Pros and Cons of Concurrent Environmental Stress Applications

Environmental stress screening (ESS), a process which precipitates latent defects, should not be confused with environmental stress testing (EST), a process that finds and corrects design flaws and establishes appropriate ESS parameters. EST should always precede ESS because it serves as the basis for defining the levels of ESS.

“But calling stress screening a test is a misnomer because it implies pass/fail,” said Domini Acquarulo, Vice President Sales and Marketing at Ling Dynamic Systems. “The only failure in stress screening is not finding a flawed item, permitting it to proceed to assembly or shipment. Stress screening is a production process.”

Failure-precipitating stresses may be produced by mechanical vibration or shock; temperature burn-in, cycling or shock; electrical overexcitation; or a combination of these. Stress-screen ingredients and parameters are product- and process-dependent.

Random vibration typically precipitates defects caused by poor solder or connection problems. Temperature shock typically precipitates component defects.

Since ESS interjected between manufacturing and shipment delays billing, ESS execution time is critical. So, when ESS requires both vibration and temperature stresses, the question arises about how feasible it is to perform both simultaneously. A corollary consideration is the benefits or detriments of simultaneous application of heat and vibration.

Opinions differ on both questions. And if operations are concurrent, should the chamber and vibration equipment be purchased from one source? Unfortunately, there is no consensus on this subject either. But to help you make better informed decisions, we will examine the pros and cons.

Sequential or Concurrent Stresses

When temperature and vibration stresses must be applied, compare the application times and examine their effect on equipment utilization. “It is not practical to tie up the shaker for 10 hours under or inside a chamber when only 10 minutes of vibration is required,” said Robert Turner, President of Environmental Screening Technology. And according to published data, for many applications it is doubtful that applying combined environmental stresses is more efficient than applying them sequentially, he added.

QualMark, however, finds that combining temperature cycling and multi-axes vibration is the most efficient defect-precipitating environment, according to J. Wayne Farlow, the company’s Vice President, Marketing and Sales. “Several key defects are only found when rapid temperature changes and vibration stresses are applied simultaneously. In addition, using an integrated system helps you apply complex temperature and vibration profiles simultaneously using a single control system,” he added.

The major advantages provided by combined systems were outlined by Dean Michels, Director of Marketing at Hanse Environmental:

o Certain failures are found only with a combination of stresses.

o Less floor space is required.

o Simultaneous datalogging is facilitated.

o Single test setup saves time.

o Faster results or feedback are obtainable.

A combined system does have a drawback. The equipment used to subject the deliverable products to ESS must operate in and survive potentially incongruous multiple vibration and temperature environments. For example, can the shaker table operate from -100° C to +200° C, or can the chamber support the mechanical or cooling requirements of the shaker?

Implementing a Combined System

Products which will be subjected to a range of varying temperatures are commonly placed in a temperature chamber. If shock or vibration will be simultaneously applied, a shaker table is usually installed in the chamber. But this is not the only way to generate combined temperature and vibration stresses.

Subjecting small objects to ESS can often be more cost-effectively achieved by mounting a small temperature-controlled hollow cube on the shaker. Several companies, such as Environmental Stress Systems and Temptronic, provide such implementations as standard or custom products.

“Our relatively light Custom Thermal Test Enclosure (CTTE) accessory has been successfully interfaced with various types of vibration test equipment,” explained Ken Cole, Director of Customer Support at Temptronic. “The CTTE mounts directly to the vibration equipment and is supplied with temperature-controlled air through a flexible hose.” With this type of implementation, the vibration equipment obviously does not have to withstand the temperature excursions required for ESS.

Single-Source or Mix-and-Match

Thermal cycling and vibration screening are very different from each other and should be treated, as well as optimized, separately. “No one vendor has the expertise required to deliver the entire system,” asserted Gino DiBartolomeo, President of Environmental Stress Systems.

“For instance, automated tri-temp systems typically require three or more vendors,” Mr. DiBartolomeo continued. “The shaker, along with the amplifiers and related equipment, is provided by one company; a handler firm builds the transport system; we provide the chambers, and an integrator writes the control software.”

