Attach_5_97X0011.pdf

Boeing

Process Guidelines for

Environmental Stress Screening

(ESS)

of Electronic Equipment

MDC 97X0011

February 1998

1. Introduction_________________________________________________________5

1.1 Purpose of This Manual ________________________________________________ 5

1.2 Environmental Stress Screening Objective _________________________________ 5

1.3 Background __________________________________________________________ 6

2. Boeing Preferred Supplier Role in Establishing an ESS Program______________9

2.1 Gold Suppliers ________________________________________________________ 9

2.2 Silver Certified Suppliers ______________________________________________ 10

2.3 Bronze Certified Suppliers _____________________________________________ 10

2.4 Uncertified Suppliers __________________________________________________ 11

3. Traditional Thermal Stress Screening ___________________________________13

3.1 Temperature and Related Failure Mechanisms ____________________________ 14

3.2 Equipment Designed for Forced Cooling__________________________________ 14

3.3 Temperature Extremes ________________________________________________ 14

3.4 Cycle Duration _______________________________________________________ 15

3.5 Temperature Rate of Change ___________________________________________ 16

3.6 Number of Thermal Cycles _____________________________________________ 17

3.7 Equipment Electrical Operational Duty Cycle _____________________________ 18

3.8 Humidity Control_____________________________________________________ 18

3.9 Failure Free Cycling __________________________________________________ 18

3.10 System Level ESS_____________________________________________________ 18

4. Random Vibration ___________________________________________________21

4.1 Introduction to Random Vibration & ESS ________________________________ 21

4.2 Vibration Level and Spectrum __________________________________________ 21

4.3 Duration and Number of Axes __________________________________________ 23

4.3.1 Duration________________________________________________________________ 23

4.3.2 Number of Screening Axes _________________________________________________ 24

4.4 Equipment Electrical Operating Duty Cycle_______________________________ 24

4.5 Lessons Learned During Random Vibration ______________________________ 25

5. Highly Accelerated Stress Screening ____________________________________27

5.1 HASS Objectives _____________________________________________________ 27

5.2 HASS Environmental Limits ___________________________________________ 27

5.3 Criteria for HASS Acceptance __________________________________________ 28

6. ESS Optimization ___________________________________________________29

6.1 Introduction _________________________________________________________ 29

6.2 Data Collection_______________________________________________________ 29

6.2.1 Manufacturing Process Capability____________________________________________ 29

6.2.2 ESS Results _____________________________________________________________ 31

6.2.2.1 Screening Metrics ______________________________________________________ 31

6.2.2.2 Failure Reporting_______________________________________________________ 31

6.2.3 Field Data ______________________________________________________________ 32

6.2.3.1 Boeing Flight Line______________________________________________________ 32

6.2.3.2 Customer Operations ____________________________________________________ 32

6.3 ESS Optimization Process______________________________________________ 35

6.3.1 Screen Type, Duration, Number of Cycles, Environmental Magnitude, etc. ___________ 35

6.3.2 Sample Screening ________________________________________________________ 37

6.4 ESS Effectiveness & Cost Analyses ______________________________________ 38

6.4.1 Using Screen Metrics______________________________________________________ 38

6.4.2 Comparing ESS Results to Manufacturing Variability ____________________________ 38

6.4.3 Comparing ESS to Flight Line and Field Performance ____________________________ 38

7. ESS Following Equipment Repair ______________________________________41

8. References _________________________________________________________43

9. Definitions _________________________________________________________45

1. Introduction

1.1 Purpose of This Manual

Environmental stress screening (ESS) is one of the processes which must be defined in this new era of acquisition reform. In many cases, the ESS requirements imposed by Boeing subcontracts are considered excessive based on the inherent reliability of modern electronic components and the achieved levels of production quality. As a result, unnecessary screens are being performed which drive up costs. This manual is a guideline for Boeing’s Suppliers to use once an ESS process is deemed necessary. It will apply to new subcontracts and revisions to existing subcontracts. It is Boeing’s intent that our Suppliers will use this manual to establish a tailored ESS process which streamlines their manufacturing capabilities while maintaining the quality of their delivered equipment at or above a predetermined threshold. It is not intended for this manual to be used as a baseline requirement.

The information provided by this document can and should be used by Suppliers to establish an ESS process of their own - one based on knowledge of the equipment, the failure mechanisms, their internal processes, warranties, and the equipment cost. If the Supplier determines that another approach to ESS, or an approach which does not even include ESS, is the most cost effective approach and does not result in unacceptable program risk, it should be proposed as another way of doing business. Boeing approval of Supplier ESS approaches will be based on the Supplier’s certification level (gold, silver, bronze, uncertified) and technical and business evaluations of the risks and benefits.

