GB2090962A

GB2090962A - Ovens for heat testing electrical and electronic components

Info

Publication number: GB2090962A
Application number: GB8100526A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-08
Filing date: 1981-01-08
Publication date: 1982-07-21
Also published as: GB2090962B

Abstract

An oven, which is used for the testing of electrical and electronic components, which are required to withstand relatively high temperatures in operation, or which might be subjected to thermal shock, comprises a forced draught internal air circulation system, and temperature regulation means adapted to cause the air circulation system to operate either in a simple circulation mode in which there is no substantial change in the air in the oven, or in a cooling mode in which there is a change of air in the oven. The temperature regulation means may comprise a sensing device in the oven and a power driven mechanism 50 controlling the opening and closing of a damper. In the preferred arrangement, the damper divides an opening into an inlet and an outlet. <IMAGE>

Description

SPECIFiCATION Improvements in or relating to burn-in ovens Burn-in ovens are used for the testing of electrical and electronic components, which are required to withstand relatively high temperatures in operation, or which might be subjected to thermal shock. For the purpose of testing, the components are fitted to testing units, which can be placed in the oven, and connected, whilst in the oven, to a source of electrical current, so that the components can be operated continuously whilst they are in the oven. The oven is also provided with apparatus (usually electrical) for supplying heat to the interior of the oven.For test purposes, it is usual to place a series of units, each fitted with numerous electrical and/or electronic components in the oven, and then, whilst the components are continuously operated, the heating apparatus is also operated, so thatthe components are subjected to an elevated temperature, for a predetermined period of time.

In practice, this elevated temperature is usually considerably higher than anything likely to be encountered by the components in practical use.

By testing electrical and electronic components in a burn-in oven before the components are subjected to normal operational use, it is possible to greatly reduce the incidence of early component failure in the operative environment.

For effective testing, it is desirable to be able to control the temperature in the burn-in oven quite closely (for example within plus or minus 50C). It is also desirable, to obtain an even distribution of heat throughout the interior of the oven, and preferably, this should be maintained within plus or minus 30C.

A problem which arises with burn-in ovens, is that most if not all of the electrical components themselves generate heat due to continuous operation, and where a relatively large number of components are being tested in the oven, the heat given off by the components, can be quite large in relation to the heat supplied by the heating apparatus. Now for temperature control purposes, the oven is fitted with a thermostat, which automatically controls the operation of the heating apparatus, but where the heat given off by the components themselves is relatively large, the thermostat control over the heating apparatus is unable to provide a fine control over the temperature within the oven. Indeed, it is possible for over heating to occur such as would destroy some or all of the components, entirely due to the heat given off by the components themselves.It is the primary object of the present invention to provide a burn-in oven, which is better able to cope with the problem created by. the heat given off by the components themselves than conventional burn-in ovens.

According to this invention a burn-in oven has a forced draught internal air circulation system and temperature regulation means adapted to cause the air circulation system to operate in a simple circulation mode in which there is no substantial change in the air in the oven, or in a cooling mode, in which there is a change of air in the oven, so that heat is extracted from the interior of the oven by the egress of warm air and the ingress of cool air. In this context, cool air is air at a temperature lower than that prevailing in the oven.

Preferably the oven has an air opening and a damper controlling the air opening for changing the air circulation system between the simple circulation mode and the cooling mode. It is further preferred that, when in an open position, the damper effectively divides the air opening into an air inlet and an air outlet.

It is further preferred that the temperature regulation means is adapted to move the damper towards the closed position whenever the temperature in the heating chamber is below a predetermined desired operative temperature and to move the damper towards the open position whenever the temperature in the heating chamber exceeds the operative temperature. Hence, so long as the heating chamber has not attained the operative temperature, the damper will close and remain closed, so that there is no substantial flow of air into or out of the oven. However, as soon as the operating temperature is attained, the damper opens and provides the air inlet and air outlet.

Cool air (as herein described) is then able to flow into the oven and warm or hot air is able to flow out of the oven. In this way there is forced convection of heat from the heating chamber until the damper closes. In practice, the damper will normally take up a partially open position and will be constantly moving slightly to provide a close control over the temperature in the oven.

The temperature regulation means preferably comprises a temperature sensing device in the oven and a power driven mechanism controlled by the temperature sensing device and adapted to control the opening and closing of the damper.

Preferably the power driven mechanism is biased towards the fully open position of the damper so that in the event of failure of the power supply to the mechanism, the air circulation system will be put into the cooling mode.

It is further preferred to provide an excess temperature control arrangement adapted, when an excess temperature in the heating chamber is detected, to cause the damper to move relatively rapidly into the fully open position. Further, it is preferred that the excess temperature control arrangement is self-latching so that it can only be cancelled by a manually actuated override system.

