Gabor Martell, Managing Director of Montford Instruments, explains the principle of HARASS testing of products incorporating electronics.

Thermal stress screening is an established way of exposing hidden faults which do not show up during normal testing. This happens because sub-standard components and connections often function correctly at first and so are passed by automatic test equipment (ATE), but fail soon after being subjected to the stresses of handling, transportation and normal use. So large sums of money are spent powering up equipment during thermal cycling in order to boost fault detection rates.

Thermal stress screening subjects devices to repeated rapid heating and cooling, causing expansion and contraction within their components and connections. This builds up internal stresses which cause potentially defective devices to fail prematurely, before they are shipped.

Even so, some hidden faults can still be missed. Until now, the most common methods to boost detection rates have involved powering up the boards and testing them during temperature cycling, in order to catch faults which are intermittent.

This requires expensive racking and cabling systems inside the stress screening enclosure, perhaps more than doubling the cost. But unfortunately, because the cable connectors themselves are subjected to constant temperature cycling and repeated insertions, after a while they often only make intermittent contact, rendering the whole stress screening facility unreliable.

HARASS, which stands for Highly Accelerated Rapid Airflow Stress Screening, is a new technique developed with a major international telecommunications company. it provides increased detection without the disadvantages of powered-up testing.

Conventional thermal stress screening cycles air temperature, but with some equipment, the boards can lag so far behind that components see very little temperature change. Getting the board temperatures to change more quickly results in more defects being uncovered. HARASS takes this to new limits, using highly accelerated airflow and sophisticated temperature control.

Because HARASS transfers temperature to the boards more quickly than conventional stress screening, it is much more effective at uncovering faults, including those which are intermittent. In many cases this removes the need for powered up testing.

One worry often expressed is that since a board consists of components of differing mass, there is a risk of over-stressing and damaging those which track the air temperature most effectively. HARASS overcomes this by directly monitoring the temperature of the fastest-responding component and automatically reversing the applied temperature the instant that the component reaches its set point.

HARASS testing is non-random and non-combined, which means that identical tests can be repeated on successive board samples and the cause of failures traced back to specific tests. These features make HARASS ideal not only for production screening, but also for confirming that the production process is under control, for finding out why failures and investigating how successfully they have been rectified.

The effectiveness with which temperature is transferred is measured by the speed at which the temperature changes in the centres of a number of sample blocks of solid aluminium. This value is expressed in C?lk/minute. Montford instruments has produced several 20Cdeg Blk/minute HARASS chambers.

Features include:

• Interactive control to eliminate wasteful dwell periods
• Inherently dry atmosphere which prevents condensation on the boards
• Small footprint requiring small floor area
• Quiet operation

• Montford Instruments
• Tel:
• Fax:

GEORGE COLEMAN managing director of Sharetree Systems explains the rules of engagement for selecting test chambers and overviews what’s on offer on the market.

Temperature test chambers are used for many different purposes ranging from fixed temperature testing of static products to fast thermal cycling of powered products that dissipate significant amounts of heat. An under-specified chamber will fail to perform the task correctly while an over-specified chamber will cost more than is necessary. Each type of application requires a different chamber design if optimum performance is to be achieved.

The simplest chambers are essentially an insulated box with a heater and rely on convection to distribute the heat throughout the chamber. Although the temperature will not be very even over the chamber volume this may not be important if only a single product is being tested occupying a small part of the chamber. The temperature is kept at the required level by using a basic on/off temperature controller. Better temperature control can be achieved by using proportional control of the heater.

Where full use needs to be made of the chamber interior (for example for a batch of products) much tighter temperature distribution becomes necessary. This can only be achieved by using forced air circulation to circulate the air more quickly. Axial fans are suitable for lightly loaded chambers but higher-pressure centrifugal fans are necessary in densely loaded situations. Faster air circulation increases the heat transfer rate between the load and the chamber air and therefore reduces the time to reach thermal stability after loading.