1.2 Environmental Stress Screening Objective

The fundamental objective of ESS is to induce latent defects present in electronic equipment to appear as hard, detectable failures prior to delivering the equipment to the customer. Latent defects are defects which are introduced by the manufacturing process that may not be detectable by a single performance test in a benign environment. Subjecting the equipment to environmental stresses over time is an effective means to identify these latent defects. Failures resulting from these latent defects during ESS or initial operational use are commonly called infant mortality failures.

Boeing currently has many contracts which require Suppliers to perform ESS on their equipment. These ESS programs have evolved from requirements and guidance from many different sources, resulting in a hodgepodge of requirements which may have dubious screening qualities, are difficult to change, and can drive program costs upward. Furthermore, many of the publications used as guidance on these existing programs are now obsolete or have become unsupported military standards. Therefore, an internal Boeing process is needed to provide Suppliers with guidance for establishing an ESS program for future contracts which may require ESS. This process is embodied by this handbook and includes:

- Guidance for determining the role Boeing Preferred Suppliers have in establishing an ESS program for their equipment,

- Methodology to design an initial ESS program for Suppliers needing assistance,

- A process for monitoring, assessing and optimizing the initial ESS program as results are obtained.

The ESS program should be established during the development phase of the equipment so that the most effective screening levels are determined before high rate production occurs. Historically, once the ESS has been established and performed on the equipment passing Qualification testing, the ESS became locked in for the duration of the program. This is contrary to the ESS objective. The ESS program must be structured as a dynamic one. Procedures should be adjusted, as indicated by screening results, to maximize finding defects in the most cost efficient manner.

This handbook is applicable to electronic assemblies used in missile avionics, aircraft avionics, and ground support systems.

1.3 Background

A low, finite number of material defects, part defects, and assembly defects are considered normal for manufacturing processes involving people or machines. To sustain the level of reliability inherent in the design, these “latent” defects must be discovered and corrected before the equipment leaves the factory.

Otherwise, these defects will manifest as equipment failures in early operational use, resulting in potential safety degradations, undesirable support cost increases, and unhappy customers.

It is important to recognize that screening and inspection can not enhance the inherent equipment reliability (unless the screening results are used to change the design or manufacturing process). However, the initial operational reliability of equipment can be improved by reducing the number of defects introduced during the manufacturing process and/or by precipitating the defects prior to shipment through inspection and ESS. The former approach is the most desirable, since it results in an overall reduction in production cost.

However, if the defect causes cannot be eliminated, ESS may precipitate the infant mortality failures before the equipment is fielded. ESS accomplishes this by subjecting the equipment to various “stresses” which act on the weak or defective components of the equipment. Normally, these stresses take the form of a combination of temperature, vibration and electrical power cycling. Ideally, the equipment Supplier will know what types of latent failures are likely to occur in the equipment, and what types of stresses will cause these latent failures to appear (or precipitate). The Supplier can then design an optimum ESS program. Historical knowledge of similar equipment manufactured by the supplier will play a major role in optimizing the ESS. Historical knowledge is defined as understanding the failure modes and mechanisms of the equipment, understanding the environments which precipitate these failures, and understanding the similarity of materials and manufacturing processes between the historical equipment and the equipment the ESS is being designed for. If this knowledge is not available, a more conservative ESS program must be implemented.

It is also important to note that the environment the equipment will be used in is not necessarily the appropriate ESS environment. For example, if a known failure mode can be precipitated in ESS by elevating the screen temperature to 55oC, even though the equipment will never be used above 20oC, the screen temperature of 55oC should be used. The “traditional” ESS wisdom blesses this approach given two conditions - the elevated temperature does not drive the equipment design and the elevated temperature does not induce additional failure modes. The purveyors of highly accelerated stress screening would not agree with these two conditions, however. Subsequent sections will discuss this in more detail.

Alternatively, if the failure modes can be identified at 20oC, but the equipment must operate at -55oC, the ESS should be conducted at 20oC. The important thing to learn from this discussion is ESS is not and should not be used as an acceptance or qualification test - it is a manufacturing screen. Granted, the equipment is normally monitored during ESS, and failures are generally identified when some specified performance level goes out of tolerance – in other words, the functions are being tested. But, the overall results of ESS should not be used as a test to prove anything other than the quality of the manufacturing process. Combining the objectives of acceptance/qualification testing with the objectives of an ESS program will compromise both.

