WO1991004824A1

WO1991004824A1 - Combined i.r./convection reflow soldering system

Info

Publication number: WO1991004824A1
Application number: PCT/CA1990/000310
Authority: WO
Inventors: John Gileta
Original assignee: Electrovert Ltd.
Priority date: 1989-09-28
Filing date: 1990-09-25
Publication date: 1991-04-18

Abstract

A soldering machine (10) is disclosed for reflowing solder on surface mount devices (SMD's) which are printed circuit boards (PCB's) without through holes. In addition to the usual infrared heaters forced air heating is provided. This is achieved by providing holes (24) in the upper and lower heaters (16, 18) together with appropriate ducts (56) and blowers (58) so that air is passed through the upper heaters (16) across the conveyor (12) and then through the lower heaters (18) for recirculation back to the upper heaters (16). Pressure equalization chambers (32) immediately prior to at least the upper heaters (16) are provided to ensure uniform flow across the heaters.

Description

COMBINED I.R./ CONVECTION REFLOW SOLDERING SYSTEM

Background of the Invention This invention relates to automatic soldering machines for soldering components on printed circuit boards. Some PCB's do not have through holes and these are known as surface mount devices (SMD's). A preferred soldering technique for SMD's is infrared (IR^ reflow in which solder paste on the PCB's is caused to reflow upon the application of heat. More particularly, heating takes place in separately controlled zones within the machine permitting precise temperature profiling. The boards pass by wire mesh conveyor first into a preheat zone which gently heats the solder paste to drive off volatile solvents and avoid thermal shock to the PCB's. After preheating, the PCB's pass into the main heating zones where they are raised to reflow temperature. On exiting from the heating zones the boards are cooled, typically by blowing air up through the conveyor mesh, to solidify the solder joints. In a development of this technique, automatic soldering machines have been designed in which the IR heating is supplemented with forced convection. In one such machine marketed by the assignee of the present application, blowers are provided above a top row of heaters which are perforated so that hot air is blown on to the PCB's. While this system does reduce "hot spots" caused by irregular product geometries there is room for further improvement.

Summary of the Invention It is an object of the present application to provide a combined IR and forced convection soldering machine which provides improved (more uniform) gas flow distribution across all heating panels.

It is another object' to provide such a machine in which the gas temperature in the process chamber is more uniform.

It is a further object to provide such a machine which provides greater process flexibility. According to one important aspect of the invention, an IR heater panel is perforated so that gas (preferably air) ca be forced therethrough and, in addition, a pressure equalization chamber is provided adjacent the upstream side of the panel such that the gas flow through all of the holes in the panel is substantially uniform. Such a chamber is advantageously obtained by spacing from the heater panel a panel of aluminum or other material having through holes which are not in alignment with the holes of the heater panel.

Alternatively, the holes in the aluminum panel could be made bigger than the holes in the heater so that, even if the two sets of holes are in alignment, the gas would not pass straight through the chamber but would still exert an equalization effect. The upstream side of the aluminum panel is made to form the floor of a gas inlet chamber.

In a preferred embodiment of the invention two or more such heater panels are provided each with its own pressure equalization chamber. They all may communicate with the same gas inlet chamber or, for greater control, each may have its own respective gas inlet chamber.

All of the heater panels, top and bottom, may be similarly constructed and configured with through holes and pressure equalization chambers with blowers provided under the bottom heaters to suck gas through the process chamber and then recirculate it to the gas inlet chamber(s) of the upper heaters. With such a direction of gas flow the bottom heaters have, of course, a gas outlet chamber(s) rather than a gas inlet chamber. Furthermore, in such a case, the pressure equalization chambers of the bottom heaters may be dispensed with as they do not greatly affect the gas flow. Where the gas used is air, ambient air can be brought into the air recirculation system as desired. In order to tailor the temperature profile each heater may have its own blower(s) which can be adjusted as desired or some of the top heaters could be blocked off from the recirculating gas. Brief Description of the Drawings Figure 1 is a longitudinal sectional view of an IR reflow soldering machine according to the invention; Figure 2 is a view similar to Figure 1 but showing a modified version of the IR machine;