Some means of cooling the chamber will be required for testing below ambient temperature. The choice is between mechanical and cryogenic cooling. Mechanical systems use the evaporation of a refrigerant (in a cooling coil) to cool the chamber with a compressor and heat exchanger converting the gas back to liquid in a closed cycle to form a self-contained system. A typical single-stage system will achieve temperatures down to about -40degC while multi-stage cascade systems are available for lower temperatures. Cryogenic systems also evaporate a liquid but the gas is then vented to atmosphere leading to a simple but powerful system. Very low temperatures are easily achieved due to the low boiling points at room temperature of the liquid nitrogen or liquid carbon dioxide used in these systems. However special delivery storage and distribution arrangements are necessary for the cryogenic liquid and running costs can be high.

Electrically powered products may dissipate significant amounts of power into the chamber and this has two main effects that need to be considered. The product heat will raise its temperature above the surrounding air by an amount dependent on the air speed past the product. Therefore in order for all products to be tested under the same conditions it is important to achieve an even air flow distribution as well as an even temperature distribution in the chamber. A more powerful cooling plant may also be required to remove the product dissipation.

A temperature programmer is required for cycling applications. This allows two or more temperatures to be pre-set together with the time at each temperature. In addition the transition between temperatures can usually be specified (as a linear or step change) and the total number of cycles required. During cycling heat has to be put into or taken out of the load and it is again important that the air flow is evenly distributed so that all parts of the load are subjected to similar test conditions.

Thermal stress chambers are a special type of thermal cycling chamber designed to subject products to mechanical stress by applying very fast temperature changes to the product. This stress causes any inherent weaknesses to fail allowing sub-standard products to be screened out before being shipped to the end customer. The temperature of the product must be changed rapidly not just that of the surrounding air. Hence the best possible thermal coupling is needed between the product and the chamber air which in turn implies fast air speeds (3-5m/s). A powerful circulating fan is therefore essential for a thermal stress chamber and standard temperature cycling chambers are unsuitable for this purpose.

In general any chamber interior should be fully seam-welded and have good door seals to prevent air leakage. This helps minimise heat loss at high temperatures and ensures that moist air does not enter the chamber at low temperatures causing frost build up. Heat losses will be further reduced if the chamber has efficient heat breaks and insulation between the inner and outer chamber. A stainless-steel chamber interior is best to avoid possible corrosion problems and is also compatible with cryogenic cooling.

It’s worth remembering that test chambers usually need to be left unattended for long periods (overnight or at weekends) and must therefore be failsafe in the event of a component failure. The chamber should be protected from control failure with an independent over-temperature trip and should include safeguards against mains and phase failure. Chambers with mechanical cooling plant should in addition have pressure and thermal protection fitted to the compressor system. Cryogenic chambers must be fitted with an under-temperature trip operating a back-up valve to avoid excessively low temperatures.

The products under test are frequently of high value and will usually justify additional protection features. Additional high and low temperature thermostats should be specified if required. These can be set to protect the load and are inset from the chamber protection thermostat levels.

But the most important safety features of course are those that protect the operator. Measures such as protection from exposed mains voltages and adequate earth bonding are mandatory under the various EC Directives and conformance is indicated by a CE mark. Measures may also be necessary to protect operators from excessively hot or cold temperatures. The latter is especially important for cryogenically cooled chambers. This can be achieved for instance by fitting a temperature controlled electric door lock.

• Sharetree Systems
• George Coleman
• Tel: 01453 828642

June 1999

GEORGE COLEMAN of Sharetree Systems explains which criteria are important when evaluating highly accelerated life testing (HALT) techniques.

A highly accelerated life test (HALT) evaluation exercise should be applied to a new production before production release. The exercise will increase (or prove) the robustness of the product and establish the basis for a production highly accelerated stress screening (HASS) process. It should initially be carried out on pre-production samples of all versions of the product.

It is important that key company personnel understand the reasons for undertaking a HALT evaluation exercise and the advantages of any subsequent HASS process. This need is best met through an initial tutorial. The tutorial presentation should cover the theoretical basis for various of stress testing and its practical application during the development and production phases of a product. The most effective stress methods are usually rapid temperature cycling and vibration through power cycling and power margining may also be useful.