Introducing Commercial-Off-The-Shelf (COTS) components and assemblies in military products will result in both good and bad equipment attributes in terms of reliability. In the general sense, COTS are built in large numbers, relative to typical military equipment build rates, using state of the art commercial processes. This will theoretically decrease the number of manufacturing defects in the equipment, and thus, decrease the need for ESS. COTS also typiclly use the most current technologies available for production equipment. Historically, newer electronic technologies are more reliable than their predecessors. However, the variability of component attributes in COTS equipment are not necessarily controlled or measured. Furthermore, many of the components are not rated at the temperatures normally specified in military electronic applications. This increases the importance of using ESS as an essential part of the production process to ensure normal COTS variability (potential latent defects) does not result in an out of tolerance condition in our applications.

The importance of performance monitoring and data collection during ESS can not be over emphasized.

Without proper performance monitoring, one will never know whether failures are occurring (or not occurring) or when they are occurring. A complete performance test should always be run at the beginning of ESS to verify the equipment is good. Since many equipment failure modes are transient, performance monitoring during ESS is crucial to detecting latent defects. The use of continuous Built-In-Test (BIT) during ESS is one solution to detect transient equipment faults which could result in a mission failure or abort in operational use. External test equipment performance monitoring can also be used when BIT is not available or in addition to BIT. BIT “fault” declarations which cannot be traced to failed equipment (could not duplicates (CND) or false alarms) are a source of considerable waste of customer resources and Boeing customer dissatisfaction. These fault declarations must be treated as real failures and not written off. There is a reason for these faults and it must be determined and corrected. At the conclusion of ESS, a final performance test must be performed to ensure the equipment still complies with the specification.

Finally, an effective failure reporting and corrective action process is a fundamental requirement of an ESS program. Without this process, the Supplier will not have the information necessary to:

a) Determine if ESS failures are truly manufacturing “escapes”, i.e., the random occurrence of defects which just happen and can not be “designed out” of the system or manufacturing process cost effectively,

b) Determine if the failures are a result of a manufacturing process gone awry that needs to be modified, or,

c) Determine if an ESS process method is inducing failures and needs to be modified.

This insight into failures will allow the most cost effective ESS to be developed and maintained.

2. Boeing Preferred Supplier Role in Establishing an ESS Program

Boeing has sought to increase the independence of Suppliers who have achieved and are maintaining Boeing Supplier Certification, particularly at the Gold and Silver levels. Consistent with this initiative, Boeing will allow certified Suppliers the following latitude in establishing, running and optimizing their ESS program.

2.1 Gold Suppliers

Suppliers who have achieved a Boeing Gold Certification level may establish, run and optimize their ESS program without Boeing intervention. Boeing will require that these Suppliers follow a documented process for establishing, running and optimizing the ESS program. Boeing will also require notification of ESS program changes, including the serial number when the change will occur and the effective date of the change.

Boeing will monitor the Supplier’s equipment performance during contractual tests, during production aircraft integration and flights at Boeing, and during initial operational use by the customer. Based on the equipment’s performance, Boeing will require the following:

1. If the number of failures occurring in contractual tests exceeds the number allowed, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of notification by Boeing that a plan is necessary. This plan will require Boeing approval.

2. Following the first failure at Boeing during aircraft integration or aircraft delivery flights, the Supplier will be required to supply a copy of the ESS Program Plan to Boeing. For a period of one year from the time of failure, ESS Program changes will be submitted to Boeing along with justification for the changes. If there are no further failures during this one year period, the Supplier will be allowed to perform ESS according to his Certification status at that time. If two failures occur in a one year window, ESS program changes will require Boeing approval. If there are no further failures during this one year period, the Supplier will be allowed to perform ESS according to his Certification status at that time. If more than two failures occur, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of notification by Boeing that a plan is necessary. Boeing will have the authority to specify changes to the ESS Program, as necessary. At the end of one year, the Supplier will be allowed to perform ESS according to his Certification status at that time.

3. If failures during initial customer use (first 200 hours of operation) exceed the specified number allowed, the Supplier will be required to supply a copy of the ESS Program Plan to Boeing. Within fourteen days of being notified by Boeing, the Supplier will be required to submit an ESS Program corrective action plan to Boeing. Boeing will have the authority to specify changes to the ESS Program, as necessary, until the equipment is meeting its specified reliability during this initial operating period. For a period of one year from the time the equipment begins to meet its specified reliability, ESS Program changes will be submitted to Boeing along with justification for the changes. If the equipment is still meeting its specified reliability after the year long period, the Supplier will be allowed to perform ESS according to his Certification status at that time.

2.2 Silver Certified Suppliers

Suppliers who have achieved a Boeing Silver Certification level may establish, run and optimize their ESS program without Boeing intervention. Boeing will require that these Suppliers follow a documented process for establishing, running and optimizing the ESS program. Boeing will require insight into this process and will be provided a copy of the ESS Program Plan. Program changes and notification of program changes, including the serial number when the change will occur and the effective date of the change, will be submitted to Boeing along with justification for the changes.

