RU1790731C - Ir furnace for soldering of electronic circuits on printed circuit boards - Google Patents

Abstract

SUBSTANCE: infrared heaters 4-6 are placed in a heating chamber by means of a rack 15 installed along the furnace conveyor with the formation of a gap between the walls of the heating chamber and the conveyor and with the possibility of changing this gap. A pipe 7 for removing volatile products is installed at the outlet of the heating chamber and is slit-shaped. The heaters are made in the form of a zigzag wire with a variable ridge pitch, pressed into the insulating layers of micanite. The implementation and installation of heaters and the design of the furnace reduce its inertia, reduce the time for its readjustment when changing the standard sizes of circuit boards and electrical circuits, and reduce energy costs. 2 ill.

Description

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The invention is intended for group soldering of the leads of electronic components pre-placed on the contact pads of an electronic circuit with solder paste deposited on them. Method and equipment for melting a solder paste using infrared radiation are widely known in radio electronics.

At present, the most optimal IR-soldering mode is heating using unfocused infrared emitters operating in the mid-wave and far-wave PC spectral regions. Medium and long infrared waves are readily absorbed by a large number of materials, do not have such strong color selectivity as near-infrared waves emitted, for example, by focused halogen quartz lamps. Unfocused panel emitters operating in the middle and far ranges provide simultaneous uniform heating of circuit components made of various materials. Their energy consumption is much lower than that of focused emitters, and their temperatures are different: 450 ° С - for unfocused ones, up to 2480 ° С - for focused ones.

A standard furnace using unfocused emitters comprises a tunnel-type heating chamber divided into heating zones, each of which contains unfocused emitters. The infrared oven also has a conveyor with which the processed printed circuit board passes successively between the lower and upper radiators through all the heating zones, the first of which, as a rule, is the zone of preheating of the printed circuit board to a temperature of about 160 ° C, the second serves to PCB exposure for a while at the same temperature. During this time, the temperature of the various components of the circuit is equalized and the volatile components in the solder paste evaporate. In the third zone, the temperature of the circuit board is brought to a soldering temperature of 210-230 ° C. The cooling zone of the printed circuit board after soldering is located outside the heating chamber and consists of fans.

The various thermal zones are separated from each other by insulating or reflecting walls to stabilize certain conditions within each zone,

To remove harmful volatile substances resulting from the melting of the solder paste and heating the fiberglass to the brazing temperature of the solder paste,

between the preheating zone and the holding zone there is a slit-like tube forming the thrust necessary to discharge the contaminated air into the ventilation duct. With this arrangement of the slit-shaped tube, hot air from the soldering zone moves to the preheating zone, which, on the one hand, reduces the efficiency of the emitters in the soldering zone, and, on the other hand, makes the temperature soak zone located between the preheating zone and poorly regulated soldering area.

The panel emitter used in the prototype furnace has a primary emitter located between 1 insulating layer and a secondary emitter. The primary emitter is a flat, spiral or corrugated wire or foil (resistive element). The insulating layer is made of a dielectric material reflecting infrared radiation, for example alumina or silicon dioxide.

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The disadvantage of the panel emitters used in the prototype is the complexity and relative high cost of their manufacture. In addition, these emitters5 occupy a large volume. A thick insulating layer of alumina or silicon dioxide, as well as the contact of the emitter with the camera body increase the temperature inertia of the furnace and do not

0 allow you to quickly change the temperature regime.

The aim of the invention is to increase productivity by reducing the inertia of the furnace and changeover time when changing the sizes of the processed products, as well as reducing energy costs.

The goal is achieved by using flat heaters in the design of the IR oven, in which the zigzag heating wire is pressed into thin insulating layers of heat-resistant micanite made from mica waste.

5 This heater, having a thickness of about 3 mm, does not require any additional insulation, as well as the use of a secondary radiator, which not only simplifies and cheapens the design of the heater, but also reduces its heating time.

The goal is also achieved by the fact that the design proposes to use one heating chamber devoid of dividing zones of insulating and reflecting walls, and flat heaters to be placed on a rack frame inside the furnace volume, which makes it easy to reconfigure the heaters. At the same time, flat heaters are located in the furnace volume so that there is an air gap between the heaters and the chamber walls, which helps to achieve a rational convective distribution of heated air and reduce the thermal inertia of the IR furnace during its operational readjustment.

