Solder reflow apparatus

A reflow apparatus for soldering has a tunnel and a pipe which form an endless passageway for hot gas. Infrared panel heaters are disposed inside the tunnel for heating printed circuit boards which travel through the tunnel on a conveyor. Hot gas is made to circulate through the passageway by a flow-producing device. A filter is disposed in the passageway for removing particles from fumes which are formed during soldering.

BACKGROUND OF THE INVENTION 
This invention relates to a solder reflow apparatus which is used for 
soldering of printed circuit boards using solder cream. 
In general, solder reflow apparatuses can be classified as radiation types 
and hot gas types. A radiation-type reflow apparatus is one in which a 
large number of panel heaters are disposed in the upper and lower portions 
of a tunnel, and printed circuit boards are heated by the heat radiated 
from the panel heaters. The inside of the apparatus can be heated to a 
suitable temperature for soldering by controlling the current to be 
supplied to the panel heaters. Recently, panel heaters which emit far 
infrared radiation have come to be used in reflow apparatuss. Far infrared 
radiation heaters are efficient at heating objects and are said to be 
suitable for the soldering of printed circuit boards in solder reflow 
apparatuses. However, far infrared radiation has the drawback that as 
printed circuit boards and electronic components become ever thinner and 
more sensitive to heat, it becomes easier for even slight overheating by 
the radiation to produce thermal damage. Another problem with 
radiation-type reflow apparatuses relates to the fact that panel heaters 
which emit far infrared radiation are made of electrically resistive 
materials and are heated up by the application of electric current. The 
temperature of the material which surrounds the heater does not 
immediately change in response to changes in the heater current. Namely, 
even if the current to a panel heater is cut off when the heated portion 
reaches a prescribed temperature, the temperature of the material 
surrounding the heater may continue to rise for a short while. On the 
other hand, if the temperature of the heated portion is below a prescribed 
temperature and current is passed through the heater, the temperature of 
the material surrounding the heater may be falling and will not 
immediately start to rise. 
A solder reflow apparatus of the hot gas type is one in which hot gas is 
always circulating past a heater, so there is the advantage that the 
heating temperature can always be maintained constant. For this reason, in 
recent years, hot gas reflow furnaces of the tunnel type have come to be 
much used. 
Due to recent advances in the technology of mounting components on printed 
circuit boards, devices which formerly had to be mounted on separate 
printed circuit boards can now be mounted on a single printed circuit 
board. It has also become possible to mount electronic parts such as power 
transistors on printed circuit boards. However, the packaging density of 
such printed circuit boards tends to be nonuniform over the area thereof, 
and this fact along with the presence of large components results in the 
heat capacity locally varying over the printed circuit board. If such 
printed circuit boards are soldered using far infrared radiation, the 
temperature of portions having a high heat capacity will not rise as much 
as other portions, causing incomplete melting of the solder and poor 
electrical connections. On the other hand, in those portions of the 
printed circuit board having a low heat capacity, the temperature will 
rise too high, causing scorching of the printed circuit board and heat 
damage to electronic parts. 
It has been found that uneven heating of a printed circuit board can be 
prevented by passing hot gas through a tunnel so as to achieve a uniform 
temperature within the tunnel. Various devices have been proposed for 
passing a hot gas through tunnels for this purpose. See, for example, 
Japanese Laid-Open Utility Model Application No. 59-61567, Japanese 
Laid-Open Patent Application No. 59-220282, and Japanese Laid-Open Patent 
Application No. 61-141199. However, each of these devices has various 
problems. 
The solder reflow apparatus disclosed in Japanese Laid-Open Utility Model 
Application No. 59-61567 has a fan mounted on the upper portion of a 
tunnel, so hot gas is blown only downwards, and there is no effect from 
the hot gas except in the region immediately below the fan. Furthermore, 
as the hot gas circulates only in the vertical direction of the tunnel, it 
is difficult to adjust the temperature merely by adjusting the rotational 
speed of the fan. 