Therefore, once an excess temperature is detected, the damper is moved into the position where a maximum cooling effect is obtained and this cooling effect will continue until the override system is actuated.

In a preferred construction, the oven has a heating chamber; an air duct extending vertically adjacent to the heating chamber and communicating at its upper and lower ends respectively with upper and lower portions of the heating chamber, and a fan adapted to circulate air through the heating chamber and the duct, the air opening being formed into the duct and the arrangement of the damper being such that when in the closed position it substantially closes the air opening, but when in an open position it deflects at least some of the air stream in the duct out through the part of the air opening on one side of the damper and allows air from outside the oven to flow into the duct on the other side of the damper.It is preferred to arrange the duct along side the heating chamber and to provide upper and lower plenum chambers respectively above and below the heating chamber, the upper and lower ends of the duct opening into these plenum chambers.

According to another preferred feature of the invention a power driven fan is provided for circulating air in the oven. Preferably the fan is located in the upper plenum chamber and in the preferred arrangement the air opening is located at approximately the same level as the fan, so that when the damper is opened, warm air in the oven is blown straight from the fan out through the air opening on one side of the damper. It will be appreciated therefore that the fan is operative in the simple circulation mode to produce the forced circulation of air in the oven and in the cooling mode to power the cooling action. The fan therefore has a dual function dependent upon the mode which in turn is controlled by the temperature regulating means.

It is further preferred that the fan occupies a major portion of the horizontal cross-section of the oven so that the upward induced airflow in the oven is distributed over substantially the entire horizontal cross-section of the oven.

One construction of a burn-in oven, and its method of operation, will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical section through the oven, and Figure 2 is a view to a larger scale of a damper and its control system.

As seen in the drawings, the oven has front and rear walls 10 and 12, a top 14 and a bottom 16.

The oven also has side walls one of which is visible at 18, and all these walls are heavily insulated as indicated at 20. A door (not shown) is provided for closing an opening 22 in the front wall of the oven. Thus, the general form of the oven is that of a rectangular heating chamber in which components to be tested can be placed, and the general construction of the oven is conventional, and needs no further description. It should be mentioned however, that the oven may be provided with racking (not shown) whereby standard units equipped with a multiplicity of electrical or electronic components to be tested, can be slid into position in the oven, and there is an electrical supply, which can be connected in the oven, to each of these units, so that all the components can be continuously operated, whilst they are subjected to an elevated temperature within the oven.Alternatively, the door may be adapted to receive these standard units, in which case one end of each unit may be exposed on the outside of the door and the electrical power supply established on the outside of the door to each of the standard units.

The heating chamber is defined by a false floor 24 in which there are a number of openings 26, a rear wall 28, and a roof 30 in which there is a large opening protected by a wire mesh grid 44. A vertical air duct 34 is formed between the rear wall 28 and the insulation 20 inside the rear wall 12 of the oven, and this air duct extends the full depth of the heating chamber. In addition, the space 35 below the false floor 24 forms a lower plenum chamber, and the space 36 above the roof 30 forms an upper plenum chamber, the vertical duct 34 communicating at its upper and lower ends respectively with the upper plenum chamber 36 and the lower plenum chamber 35. In the upper plenum chamber 36 there is a comparatively large fan 38 mounted on the lower end of the output shaft 40 of an electric motor 42 bolted to the top wall 14 of the oven.The shaft 40 passes through a sealing arrangement (not shown) in the insulated top wall 14. It is to be noted, that the diameter of the fan 38 is such, that the fan occupies a substantial proportion of the horizontal cross-section of the interior of the oven, and in fact, the opening which is protected by the wire mesh grid 44 is of approximately the same diameter as the fan.

The arrangement of the fan 38, is such, that when it is in operation, it induces an upward draught of air in the heating chamber of the oven, this air being then directed radially outward from the fan as indicated by the arrows A, to the upper end of the vertical duct 34. Because the fan 38 tends to draw air into the heating chamber through the openings 26 in the false floor 24 as indicated by the arrows B, so long as the fan is in operation, there is a forced circulation of air, from the fan in the direction of the arrows A, thence down the vertical duct 34, into the plenum chamber 35, up through the holes in the false floor 24, up through the heating chamber, and then through the perforated screen 44 to the fan 38.

Moreover, because of the large swept area of the fan 38, relatively to the horizontal cross-section of the heating chamber, the upward flow of air through the heating chamber is quite evenly distributed across the heating chamber, and there is little or no tendency for the accumulation of "stagnant" areas within the heating chamber. This of course assists in ensuring that there is even temperature distribution throughout the volume of the heating chamber. One or more electric resistance heating elements such as that shown at 37 are located in the upper end of the duct 34 in horizontal alignment with the fan 38.