Figure 3 is a view similar to Figures 1 and 2 but showing another embodiment of the invention;

Figure 4 illustrates the temperature profile existing along the machine of Figure 1; Figure 5 illustrates the temperature profile existing along the machine of Figure 2;

Figure 6 is a plan view of the heater assembly illustrating a non-aligned hole arrangement;

Figure 7 is a plan view of the heater assembly illustrating an aligned hole arrangement.

Description of Preferred Embodiments Referring firstly to Figure 1, an IR soldering machine 10 according to the invention includes a wire mesh conveyor 12 for carrying printed circuit boards (not shown) of the SMD type from left to right through a series of heater zones 14 enclosed within a sheet metal housing the ends 15 of which are shown.

The heating zones 14 include a row of IR heating elements 16 above the conveyor 12 and a row of heating elements 18 below the conveyor, the heating elements extending transversely across the width of the conveyor and together extending axially along the length of the conveyor except for an inlet section 20 of the conveyor and an outlet section 22 of the conveyor 12 which are not disposed between the heating elements 16 and 18.

Each eating element 16, which may be of the type available s Electrovert Part No. 3-0759-145-01-4, is a heating panel, aving a plurality of through holes 24 located uniformly across the panel. Associated with each heating element 16 is a perforated aluminum plate 26 which is spaced slightly from the lower face of the heating element or otherwise insulated electrically from the heating element, the holes 28 of plate 26 registering with the holes 24 of the perforated heating panel. Also associated with each heating element 16 is a perforated aluminum plate 30 spaced above the heating element 16 to form a chamber 32 defined between plate 30, heating element 16 and vertical spacing plates 34 located between each pair of heating elements 16 and extending as far as aluminum plates 26 and 30. (Of course, the first and last chambers 32 have as one wall a portion of the end wall 15 of the housing. ) The vertical spacing plate may be made of aluminum or other sheet metal.

The holes 36 in perforated aluminum plate 30 are also located across the plate 30 but they are deliberately offset as seen in Figure 6 with respect to the holes 24 of the heating element 16. However, as seen in Figure 7, the two sets of holes could be formed in alignment in which case the holes 36 are made larger in diameter than the holes 24.

Instead of a plurality (six in this example) of individual plates 26, a single perforated aluminum plate extending the entire length of the heating zones could be used and, in that case, the lower ends of the spacing plates 34 would be flush with the upper surface of that single aluminum plate. Similarly, plates 30 could be replaced with a single, unitary plate. Furthermore, although some beneficial effect may arise from constructing plates 30 of aluminum, other materials could be used.

The entire upper surfaces of the perforated plates 30 are enclosed by a housing portion 38 which defines a single chamber 40 above plates 30. The upper surface of housing portion 38 is provided with a plurality, four in the example shown, of hot air inlets 42.

Each heating element 18 is identical to heating elements 16 and a perforated aluminum plate 44, chamber 46 and perforated plate 48, corresponding to plate 26, chamber 32 and plate 26 respectively, are provided and arranged in a similar but mirror image manner such that aluminum plate 44 is uppermost, followed by heating element 18, chamber 46 and plate 48. The entire lower surfaces of the perforated plates 48 are enclosed in a housing portion 50 which defines a single chamber 52 below plates 48. Two hot air outlets 54 are provided in the lower surface of housing portion 50.

Each outlet 54 branches into two ducts 56 respectively connected to two of the hot air inlets 42 at the top of the machine. In this way a hot air return is established to all of the hot air inlets 42. Situated in each duct 56 near its junction with outlet 54 is a fan 58 arranged to suck hot air down through outlet 54 and blow the hot air back to the respective hot air inlet 42. Located between each pair of fans 58 is a fresh air inlet 60 which has a damper 62 which can be opened as desired to allow ambient air into the ducts 56 for make-up purposes or to permit rapid cool down.