In addition, the ventilation slit pipe is proposed to be placed outside the heating chamber, but close to it, immediately behind the outlet of the heating chamber, which allows the use of hot air generated in the first heating zones to additionally heat the processed electronic unit on printed circuit board in the soldering zone and, due to this, to reduce the energy consumption for the production of soldering.

In FIG. 1 schematically shows a general view of an infrared oven; in FIG. 2 is a view of a radiation-thermal planar heater.

The infrared oven comprises a heating chamber 1 limited by a casing 2; a conveyor 3 extending inside the heating chamber 1 (along the chamber); pairs of flat radiation-thermal heaters 4, 5, 6 located below and above the conveyor 3 along the heating chamber 1 and the conveyor 3; slit-shaped pipe 7 located above the plane of the conveyor 3 behind the outlet of the heating chamber 1 close to the heating chamber 1; cooling fans 8 located above the plane of the container 3 behind the nozzle 7 and temperature controllers 9. In addition, two parallel profiles 14 pass through the heating chamber, through the inlet 12 and the outlet 13 on the sides of the conveyor 3, on which they are vertically fixed , with the possibility of replacement, frame racks 1S, used for fastening flat radiation thermal heaters 4, 5, 6. The height of the rack 15 defines the gap between any of the heaters 4, 5 or 6 and the casing 2, as well as between the heater plane conveyor pa 3. The profiles 14 and 15 comprise a rack pinion cage for attachment heaters.

Flat radiation-thermal heaters 4, 5, 6 contain: an upper layer of micanite 14, a lower layer of micanite and a metal resistive wire 16 laid in a zigzag pattern with such a distribution density that, when using this radiation-thermal heater, in the corresponding heating zone IR the oven achieves uniformly the same temperature distribution on the electronic assembly being processed on the printed circuit board. The resistive wire 16 is pressed or laid without pressing between layers 14 and 15 of micanite or other similar material, for example, eating from the bottom of the formation.

An infrared oven for soldering electronic components on printed circuit boards by reflowing solder paste operates as follows.

A printed circuit board 10 located on the carrier 11. is mounted on a conveyor 3. which moves the carrier 11 with the printed circuit board 10 into the heating chamber 1 through the inlet 12. In the heating chamber 1, the carrier 11 with the printed circuit board 10 is moved first between the flat radiation-thermal heaters 4, the temperature of which is maintained by the regulators 9, provides heating of the printed circuit board 10 to a temperature of 150-170 ° C, then, driven by the conveyor 3, the carrier 11 and the printed circuit board 10 pass between the radiation term eskimm heaters 5, supporting plate 10 reached the temperature. Further, the carrier 11 and the printed circuit board 10 are advanced between the radiation-thermal heaters 6, bringing the temperature of the printed circuit board 10 to the freezing temperature of the solder paste 210-230 ° C. and through the outlet 13, leave the heating chamber 1. The carrier 11 moved by the conveyor 3 and the printed circuit board 10. leaving the heating chamber 1, pass under the fans 8, which cool the printed circuit board with the soldered components.

The removal of harmful volatile components formed during soldering from the heating chamber 1 results in an outlet 13 due to the formation formed in the slit-like nozzle 7.

Claims (2)

SUMMARY OF THE INVENTION 1. An infrared furnace for soldering electronic circuits on printed circuit boards, comprising a tunnel heating chamber in series and connected to each other by means of a conveyor with slotted inlet and outlet openings and infrared heaters located along the heating zones and a cooling chamber, a pipe for removing volatile products, characterized in that, in order to increase productivity by reducing the inertia of the furnace and the time of readjustment when changing sizes of processed products and cost reduction, the heaters are made in the form of a zigzag wire pressed into the insulating layers of micanite, while the heaters are placed in the chamber by means of a rack frame installed along the conveyor with the formation of a gap between the walls of the chamber and the conveyor and with the possibility of changing this gap, and the pipe made slit-like and installed at the outlet of the heating chamber. 2. The furnace according to claim 1, wherein the heater wire is laid with a varying ridge pitch.