After the briefing stage the approach should consist of the following stages:

• Development of the evaluation program
• Collection and consideration of existing information
• Selection of vibration method and vibration survey
• Vibration HALT evaluation
• Thermal product survey
• Thermal HALT evaluation
• Summary of results and desirable design changes
• Recommended production HASS process

It is possible to perform HALT evaluation using a combined vibration system and thermal stress chamber. In order to understand the failure mechanisms however it is usual to apply the two screening methods independently. After completing an HALT exercise on pre-production units it is entirely feasible to establish a production screening process which will combine the thermal and vibration screens at appropriate levels.

A small in-house design team should be formed to guide the HALT evaluation exercise which should include representatives form all departments – engineering production quality assurance and so on. When starting the investigation a number of decisions will need to be made before the HALT evaluation exercise can be implemented – for example the number of samples to be tested test conditions number of stress steps. These will depend on factors including product complexity method of construction and known or suspected weaknesses.

The product build standard should be as close as possible to the production version. Where a lot of build variants are possible the exercise should be planned to minimise the amount of testing by evaluating the most representative versions.

Methods of electrically powering and monitoring the product will need to be agreed in order to be able to detect intermittent operation during vibration or thermal stress. Electrical loading of power supply outputs is recommended during thermal evaluation – to maximise the stress in certain power components especially during power cycling.

Full use should be made of existing information on failure modes where available. Examples are results from previous testing in-house inspection and rework test results and field data on related products.

This information is a valuable guide to the most appropriate stress conditions and a useful check on the relevance of failure modes to those found under actual use conditions. Special attention should be given to design features differing significantly from previous products.

Similarly at the end of the HALT evaluation it is important to summarise the results and to discuss them (if not already done) with the in-house team. Desirable design changes may be impractical for a number of reasons such as economic or production constraints and alternative solutions may need to be agreed.

Once the design changes have been implemented the product should be tested under similar stress conditions to prove the effectiveness of the changes. A long-term life test may also be carried out to ensure that the proposed stress screen does not significantly reduce the useful life of good units.

Careful consideration should be given to the failure modes detected and to potential process problems when deciding the best production stress screening (HASS) method. In general stress levels for production will be somewhat below the maximum levels achieved without failures during the evaluation exercise to allow for process variables. Many other factors like procurement and operating costs will also need to be examined before choosing the optimum HASS facility.

Sharetree Systems has launched a range of standard test chambers designed for temperature cycling and climatic test conditions with a temperature of +150deg to -40degC or -70degC The Cyclone chambers are available in four sizes from 500 to 1000mm.

Cyclone chambers are suitable for both constant temperature and temperature cycling applications. Evenly distributed airflow ensures consistent conditions throughout the chamber volume and therefore more meaningful results.

Close control of temperature/humidity conditions is achieved by means of a microprocessor-controlled dual loop programmer. An electronic capacitive humidity probe is fitted for high reliability and minimum maintenance.

Humidity generation requires a supply of de-ionised water. This is supplied from the built-in reservoir which may be filled automatically from a mains supply. Chamber condensate is collected by the drainage system and normally run to waste to avoid contamination build-up. However a recirculation pump can be fitted if required.

Options and accessories include boost heating and liquid nitrogen cooling. Computer control may be used to set up and monitor chamber conditions which is particularly useful for simplifying the programming of complex cycles.

• Sharetree Systems
• George Coleman

June 1998

The increasing number of industrial applications for machine vision systems are making the technology an integrated part in many production environments. IAN PURCELL, Marketing Manager of Matsushita Automation Controls looks at the different hardware options.

The last ten years have seen a dramatic change in machine vision systems. Much of the development can be attributed to technological advances in CPUs and computer interfaces. Thanks to these advantages and the quality demands of end customers, machine vision systems have become one of the key strategic tools of the production industry.

Today’s machine vision systems offering 100% quality assurance by providing high-speed quality information have now become a cost-effective option for industries involved in mass production. Applications such as fill levels in the food and beverage industry and date codes in the pharmaceutical industry are typical of their use.

Most applications involve a CCD camera, computer based hardware, image processing software and a lighting fixture. The camera, normally positioned above or to the side of the part to be inspected, captures and sends an image at the request of the hardware. The image is stored and analysed to determine its conformance to pre-set criteria and limits. This may be the presence of components within an assembly, the orientation of a part or simply dimensional measurement results.