As indicated above, the structure and configuration of plate 44, chamber 46 and plate 48 can be the same as their counterparts above the conveyor 12 but, as will be explained hereinafter, the plate 48 could in fact be dispensed with.

At the inlet and outlet ends of the conveyor are located exhaust ducts 64 and 66 which are intended to remove noxious process fumes which otherwise could spill out of the machine into the environment. As such exhaust systems are common in the industry and form no part of the present invention they will not be described in any greater detail. However, it should be noted that various exhaust systems have previously been proposed and one which has the added function of cooling the PCB's as they leave the heating zones could advantageously be substituted for simple exhaust ;duct 62 at the conveyor outlet side. Such a combined cooling and exhaust system is disclosed in United States Patent No. 4,912,857 issued on April 3, 1990.

In use, hot air enters upper chamber 40 via the four inlets 42. The hot air then passes through the perforated aluminum plates 30 into chambers 32. The plates 30 dampen the high velocity air flow and cause the air flow to become more uniform. As well, as there is considerable mass in the aluminum plates 30 the temperature of the air in the various chambers 32 remains uniform.

Chambers 32 act as pressure equalizers, particularly as the holes in heating elements 16 and aluminum plates 26 are not in alignment with the holes in plates 30 such that the air flow will not have a tendency to pass straight through chambers 32. Thus, the air flow through the heating elements 16 and aluminum plates 26 is substantially uniform. As the air passes through the heating elements and aluminum plates 26 it receives a boost in temperature before it makes contact with the circuit boards on the conveyor 12.

The hot air makes vigorous contact with all surfaces of the components on the printed circuit boards as it passes down to the lower heaters 18 and has the effect of removing any "hot spots" brought about by the heat radiated from the surfaces of aluminum plates 26 and 44. The reflow process is, therefore, achieved uniformly across the printed circuit boards.

As the air passes through the lower perforated aluminum plates 44 and heating elements 18 its lost heat is replenished. The reheated air is then sucked out of chamber 52 and blown back through ducts 56 to the inlets 42 to complete the recirculation cycle. As indicated hereinbefore, ambient air can be introduced into the cycle as necessary by opening dampers 62.

It is to be noted that because heating elements 18 and plates 44 are located relatively near the suction side of fans 58, the air passing through elements 18 and plates 44 is being sucked rather than blown. As air flow which is sucked through a perforated surface tends to be relatively uniform, unlike when it is blown, the perforated plates 48 may be dispensed with.

In the machine of Figure 1 steady state temperature can be reached quickly ι.rom cold or when new settings are chosen. Also, air distribution is uniform across all the heating panels and the air temperature in the conveyor space (process chamber) is uniform. It is noted that the continuous air flow transversely across the conveyor operates as an air curtain preventing or at least greatly reducing any adverse effects on the process that might otherwise be brought about by virtue of turbulence generated about the exhaust ducts 64 and 66.

The fans 58 and their driving motors are located under the conveyor in a relatively cool environment and, as a consequence, the motors are not liable to burn out. The temperature profile for the machine of Figure 1 is illustrated in Figure 4 in which the abscissa represents the distance from the inlet side of the machine to the outlet side and the ordinate represents the temperature within the process chamber. The actual profile shown is not the most desired for this type of process. A more desirable profile, as illustrated in Figure 5, is one in which there is a steadily rising portion 70 followed by a constant temperature portion 72 in turn followed by a peak portion 74 just before the final cooling and exhaust portion 76. One way of achieving the profile of Figure 5 using the general concepts embodied in the machine of Figure 1 is to modify the machine as shown in Figure 2.

