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News Release from: QualMark | Subject: HALT/HASS test chambers
Edited by the Electronicstalk Editorial
Team on 19 December 2006
Read between the lines of test chamber
specs
Determining which HALT or HASS chamber can best meet your reliability programme requirements takes more than a quick glance of spec sheets.
For those engineers who are not accelerated testing experts, but yet surveying the market for the best value in HALT and HASS chambers, the industry is full of bewildering and confusing "specmanship" claims A larger value of itself does not guarantee better performance in actual use, and sole reliance on the data supplied from most specification sheets is a recipe for disaster
This article was originally published on Electronicstalk on 2 Oct 2006 at 8.00am (UK)
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The subtleties of accelerated testing chambers cannot always be easily boiled down to readily quantifiable attributes.
Instead, it takes an educated eye to see beyond the printed figures to understand the true performance factors of any given chamber.
HALT (highly accelerated life testing) and HASS (highly accelerated stress screening) have been recognised as one of the fastest and most effective new disciplines for design verification testing and production screening - allowing a broad range of industries like consumer electronics, medical, automotive, military and aerospace to bring products to market quickly with reduced design and warranty costs.
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But modern accelerated chambers come with their own set of parameters which go beyond those of traditional electro-dynamic (ED) shakers that are designed to test to a design spec.
Therefore, consider data such as "thermal ramp rates" and "vibration frequency and energy" as merely the starting points for comparing the new repetitive stress (RS) chambers used for HALT and HASS that incorporate stresses in excess of that found in the field.
Ideally, the best way to choose a chamber is to take a design under test (DUT), seed it with known failures, and have each of the manufacturers under consideration subject the product through an abbreviated HALT test.
The most effective chamber will be the one that finds most or all of the failure modes.
Secondly, a visit to each manufacture allows an engineer to observe how each chamber is designed and built, and to discover what support resources are available to help initiate and optimise a HALT/HASS programme.
Since neither of these approaches is seldom practical, the next-best way to begin evaluation is to obtain as much detailed technical information as possible.
Digest it with the following considerations in mind.
"Almost by definition, a HALT chamber must have very rapid thermal transition rates that border on being classified as thermal shock", says Tom Peters, Senior Application Engineer for QualMark.
"This is important because the faster the rate, the greater the stress you're going to apply to the product".
"As an example, some of our tables can go from -100 to +200C, at 60C per minute".
Denver, Colorado-based QualMark Corp is a leader in designing, marketing, and manufacturing accelerated testing systems.
By virtue of conducting more than 4000 tests within in its own lab facilities, and installing and maintaining over 700 chambers in 30 countries, QualMark has earned the position as the knowledge leader in accelerated testing methods.
With five of years of accelerated testing experience behind him, Peters points out that engineers should delve beyond the temperature ramp rate, as this figure may reflect only the temperature of the air within the chamber, as opposed to the temperature of the product within the chamber.
"Some manufacturers specify a ramp rate based on a thermocouple hanging in midair within the empty chamber, but for the application of HALT and HASS, you must be able to move the product temperature at a very high rate of change in order to detect weaknesses in a compressed time environment", Peters reiterates.
When considering airflow, the typical velocity in a standard thermal humidity chamber is around 2 to 3.5m/s, whereas the air velocity in high performance, purpose-built HALT chambers approaches 20m/s.
This difference in air velocity is crucial to the thermal ramp rate performance of the chamber, but still does not tell the whole story.
"The air management system is the critical element affecting the thermal performance of a chamber", stresses Peters.
"The air boundary layer on the components and assemblies must be overcome in order to rapidly change product temperatures, as required in HALT/HASS processes".
Here, an evaluation of the construction of the chamber helps determine effectiveness.
For example, the carefully engineered use of a plenum and ducting helps maximise air volume over the product by creating a turbulent - as opposed to laminar airflow.
Turbulent airflow extracts the greatest BTU change rate on a product.
Air management also plays a significant part in the thermal efficiency of a chamber.
If more BTUs are more quickly transferred to the DUT, then less energy is required to run the chamber.
For example, QualMark makes a HALT chamber that achieves a rated ramp rate of 70C/min with 100A service.
If another comparable chamber requires 160A to achieve the same ramp rate, then it will incur greater utility costs.
"One way to compare thermal efficiency is to develop a standard HALT test profile and then ask each manufacture to calculate the energy consumption for the standardised test", adds Peters.
Random vibration specifications are often the most confusing of any data published for accelerated testing chambers.
The maximum vibration of a HALT/HASS system is usually expressed in units of G RMS.
This value represents the root mean square value of the acceleration (measured in Gs) of the vibration system at maximum control set-point over some defined frequency band.
This specification is usually based on an unloaded table, but as Peters points out: G RMS data on an empty table are useful only for an empty table.
What is needed to properly evaluate a loaded vibration system is information about the energy distribution within the specified frequency range, as this can vary wildly for the same level of acceleration.
"If plotted - with frequency along the x axis, and g RMS along the y - the ideal waveform would be a horizontal straight line - ie all energy is equal across the entire frequency range", explains Peters.
"While it will never be perfectly straight, you want that line to be as flat as possible".
"A frequency distribution of +/-6dB from 200 to 2000Hz would indicate a well-designed repetitive stress table".
On the other hand, the worst scenario is to have major gaps (often referred to as "picket fencing") in energy across the frequency spectrum.
This can happen on poorly designed systems and can also occur on systems that are dependent on pneumatic modulation in an attempt to get good frequency distribution when they are run near maximum vibration levels.
"The whole idea of vibration is to excite all the resonant frequencies of a product, but if you have energy missing in big gaps, then you're going to miss problems", says Peters.
Typically, electronic and electromechanical products have resonances from below 1000Hz for discrete components such as transformers, heatsinks, torroids etc, to above 4000Hz for SMT (surface mount technology) components.
In order for a vibration system to be effective across a wide range of applications, it must produce energy across a broad spectrum of frequencies - ideally from 10 to 5000Hz.
"One way of addressing this issue is by having several different types of hammers underneath the RS table, each with different repetition rates to ensure thorough fill", observes Peters.
"A very low rep rate, say 10Hz intervals, ensures energy at 10, 20, 30, 40, 50, 60Hz etc".
"In this manner you get a complete spectrum with very little, if any, gaps".
When addressing the issue of vibration consistency across the x, y, and z axes, ideally a table will have fairly equal energy in each of the three axis vectors.
This would be represented by a ratio between any two of the axes approaching 1:1.
However, as for other specifications, this data may not prove useful if they are expressed using an empty, flat table.
In reality, the ratio of x and y to z will vary with the load, and with the height that the DUT is fixed above the table.
"If you think of the RS table as the hull of a sailboat, and the mast the distance above the table that your product is fixtured to, you can easily visualise that the higher above the table, the greater will be the x and y motion relative to the table surface", illustrates Peters.
"So if the x, y and z ratios are expressed at a set distance above the table surface, then this gives you a better picture of the actual energy in each axis that your product will experience".
As the majority of failures in the field are caused by thermal and vibration stresses, getting behind the numbers on these particular specifications can provide the greatest return on HALT/HASS programme dollars.
Ultimately, the most useful specifications to make valid chamber comparisons would be those that are based on a mass that closely represents your product.
At least with technically definitive manufacturer's data, an engineer can make an informed decision as to which chamber will best help the organisation attain its reliability goals.
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