Serial or parallel data available after each inspection can be used to integrate the vision system into a production line. Two main levels of information are available; the first comprises digital outputs that can be used to activate a reject mechanism and provide simple statistics on the number of good and bad parts over a period of time. The serial alternative can communicate all checking results via a network to a host or other computer based system and can transfer all checking results to allow full statistical process control (SPC) to be employed. In terms of performance, a vision system is much faster and more accurate than a human inspector, and ensures 100% quality assurance.

The advances in computer systems and software have enabled easy to use graphical user interfaces (GUIs) to be designed. These allow an application to be set-up in only a few hours – even by users with little or no previous experience of these systems. In the case of the Matsushita user-interface for the M100 and M200 systems, an eight-button keypad and an on-screen window and menu system is used to configure the systems. Usually, after only a few hours training, the customer can set-up an M100 or M200 system on-line and be able to troubleshoot any problems that may occur during the working life of the vision system.

PC-based machine vision systems using standard operating systems like Windows 95 or Windows NT 4.0 with applications written in Visual C++ or Visual Basic also allow straightforward programming or configuration in the form of a “point-and-click” application programmers’ interface (API). The powerful Matsushita PC based P400 vision system has an API developed in Visual C++ that is user-friendly and allows rapid development of the checking parameters required by the system for the inspection of a part.

There are several common types of machine vision system that combine hardware and software elements.

The Intelligent Camera systems are the simplest and often the most cost-effective. Using a digital signal processor (DSP) and a small CCD sensor, the application is usually developed on a PC and downloaded to the camera unit to allow simple tasks like presence detection and measurements to be performed. Although these systems are fairly compact, all the electronics are built in to the camera unit – as the name suggests. Since they do not have a remote camera option the size can often prohibit installation, especially in a confined area. Also in most cases, since the application for an intelligent camera is developed on a PC, the system does not usually allow the connection of an external monitor that allows online monitoring of the application. This can be an essential feature of an installation allowing an operator to visually monitor the inspection process.

The need for a robust and simple to use ‘intelligent sensor’ led to the development of a type of machine vision system known as “proprietary hardware”. Using specially designed hardware and GUI, this type of system is well suited to the industrial environment. Often in the style and around the same size as a 24 I/O “brick” size PLC, these systems use remote cameras, a remote monitor and are configured using on screen windows and menus via a small keypad.

The M100 and M200 conform to this style and were initially designed to carry out automated inspection functions within Matsushita’s own factories around the world. Modular in design, they have firmware modules that include relevant checkers for certain types of inspection, such as presence checking, measurement, feature extraction and pattern matching.

The use of reduced instruction set computer (RISC) processors and applications-specific integrated circuits (ASICs) allow rapid capture and processing of images in real time. Dedicated digital inputs for sensors to trigger the system when an inspection is required and outputs for strobe light control and communication of pass and fail signals are coupled with a RS232 port that can be used to communicate every result from the inspection routine to a host.

In the last few years, PC based machine vision systems have moved away from highly complicated bundles of various pieces of computer hardware and software that required an understanding of computer science. The new generation of PC systems are now enjoying a rebirth and are a viable option. Generally more expensive, PC-based systems now offer greater flexibility and allow more complex inspections to be carried out. Using a PC as a machine vision system also simplifies connection to a host or communication of results to an SPC package – which may be running on the same PC.

The PC CPU technology, with speeds up to 300Mhz and fast PCI frame grabbers have led to the P400 being developed by Matsushita. An API that uses icons, drop down menus and status displays allows the user to set up the application parameters. Able to perform high level inspections like pattern matching and feature extraction in grey scale, the P400 allows connection of up to 12 cameras and can be networked to other devices to establish full integration in to a process.

Intelligent Camera PC Based Proprietary Hardware

 Table 1 General comparison table of common machine vision system types

April 1998

Vibration and shock can cause as many problems for sensitive instruments and other extreme environments such as heat and pressure. So it is often vital to invest in vibration control equipment.

