GB2336914A - Environmental stress screening - Google Patents

Abstract

An assembly is repetitively alternately heated using very hot air and cooled by the evaporation of liquid nitrogen. The resulting thermally-produced mechanical stresses convert any latent mechanical defects into actual "hard" defects. To ensure that the safe temperature limits of the constituent devices of the assembly are not exceeded, a discrete temperature-sensing element is used comprising a temperature sensor (14, fig. 2) arranged within a thermally-conductive body (10) having a thermal response comparable with the most sensitive device of the assembly. This element is used to control the switching between heating and cooling cycles. To allow a number of different types of assembly to be stressed in the same enclosure, an array of temperature-sensing elements 1-5 may be provided, each having different thermal responses. The element having the response closest to the most sensitive device of the assemblies to be tested is then selected and used for cycle control.

Description

1 Environmental Stress Screeninp, This invention relates to environmental

stress screening.

2336914 As is known to those skilled in the art, where it is necessary to ensure that equipment will not fail in service due to a latent mechanical defect, it is the practice to subject assemblies to screening with a view to converting any latent defects into actual defects.

Such stress screening includes subjecting the assembly to thermal cycling whereby the equipment is repetitively alternately subjected to high and low temperatures. The purpose thermal cycling is not to see how the equipment behaves at extreme temperatures but to produce mechanical stresses in the assembly arising from the differential thermal expansion between different parts of the assembly. In general the assembly is not powered and is not maintained at the extreme temperatures for any appreciable time. As soon as the upper temperature limit is reached, the assembly is cooled and as soon as the lower limit is reached the assembly is heated, and so on.

The number of cycles necdssary to attain a predetermined degree of confidence that a latent fault has been converted to a hard fault can be determined using the so-called "Hughes Equation". As is known to those skilled in the art, this equation generates a factor called "Screen Strength" which can be defined as " the probability that a screening process will turn all latent defects into hard failures if they are capable of being turned into.hard failures by that process". A Screen Strength of 'T' means that all latent defects will be turned into actual defects, and a Screen Strength of "0" means that 2 none of the latent defects will be turned into actual defects. The greater the number of temperature cycles, the greater the Screen Strength. It is immaterial how long a component is maintained at a temperature extreme, it is only necessary for the extreme to be reached. Thus to minimise the time necessary to attain a desired Screen Strength, the component should be cycled as rapidly as possible between its temperature extremes.

One known type of stress cycle method utilises a chamber in which extremely rapid heating is effected by hot air using powerful fans and high wattage heating elements to rapidly fill the chamber with very hot air. An air temperature in the order of 200 C may be obtained. Rapid cooling is effected by turning off the heating elements and introducing liquid nitrogen into the chamber. As liquid nitrogen boils at -19CC, the air temperature of the interior of the chamber falls very rapidly indeed.

The high temperature of the hot air and the low temperature of gas close to the boiling point of liquid nitrogen will in general lie outside the safe storage temperature limits of at least some of the components of an assembly being stressed. Relying on the fact that it takes a finite time for heat to flow in either direction between the ambient air surrounding the body and the interior of the body, by keeping the cycle time sufficiently short it can be ensured that no device has either of its storage temperature limits exceeded.

This is illustrated in figure 1 which shows the variation of temperature with time in response to step changes in ambient temperature at a point in the interior of a thermally- 3 conductive body having a finite thermal capacity. The horizontal scale represents time.

Time slots H denote periods during which the ambient temperature is very high, typically in excess of 100T, and time slots L periods during which ambient temperature is very low, in the order of -80T to -100T. On the vertical scale, THrepresents the high ambient temperature, TL the low ambient temperature, +Ts and -Ts the upper and lower storage temperatures respectively of the body, and T, the initial ambient temperature in the absence of heating or cooling. The solid trace represents the temperature within the body.

