GB2368629A

GB2368629A - An industrial oven

Info

Publication number: GB2368629A
Application number: GB0022174A
Authority: GB
Inventors: Paul James Morton; John Peter Morton; Kevin Peter Whitehouse
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-09-08
Filing date: 2000-09-08
Publication date: 2002-05-08
Also published as: GB0022174D0

Abstract

An industrial oven has a conveyor means 1, which transports a product via infrared heating assemblies 2,3. The heating assemblies comprise a planar array of heater elements 4. Downstream of the heater assemblies, a heated inert gas such as nitrogen, flows into the oven via the gas inlets 7,8, along a surface 6 and over the products carried on the conveyor means in a generally laminar flow in order to minimise turbulence. The gas may flow in an opposite direction to the motion of the conveyor means. A secondary flow of an unheated inert gas may then be supplied via a secondary set of inlets 9,10 at an angle such that laminar flow is achieved in the direction of the motion of the conveyor means.

Description

INDUSTRIAL OVENS

Field of the Invention The invention relates to industrial ovens, particularly but not exclusively, to industrial ovens used for heating electronic circuits and/or components, such as reflow ovens, so as to melt solder.

Background to the Invention In the manufacture of electronic devices, it is common for circuit elements to be provided with solder so that connections can be made to them. Solder has traditionally consisted of a lead/tin eutectic alloy with a melting point of 1 83'C. It is important that the solder is melted according to a recognised time/temperature profile in order that a soldered joint or the provision of a solder pad, for example, is made without defects. Where a number of joints or pads are to be made on a single circuit or in a plurality of circuits, it is important that every solder joint etc on every circuit is heated similarly to achieve uniformity.

A typical profile of temperature against time is shown in Figure 1 of the drawings. It can be seen that there are discrete zones. In the preheat zone, the temperature of the solder is raised rapidly but in a controlled manner. The objective is to preheat the solder fully to between 100 C and 150C but at a rate not exceeding 14C per second so as to avoid thermal shock which would otherwise damage components on a circuit.

In the second zone, commonly referred to as the soak, dryout or preflow zone, the temperature is raised steadily to enable the conditions to stabilise and to ensure that the solder has dried out thoroughly before progression into the next zone. The soak etc zone is particularly important when solder pastes are used rather than conventional tin/lead solder or when solder balls are used to create contacts.

In the reflow zone, the temperature is raised rapidly to a peak, after which the temperature is allowed to decrease. The peak needs to be above the melting temperature of the solder by a suitable margin, typically of 2axe. This margin also needs to be maintained for a time sufficient for the solder to enter its liquid phase and to flow as required. This time is known as the wetting time. In the case of tin/lead solder, the peak

temperature may be in the region of 210 C and the wetting time in the region of 30 to 60 seconds. An excessive wetting time may lead to the formation of an intermetallic layer between the solder and the underlying material, such as copper, which in turn can lead to brittle joints. Equally, excessively slow cool down after the peak can lead to the solder joint having a large grain structure, resulting in a potentially weaker solder joint.

A further problem is likely to arise in the near future in that the use of lead in conventional lead-based eutectic solder alloys is becoming less attractive environmentally and is likely to be phased out or prohibited. Other eutectic alloys will need to be used and these carry the penalty of having a higher melting point temperature. The temperature profile of ovens will therefore need to be raised.

In addition, it is imperative that the whole operation is carried out in an inert atmosphere to prevent oxygen in the ambient air from causing oxidation of any of the surfaces to which the solder is to be adhered. This will lead, in turn, to unsatisfactory joints or solder pads. Traditional reflow ovens attempt to overcome this problem by passing a heated inert gas, such as nitrogen, over the electronic components in order to melt the solder and provide an inert atmosphere so as to exclude air. This operation is carried out on the basis of the heat capacity of the nitrogen alone. However, this heat capacity is not normally sufficient and measures have been taken in prior art ovens to increase the ability of the oven to achieve the required temperature profile. These in clude accelerated nitrogen flow which had the adverse effect of causing distortion of the circuit (s) being processed. The direct impact of hot nitrogen onto the circuit (s) also led to spatter of the solder within the oven, leading to the need for regular cleaning and consequent unsatisfactory quality and increased downtime.