Boeing will monitor the Supplier’s equipment performance during contractual tests, during production aircraft integration and flights at Boeing, and during initial operational use by the customer. Based on the within fourteen days of notification by Boeing that a plan is necessary. This plan will require Boeing approval.

2. After two failures occur in a one year window during aircraft integration or flight testing at Boeing, ESS program changes will require Boeing approval. If there are no further failures during this one year period, the Supplier will be allowed to perform ESS according to his Certification status at that time. If more than two failures occur, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of notification by Boeing that a plan is necessary. Boeing will have the authority to specify changes to the ESS Program, as necessary. At the end of one year, the Supplier will be allowed to perform ESS according to his Certification status at that time.

3. If failures during initial customer use (first 200 hours of operation) exceed the specified number allowed, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of being notified. Boeing will have the authority to specify changes to the ESS Program, as necessary, until the equipment is meeting its specified reliability during this initial operating period. For a period of one year from the time the equipment begins to meet its specified reliability, ESS Program changes will be submitted to Boeing along with justification for the changes. If the

2.3 Bronze Certified Suppliers

Boeing will require Bronze Certified Suppliers to follow a documented process for establishing and optimizing the ESS program which has been approved by Boeing. Program changes and notification of program changes, including the serial number when the change will occur and the effective date of the change, must be submitted to Boeing along with justification for the changes. Boeing will have approval rights for these changes.

Boeing will monitor the Supplier’s equipment performance during contractual tests, during production aircraft integration and flights at Boeing, and during initial operational use by the customer. Based on the within fourteen days of notification by Boeing that a plan is necessary. This plan will require Boeing approval.

2. If more than two failures occur at Boeing during aircraft integration or aircraft delivery flights, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of notification by Boeing that a plan is necessary. Boeing will have the authority to specify changes to the ESS Program, as necessary. At the end of one year, the Supplier will be allowed to perform ESS according to his Certification status at that time.

3. If failures during initial customer use (first 200 hours of operation) exceed the specified number allowed, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of being notified. Boeing will have the authority to specify changes to the ESS Program, as necessary, until the equipment is meeting its specified reliability during this initial operating period. For a period of one year from the time the equipment begins to meet its specified reliability, ESS Program changes will be submitted to Boeing along with justification for the changes. If the

2.4 Uncertified Suppliers

Non-certified Suppliers must work closely with Boeing to develop an approved ESS program. This document will help establish the initial ESS program requirements. This ESS Program will be approved by Boeing. All proposed changes to the approved ESS Program must be approved by Boeing.

Boeing will monitor the Supplier’s equipment performance during contractual tests, during production aircraft integration and flights at Boeing, and during initial operational use by the customer. Based on the within fourteen days. This plan will require Boeing approval.

2. If failures during initial customer use (first 200 hours of operation) exceed the specified number allowed, the Supplier will be required to submit an ESS Program corrective action plan to Boeing within fourteen days of being notified. Boeing will have the authority to specify changes to the ESS Program, as necessary, until the equipment is meeting its specified reliability during this initial operating period. If the equipment is still meeting its specified reliability after the year long period, the Supplier will be allowed to perform ESS according to his Certification status at that time.

3. Traditional Thermal Stress Screening

The thermal portion of ESS generally consists of cycling the equipment between low and high temperature “extremes” while monitoring performance. The variables to consider during thermal cycling include:

• The magnitude of the extremes

• Number of cycles

• Length of cycles

• Dwell time at the extremes

• Temperature rate of change between the extremes

• Equipment electrical power duty cycle

• Relative Humidity of the Chamber

Boeing and the Supplier should always use knowledge of the equipment, its assembly process and expected failure modes to establish the ESS program. When there is no historical information relative to the equipment’s ESS experience and the expertise is not available to analyze the equipment to determine the probable failure modes and the associated stresses which will precipitate these failures, the following sections provide information to help choose initial values for these variables. Figure 3-1 should be referenced when reviewing these sections. This Figure is an example of a “typical” ESS thermal cycle.

-30 -20 -10

0 50 100 150 200 250 300 350 400 450

Time

D eg re e

C

Electrical Power Cycle

On Off

10 C/min Rate of Change

Maximum Temp Minimum Temp

Hot Dwell

Cold Dwell

Figure 3-1. “Typical” ESS Thermal Cycle

3.1 Temperature and Related Failure Mechanisms

This section will provide some insight into the role temperature plays in ESS. Entire books have been written on this subject and should be consulted for more details.