At time t the body is at ambient temperature TA. At time t, hot air is blown across the body, raising the ambient temperature substantially instantaneously to TH. As the body has a finite thermal capacity and a finite thermal conductivity, and gases are poor conductors of heat, the temperature of the interior of the body cannot follow the step change in ambient temperature, but rises exponentially towards T.. Over the initial part of the curve, this rise is approximately linear. Thus the interior temperature of the body rises linearly until it reaches temperature +Ts at time t2. If heating were continued, the temperature would continue to rise along the chain-dashed path, causing damage to the body. Thus heating is discontinued at time t2and cooling initiated by blowing very cold air across the body. This causes the internal temperature of the body to fall until 20 temperature -Ts is reached at time t3. If cooling were continued, the temperature would continue to fall exponentially towards TL, causing damage to the body. Thus at time t3 cooling is discontinued and hot air at temperature THis again blown across the body. The temperature now rises, reaching +Ts again at time t4. Heating is now discontinued 4 and cooling resumed.

The sequence of alternate heating and cooling is continued for as long as is necessary to attain the desired degree of confidence that any latent defects have been converted into hard defects. It will be appreciated that, although the ambient temperature extremes lie well, outside the storage temperature Emits for the body, the actual body temperature remains within those limits. Very high and very low ambient temperatures are used to efrect as rapid a transition as possible between high and low body temperatures so as to minimise the time necessary for the body to be subjected to the necessary number of cycles. Because the body remains within its specified temperature limits, cycling has no appreciable effect on the life of a fault-free body. Only a body having a latent fault is affected by transforming a latent fault (which would otherwise only have appeared in service) into a hard fault (which can be detected by post-stress testing).

For a given chamber having a given chamber performance rating, for each component of an assembly it is necessary to determine the longest heating and cooling times to which that component may be exposed to the respective air temperature extremes without exceeding its storage temperature limits for that device. Chamber performance has to be taken into account because heat transfer is affected by air speed as well. as air temperature. In this specification, chamber performance rating is defined as a measure of the rate at which a chamber is able to change the temperature of a given temperature sensing element.

The component having the shortest time is then the critical component, as a cycle time longer than that determined for this component would result in at least one of its safe temperature limits being exceeded. To ensure that this does not happen the temperature of this critical component is monitored and used to effect the changeover from cooling to heating and vice versa.

In general it will be necessary to modify the critical component to mount a temperature sensor at a suitable position in its interior. Such modification is generally destructive and means that the assembly of which the critical component forms a part is lost to production and thereafter can only be used as a reference assembly for controlling the cycle time. While the loss of one assembly may be of no importance in a production ran of hundreds or thousands of items, such a loss from a small batch may have a significant impact on production costs.

Having determined the length of each heating and cooling cycle, a set of temperature changes of all the components has to be determined and from this set of temperature changes, a set of numbers of cycles necessary to attain the desired degree of confidence that a latent fault will be converted to a hard fault has to be calculated. The greatest number of cycles thus determined gives the number of cycles to which the assembly 20 must be stressed to adequately stress all its constituent components.

While this prior art arrangement has proved satisfactory in production for stressing identical items manufactured in large quantities, it has a number of disadvantages when

6 applied to batch production of items. This is particularly the case where there is a relatively large number of different types of assembly, and the number of items of each type in a batch is relatively low.

The prior art practice of providing a temperature sensor on the critical component of an assembly has meant that it has not been considered possible to stress different types of assembly simultaneously in the same chamber. This has led to the need to provide a large number of chambers to complete stressing of the assemblies in a reasonable time, the alternative of stressing each batch sequentially resulting in a production bottleneck.

The present invention arose from an attempt to ameliorate the disadvantages of the prior art.

The invention provides a method of stress screening in which assemblies to be screened are repetitively alternately subjected to high and low extremes of ambient temperatures in heating and cooling cycles in a screening chamber. each assembly comprising a plurality of components, the method comprising the steps of, (a) for each component to be screened, determining the longest cycle time which may be employed in that screening chamber without exceeding its safe temperature limit, thereby producing a set of cycle times; (b) determining the shortest cycle time of the set; (c) identifying as the critical component the component having the shortest cycle time; (d) providing a plurality of temperature-sensing elements, each comprising a respective thermally conductive body, each body having a respective temperature-measuring sensor arranged 7 to sense the temperature at a point in the interior thereof, (c) selecting that one of the plurality of elements whose thermal properties are closest to those of the critical component; and (f) using the temperature sensor of the selected element to effect changeover between heating and cooling cycles.