Vibration even when undetectable to human senses can upset the accuracy of sensitive instruments. The more damaging vibration tends to be at low frequency and is sometimes hardest to detect. Problems perhaps involving precision work or electronics are often attributed to other causes when the real culprit is ground-borne vibration even from far afield.

Fabreeka’s Precision Aire Table (PAT) comprises what is essentially a substitute support frame and floats the instrument on a cushion of compressed air. More specifically it is mounted on a series of pressurised bellow-like diaphragms which isolate the unit from direct ground contact.

Apart of safeguarding accuracy its use in factory applications can also improve on-line efficiency. It enables instruments such as the coordinate measuring machine shown in Fig 1 to be sited where they are needed at the point of production where shock vibration can be most severe.

Vibration can be deceptive and merely suppressing it is not the complete answer. Eliminating it – a different matter from dampening it down – is based on diagnosing its frequency and being able to neutralise it – in effect tuning it out. Called natural frequency different concretes and their underlying bedrock each transmit their own distinctive signature. Ignore it or worse misjudge it and the resultant tuning fork effect may even magnify the problem. An initial vibration survey is crucial and determines the right vibration to guild into the stand.

In efficiency terms measured by the reduction in transmitted vibration the PAT can achieve close to 100% even in ground-shaking conditions where instruments can still record micron differences with unwavering accuracy.

Questions About Vibration

What does it mean to isolate vibration?

Isolation refers to a reduction in transmitted vibratory forces or amplitude of motion.

How is isolation achieved?

It is achieved by placing an isolator (elastic element) between the unit vibrating and its support. This allows the inertia of the unit to oppose and thereby reduce the vibratory motion transmitted to the support.

What characteristics must an isolator have?

It must be (and remain ) elastic for the life of the installation. It must have the capacity to support the static weight of the unit as well as the unbalanced dynamic force. It must have a natural frequency lower than the offending unit’s disturbing frequency.

How do we determine the natural frequency of an isolator?

It is determined by a mathematical relationship depending on dynamic spring rate and the static weight of the isolated unit. Isolator manufacturers have this information readily available in their publications.

What natural frequency should an isolator have?

It depends on the desired percent reduction in transmitted vibration referred to as transmissibility and is governed by the ratio of disturbing frequency to isolator natural frequency. The larger this ratio the greater the reduction. Isolation begins at a ratio of 1.414.

How does this frequency ratio produce isolation?

It causes the isolated unit to be “out-of-phase” with the vibrating force thereby opposing it by virtue of its inertia. The greater the ratio the closer it gets to a 180deg “out-of-phase” condition. (Zero transmissibility)

Are there other ways to control or reduce vibration?

Yes there are:

• You can correct the cause of the problem namely the unbalanced condition that exists by dynamically balancing the machines rotating or reciprocating parts. However this may not be practical to do or cannot be done.
• The speed of the unit could be reduced to lower the disturbing force since the unbalanced centrifugal force is directly proportional to the square of the speed of the machine. Many times this cannot be done because the machine must run at the higher speed.
• If the option exists you can select to use a rotary rather than a reciprocating unit since a rotary unit generates less unbalanced force than a reciprocating unit.
• A relocation of the offending machine may be a solution if the vibration is not harmful to the machine itself but is a problem in the surrounding area.
• In the case of a sensitive piece of equipment being disturbed from an outside vibrating source you can modify the response of the equipment through structural changes for example by adding mass or increasing stiffness.

Does the use of an isolator change the disturbing frequency of the unit?

No it does not. It simply allows the inertia of the unit to oppose the dynamic unbalanced force or motion of the unit when the frequency ratio of the machine and “the-machine-on-its-isolators” exceeds 1.414.

What is natural frequency?

All physical systems have a natural frequency which is determined by the systems’ spring rate and weight. The natural frequency of a system is its response after being displaced from its equilibrium (static) position. It responds by vibrating freely until it dissipates all of the kinetic energy it received and stored (potential energy) due to its elastic nature. This free vibration is its natural response and consequently is referred to as its “natural frequency”. It is this natural response that opposes the unbalanced dynamic force generated by the equipment. The larger the difference in the frequency ratio i.e. disturbing-to-isolator the greater the reduction in force transmission to the machine support.