Thermal cycling precipitates latent defects in several ways. First, temperature change causes material to expand and contract at different rates. The coefficient of thermal expansion (CTE) quantifies this property in units of micro-inches per degree C. If two or more dissimilar materials are joined together, the different CTEs induce stress at the material interfaces when the temperature is changed. If the interface has not been assembled properly, the stresses can induce a failure. This failure mechanism will help identify latent defects in several areas. The following list are examples, albeit incomplete, of failures induced by CTE mismatch:

• Solder joints are stressed due to the different CTEs of the component and the circuit board laminates. Solder joints with insufficient solder, voids, or poorly wetted surfaces may not be able to withstand these stresses, thus precipitating a latent defect to failure.

• The CTE mismatch between copper plated vias and the circuit board resins can induce failures if the plating is too thin or not properly formed.

• Bolted or screwed assemblies can work loose due to CTE mismatch when improperly restrained metallic fasteners are used to hold composite or plastic components together.

The other significant thermal effect is on the electrical performance of the equipment. Assuming the equipment has been designed to operate over the specified temperature range, the random variability of the component electrical properties is of concern. A few examples of temperature effects on electrical performance are:

• Capacitance and resistance values shift, effecting timing, energy storage capability of capacitors, and circuit impedance.

• The mobility of electrons in semiconducting materials change with temperature, effecting the timing of signals. When sufficient time shift accumulates through a circuit, an out of tolerance condition can exist.

• Oscillators can become unstable at temperature extremes.

3.2 Equipment Designed for Forced Cooling

Cooling air or liquid to operating equipment “buffers” the equipment from the real ambient conditions.

The purpose of ESS is to stress the equipment, not buffer the equipment thermally. Therefore, the function of cooling air or liquids should be altered during ESS. The cooling medium should be used to help the thermal chamber stress the equipment. Since the thermal path between the equipment and the cooling medium is generally more direct than the thermal path to the chamber, more efficient screens can be implemented. Thermal rates of change can be enhanced and screening time may be reduced.

3.3 Temperature Extremes

The following information should be used to establish initial temperature extremes when historical data or other information is not available. Before this discussion begins, several definitions are necessary. The maximum and minimum temperatures discussed in this document refer to the equipment temperature – not the temperature of the environmental chamber. The chamber extremes are derivations based on the target equipment extremes.

The temperature range should be selected to provide the maximum excursion within the specified performance limits of the equipment. Normally, the maximum temperature for the equipment should not exceed the lowest maximum rating of the parts and materials in the equipment. Internal heating of the equipment must be considered when determining the maximum chamber temperature to assure that the maximum ratings are not exceeded. Likewise, the minimum temperature for the equipment should not be lower than the highest of the minimum ratings of the parts and materials in the equipment.

Guidelines for temperature-cycling temperature ranges, lacking a specific analysis, are as follows:

Platform Special Conditions Temperature Range

Equipment installed in military fighter aircraft and helicopters

ECS forced-air cooled equipment installed in avionics bays

-40 OC to +71 OC

Equipment installed in the aircraft crew station

-40 OC to +95 OC

Ambient-cooled equipment installed in avionics bays

-40 OC to +71 OC

Equipment installed on aircraft external surfaces or externally mounted

-54 OC to +71 OC missiles

-54 OC to +71 OC manned maintenance areas

0 OC to +55 OC

3.4 Cycle Duration

The following information should be used to establish initial dwell times when historical data or other information is not available.

The thermal cycle duration is a function of the thermal inertia of the equipment, the cooling air supplied to the equipment, and the test chamber air temperature rate of change. Dwell time at each temperature extreme should be of sufficient duration to temperature stabilize the equipment within some pre-defined tolerance. Normally, the equipment is considered stabilized when the temperature of the component with the longest thermal lag is exhibiting a temperature rate-of-change of no more than X degrees centigrade per hour and is within Y OC of the target temperature. Typical values for these two variables are:

5oC < X < 10oC

2oC < Y < 5oC

The dwell time is obtained by subjecting the equipment to a thermal survey to determine when the components have stabilized at the temperature extremes and what their stabilized temperature is (the target temperature). This information should be used to verify component temperatures are within their specified ranges, as mentioned in the previous paragraph. To simplify production operations, the dwell times should be adjusted such that a discrete number of cycles can be performed in the number of hours production takes place in a day, i.e., if operations are eight hours long, the cycles should be two, four or eight hours long.

Guidelines for temperature-cycle durations, lacking a specific analysis, are as follows:

Platform Special Conditions Cycle Duration helicopters

ECS forced-air cooled equipment installed in avionics bays or equipment installed in the crew station

2 hours on 1 hour off

1 hour on .5 hour off

3.5 Temperature Rate of Change

The following information should be used to establish initial temperature rates of change when historical data or other information are not available. The temperature rates of change quoted are for the equipment, not the chamber.