This avoids both the need to use one of the assemblies as a temperature reference and the need to make and break connections to any assembly when loading assemblies into the chamber in which they are to be stressed. All assemblies can then be used for production.

The method further provides the steps of providing a set of data covering each component to be stressed, the data relating to a given chamber performance rating as hereinbefore defined and a predetermined component temperature range, the data including an appropriate temperature-sensitive element and the relationships between numbers of temperature cycles and screen strengths; using the set of data to determine for each component the actual limits of temperature each component will experience when subjected to the temperature cycle duration determined in step (b); using the set of data and the cycle duration determined in step (b) to determine for each component the number of cycles necessary to attain a selected screen strength; determining the greatest number of cycles determined in step (i); and subjecting the assembly to the said greatest number of cycles determined in step (i).

This is particularly advantageous when a mixed batch of assemblies is to be stressed 8 simultaneously in a single chamber. Rather than adopting the prior art procedure of performing individual calculations, the set of data can be consulted to detern-dne a temperature sensor appropriate to that particular combination of assemblies and the number of cycles necessary to attain the desired screen strength. The assemblies can then be stressed simultaneously rather than sequentially.

Even when only a single type of assembly has to be stressed, the method in accordance with the invention can simplify the task of determining the appropriate sensor and the necessary number of cycles by avoiding much of the need to perform calculations.

is In accordance with a finther aspect of the invention, the invention provides an array of temperature-sensing elements comprising a plurality of thermally-conductive bodies having mutually different thermal characteristics, each body having a temperature sensor arranged to sense the temperature at a predetermined position within its associated body.

This avoids the need to modify any of the assemblies being stressed to accommodate a temperature sensor. The assemblies are simply loaded into the stress chamber and subjected to thermal cycling. The array can remain in the thermal stress chamber, selection of the element required for the particular combination of assemblies being performed electrically by switching the output from the chosen sensor to the appropriate cycle control circuit.

Embodiments of the invention will now be described by way of non-limiting example 9 only with reference to the drawings, in which Fig 1 shows the variation of temperature with time at the interior of a body when subjected to step changes in ambient temperature; Fig 2 shows a sectioned view of a temperature-sensing element for use with the invention; and Fig 3 shows an isometric view of an array of temperaturesensing elements for use with the invention.

Referring now to figure 2, a temperature-sensing element 1 comprises a generally cylindrical body 10 having an axial bore 12 extending therethrough. A temperatureresponsive sensor 14 is arranged at the midpoint of bore 12. Electrical connections to the temperature sensor 14 are made by first and second insulated conductors 16, 18. Ile body 10 is made of a thermally conductive medium. The dimensions of the body 10 and 15 the material of which it is made will determine how quickly the temperature of the temperature sensor 14 responds the changes in ambient temperature. The larger the body, the slower the response, and the better the thermal conductivity, the faster the response.

The temperature-sensing element 1 should ideally have the same thermal response as the critical component of the assembly being screened. The necessary dimensions and material can be chosen to achieve this, or a number of elements 1 constructed, each differing in size andlor material, and the device having thermal properties closest to those of the critical component chosen. As it is unlikely that there will be an element whose properties exactly match the critical component, an element having a slightly faster response should be chosen in preference to one having a slightly slower response, as this will ensure that the temperature excursions suffered by the critical component are smaller than those suffered by the temperature sensor of the chosen temperature-sensing element, thereby providing an extra margin of safety.