Japanese Laid-Open Patent Application No. 59-220282 discloses a solder 
reflow apparatus in which a gaseous heating medium is introduced from the 
outside of the furnace and is preheated by an auxiliary heater or the 
like. In that device, it is possible to control the temperature of the hot 
gaseous medium which flows through the tunnel. However, after passing 
through the tunnel, the gaseous heating medium is discharged to the 
outside of the tunnel through a discharge port. Accordingly, this solder 
reflow apparatus is expensive to manufacture, and it is uneconomical to 
operate because electricity is used to heat the gaseous heating medium and 
because the gaseous heating medium is discarded. Furthermore, if the 
gaseous heating medium is discharged into a room, the room becomes filled 
with fumes which smell of flux and pose an air pollution problem. 
Japanese Laid-Open Patent Application No. 61-141199 discloses a solder 
reflow apparatus in which air enters through a lower air intake port and 
is heated by a heater. The hot air is discharged after it has been used 
for heating, so that invention suffers from the same problems of poor 
economy and of causing air pollution as does Japanese Laid-Open Patent 
Application No. 50-220282. 
Japanese Laid-Open Patent Application No. 63-215371 corresponding to U.S. 
Pat. No. 4,771,929 discloses circulation of part of the hot gas withdrawn 
from the furnace. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a novel 
solder reflow apparatus which can solve the above-described drawbacks of 
conventional hot gas reflow apparatuses. 
It is another object of the present invention to provide a solder reflow 
apparatus which allows the easy adjustment of the temperature of gases 
within the furnace. 
It is yet another object of the present invention to provide a solder 
reflow apparatus which does not discharge gases or unpleasant odors to the 
outside thereof. 
It is still another object of the present invention to provide a solder 
reflow apparatus which consumes little energy. 
These objectives could conceivably be met by a solder reflow apparatus 
which employs a circulating heated gas. However, such solder reflow 
apparatuses of the gas-circulating type suffer from problems related to 
the nature of solder cream which is used to solder printed circuit boards 
in reflow apparatuses. 
Solder cream is a viscous cream which is formed by mixing a flux and solder 
powder. The flux is made by liquefying a solid component such as pine 
resin, a thixotropic agent, or an activator using a solvent. When cream 
solder is heated, fumes are formed which contain not only the solvent but 
also the solid component of the flux, and the inside of the solder reflow 
furnace becomes filled with the fumes. In a solder reflow apparatus of the 
gas-circulating type, the fumes are circulated together with the hot gas. 
As the concentration of the fumes inside the furnace increases, the thick 
fumes contact the walls of the oven and the blower. The contact with these 
parts cools the fumes and causes the solid components to resolidify and 
adhere to the surface which they contact. Over a long period of time, the 
adhered solids accumulate to a considerable thickness, and at times, they 
peel and fall off. If the falling solids fall onto a printed circuit board 
which is passing through the reflow furnace, not only will the printed 
circuit board become dirty, but the falling solids can cause a decrease in 
electrical resistance, corrosion, and other problems which can adversely 
affect the electrical components on the printed circuit board. In 
addition, if solids originating from flux fumes adhere to the blower which 
circulates hot gas through the furnace, the operation of the blower can be 
impaired, and solids which have built up on the blower can be thrown by 
the blower with great force at printed circuit boards passing through the 
oven, resulting in damage to the printed circuit boards. 
In the present invention, in order to solve this problem, a filter is 
disposed in an endless passageway for circulating hot gas along which an 
object to be soldered is passed. Solid particles in fumes which are 
generated during soldering can be removed from the circulating hot gas by 
the filter and prevented from adhering to the inside of the furnace. 
Therefore, the hot gas which circulates through the furnace is always kept 
clean, and damage to objects being soldered by falling solids or damage to 
blowers and other parts due to solids adhering thereto can be prevented. 
In addition, the hot gas always remains inside the solder reflow apparatus 
and is repeatedly reused, so the amount of heat required to heat the gas 
is reduced. 
Furthermore, because the hot gas remains in the solder reflow apparatus, no 
noxious fumes are discharged to the outside of the solder reflow 
apparatus. 