Platform Special Conditions Temperature Rate of Change (oC min helicopters

ECS forced-air cooled equipment installed in avionics bays or equipment installed in the crew station

10oC - Cold to Hot 5oC - Hot to Cold

10oC - Hot to Cold

Equipment mounted in

10oC - Hot to Cold

5oC - Cold to Hot

3.6 Number of Thermal Cycles

The following information should be used to establish the initial number of thermal cycles when historical data or other information is not available.

The number of temperature cycles should be based on the complexity of the equipment, a relative indicator of the number of opportunities for latent defects to occur. Guidelines for selecting the initial number of cycles, lacking a specific analysis, are:

Equipment Complexity (Number of Discrete Components)

Number of Cycles

Simple - < 500 parts 6

Moderate - 500 < parts < 3000 8

Complex - > 3000 parts

3.7 Equipment Electrical Operational Duty Cycle

The non-operating portion of ESS is the least productive portion of the screening process and should be minimized. It is important to power up the equipment at the low temperature extreme so the stresses induced by internal heating will be maximized.

Non-operational ESS has been used to rapidly temperature cycle printed circuit assemblies prior to integrating them into a system. This cycling was performed without attempting to reach board temperature stabilization. This type of ESS has not proven to be effective and should generally not be invoked unless there is a specific failure mode that can cost effectively be identified in this manner. If this type of ESS is used, it is essential that a performance test be performed prior to the non-operational temperature cycling and then following the cycling, so that failure rates of the assemblies before and after the screening can be compared. In the past, some equipment was not performance tested prior to temperature cycling.

Subsequently, all the failures were found after the temperature cycling. The screening was given credit for precipitating the failures, even when the failures could have been detected prior to the thermal screening.

This process yields a very optimistic and unrealistic view of the screen’s effectiveness.

3.8 Humidity Control

Humidity is not normally considered as a screening environment. However, it can be used as a means to verify proper sealing and application of conformal coating. While the value of relative humidity could be tailored to achieve some specific goal with a piece of equipment, the most important thing to do during ESS is to not desiccate the chamber during testing – let the humidity float at the levels present in the local environment.

3.9 Failure Free Cycling

The concept of failure free screening was developed to remove unproductive reporting requirements (created by differences of opinion between Boeing and Suppliers over when a reportable failure, or when one requiring corrective action, had occurred during ESS) and to assure a minimal level of confidence that supplied equipment would not fail in the initial operational use environment or in the production reliability acceptance test (which immediately follows ESS for some equipment).

Normally, equipment is subjected to a reasonable number of failure-free (i.e. no faults) cycles and then subjected to an ambient temperature performance test, which must be completed failure free, also. If no failures are found during the performance test, the equipment is ready for the reliability acceptance test or be shipped to the user. If a failure occurred during the “failure-free” cycles or was detected during the performance test, the entire failure free sequence is repeated until completed without failure. The concept prevents “lemons” from being delivered to reliability testing or the user, and forces the Supplier to develop timely and effective corrective action to correct problems.

As an initial starting point, the last two (2) ESS cycles prior to the performance test should be failure-free.

Chapter 4 provides more detail on optimizing the number of cycles to run.

3.10 System Level ESS

Thermal screening at the system level for complex systems, such as a fire control radar, is preferred in lieu of screening individual “black boxes”. Screening at the system level can reduce overall screening costs in several ways. First, it eliminates the need for specialized test equipment for each black box. Second, it provides the capability to detect marginal signals/timing problems at the system level that may not be detected at the box level. Temperature profiles and number of cycles at the system level should be determined by the same methodology described in the previous paragraphs. It is important to recognize that program planning will be required to assure that all of the individual black boxes, including spares, are subjected to system level thermal screening.

4. Random Vibration

4.1 Introduction to Random Vibration & ESS

In 1957, a Reliability of Electronic Equipment advisory group to the DOD R&D Board developed a plan to incorporate environmental screen profiles in reliability demonstration testing and acceptance testing of electronic hardware. For the vibration acceptance testing, the environment was limited to sinusoidal excitation of 2 g’s at a constant non-resonant frequency between 20 and 60 Hz. Testing using this type of vibration continued until the emergence of random vibration testing techniques during the Mercury and Gemini spacecraft programs. It was speculated that random vibration could be more effective in screening electronic equipment.

Grumman Aerospace undertook an investigation in the early 70s of the merits of sine and random vibration testing. Their assessment confirmed that random vibration indeed provided a much better manufacturing defect screening environment than was possible with single frequency sine test. The improved screening effectiveness was attributed to the ability of random vibration to simultaneously excite many frequencies associated with multiple modes of an equipment design.

