature variations. Further, when the circuit board (ceramic substrate) 1 carried into the soldering furnace is preheated to the soldering temperature by connecting to the heating block 14 in the heating zone, the board of the circuit board 1 is unevenly heated during the transportation. The surface is warped by about 100 to 200 μm and deformed, and there are portions that contact the circuit board 1 and the upper surfaces of the heating and cooling blocks 14 and 15 and portions that do not contact the upper surface. Therefore, heat is not transferred evenly between the heating / cooling block and the circuit board 1 placed on the upper surface of the block over the entire area of the board, so that the temperature of the circuit board 1 conveyed in the furnace becomes uneven. Become like

In this case, if the circuit board 1 has a temperature unevenness in the process of solidifying the solder from the liquid phase to the solid phase, particularly in the transport region in the temperature lowering zone in the latter half of the furnace, the solder layer The whole does not solidify at the same time, and the solder in the last solidified portion flows by the volume contraction of the previously solidified solder portion so as to be pulled by the surface tension. As a result, as shown in FIG. Undercut 3a is generated in solder layer 3 below. Due to the occurrence of the undercut 3a, a good solder fillet is not formed, and stress concentration occurs in this portion due to a notch effect acting on the portion of the undercut 3a, so that the solder joint strength is reduced.

Further, as in the power transistor module described above, 100
When heat shocks exceeding ℃ are repeatedly applied,
Starting from the notch of the undercut 3a, there is a possibility that a crack may occur in the solder layer 3 and the chip element 2 may develop into a fatal trouble such as peeling off from the circuit board 1.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to solve the above-mentioned problems, and to provide a soldering step for solder-mounting a chip element on a circuit board. It is an object of the present invention to provide an improved method and apparatus for soldering an electronic component so that a good solder fillet free from undercuts, which can cause defects, can be formed.

[0015]

In order to achieve the above object, according to the soldering method of the present invention, a circuit board and a preliminary solder are applied by using a soldering furnace having a hot plate laid in the furnace. The chip element is loaded into the furnace, the two are superimposed, and the solder is melted and solidified in the course of the transfer to perform the soldering. In the transfer area where the solder solidifies from the liquid phase to the solid phase in the furnace The circuit board is supported in a point contact manner from the lower surface side (claim 1).

According to the above-described soldering method, the circuit board is supported in point contact with the hot plate in the transport region where the preliminary solder melted in the temperature lowering zone in the furnace solidifies from the liquid phase to the solid phase. As a result, the supporting area of the substrate, that is, the heat transfer area between the supporting body and the supporting body is extremely small as compared with a supporting state in which the bottom surface of the substrate is superimposed on the upper surface of the hot plate and brought into surface contact. The heat transfer is in a state close to thermal insulation. Therefore, the circuit board is hardly affected by the temperature distribution on the plate side in terms of heat transfer, and the temperature unevenness of the board due to this is not caused. As a result, the substrate temperature of the circuit board is determined in relation to the ambient gas atmosphere (uniform temperature) in the furnace, so that the entire area of the solder layer joining the chip elements solidifies from the liquid phase to the solid phase almost simultaneously. become. As a result, an undercut does not occur in the solder layer under the chip, and a good solder fillet is formed.

In the soldering apparatus of the present invention used for carrying out the soldering method, a heating block is provided in a first half temperature rising zone and a cooling block is provided in a second half cooling zone along a substrate transfer path in a tunnel-shaped chamber. The circuit board and the chip element to which the pre-soldering is performed are carried into the soldering furnace constructed by laying, and the two are superimposed and transferred between the heating and cooling blocks. The soldering is performed by melting and solidifying the solder on the way. In the transfer area of the cooling zone in the furnace of the soldering furnace, the cooling block laid at the point where the solder solidifies from the liquid phase to the solid phase The substrate supporting surface is constituted by a sword mountain in which a number of needle-like columns are arranged, and the circuit board is supported on the sword mountain to support in a point contact manner.

With such a configuration, in a region where the circuit board to which the chip element is soldered moves in the temperature lowering zone in the furnace,
When the pre-solder reaches the point where the pre-solder solidifies from the liquid phase to the solid phase, the substrate rests on the sword and is supported in a point contact manner. This allows the solidification of the solder to proceed with almost no thermal effect between the block. Therefore, by employing this soldering apparatus, as described in the section of the soldering method, a solder fillet without undercut is formed below the chip.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 (a) and 1 (b). In the drawings of the embodiment, the same members corresponding to FIGS. 3 to 6 are denoted by the same reference numerals. That is, the soldering device shown in the illustrated embodiment is
Basically, the configuration is the same as that of the conventional apparatus described with reference to FIGS. 4 to 6, but among the cooling blocks 15 laid in the temperature lowering zone in the furnace, in particular, the preliminary solder melted in the soldering part 9 is solidified from the liquid phase. Cooling block 15a laid at the point where it solidifies into a phase
With respect to the first embodiment, a water cooling tube is not provided in the block, and a large number of needle-like columns 18a are planted on the upper surface side of the block.
8, the assembly of the circuit board 1 and the chip element 2 is supported on the sword 18 in a point contact manner.