Consequently, as the air is circulated, it is heated where it passes in contact with the heating element(s). It will be appreciated, that the induced draught through the oven is of course additional to any draught which would arise by convection.

An air opening 32 is formed in the rear wall 12 of the oven, adjacent to the upper end of the vertical duct 34, and the upper part of this air opening is in horizontal alignment with the fan 38.

A sheet metal damper 46 is provided for controlling the opening 32, this damper being pivoted at 48. In the closed position, the damper 46 completely closes the opening 32, so that there is then no provision for air to flow either into or out of the oven. This represents the simple circulation mode of operation of the oven. When the damper is opened (and the fully open position is shown in Figure 2) the opening 32 is divided into an air inlet 33 below the damper and an air outlet 35 above the damper. Moreover, because of the location of the damper in the opening 32, the air outlet 35 is smaller than the inlet 33.Also, in the open position, the damper 46 forms a deflector, so that air flowing rearwardly from the fan 38 in the direction of the arrow A, is guided over the top of the damper and then through the air outlet 35, whereas any air attempting to flow in through the inlet 33, is deflected downwardly by the damper, into the vertical duct 34.

A small electric motor driven actuator 50 is provided secured on top of the oven, and this actuator is of a kind which is resiliently loaded (for example by means of a spring) into a de-energised position, but which is capable, when power is applied to it, of turning its output shaft through 900 from the start (de-energised) position. Stop means are provided to limit the angular motion of the actuator, but the electric motor in the actuator is of a kind which is able to operate continuously under overload conditions, so that once the actuator has turned through its maximum permitted angle of motion, current may still be applied to the actuator, which will then simply retain its shaft in the fully turned position.

A disc 52 is secured on the actuator shaft, and the top end of a connecting rod 54 is attached to one of a series of holes in the disc 52. The lower end of the rod 54 is attached to the damper 46.

The connecting rod arrangement is such that the damper is held in the open position (as illustrated) when the actuator is in the start (de-energised) position, and since the actuator will always return to this position, whenever current ceases to be supplied to it, it follows that the damper will always return to the open position unless the actuator is energised.

The oven is provided with temperature sensing means (not shown) and with two temperature responsive switches, each of which can be pre-set to operate at a pre-selected temperature. The first of these switches ia adapted to energise the electric motor of the actuator 50 when it is closed, but to cut off the electric supply to the actuator 50 whenever it is opened by attainment of the preselected temperature. In practice, this switch is set to open at the desired operating temperature of the oven.

The second switch is arranged in series with the first switch, and consequently, will only allow current to the motor of the actuator 50, so long as it is closed. Furthermore, this second switch is arranged to be self-latching in the open condition, and is pre-set to open at an excess temperature somewhat higher than the desired operating temperature.

When the oven is to be operated, the units carrying the components to be tested are placed in the oven, and the necessary electrical connections for the components are established. The door of the oven is closed, and then electric current is supplied to the heating element(s) 37, the fan motor 42 and the motor of the actuator 50. As a result, the damper 46 is turned by the actuator, towards the closed position, and eventually arrives at the fully closed position. The oven then operates in the simple circulation mode, and the temperature in the heating chamber begins to rise.

The electrical and/or electronic components which are under test, are then exposed to the warm air stream passing upwardly through the heating chamber. In this mode, and at this stage in the process, because the damper 46 is fully closed, there is no flow of air into or out of the oven.

As soon as the desired operating temperature is attained, the first temperature responsive switch causes the heater(s) 37 to be switched off, and the actuator 50 to be de-energised. Heat is no longer supplied to the interior of the oven by the heater(s) 37, although of course heat may be given off by the components which are under test, and this would continue to raise the temperature in the heating chamber. However, because of the deenergisation of the actuator 50, the damper 46 begins to open under the biasing of the actuator.

Hot air is then forced to flow out through the air outlet 35, and because that outlet is of relatively small dimensions, the outflowing air travels at considerable velocity. Cold air from outside the oven is able to flow in through the inlet opening 33, where it is deflected by the damper 46 down into the duct 34. The fan 38 is still operative, and the cold air flowing into the duct 34 is drawn through the lower plenum chamber 35 upwardly through the heating chamber towards the fan, and this cold air exercises a cooling effect on the interior of the oven. The relatively high exit velocity of the hot air ensures that it is guided well clear of the air flowing into the air inlet, so that there is little chance of the in-flowing air being heated by contact with the exhaust air.

When the cooling effect has reduced the temperature below the desired operating temperature, the first temperature responsive switch closes, and causes current to be supplied to the heater(s) 37 and energises the actuator 50.

The damper 46 therefore begins to close, and the cooling effect permitted by the opening of the damper is reduced.