How does dissipation of energy by the responding system occur?

Damping is a term used to describe the physical phenomenon of converting the energy of motion (kinetic) to another form namely heat. This is accomplished through frictional resistance. The isolator itself may exhibit damping in the form of internal friction or hysteresis. If the isolator material has very low inherent damping then another physical element is added to the isolator system to give the damping required.

What are the benefits of having damping in an isolator system?

In a “free vibration” system it reduces the time required for the system to come to rest. In a “forced vibration” system it reduces the maximum force developed and transmitted at resonance.

What is resonance?

Resonance is an exact frequency match between the disturbing frequency and the isolator natural frequency. They are “in-tune” with each other. This produces a perfect “in-phase” frequency relationship that causes maximum amplification of transmitted vibration. Obviously this condition must be avoided. However it cannot be avoided entirely since an isolated unit will pass through resonance as it “runs-up-to” and “coasts-down-from” its normal running speed. If the unit “runs-up-to” and “down-from” speed quickly the machine’s inertia acts to minimise amplification. However if the unit passes slowly through resonance damping must be provided.

• Fabreeka International
• 01274 531333

October 1998

It’s a myth that electronic weighing systems are inappropriate for use in hazardous areas says CHARLES WARREN Industrial Product and Marketing Manager of Avery Berkel.

With increased concerns for the health and safety of personnel and plant more and more areas previously classified as “safe” are now being reclassified under European safety regulations as hazardous. The use of instrumentation in so-called hazardous areas is strictly regulated by industry-standard Codes of Practice.

Tracking the transition to fast high accuracy digital weighing by industry over the last decade or so it is apparent that the process industry has largely relied on mechanical equipment for hazardous applications. There has been no ideal safe digital weighing solution suitable for major parts of the industry.

Where electronic weighing has been applied in hazardous areas several methods of protection have emerged (see panel). Depending on the application each varies in effectiveness and some methods can present serious drawbacks. All involve extensive installation work and continue to incur high costs for on-going servicing and maintenance.

Intrinsically safe weighing equipment

Intrinsic safety – an energy limitation approach – is the preferred method of protection against explosion. It is the only technique permitted for use in Zone 0 areas because the intrinsically-safe circuit produces no sparks or thermal effects capable of causing ignition.

Avery Berkel’s latest intrinsically safe hazardous area instrumentation offers a typical accuracy of +/-1 part in 6000 and has been developed to suit hazardous zones 1 and 2. Linked to platform scales load cell weighers or vessel weighing systems this instrumentation uses low power technology for safe weighing in hazardous areas. It is approved by DEMKO (The Danish board for the approval of electrical equipment) and SIRA to CENELEC standards.

This indicator family may be connected to a wide range of CENELEC-approved load cell weighing platforms and systems providing a standalone weighing or filling installation without enclosures gas/air supplies or cable communications back to the safe area. Units are made from stainless steel treated by pickling and sometimes by electropolishing. This decreases the risk of cross-contamination and helps to ensure the scale’s durability.

As there is sufficient energy in the instrument’s circuits to cause an incendiary spark installation and maintenance may be carried out enclosures may be opened and printed circuits may be interchanged all in the hazardous area and without stopping the process. Indicators may be AC or battery-powered and include a mobile weigher.

Avery Berkel has also launched the RDREx range of rugged electronic machines for heavy-duty hazardous area weighing challenging the notion that only mechanical machines are suitable for cost-effective weighing in explosive environments.

The above area classification deals only with risks associated with inflammable gases and vapours and by implication flammable mists. It does not deal with dusts for which an area classification code is being developed.

An area that is not classified Zone 0. 1 or 2 is deemed to be non-hazardous and normal electrical techniques generally apply with the use of normal industrial electrical equipment.

BS EN 600769-10:1996 is the latest publication dealing with the classification of hazardous areas for gases and vapours.

Ingress Protection (IP) Codes

(BS EN 60529:1992)

As well as the Hazardous Area safety rating process control instrumentation is also given a coding for the degree of “hermeticity” to be expected when used with common process chemicals and solvents – there is often a requirement for these to be hose-proof and dust-tight too.