A solder reflow apparatus of the tunnel type in accordance with the present 
invention comprises a tunnel provided with a preheating zone and a main 
heating zone, an endless passageway for circulating a hot gas through at 
least a portion of the tunnel, flow-producing means for causing a gas to 
circulate along the passageway, a conveyor for transporting an object to 
be soldered along a portion of the passageway, and a filter which is 
disposed along the passageway for filtering the gas as it circulates 
through the passageway. 
In one aspect of the invention, the tunnel has a gas supply port and a gas 
recovery port, and the passageway comprises a pipe which is connected 
between the gas supply port and the gas recovery port on the outside of 
the tunnel. The gas supply port and the gas recovery port are disposed at 
different points along the length of the tunnel so that the hot gas will 
flow through the tunnel in the axial direction thereof and parallel to the 
direction of movement of the conveyor. 
In another aspect of the present invention, the passageway is provided 
within a tunnel through which the conveyor passes and a hood which is 
disposed inside the tunnel above the conveyor and which has an inlet in 
the upper portion thereof and an outlet which confronts the conveyor. The 
filter is mounted in the hood, and the flow-producing means is a blower 
which is disposed inside the hood. Instead of circulating in the axial 
direction of the tunnel, the hot gas circulates in the vertical direction 
while it passes through the hood and the filter. 
A heater is provided within a tunnel for heating an object as it is 
transported by the conveyor. The heater is not restricted to any certain 
type, but infrared and far infrared heaters are particularly suitable 
because of their heating efficiency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinbelow, a number of preferred embodiments of a solder reflow apparatus 
in accordance with the present invention will be described while referring 
to the accompanying drawings, of which FIGS. 1 and 2 illustrate a first 
embodiment. As shown in these figures, the solder reflow apparatus is of 
the tunnel-type comprising a tunnel 1 which is furnished with a plurality 
of infrared heaters 2 and 2' in the upper and lower portions thereof. The 
heaters 2' on the left side of the tunnel 1 are preheaters, while the 
heaters 2 on the right side of the tunnel 1 are main heaters. Thus, the 
solder reflow apparatus comprises a preheating zone provided with the 
heaters 2' and a solder reflow zone, i.e., main heating zone provided with 
the heaters 2. A cooling zone may also be provided. A conventional 
conveyor 3 transports printed circuit boards P between the upper and lower 
heaters 2 and 2' in the direction shown by arrow A. A gas supply port 4 
and a gas recovery port 5 are formed at opposite axial ends of the tunnel 
1. In this embodiment, the gas supply port 4 is near the exit of the 
tunnel 1 while the gas recovery port 5 is disposed near the entrance. The 
gas supply port 4 and the gas recovery port 5 are disposed at opposite 
ends of the tunnel 1 so that hot gas will circulate in a direction 
parallel and opposite to the direction of movement of the printed circuit 
boards P, whereby a uniform temperature distribution of the printed 
circuit boards P can be attained. 
The gas supply port 4 and the gas recovery port 5 are connected with one 
another by a gas-circulating conduit, e.g. pipe 6 which extends around the 
outside of the tunnel 1. A flow-producing device 7 such as a pump or a 
blower is installed in the pipe 6. The flow-producing device 7 is a device 
which makes a gas flow in one direction through the pipe 6 from the gas 
recovery port 5 to the gas supply port 4. The flow speed of the gas can be 
adjusted by controlling a drive motor 8 which powers the flow-producing 
device 7. When the temperature of the hot gas within the pipe 6 exceeds a 
prescribed temperature, cool air from the outside can be introduced into 
the pipe 6 through an air inlet 9. It is also possible to introduce an 
inert gas into the tunnel 1 through the air inlet 9 and create an 
atmosphere of inert hot gas within the tunnel 1. When the temperature of 
the hot gas within the pipe 6 becomes too low, the hot gas can be heated 
by a heater 10. 
When the cream solder on the printed circuit boards P melts, volatile 
components of the flux of the cream solder vaporize, and these components 
along with solid particles form fumes. These fumes are entrained in the 
circulating gas flowing through the tunnel 1. To prevent these fumes from 
adhering to inside of the tunnel 1 and the pipe 6, the filter 11 is 
disposed in the pipe 6. A filter 11 can also be disposed in the gas 
suction port 5. 