To stress a batch of assemblies, the batch of assemblies is loaded into the stress chamber and the selected temperature sensing element 1 is also placed in the chamber in a position where it is exposed to the ambient air flow within the chamber. The conductors 16, 18 are connected to the chamber cycle controls so that the change between heating and cooling cycles is determined by the temperature at the centre of body 10 as sensed by the temperature sensor 14. The chamber controls are set to subject the assembly to the required number of cycles.

The use of a discrete temperature-sensing element avoids the need to utilise a member of the batch for temperature-sensing purposes, allowing all assemblies produced to be available for production.

When a number of different batches fall to be tested at different times, it will often be the case that the cycle time of the critical component is different for different batches. Thus it will be necessary to substitute a new temperature-sensing element appropriate to the batch to be stressed.

11 The arrangement shown in fig 3 avoids the need for any physical substitution of one element for another, with its attendant connection and disconnection. A plurality of temperature-sensing elements 1, 2, 3, 4, 5 are suspended in frame 6 by their connections leads 16, 18; 26, 28; 36, 38; 46, 48; and 56, 58 respectively. Each element is constructed generally as described with reference to fig 2, but each is of different dimensions. In the illustrated arrangement each sensor is constructed of aluminium, each body having a different size and thus a different temperature response. The largest sensor 1 will have the slowest response, the smallest sensor 5 the fastest response. The electrical connections from the sensors are fed via a multi-core cable 7 to a selector switch, not shown, by means of which any one of the sensors can be selectively connected to the cycle control circuit of the stress chamber.

In use, the assembly is mounted in a stress chamber such that air is, caused to pass freely over the sensors 1-5. The assembly remains in the chamber, a sensor appropriate to the critical component in a batch to be stressed being selected using the selector switch Thus it is only necessary to load a batch of assemblies into the chamber and select the appropriate sensor. No electrical connections need to be made or broken.

It may be the case that production consists of a large number of small batches of different assemblies. As noted above, this has generally let to the expense of providing a large number of chambers, the alternative being a production bottleneck. In accordance with the invention the disadvantage of the prior art can be ameliorated by stressing different batches simultaneously.

12 In a method in accordance with the invention, for a given chamber performance rating, a set of data is produced covering all the constituent items of the various products which will need to be screened. For each item the data includes its appropriate temperature sensor, its maximum and minimum storage temperature, the relationship between the temperature range over which the device is cycled and the stress screen strength, and relationships between numbers of cycles and screen strengths To screen an assembly, the critical cycle time for each device is determined and the device having the shortest critical cycle time identified. That temperature-sensing element having a cycle time equal to or shorter than the critical cycle time is selected and the cycle time of the selected element identified. The selected element is then used in the chamber, either by installing the appropriate individual element, or by selecting the appropriate element from an array of different elements using a switch as described above. For each component, and using the cycle of the selected element, the number of cycles necessary to achieve a specified stress screening level is determined. The greatest number of cycles is the number of cycles to which the assembly must be subjected to thermal cycling to ensure that all components receive at least the minimum screening. The assembly is then subjected to thermal cycling using the selected temperature sensor. It is to be noted that cycling only requires the loading of the assemblies into the 20 chamber. All the available space in the stress chamber can be used for stressing, unlike prior art methods where part of the volume had to be occupied by a dummy assembly to which electrical connections had to be made and broken.

13 The method allows more than one type of assembly to be stressed simultaneously in the same chamber. The procedure is exactly the same as for stressing a single type of assembly, the device having the shortest cycle time now being the shortest found in the group of assemblies. Where only one of the assemblies contains devices having the shortest cycle time, this will mean that the other assemblies win be subjected to more cycles than would have been necessary to achieve the desired screen strength had they been stressed separately: however, this extra cycling provides an increased screen confidence, and the fact that a longer cycle time is used is more than compensated by the ability to screen a number of batches simultaneously, thereby making full use of the screening chamber. This is particularly advantageous where the physical volume of a batch is appreciably smaller than the volume of a stress chamber.