Purchasing a vibration and thermal system from a single supplier exposes you to undue risk, commented John Raymond IV, Applications Engineer at Unholtz-Dickie. “Problems include misdiagnosis of system faults and excessive downtime, inadequate training and warranty issues. It is better to procure environmental equipment separately, letting each manufacturer bring to bear his own particular expertise and technology. All leading vibration test equipment and thermal chamber manufacturers work together on joint efforts and supply hundreds of successful installations,” Mr. Raymond concluded.

But suppliers who routinely furnish combined systems, such as QualMark and Screening Systems, obviously disagree. “The advantages include dealing with a single source who has complete responsibility for delivery, performance and service, as well as assurance that there are no system integration problems for you to resolve,” said Bill Leon, Director of Sales at Screening Systems. “In addition, operation and performance of the entire integrated system can be verified at the supplier’s plant prior to acceptance.”

Whether you should buy an integrated system or select vibration equipment and a temperature chamber separately depends on these factors:

The extent to which you want to make use of environmental equipment available at your plant.

Your personnel’s integration skills and experience.

The degree of confidence you have in the integration capabilities of your vendors.

If you are the integrator, don’t overlook mechanical interfaces such as dimensional fit, alignment, seals and gaskets, as well as safety features and control system compatibilities, datalogging and processing.

ESS Products

Chamber Has Slide-Out Features

For Tri-Axial Vibration Table

The QRS™-600V Power Screen System consists of a thermal chamber that features separate heating and cooling compartments and a triaxial quasirandom vibration system. The Auto-Height™ chamber ceiling is designed to optimize performance and save energy. Programming, operation and control are facilitated with the SSC-2000 Controller. The 44″ x 44″ shaker table employs 16 precision pneumatic actuators. Vibration levels of 30 grms are attainable and loads up to 600 lb are accommodated. The temperature chamber provides transition rates to 60° C/min. Screening Systems, (714) 855-1751.

Air-Cooled System Features

Lightweight Armature

The combination of the Series 800 Shaker, the SPA-K Amplifier and the DVC 4000 or DSC4 Controllers is suited for a range of ESS applications. Shakers cover a frequency range from 0 to 3,000 Hz. Force ratings extend from 2,200 lbf to 8,000 lbf, and the amplifier series has a power range from 5 kVA to 50 kVA. The DVC 4000 Controller includes sine, random and transient control. The DSC4 Controller provides 16 user-defined sine vibration programs. Ling Dynamic Systems, (800) GO TO LDS.

System Provides Concurrent

Vibration, Temperature Changes

The OVS-1, -2, -3 and -4 temperature and vibration screening systems feature work spaces ranging from 18″ x 17″ x 15″ to 55″ x 54″ x 54″, accommodating vibration tables of 12″ x 12″ to 48″ x 48″. The tables support 27 to 340 lb. The company’s OmniAxial™ Vibration System excites three linear axes and three rotations with broadband random vibration over a frequency range from 2 Hz to 10,000 Hz. The UltraRate™ Thermal System allows temperature ramping to 60° C/min over a range from -100° C to +200° C. QualMark, (303) 254-8800.

Control System Supports

Range of Vibration Equipment

The TA-, S- and R-Series of Vibration Test Systems are available with ratings ranging from 25 to 1,000 lbf, 100 to 6,000 lbf and 3,500 to 12,000 lbf, respectively. Real-time control is provided by the Windows™-based UD-VWIN Digital Vibration Control System, which uses digital signal processing and targeted control algorithms. Random, swept/discrete sine, transient, random-on-random and multitone sine-on-random vibration environments are supported. Multichannel parallel inputs, each with 16-bit resolution, are provided. The graphical interface supports eight independently configurable displays. Unholtz-Dickie, (203) 265-3929.

Configurable System Burns In

Memory or Logic Devices

A new multiple temperature zone system can be configured to burn-in and test either memory or logic devices. It is a member of the Automatic Burn-In/Environmental System (ABES) family. The oven features up to four individual heat-only or hot-cold temperature zones. Memory burn-in configurations provide 14-MHz operation and accommodate 128 data I/O lines. Logic burn-in offers on-the-fly timing, five programmable voltages and 256 I/O drivers per board. Micro Control, (612) 786-8750.