Next, the properties of a solder reflow apparatus according to the present 
invention will be described. 
FIG. 3 shows the temperature profile of printed circuit boards which were 
soldered using the embodiment of FIG. 2, while FIG. 4 shows the 
temperature profile for the same solder reflow apparatus when only 
infrared heaters were used without a circulating hot gas. 
As is clear from FIG. 4, when heating is performed using only infrared 
heaters, during the main stage of heating in which the cream solder is 
melted, there is a difference "d" of up to 50.degree. C. between the 
minimum temperature 12 and the maximum temperature 13 of the printed 
circuit boards P. However, using the solder reflow apparatus of the 
present invention in which hot gas is circulated through the tunnel 1, the 
difference "d'" between the minimum and maximum temperatures is at most 
20.degree. C. 
In this embodiment, hot gas is brought to the outside of the tunnel 1 
through a pipe 6 of which part of the passageway for hot gas is composed. 
When the temperature of the hot gas becomes too high, outside air can be 
introduced into the pipe 6, and if the temperature of the hot gas is too 
low, the hot gas can be heated by the heater 10, which can be disposed 
either on the inside or the outside of the pipe 6. Thus, the temperature 
of the hot gas can be easily controlled. 
The provision of the filter 11 makes it possible to constantly remove fumes 
which are contained in the circulating hot gas. Therefore, the heated gas 
which is blown at the printed circuit boards P is always clean. 
Furthermore, fumes can be prevented from adhering to the printed circuit 
board P or to the inside of the tunnel 1 or the pipe 6. 
FIG. 5 is a longitudinal cross-sectional view of a second embodiment of the 
present invention. In this embodiment, far infrared heaters 2 and 2' such 
as those disclosed in U.S. patent application Ser. No. 156,632 are mounted 
inside a tunnel 1 above and below a conveyor 3. The main heaters 2, which 
have a porous surface, are each disposed over a gas supply port 4. The 
porous surface of the main heaters 2 may be made of a ceramic, so they 
generate far infrared rays which can heat the boards P efficiently. A 
gentle flow of hot gas is discharged into the tunnel 1 from the porous 
surface of the main heaters 2, so there is no danger of electronic 
components being blow off a printed circuit board by a blast of hot gas. 
This embodiment is otherwise the same as the embodiment of FIG. 2. 
FIG. 6 is a longitudinal cross-sectional view of a third embodiment of the 
present invention. In this embodiment, a gas recovery port 5 is formed 
near the lengthwise center of a tunnel 1. A gas supply port 4 is formed in 
the upper portion of the tunnel 1 between two preheaters 2', and another 
gas supply port 4 is formed near the exit of the tunnel 1. A filter 11 is 
installed in the gas recovery port 5. The gas recovery port 5 is connected 
to both gas supply ports 4 by two pipes 6 which extend to the outside of 
the tunnel 1 and branch from a flow-producing device 7. Each pipe 6 is 
equipped with its own outside air inlet 9 and heater 10 so that the 
temperature of the gas flowing through each pipe 6 can be controlled 
independently of the temperature in the other pipe 6. Hot gas at a first 
temperature, such as 150.degree. C., is blown from the gas supply port 4 
near the exit, and hot gas at a second temperature, such as 220.degree. 
C., is blown from the other gas supply port 4 between the two preheaters 
2'. Both streams of hot gas flow towards the center of the tunnel 1 and 
into the gas recovery port 5. The use of two streams of hot gas of 
different temperatures reduces the difference between the maximum and 
minimum temperatures of the printed circuit boards P within the tunnel 1 
to at most 10.degree. C. The hot gas at 220.degree. C. increases the 
temperature of the printed circuit boards P in the preheating zone 
provided with the preheaters 2' when they are passing therebetween, and 
the hot gas at 150.degree. C. which blows over the printed circuit boards 
P in the main heating zone provided with the main heaters 2 transfers heat 
from the high temperature portion of the printed circuit board P to the 
low temperature portion thereof, thereby decreasing the temperature 
difference between the two portions. 
