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more than technology ” High Accelerated Life Test : Translating Theory in to Design Solutions for the Implementation Tool” “solid-edge design”
INTRODUCTION The Environmental Test Chambers are today compulsory for developing any product reliability, The one which is becoming more and more popular is the chamber for : Highly Accelerated Environmental Test or “HALT”. The chamber is easy recognized by high environmental performance and integrated shaker, but… this is not all what is required to an environmental chamber to perform accelerated tests
ACCELERATED TESTING The theory behind the accelerated testing has been explored in many academic contexts, the advantages have been broadcasted all over the world for the benefit of every one The object is now how to translate the accelerated test requirements in the proper technical solutions for the tool which has to implement the test : the environmental test chamber. basic philosofy for accelerating environmental tests : to provide high stress to the product by rapid cycling between extreme environmental conditions and by strong stimulation of the full spectrum failure modes.
HALT requires Outstanding Performance 1 - Extreme change rate chamber 2 - Multi-axis Random Vibrations 3 - Maximum Vibration Transmissibility 4 - Maximum Thermal Transmittance 5 - Adjustable Air Flow 6 - Reliability and Usability 7 - Supplier qualification
1 - Extreme Temperature Change Rates When temperature rate goes to extreme (20 - 30 °C/min), the cost effective solution for the cooling ramps is to use the evaporation of LN 2 (Liquid Nitrogen). Being the evaporation temperature at ambient pressure -196°C and the calorific value 40 -50 Kcal/Kg, it gives the LN 2 a very high cooling power to rapidly cool down the systems which include : the air inside the chamber, the walls and components of the chamber and…. the specimen.
1 - Extreme Temperature Change Rates Considering also a price which all over the world is under the 0. 25 USD per Kg, the LN 2 solution is convenient when the LN 2 supply facility is already available in the laboratory. Traditional compressor refrigeration system for this application would require tenths of HP, even if the electrical energy consumption cost will save some money in comparison with the LN 2 consumption.
1 - Extreme Temperature Change Rates As far as heating the solution refers to the traditional technology of electrical heat dissipation from resitors : easy to implement and fast to be controlled. The associated power has to be adequately increased, for a 1 m 3 chamber with 20 kg of electronic specimen to be heated at 60°C/min, the dissipated, and electrically supplied, power is in the range of 100 Kw !
2 - Multi Axis random Vibration Accelerating the test means also to test "in parallel" as it can be done with a multiaxial shaker which wich avoids rotation of the specimen and test repeatition. Furthermore it has been demonstrated that some product failures are detected by : - correlation effect of multiple axes vibration ; - application of a vibration signal which is random as far as possible (flat and extended frequency band). The cost-effective solution is to use multiple pneumatic actuators.
2 - Multi Axis random Vibration This experimented system satisfy the technical requirements but also is available at costs which are considerably less than complex 6 Do. F system which use different technology (i. e electrodynamic). Vibration levels up to 60 G’s.
Multi-Axis Repetitive Shock Vibration The instateneous pressure of the supplied air controls the "movement speed" of each actuator, viceversa when the actuator works it affects the instaneous air pressure and consequently the "speed" of other actuators, pratically each of the actuators moves randomly and in different direction. Not only the intensity of the acceleration vector is random but also the direction
Multi Axis random Vibration Wide frequency band in the PSD A potential disadvantage is that the vibrations are basically not controllable as far as spectrum shape (they are random only) but they can be controlled only in total rms level. Nevertheless this is what HALT requires.
3 - Maximum Vibration transmissibility Mounting products on Aluminum standoffs provides maximum transmissibility of vibration input from table
4 - Powerful Airflow Thermal power is nothing without the ability to transfer the power to the real subject of the test : the specimen. One of the most important parameter affecting thermal transfer is the “thermal resistance” between the air and specimen. It depends from material and shape of the specimen, density (temperature and pressure) and humidity of the air, airspeed. The factor controllable by the chamber is the airspeed : the higher is the airspeed the lower is thermal resistance and the most part of thermal energy is transferred to the specimen.
4 - Powerful Airflow High airspeed for a given volume requires powerful fans (2 - 4 HP of total power), likely two fans which in combination with a flexible configuration of air ducts give a good airflow distribution.
5 - Adjustable Airflow Precise control of air direction provides faster product change rates Up to four ducts can direct air onto different products
Adjustable Air Flow Air slot can be open or closed When slot is closed, air is directed to the ducts allowing air to be directed onto the product Allows flexibility in controlling airflow to maximize air velocity across the product
Adjustable Air Slot
6 - system reliability & safety : Vibration Top of the line Teflon Coated Impactors for long life and high MTBF
6 - system reliability & safety : CE Mark The system has to comply with the latest requirements of the European Community. The package includes : proper grounding procedures, specified components, screw type terminal connections, Electromagnetic shielding/immunity as well as filters for the heating system to reduce electrical noise
6 - “Usability” Convenient Table Height Front and Rear doors for unrestricted product access 18” X 18” Windows for observation Access Ports for product interconnect, and product monitoring sensors Access Panels
7 - Supplier Qualification Broad product line and experience in environmental chambers Turn-Key System : chamber, controls, and vibration system. Local Sales and Service. Provide factory trained professionals. In house design and manufacturing of the system, from sheetmetal to electronics, electrical subsystems, etc. This assures full control of the production.
Efficiency considerations Saving energy is today an "ambientalist" issue but also affect very much the money budget to run a particular chamber like an accelerated test chamber which use 100 k. W of electrical power and hundreds of kg of LN 2.
Efficiency considerations do not waste energy special care has to devoted to : - minimize thermal capacity of the chamber internal elements in order do not absorb useless energy especially during the temperature transients - minimize thermal capacity of the chamber walls and of the shaker table having the necessary thermal insulation; - improve thermal transfer characteristics with an adequate airspeed and good air circulation around the specimen ;
Efficiency considerations - minimize the pressure drop and turbolence along the airflow circuit in such a way to reduce the dissipated power, the fan motor power and its heating transferred inside the chamber. Careful consideration the above points can considerably reduce the energy consumption
Chamber Typical Performance -100°C / +200°C Temperature variation rate: 60°C/min cooling from +150°C to – 60°C /min heating up from – 60°C to +150°C Air speed: 20 m/sec Acceleration: Up to 60 grms Frequency range: Up to 10000 Hz Vibration table size: 36 x 36 inches Chamber internal dimension: 1000 litres (adjustable) Vibrator features: The system provides vibration using impact excitors with 6 degrees of freedom equally distribuited il all 3 axes and in all 3 rotations Heating: 100, 000 Watts by Nicrome heaters Cooling: Liquid nitrogen expansion Noise pressure level : 76 d. B(a) Temperature Range:
HALT & HASS Chamber Mod. UHS 300 UHS 1200
HALT & HASS Chamber Mod. UHS 300 Temperature Range: Temperature variation rate: -70°C / +150°C 30°C/min cooling from +120°C to – 40°C 30°C /min heating up from – 40°C to +120°C Air speed: 5 m/sec Acceleration: Up to 60 grms Frequency range: Up to 2000 Hz Vibration table size: 24 x 24 inches Chamber internal dimension: 300 litres Vibrator features: The system provides vibratio using impact excitors with 6 degrees of freedom equally distribuited il all 3 axes and in all 3 rotations Heating: Nicrome heaters Cooling: Liquid nitrogen expansion Power: 45 k. W