In order to determine the location of the blade 18, a thermocouple is mounted on the chip mounting position of the circuit board 1 and is actually loaded into the soldering furnace from the entrance side, and at each point during the transportation. The change in substrate temperature is measured with a thermocouple.
After the substrate passes through the heating zone and enters the cooling zone, the position where the measured temperature of the thermocouple becomes a temperature at which the solder solidifies is defined as the installation location of the sword mountain 18.

With this configuration, after the chip element 2 is soldered to the substrate 1 at the soldering section 9 in the main chamber 4 in the soldering step of solder mounting the chip element 2 on the circuit board 1, the substrate 1 is placed in a furnace. When it moves to the temperature lowering zone in the latter half of the inside and reaches the sword mountain 18 installed in the cooling block 15a, it solidifies from the solder liquid phase to the solid phase at this position.

FIG. 2 shows the temperature characteristics of the solder in the temperature lowering zone. In FIG. 2, the horizontal axis of the graph represents the elapsed time since the circuit board 1 was carried into the soldering furnace, and the vertical axis represents the temperature of the solder layer 3 joining the substrate 1 and the chip element 2. The temperature t1 is the liquidus temperature of the solder 3, t2 is the solidus temperature, time T1 is the time when the temperature of the solder layer 3 has reached the liquidus temperature t1, and T2 is the solidus temperature t.
The solidification of the solder proceeds as the temperature of the solder layer 3 shown by the characteristic line A decreases from t1 to t2.

By the way, as described above, in the configuration of the illustrated embodiment, the circuit board 1 is mounted on the cooling block 15a as shown in FIG. And is supported in point contact. Accordingly, the substrate 1 is hardly affected by the heat transfer between the substrate 1 and the cooling block 15a, but rather is uniformly cooled by the gas atmosphere in the furnace, and follows the process of lowering the solder temperature from t1 to t2. As a result, the circuit board 1 can be
The unevenness in the temperature of the substrate caused when the substrate is brought into surface contact with the cooling block is eliminated, and the solidification proceeds simultaneously in the entire region of the solder layer 3. As a result, a good solder fillet without undercut is formed under the chip element 2 as shown in FIG.

[0024]

As described above, according to the soldering method and apparatus of the present invention, in the soldering step of carrying a chip element onto a circuit board by carrying it into a soldering furnace, the soldered board is removed. In the process of transferring the solder to a solid phase from a liquid phase to a solid phase by transferring to a temperature lowering zone, it is possible to suppress the occurrence of temperature unevenness in the circuit board and simultaneously solidify the entire solder layer. As a result, a good solder fillet without undercut is formed under the chip element, and high quality soldering without defects can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a main part of a soldering apparatus according to an embodiment of the present invention, (a) is a configuration diagram in a furnace of a soldering furnace,
(b) is a cross-sectional view of the cooling block with a sword peak in (a).

FIG. 2 is a characteristic diagram showing a change in solder temperature when a circuit board passes through a cooling zone in a soldering furnace.

3A and 3B are assembly diagrams of a power transistor module to which the present invention is applied; FIG. 3A is an assembly diagram of the entire module by normal soldering; FIG. Partial view showing a certain soldering state

FIG. 4 is a plan view schematically showing the overall configuration of the soldering apparatus.

FIG. 5 is a detailed view of the soldering furnace in FIG. 4,
(a) is a cross-sectional side view along the transport path, and (b) is a temperature distribution diagram inside the furnace.

6 is a principle view of a walking beam type substrate transfer mechanism incorporated in the soldering furnace of FIG. 4, (a) is a side view,
(b) is a front view

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Circuit board 2 Chip element 3 Solder layer 4 Main chamber of soldering furnace 9 Soldering part 14 Heating block 15 Cooling block 17 Substrate support beam of walking beam type substrate transfer mechanism 18 Sword mountain 18a Needle-shaped support

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takeshi Yanagisawa Fuji Electric Co., Ltd. 1-1-1, Tanabe-Nitta, Kawasaki-ku, Kawasaki-shi

Claims (2)

[Claims] 1. A method for soldering an electronic component in which a chip element is solder-mounted on a conductor pattern of a circuit board, using a soldering furnace in which a hot plate is laid in a furnace, using a circuit board and a pre-soldered chip. The element is carried into the furnace, the two are superimposed, and the solder is melted and solidified in the course of the conveyance to perform soldering. A method of soldering an electronic component, wherein a circuit board is supported in a point contact manner from a lower surface side. 2. A soldering apparatus used for carrying out the soldering method according to claim 1, wherein a heating block is provided in a first half of the heating zone and a cooling block is provided in a second half of the cooling zone along a substrate transfer path in the tunnel-shaped chamber. The circuit board and the chip element subjected to the preliminary soldering are carried into the furnace in a soldering furnace configured by laying blocks, and the two are superimposed and transferred between the heating and cooling blocks. The soldering is performed by melting and solidifying the solder in the middle of the transfer. In the transfer area of the cooling zone in the furnace of the soldering furnace, cooling is laid at the point where the solder solidifies from the liquid phase to the solid phase. An electronic component soldering device, wherein a substrate supporting surface of a block is constituted by a sword mountain in which a large number of needle-like columns are arranged.