In each of the preceding embodiments, hot gas is made to circulate through 
a tunnel 1 in the axial direction thereof FIGS. 7 and 8 illustrate a 
fourth embodiment of the present invention in which hot gas circulates 
vertically within the tunnel furnace. As shown in FIG. 7, which is a 
longitudinal cross-sectional view of this embodiment, a tunnel 1 is 
divided into a preheating zone 20, a main heating zone 21, and a cooling 
zone 22. A conventional conveyor 3 which is powered by an unillustrated 
drive mechanism passes through the tunnel 1 and transports printed circuit 
boards P in the direction of arrow A. A plurality of preliminary heaters 
2' and main heaters 2 are disposed in the bottom portion of the tunnel 1 
so as to heat the printed circuit boards P from below. A plurality of 
hoods 23 are mounted in the upper portion of the tunnel 1 above the 
conveyor 3 in the preliminary heating zone 20 and the main heating zone 
21. Each hood 23 extends downwards from the top of the tunnel 1 towards 
the conveyor 3 and has an inlet 23a in the upper portion thereof and an 
outlet 23b in the bottom portion thereof which confronts the conveyor 3. A 
removable filter 26 for filtering hot gas is mounted in the inlet 23a. The 
filter 26 can be cleaned by washing when it becomes clogged. A blower 24 
for circulating hot gas extends into each hood 23 from above. The blowers 
24 are driven by corresponding electric motors 25 which are mounted atop 
the tunnel 1. The cooling zone 22 is not equipped with a hood 23 but has 
another blower for blowing cooling air onto the printed circuit board on 
the conveyor 3. Cool air from the outside of the tunnel 1 is drawn into 
the tunnel 1 through a plurality of holes 27 for cooling air which are 
formed in the top surface of the tunnel 1 in the cooling zone 22. This 
cool air is blown at the printed circuit boards P in the cooling zone 22 
in order to cool them. 
In this embodiment, as shown in FIG. 8, which is a transverse 
cross-sectional view thereof, a passageway for the circulation of hot gas 
is formed by the inside of the hood 23 and the space between the outside 
of the hood 23 and the inside of the tunnel 1. Gas inside the tunnel 1 is 
heated by the heaters 2 and 2'. The blower 24 blows hot gas downwards, as 
shown by the arrows, through the outlet 23b of the hood 23 towards the 
conveyor 3. After leaving the hood 23, the hot gas flows upwards between 
the outside of the hood 23 and the side walls of the tunnel 1 and rises to 
the top of the tunnel 1, where it reenters the hood 23 through the inlet 
23a. The hot gas flowing over the printed circuit boards P melts the 
solder cream which is coated on the boards P, and the melted flux in the 
solder generates fumes which are entrained in the hot gas. As the hot gas 
passes through the inlet 23a of the hood 23, it is cleaned by the filter 
26, and any large fume particles which are entrained in the hot gas are 
collected by the filter 26. Therefore, the hot gas which is discharged 
from the outlet 23b is kept clean. 
There is no restriction on the location of the filter 26, and FIG. 9 
illustrates a fifth embodiment in a cross-sectional view similar to FIG. 
8, in which a filter 26 is installed in the outlet 23b of a hood 23. The 
structure of this embodiment is otherwise identical to that of the 
previous embodiment. Installing the filter 26 at the lower end of the hood 
23 reduces the strength of the stream of hot gas which is blown at the 
printed circuit boards P and prevents the printed circuit boards P from 
being damaged by a blast of hot gas. 
In the embodiments of FIGS. 7-9, the circulating hot gas always remains 
inside the tunnel 1. However, it is also possible to circulate the hot gas 
from the lower end to the upper end of the hood 23 by a pipe which extends 
to the outside of the tunnel 1 in a manner similar to the previous 
embodiments. 
The structure of a filter employed in the present invention is not 
restricted to a specific one so far as it can efficiently remove solid 
particles from the circulating hot gas or it can remove fumes from the hot 
gas when they solidify on the surface thereof. An example of the filter is 
a net of stainless steel mesh. Preferably, two or three nets of stainless 
steel mesh are placed on one another to form a panel filter. 
Although the present invention has been described with preferred 
embodiments it is to be understood that variations and modifications may 
be employed without departing from the concept of the invention as defined 
in the following claims.