Traditional methods of protection for weighing equipment

Flameproofing

• Known as explosion-proofing in the USA this technique involves housing standard instrumentation in a “bomb-proof” enclosure. Small explosions which may be caused by the electrical equipment inside allowed to take place within the enclosure under carefully controlled conditions.
• Flameproofing is permitted for Zone 1 and Zone 2 areas only and the safety of the equipment is crucially reliant on careful maintenance.
• Equipment is heavy and cumbersome to install and allows only a limited view of the weight display through very thick safety glass and very limited access to the glass.

Sand filling

• Sand or quartz filling – thus limiting the spread of ignition – is limited to Zone 2 applications.
• Has found popularity within some European countries.
• Safety of equipment is dependent on stringent maintenance procedures.
• Involves return-to-factory filling of sand/quartz so downtime is a problem.

Gas-purged

• Gas-purged enclosures house the weighing instrumentation within a pressurised box.
• Safety of the system depends on supplies of clean air or inert gas continuously feeding through the enclosure backed up by good fail-safe start-up and shut-down procedures.
• Equipment is bulky and difficult to install with costly runs of cabling and pipes for gas or air supplies.
• Gas or air is expensive to supply.
• Weighing is impeded by difficult access to instrumentation controls.

Barrier methods

• Consists of load cell equipment situated in the hazardous area with cabling back to standard instrumentation in the safe area connected via a Zener barrier box.
• Work by preventing large amounts of energy getting into the hazardous area.
• Permitted in Zones 1 and 1 only.
• As instrumentation is remote from the weighing operation it is customary to install a special weight display unit next to the load cell platform necessitating yet more barriered circuits back to the hazardous area (or the use of flameproof equipment).
• Installation is complex and expensive – often involving long runs of cabling.
• Weighing resolution is degraded by the presence of barriers in the system.
• Control of the instrument is very limited from within the hazardous area.

• Avery Berkel
• Charles Warren
• 0121 558 1112

February 1999

A major in-service failure which necessitates the return of all systems for rectification is expensive and embarrassing. Vibration screening is one way to improve reliability.

On average, up to about 40% of all screenable defects are the rsult of other than component or device failures. The earlier a defect is detected, the lower will be the costs of repair and test. The application of effective screening techniques may result in an initial net increase of in-house failure occurrence of between 30-75%, while in-field failures may be decreased by between 20-90%.

Before ever considering the effects of vibration on a PCB it is important to understand that the printed circuit board at resonance will have a bending mode. With the use of surface mount components the actual centre of board deflection must be restricted otherwise failure will occur.

Fitting a large device will cause changes in local stiffness and will modify the first bending mode. The resonance characteristics will be affected by the type of edge conditions – or if pillars are utilised, the number and position of them.

Often overlooked or left until too late in the design cycle, the vibration screening fixture is as important to the production screening process as the vibration controller or the shaker system. The effect of a badly designed fixture can be an over test condition, resulting in unnecessary failures.

There is a need for a vibration assessment to be conducted as early as possible in the design cycle. This enables modifications and redesigns to be conducted before the design is frozen or the start of production when costs are lower.

A vibration survey is done at ambient and using swept sine. By sweeping over the required frequency range at a level high enough to excite the board and evaluating the responses from the auxiliary or monitor accelerometers, it is possible to assess the survivability. The vibration survey is used to:

• verify the resonant frequency of the module
• assess the Q factors at resonance
• ensure the displacements do not exceed the recommendations for the chosen technology

Results are generally obtained by controlling on the vibration fixture and attaching auxiliary accelerometers to the PCB.

The importance of a vibration survey over the full temperature range is often overlooked but it has a number of rather important functions.

• It is used to identify the changes in vibrational characteristics of the materials due the changes in stiffness
• it is also used to assess the effectiveness of the PCB mounting method.

Stepped stressing is generally performed utilising random vibration to determine

• the operating limit, beyond which the test item will not function but if the test item is brought below this level it will recover
• the upper and lower design limits beyond which damage will be done to the test item. This is generally not what the designer has designed too but is the actual achievable limits
• any design weaknesses uncovered during stepped stressing must be evaluated and corrected

With the application of the random profile at a low level, the level may be incremented in 1dB steps and using a dwell period of 5 min at each level until the upper operating limit is identified. The level is further increased until failure occurs. This may be a design weakness or a device defect. Having effected a repair the level is further increased until

• a multiple failure mode occurs
• further increase would destroy the test item.