The present invention therefore aims to overcome these problems and, at the same time, provide access to higher temperatures within the oven without the defects encountered with prior art ovens.

Summary of the Invention Accordingly, the invention provides, in its broadest aspect, an industrial oven comprising a source of infrared heating for an article presented to said source, and means for causing a substantially laminar flow of an inert gas to pass over the article.

The inert gas is preferably caused to flow along a surface provided between the heat source and the article, thereby minimising turbulence.

Preferably, the article is one of many presented successively to the heat source by means of a conveyor.

The infrared source may consist of a plurality of ceramic heaters or one or more ceramic panel heaters. In the former case, the said surface is conveniently provided by a mesh, for example made of stainless steel or IR transmissive glass, located between the ceramic heaters and the article. In the latter case, a surface of the panel heater itself may conveniently provide the said surface.

The inert gas may have a primary direction of flow opposed to the direction of travel of the conveyor. The primary inert gas is preferably heated. The oven may also contain a secondary flow of inert gas, which is not necessarily heated, and which is directed in the same direction as the conveyor. The primary flow of inert gas is conveniently directed towards the entrance to the oven. The secondary flow is preferably directed towards the exit of the oven.

The primary flow of inert gas is directed towards the said surface at such an an gle that the gas clings to the surface by the Bernoulli effect. The angle may lie between about zero degrees and about 60 degrees. The angle is preferably about 30 degrees to the horizontal.

The primary gas flow may be in the region of 500 C. The flow rate may be in the region of 2m/s. The inert gas may be nitrogen.

Brief Description of the Drawings The invention will now be described with reference to the drawings, in which: Figure 1 is a graph representing the temperature profile of a typical prior art industrial oven and the temperature profile of an oven according to the invention; and

Figure 2 is a side elevation of a schematic representation of an oven according to the invention.

Detailed Description of the Illustrated Embodiments In the following description, the invention is described in the context of reflow ovens for heating eutectic solder alloys associated with electronic circuits processed through the oven. However, the invention may have application to other fields of use, such as for example curing adhesives, encapsulants or the like. The invention is not to be read as restricted solely to reflow ovens for the electronics industry.

Referring first to Figure 2, the oven according to the invention consists of an endless belt or chain conveyor 1 arranged to pass through a housing containing opposed infrared heater assemblies 2 and 3. As shown, each of the heater assemblies 2,3 comprises a substantially planar array of individual heater elements 4, such as ceramic IR heaters. The conveyor passes between the heater assemblies 2,3 so that both sides of the articles carried on the conveyor are simultaneously heated.

Located between the heater assemblies and the conveyor are a pair of meshes 5 which are both transparent and resistant to IR heating. Suitable materials include stainless steel or certain glasses. They provide a surface 6 which is substantially parallel to, and spaced from, the conveyor. The conveyor is depicted as travelling from left to right in the drawing. The heater assemblies are located near the entrance to the oven, relative to the travelling direction of the conveyor.

Downstream of the heater assemblies are a pair of opposed inlets 7, 8 for a primary flow of inert gas. The gas may typically be nitrogen and is preferably heated to around 500 C before entry into the oven. Although hot nitrogen has been used in prior art ovens, as discussed above, it is important to appreciate that the nitrogen in the present invention is not the primary heat source for articles carried on the conveyor.

The gas enters the oven at an angle A to the conveyor somewhere between zero and 60 degrees. Whatever the actual angle, the gas is caused to enter the oven at such an angle that it clings to the upstream surfaces 6, in a manner akin to the Bernoulli effect. The gas follows a generally laminar flow along the surfaces 6 and thereby over the surfaces of the elements carried on the conveyor. The conveyor is preferably of the

chain type so that both sides of the elements can be contacted by the inert gas and can therefore be subjected to the same degree of heating from the respective heat sources.