It will be appreciated, that there is in practice repeated operation of the first switch, and as a result, the damper is continually moving in one direction or the other, to exercise a modulating control over the temperature in the oven, which in this way can be kept within an acceptable tolerance of the desired operating temperature.

Moreover, since the control of the damper is dependent on the sensed temperature, it is possible to maintain control over the temperature in the oven, despite the fact that the components under test will themselves be giving off heat. If in fact the total heat given off by the components is in itself sufficient to maintain the desired operating temperature within the oven, then the heater(s) 37 will remain permanently off, and the damper 46 will remain in a permanently partial open position.

If for any reason, the pre-set excess temperature is attained within the oven, then the second temperature responsive switch will be opened. This causes the actuator 50 to be deenergised, so that the damper moves towards the open position, but because the second switch is self-latching, the damper will in fact move to the fully open position, and will remain in that open position, irrespective of any subsequent reduction in the temperature in the oven. It will be appreciated that this provides a fail safe feature, because once the excess temperature has been attained, the oven will go automatically into a maximum cooling mode. The oven may be equipped with an audible and/or visible alarm system, in which case, the alarm is arranged to be energised by the actuation of the second temperature responsive switch. Once this switch has been actuated, it is only possible to recommence heating, by manual cancellation of the latching effect on the second temperature responsive switch.

The arrangement is particularly safe, because in the event of total power failure to the oven, although the fan 38 will cease to rotate, and hence there will only be convection circulation of air within the oven, the damper 46 will automatically move to the fully open position, in which there is a maximum cooling effect.

Claims

1. A burn-in oven having a forced draught internal air circulation system and temperature regulation means adapted to cause the air circulation system to operate in a simple circulation mode in which there is no substantial change in the air in the oven, or in a cooling mode, in which there is a change of air in the oven, so that heat is extracted from the interior of the oven by the egress of warm air and the ingress of cool air.

2. A burn-in oven as claimed in Claim 1, in which the oven has an air opening and a damper controlling the air opening for changing the air circulation system between the simple circulation mode and the cooling mode.

3. A burn-in oven as claimed in Claim 2, in which when the damper is in an open position, it effectively divides the air opening into an air inlet and an air outlet.

4. A burn-in oven as claimed in either of Claims 2 and 3, in which the temperature regulation means is adapted to move the damper towards the closed position whenever the temperature in the heating chamber is below a predetermined desired operative temperature and to move the damper towards the open position whenever the temperature in the heating chamber exceeds the operative temperature.

5. A burn-in oven as claimed in any one of Claims 2 to 4, in which the temperature regulation means comprises a temperature sensing device in the oven and a power driven mechanism controlled by the temperature sensing device and adapted to control the opening and closing of the damper.

6. A burn-in oven as claimed in Claim 5, in which the power driven mechanism is biased towards the fully open position of the damper so that in the event of failure of the power supply to the mechanism, the air circulation system will be put into the cooling mode.

7. A burn-in oven as claimed in any one of Claims 2 to 6, in which an excess temperature control arrangement is provided which is adapted when an excess temperature in the heating chamber is detected, to cause the damper to move relatively rapidly into the fully open position.

8. A burn-in oven as claimed in Claim 7, in which the excess temperature control arrangement is self-latching so that it can only be cancelled by a manually actuated over-ride system.

9. A burn-in oven comprising a heating chamber; an air duct extending vertically adjacent to the heating chamber and communicating at its upper and lower ends respectively with upper and lower portions of the heating chamber, and a fan adapted to circulate air through the heating chamber and the duct, the air opening being formed into the duct and the arrangement of the damper being such that when in the closed position it substantially closes the air opening, but when in an open position it deflects at least some of the air stream in the duct out through the part of the air opening on one side of the damper and allows air from outside the oven to flow into the duct on the other side of the damper.

10. A burn-oven as claimed in Claim 9, in which the duct is arranged alongside the heating chamber and there are provided upper and lower plenum chambers respectively above and below the heating chamber, the upper and lower ends of the duct opening into these plenum chambers.

11. A burn-in oven as claimed in any one of Claims 1 to 10, in which a power driven fan is provided for circulating air in the oven.

12. A burn-in oven as claimed in Claim 11, when dependent from Claim 10, in which the fan is located in the upper plenum chamber.

13. A burn-in oven as claimed in Claim 12, in which the air opening is located at approximately the same level as the fan, so that when the damper is opened, warm air in the oven is blown straight from the fan out through the air opening on one side of the damper.

14. A burn-in oven as claimed in any one of Claims 11 to 13, in which the fan occupies a major portion of the horizontal cross-section of the oven so that the upward induced air flow in the oven is distributed over substantially the entire horizontal cross-section of the oven.

1 5. A burn-in oven constructed and arranged substantially as herein described with reference to the accompanying drawings.