Exposure time required to identify manufacturing type flaws was also evaluated in the study. It was concluded that screening for a period of five (5) minutes per axis was adequate for equipment screened in more than a single axis, and a minimum of ten (10) minutes for a single axis screening. Selection of the number of axes for ESS was assumed to be dictated by the complexity of the equipment design features.

The observations and conclusions derived from the Grumman investigation of random vibration screening were used to develop the vibration ESS guidelines described in Reference 1. This document was one of the first of its kind to serve as a basis for establishing temperature cycling and vibration screening processes for electronic equipment.

In most cases screening has been conducted with sequential rather than simultaneous exposure to random vibration and temperature cycling. However, it was recognized in Reference 1, that for some equipment, combined vibration and temperature screening might be required and the additional cost justified.

Although random vibration ESS was intended primarily for screening at the box or assembly level, it was also understood that ESS screening at the part or component level might be required to identify and correct some types of defects in a more cost effective manner.

The intent of the discussion that follows on the key issues related to random vibration ESS is to provide the framework for establishing a meaningful, unambiguous random vibration process that meets the objectives of ESS in a cost effective manner. Details and considerations covered in Sections 4.2 through 4.6, should be used to define a set of ESS guidelines.

4.2 Vibration Level and Spectrum

One of the important elements of any ESS effort is the selection of environmental levels to conduct the screening process. The issue of random vibration screening level has been and still is one of the most confusing aspects of the process. In general, as suggested from the Grumman Study results of Reference 1, a power spectral density of 0.04 g2/Hz over the frequency of 20-2000 Hz is considered appropriate for screening of common manufacturing defects found in most electronics assemblies. The spectrum associated with this study is shown in Figure 4-1. This was arrived at by observing the relationship between response levels of assembly circuit boards and components, and the ability to identify various types of manufacturing defects.

Prior knowledge of the types of defects expected and the vibration level needed to detect them is information that is highly beneficial in developing the ESS vibration spectra. Although the screening level of 0.04 g2/hz was intended to be a general guideline and not a “rigid” requirement for the frequency range of 100 to 1Khz, it has been, and still is, often used as shown in Figure 4-1 as the “standard” random vibration ESS level. Implementing vibration ESS in this manner frequently does not meet the objectives of ESS to reduce overall costs while enhancing production efficiency and service reliability. ESS experience strongly suggests that tailoring the ESS process to the design characteristics of equipment, including establishing environment levels, is needed to achieve ESS cost effectiveness.

Development of effective environmental screening requires that the screening levels be bounded by two considerations. First, the ESS vibration levels must be sufficient to allow detection of the types of defects suspected of being present in the equipment. Simultaneously, the screening environment must not be excessive so as to reduce the service life of the assembly nor exceed vibration design levels. Because of the importance of establishing an appropriate ESS vibration level, and because the design features and characteristics of equipment tend to vary widely, it is not possible to establish a “standard” vibration screening environment. Consequently, the concept of tailoring the screening environment to a specific design becomes important in developing the random vibration ESS process.

0.001

0.01

0.1

10 100 1000 10000

Frequency (Hz)

PS

D

(G 2/

H Z

Figure 4-1. “Standard” Random Vibration Spectrum

Tailoring the ESS vibration environment to a specific assembly design requires conducting an equipment vibration survey. The vibration survey provides basic response measurement data for use in developing equipment specific ESS vibration spectra. Measurement data primarily includes defining the relationship of input to response levels as a function of frequency at key equipment response locations, identification of significant response modes of the assembly, evaluation of the vibration response characteristics for different equipment input axes, and establishing a general profile of the variation of response levels throughout the module. Typically, the survey should cover three orthogonal axes (the critical axis should be perpendicular to the plane most susceptible to vibration energy) of the equipment design being evaluated. The vibration survey information, in conjunction with knowledge of expected types of defects and the vibration level required to disclose them, forms the fundamental ingredients of developing a tailored ESS vibration process.

When design, manufacturing and ESS experience exists as described above, and is applicable to a new design, it should be used to establish suitable vibration levels along with any modification to the spectrum shape required to account for equipment response characteristics measured during a vibration survey.

When such information is not available, a somewhat different approach is necessary. Without knowledge of the types of defects and/or the relationship between particular defects and vibration response levels required for identification, a suitable starting point for establishing the vibration screening environment is to use a response power spectral density of 0.04 g2/Hz range, as shown in Figure 4-1. Module vibration survey data can then be used to determine the input spectrum required to generate the desired response power spectral density at equipment principal response frequencies. Dependent on the particular equipment design characteristics, the primary response frequency range of interest is typically from 20 to 2 kHz in establishing screening spectra. As a general rule, the vibration screening levels should never exceed module vibration design conditions. However, to avoid using up a significant portion of the module’s design life during vibration screening, one school of thought suggests the vibration levels should be kept 3 db lower than the design requirement.