Burn-In Tester Features

Modular Continuous Scanning

The Automatic Burn-in Tester is comprised of multiple line-source monitor chassis, each containing an IEEE 488.2 Slot 0 controller with an application-specific SCPI command set. Two scanning DMMs are supported per line-source monitor. The system accommodates an unlimited number of scanning monitors in blocks of 512 or 1,024 channels. The DMMs measure voltages to 400 VDC and 280 VAC. Ranges for each channel are software configurable. Scanned values are immediately available since there is no need for individual selection or conversion. Autotest, (210) 661-8661.

Temperature System Interfaces

With Any Vibration Table

Small assemblies, components or modules may be subjected to temperature plus vibration ESS in a custom thermal test enclosure mounted on a vibration table and subjected to a temperature-controlled airstream. Air is supplied by a TP04010 ThermoStream System via a flexible hose. The temperature runs from -80° C to +225° C. The TP04010 features programmable ramp, cycle and soak functions and is operable from the front panel or the IEEE 488 or optional RS-232 interface. Temptronic, (617) 969-2501.

Vibration, Shock Controller

Runs Under Windows

The Win2001 Vibration and Shock Controller features a Microsoft^® Windows-based interface. It allows simultaneous data inputs on two, four and eight channels and controls one or two shakers separately. The software includes random, sine, classical shock with time, frequency analysis, sine-on-random, random-on-random, FFT analysis, second-shaker control, and random excitation and analysis with 1,600 and 3,200 lines of resolution. MB Dynamics, (216) 292-5850.

Lab-Type Chamber Suited for

Pre-Qual, Profile Verification

The ESS5 Lab-Type Chamber provides a multipurpose stress-screening environment. Typical applications include prequalification of ESS programs and determination of cost-effective stress profiles. A roll-in cart system accepts product tray fixtures and facilitates prototype-to-production application progression. The chamber is also useful for off-line failure analysis activities and small-scale production ESS programs. ESPEC, (616) 878-0270.

Burn-In System Provides

Tight Temperature Control

The Saturn Tower Burn-in System accommodates DC/DC power converters. It maintains the case temperature of the power supplies over a temperature range of 40°C to 125°C. The system employs the patented BakPak™ closed-loop conductive heating/cooling technique to achieve precise temperature control. For example, a 100-W DC/DC UUT can be controlled to stay at a set-point case temperature within ±3°C. The Saturn Tower also performs testing during burn-in. EJ Systems, (508) 689-0136.

System Combines Infrared and

Vibration to Perform Screening

The IrIs System inspects and screens electronic assemblies such as direct chip attachments, flip-chip bump bonds and multichip modules. The system applies burst random vibration to precipitate and identify workmanship flaws. Good and bad bonds are differentiated by comparing heat-flow variation signatures obtained by sensing infrared images and subsequent signal processing. Environmental Screening Technology, (616) 772-5485.

System Generates Controlled

Temperature Over Shaker

A controlled-temperature airstream system, a shaker table and an insulated chamber combine to provide an ESS environment for small to medium-sized components and assemblies. The assemblies are manually loaded onto the shaker fixture and the insulated chamber automatically lowers itself over the head of the shaker. Six precisely controlled airstreams are fed into the chamber. Special nozzles direct the airflow over the fixture to ensure rapid thermal soaking and uniform temperature. Environmental Stress Systems, (209) 928-4900.

Controller Combines

Random and Sine Features

The SD1825 Digital Random/Sine Vibration Controller combines features of the SD1815 Digital Sine Servo and the SD1800 Random Vibration Controller. The system offers a choice of control strategies while operating in a PC graphical user interface environment. Profiles can be easily selected, downloaded and run. Random-control features include an 80+ dB dynamic range, four inputs, average and peak limit strategies, and a 5,000-Hz analysis range. Sine servo control provides sweeps to 5,000 Hz and automatic resonance reporting. Spectral Dynamics, (714) 443-5000.

Temperature-Forcing System

Suited for Small Assemblies

The T-2420BV Precision Temperature-Forcing System is light, compact and portable for use in applications where traditional systems are impractical. It occupies a space of 18″ x 18″ x 10″, weighs 50 lb and covers a temperature range from -10°C to +200°C. The vortex principle is used to provide cold air, eliminating the need for LN₂. The microprocessor-controlled system includes an automatic 10-step temperature cycle mode. IEEE 488 and RS-232-C interfaces are standard; PC software is optional. Thermonics, (408) 496-6838.

Copyright 1996 Nelson Publishing Inc.

March 1996