Downstream of the hot inert gas inlets 7,8 are a secondary set of inlets 9,10 for a secondary flow of inert gas, again typically nitrogen. The secondary flow need not be heated and indeed is preferably cold so as to aid cooling of the articles emerging from the oven on the conveyor. It is desirable for the gas to enter at an angle such that laminar flow is achieved, as for the primary inert gas, so as to reduce turbulence and the consequent ingress of oxygen.

The inert gas from both the primary and the secondary flows exits through some form of exhaust (not shown) provided at both ends of the oven. The exhausts are preferably balanced top and bottom so as not to create turbulence which could lead to undesired ingress of air into the oven. Alternatively, the whole oven could be contained within a housing which is connected to a pump to extract used inert gas from the oven. Ideally the gas would be recycled but it is usually too contaminated with waste products (eg flux) resulting from the heating process within the oven and must normally be disposed of safely.

Typical operating parameters for the oven would include a conveyor speed of around 250mm/min. As will be seen shortly, the oven can reach temperatures in excess of 325OC, possibly up to 400oC, depending on the effectiveness of the combination of the IR heating sources and the improved flow pattern of inert gas to deliver increased heat energy to the articles on the conveyor without the problems encountered in prior art ovens.

Referring now to Figure 1, the temperature profile of such a prior art oven is shown in solid line. As can be seen, a maximum temperature of around 150oC is reached at the end of the preheat zone after about 1 to 1.5 minutes, followed by a steady rise to around 180oC at the end of the soak zone. In the reflow zone, temperatures reach up to around 21 DoC after around 3.5 to 4 minutes.

By comparison, as shown in dotted line in Figure 2, the oven according to the

invention can achieve a temperature of around 200oC at the end of the preheat zone, followed by a steady rise to around 250oC to 280oC at the end of the soak zone. The peak temperature in the reflow zone can reach 325OC or even up to 400oC as previously mentioned.

The invention is particularly valuable in melting palladium solder balls to produce contact areas on circuits (e. g. flip-chips) without the need for subsequent terminations and consequent packaging.

In addition, the oven has a very small footprint, in the region of 2.5m length and 1 m width. It is therefore very economical in terms of floor space. This is a substantial advantage when the oven is to be installed in a"clean"environment which attracts a cost and space premium.

Claims

1. An industrial oven comprising a source of infrared heating for an article presented to said source, and means for causing a substantially laminar flow of an inert gas to pass over the article.

2. An oven according to Claim 1, wherein the inert gas is caused to flow along a surface provided between the heat source and the article, thereby minimising turbulence.

3. An oven according to Claim 2, wherein the infrared source consists of at least one ceramic panel heater and the said surface is provided by a surface of the or each panel heater.

4. An oven according to Claim 2, wherein the infrared source consists a plurality of ceramic heaters and the said surface is provided by a mesh located between the ceramic heaters and the article.

5. An oven according to any of the preceding claims, wherein the article is one of many presented successively to the heat source by means of a conveyor.

6. An oven according to Claim 5, wherein the inert gas has a primary direction of flow opposed to the direction of travel of the conveyor.

7. An oven according to Claim 6, wherein at least the gas flowing in the primary direction is heated.

8. An oven according to Claim 7. wherein the primary gas flow is heated to around 500oC before entry into the oven.

9. An oven according to any of claims 6 to 8, comprising means for directing a secondary flow of inert gas in the same direction as the conveyor.

10. An oven according to any of claims 6 to 9, wherein the primary flow of inert gas is also directed towards the said surface at an angle of between zero degrees and 60 degrees to the horizontal.

11. An oven according to Claim 10, wherein the angle is about 30 degrees.

12. An oven according to any of the preceding claims, wherein the gas flow rate is in the region of 2m/s.

13. An oven according to any of the preceding claims, wherein the inert gas is nitrogen.

14. An industrial oven, substantially as described with reference to, or as shown in, the drawings.