There are major benefits of following this type of procedure as

• design weaknesses are removed
• a more mature product will reach production
• fewer engineering hours re wasted during the early stages of productionisation.

MIL-STD-781B type of vibration

The screen strength even at a reasonably high level of 6g is very low and at best does not exceed a screen strength of 0.2 after a duration of 100 min.

While this specification with its fixed acceleration level of 2g and fixed frequency (between 20 and 60Hz) fulfilled a useful function in the evolution of stress screening techniques, it has been evident for some time that the fixed frequency vibration method:

• is ineffective in detecting workmanship defects
• will highlight only the most obvious defects
• is likely to cause unwarranted failure through fatigue
• is not a realistic screening technique as it bears little relationship to ‘real life’ operational stresses in that the frequency of the vibration is generally selected to avoid resonances.

As this is the result of a spectrum analysis of a flat bed table it clearly demonstrates that there are three major areas of concern

• the extremely narrow bandwidth of the Power Spectral Density
• the limited frequency range between which the actual vibration can be set (nominally 25 to 60Hz but not on a resonance)
• as the frequency is low the displacements are generally reasonably high and can induce fatigue.

Despite these failures, MIL-STD-781B is still used extensively because this method of applying vibration is cheap and reliable using only an electric motor and adjustable out of balance rotors to produce a very basic sine wave on the vibration table.

But reports from the American defence industries confirm the failure of MIL-STD-781B’s fixed level frequency as an effective screen, because it only exposes about 7% of the most obvious workmanship or manufacturing defects. On at least one occasion an unsoldered joint survived 9 thermal cycles and 7.5 hours of 2.2g at 25Hz before failure occurred because there was a reasonable mechanical contact. Utilising a sensible random spectrum, this type of defect should be found in seconds.

Realising that random vibration is less understood by engineers than any other test method it is advantageous to invite them to both hear and feel different levels on the table so they have an appreciation of what they are specifying or designing for, starting with 810D common transport and working up through tests like the NAVMAT-P9492.

Stressing levels should be high enough to precipitate infant mortalities (such as loose particles in hybrids, poorly bonded substrate, manufacturing defects, chattering relays, etc), and yet not consume a large proportion of the useful life of the equipment.

NAVMAT-P-9492 is an advisory document and not a specification. When optimising a random screen, the Acceleration or Power Spectral Density (ASD or PSD), Bandwidth and the test duration must be considered together with the axis of applied excitation.

The profile is shaped with a +3dB/oct slope between 20 and 80Hz that helps diminish the bending stresses applied to the boards. However, some modules or assemblies may require the low frequency stimulus to precipitate certain defect modes. It may therefore be necessary to run a flat response between 20 and 350Hz. If this is the case, then testing should be conducted at a much lower level and only incremented to the NAVMAT values if no damage occurs. Note that there is evidence to show hat running a lower level profile with a flat response between 20 and 350Hz is more effective than running the NAVMAT.

The -3dB/oct slope between 350 and 2000Hz was introduced to reduce the stresses applied to small components with high natural resonances.

Care must be given to the selection of a random screen because levels, profiles and durations are dependent upon the mechanical design of both modules and units. Larger units over 4 cubic feet (0.113 cubic meters) have shown that random excitation levels as low as 2g can cause serious damage while some boxes up to ONE ATR (Air Transport Racking System) sizes used on aircraft can accept 6g recommended by the NAVMAT-P-9492 without any significant deterioration or the precipitation of latent defects into detectable failures.

Caution must also be exercised when tailoring any screening process where thermal barriers, acoustic barriers, or vibration isolators are an inherent design feature. There is usually some confusion when engineers have to determine the vibration levels and durations for a production screening programme to be applied to their equipment.

• Brumby Systems
• Tel: 01453 548881
• Fax: 01453 545810
• Contact: Mark Ashley