The only difference between Figures 1 and 2 is that the first four top heaters from the left are blocked off by means of a removable blocking panel 78 which lies across the top of the first four perforated plates 30. In this way the hot air is forced through the remaining two top heaters thereby causing the desired peak in the profile. Another way of achieving a similar profile but in a more flexible manner is illustrated in Figure 3. Again, the underlying concepts of Figure 1 are to be found in Figure 3. However, the vertical spacing plates 34 are extended upwardly in the case of the upper heaters to abut the housing portion 38 thereby dividing chamber 40 into six separate chambers 40'. Also, the vertical spacing plates 34 of the bottom heaters are extended downwardly to divide chambers 52 into six separate chambers 52'.

Furthermore, instead of there being four blowers 58, six blowers 58' are provided, one for each heating zone. Each blower communicates with its respective chamber 52' by means of a respective outlet 54' and via a respective duct 56' with a respective air inlet 42' which leads into a respective chamber 40'. Thus, instead of four inlets and two outlets as in Figures 1 and 2, six inlets and six outlets are present.

The fact that each zone has its own inlet and outlet chambers 40' and 52' and its own blower 58' means that the air flow forced through each zone can be controlled by controlling the speed of the blowers. In this way any desired profile can be achieved. Although only one blower per zone is shown, as many blowers as are required for a given zone could be added.

Claims

CLAIMS :

1. A heater assembly for a reflow solder machine comprising an infrared heater having a plurality of holes therethrough, characterized by a pressure equalization chamber defined between the heater, two side walls and a perforated plate spaced from the heater.

2. A heater assembly according to claim 1, characterized in that the perforated plate has holes which are not in alignment with the holes in the heater.

3. A heater assembly according to claim 1, characterized in that the perforated plate has holes which are larger than the holes in the heater.

4. A heater assembly according to claim 2, characterized in that the perforated plate has holes which are larger than the holes in the heater.

5. A heater assembly according to claim 1, 2, 3 or 4 characterized by a blower for forcing gas into the pressure equalization chamber and thence through the holes in the heater.

6. A machine for reflow soldering components on printed circuit boards comprising a conveyor for the printed circuit boards, a plurality of upper infrared heaters arranged along and above the conveyor and a plurality of lower infrared heaters arranged along and below the conveyor, wherein at least one of the lower infrared heaters has a plurality of through holes and at least one of the upper infrareid heaters is part of a heater assembly according to claim 5, whereby gas forced through the holes in the upper heater passes across the conveyor and through the holes in the lower heater, characterized by ducts for recirculating gas passing through the at least one lower heater to the pressure equalization chamber via the blower.

7. A machine according to claim 6, characterized in that at least two of the upper infrared heaters have a plurality of holes therethrough and each of the at least two heaters has its own pressure equalization chamber.

8. A machine according to claim 6, characterized in that each of the upper infrared heaters has a plurality of holes therethrough and each of the upper heaters has its own pressure equalization chamber and characterized in that each of the lower infrared heaters has a plurality of holes therethrough.

9. A machine according to claim 8, characterized by means to selectively block off some of the pressure equalization chambers thereby preventing gas from passing through the respective heaters so as to alter the temperature profile of the machine as desired.

10. A machine according to claim 8, characterized in that there are independent blower means and duct means associated with each upper heater and an associated underlying lower heater whereby gas is recirculated from that lower heater only to the associated upper heater.

11. A machine according to claim 8 wherein the lower heaters are each provided with a pressure equalization chamber adjacent the lower side of the lower heaters.

12. A machine for reflow soldering components on printed circuit boards comprising a conveyor for the printed circuit boards, a plurality of, upper infrared heaters arranged along and above the conveyor and a plurality of lower infrared heaters arranged along and below the conveyor, wherein each of the heaters has a plurality of holes therethrough and there is provided a blower for forcing gas through the holes in the upper heaters across the conveyor and through the holes in the lower heaters characterized by ducts for recirculating gas passing through the lower heaters to the upper heaters.