It may be that, where an assembly consists of items having grossly different thermal capacities, then the very long response time of the larger items may result in impractically long numbers of cycles. For example, in an electronic assembly containing a small diode and a large iron-cored transformer, the cycle time necessary to subject the diode to its storage temperature range may have negligible effect on the internal temperature of the transformer. In such a case the relevant component would be ignored when calculating the necessary cycle times and would therefore need to stressed individually, generally prior to being incorporated into the assembly.

A number of modifications are possible in the scope of the invention. While the bodies of the temperature-sensing elements described are of cylindrical form for convenience 14 of manufacture, this is not essential. Other shapes, such as spheres or polyhedra may be employed. It is not necessary for the elements to be suspended using a frame as shown in the drawings. Both electrical connections to a sensor may be brought out from the same end of the element, eg using two conductor cable or two parallel or twisted conductors. A suitable filament attached to the opposite end may then be provided to restrain the element in position within the frame. Alternatively the elements may simply be suspended from one end by the two-conductor cable. The temperature sensing elements may conveniently be located immediately down-stream of the stress chamber's circulating fan, but may also be located at any other convenient point, such as the air outlet duct or at some point within the stress chamber where they will be exposed to air flow.

As an alternative, or in addition to, the bodies of the temperaturesensing elements being of different sizes, they may be constructed from different materials having different specific heats andlor thermal conductivities.

A temperature-sensing element in accordance with the invention can be used to determine the chamber performance rating of a given chamber.

In the present specification, references to "air" include any gas which may be suitable for use in heating or cooling, such as nitrogen.

0

Claims (8)

1. A method of stress screening in which assemblies to be screened are repetitively alternately subjected to high and low extremes of ambient temperatures in heating and cooling cycles in a screening chamber, each assembly comprising a plurality of components, the method comprising the steps of.

   (a) for each component to be screened, determining the longest cycle time which may be employed in that screening chamber without exceeding its safe temperature range, thereby producing a set of cycles times; (b) determining the shortest cycle time of the set; (c) identifying as the critical component the component having the shortest cycle time; (d) providing a plurality of temperature-sensing elements, each comprising a respective thermally-conductive body, each body having a respective temperature-measuring sensor arranged to sense the temperature at a point in the interior; (e) selecting that one of the plurality of elements whose thermal properties are closest to those of the critical component; and 16 (f) using the temperature sensor of the selected element to effect changeover between heating and cooling cycles.
2. 2. A method as claimed in claim 1, further comprising the steps of (g) providing a set of data covering each component to be stressed, the data relating to a given chamber performance rating as hereinbefore defined and a predetermined component temperature range, the data including an appropriate temperature-sensitive element and the relationships between numbers of temperature cycles and screen strengths; (h) using the set of data to determine for each component the actual limits of temperature each component will experience when subjected to the temperature cycle duration determined in step (b); (i) using the set of data and the cycle duration detennined in step (b) to determine for each component the number of cycles necessary to attain a selected screen strength; 0) determining the greatest number of cycles determined in step (i); and (k) subjecting the assembly to the said greatest number of cycles determined in step (i).
3. An array of temperature-sensing elements for use in a method according to any preceding claim, comprising a plurality of thermally-conductive bodies having mutually different thermal characteristics, each body having a temperature sensor arranged to sense the temperature at a predetermined position within its associated body.
4. 4. An array as claimed in claim 3 in which at least one body comprises a solid circular cylinder.
5. 5. An array as claimed in claim 3 in which at least one body comprises a sphere.
6. 6. An array as claimed in any one of claims 3-5 in which the bodies have different sizes.
7. 7. An array as claimed in any one of claims 3-6 in which the bodies are of different shapes.
8. 8. An array as claimed in any one of claims 3-7 in which at least one body comprises a diffrent material from another body.

   An array as claimed in any one of claims 3-8 comprising a frame and means to suspend the bodies from the frame so as to allow free circulation of air past the bodies.

   18 10. A method of stress screening substantially as described.

   is 11. An array of temperature-sensing elements substantially as described with reference to or as illustrated in figure 3 of the drawings..