Results of ESS screening using an established random vibration spectrum should frequently be reviewed and a determination made as to its screening effectiveness. The number and types of defects identified during screening along with any field reliability data should dictate whether the vibration screening levels are adequate or should be modified. Screening levels should be adjusted, if warranted, to maximize the finding of defects.

4.3 Duration and Number of Axes

Upon completion of a vibration survey, and one or more random vibration spectra have been established for ESS, the issue of duration and the number of axes to vibrate needs to be addressed. Both parameters have been investigated in ESS studies such as that described in Reference 1. Results from the Grumman Study of Reference 1 have since been supplemented by considerable experience accumulated from many years of random vibration ESS on a variety of programs. A review of various documents that have been written on the subject of ESS indicates general agreement on guidelines used for selecting screening duration as well as for determining the number of screening axes.

4.3.1 Duration

Study results and ESS experience have shown that a screen duration between 5 and 10 minutes is sufficient for detection of the types of defects commonly found in electronics equipment. Unless there are unique equipment design features or defects that would justify a longer screening duration, five (5) minutes per axes is recommended for ESS conducted in more than one axes. For designs where only a single axis of screening is required, ten (10) minutes of screening is considered appropriate.

In general, vibration ESS screening for longer durations, whether for a single or multiple axes screen, is discouraged since the service life of the equipment can potentially be reduced. However, there may be some exceptions to the screen durations indicated above, i.e., when functional screening takes longer than the vibration period. Under such circumstances, it must be demonstrated through analysis and/or screening that the additional screen duration does not compromise the equipment’s required service life. Likewise, there may also be instances, based on experience with certain types of defects, characteristic of a particular category of equipment or a peculiar manufacturing process, that ESS screening for shorter durations may be justified. As indicated in the discussion on screening levels, tailoring of screen durations is also the general rule for any unique situation.

4.3.2 Number of Screening Axes

Selection of the number of screening axes is dependent on module design characteristics, facility capability and screening strategy. In general, ESS screening is typically performed in the most critical axis (or axes) of the three major equipment axes, or, if the appropriate equipment fixture is available, all three axes can be excited simultaneously. Vibration screening in all three axes for equipment with components susceptible to multiple axis vibration is prudent.

However, some equipment designs may not require screening in all three axes. For example, when a configuration consists of circuit boards mounted in a chassis with the same planar orientation. In such cases, when defects are likely to be detected when vibration is applied in a direction normal to circuit boards, screening in that axis only is effective and recommended. Applying the same line of reasoning, there are some assembly designs for which vibration screening in only two axes is meaningful and cost effective.

Overall, the selection of the number of screening axes to be included in the vibration screening process should be determined after consideration of the structural-mechanical design features of the equipment and a review of vibration response data obtained from vibration survey tests. Once the number of relevant axes has been established, then specific ESS screen strategies and procedures can be defined. For instances when multiple axes of screening are required, there are several approaches available to implement the actual screening . If it is determined that screening in all three axes is important, for example, ESS can be accomplished by screening sequentially in each of the assembly axes. An alternate approach however, is to conduct the screen using a skewed test fixture that applies a single axis of vibration excitation to all three equipment axes simultaneously. Screening in this manner can make the test equipment setup more complex for some equipment designs, but it can result in a more time efficient process. Again, tailoring procedures based on knowledge, experience and design features of a particular assembly provides the best ESS process.

4.4 Equipment Electrical Operating Duty Cycle

Over the years, vibration screening has been performed with equipment powered as well un-powered. This issue requires insight into the types of defects expected during manufacturing and assembly of equipment, as well as some general understanding of how defects will affect equipment functional performance.

It is widely accepted that, in general, vibration screening at the assembly level is more effective with equipment powered. The detection of the type of defects such as those associated with solder joints and lead connections can go completely undetected during un-powered vibration. They can remain undetected during subsequent functional testing under ambient conditions, only to result in a failure or degraded performance after delivery. Recognizing that vibration screening under powered conditions is more efficient in uncovering defects, special procedures may be required in some cases to implement such screening. Conducting vibration screening with power applied for a given duration could, for example, result in excessive and perhaps damaging temperature buildup. To prevent such an undesirable condition, provisions for cooling may be required to allow screening under acceptable temperatures.

4.5 Lessons Learned During Random Vibration

Based on the vast experience with ESS and the many different ESS processes and criteria that have been used, the single most important core…