Measuring and testing – Vibration – Test chamber
Reexamination Certificate
2003-04-30
2004-11-02
Kwok, Helen (Department: 2856)
Measuring and testing
Vibration
Test chamber
C073S579000
Reexamination Certificate
active
06810741
ABSTRACT:
FIELD OF INVENTION
The present invention relates to vibration testing, and more particularly to methods for determining vibratory excitation spectrum tailored to physical characteristics of a structure having critical elements located thereon, such as a printed circuit board including electronic components associated with an interconnecting layout, some of which are considered as critical elements including connectors, resistors, capacitors, inductances, Integrated Circuits (IC) or Ball Grid Array components (BGA), to be subjected to vibration testing, under predetermined thermal conditions.
BACKGROUND OF INVENTION
The production of reliable electronic products requires the use of defect precipitation and detection processes such as Environmental Stress Screening (ESS). The defect detection process should take place at different integration stages during the manufacturing process. Although each electronic system and the location and types of defects vary widely, an average 70% of the defects found in electronics are a result of a defect at the Printed Circuit Board (PCB) level (solder, component defects). The other 30% of the causes of failures are found at the system assembly level (connectors, errors in assembly). ESS testing at PCB manufacturing level offers many advantages. Diagnostics for individual circuit cards can run faster and may be more specific for identifying the root cause of the fault than diagnostics at the system level. Other stresses such as voltage margining and power cycling can be tailored to each circuit card type to maximize precipitation and detection of defects. Moreover, finding a problem at the circuit card level is usually less expensive than at the system assembly level.
The use of vibration and thermal stresses to find latent defects was first advocated and promoted in the military field. As mentioned by D. S. Steinberg in “
Vibration analysis of electronic equipment
”, Second edition ed. New York: John Wiley & sons, 1988, pp. 443, electronic chassis assemblies that have high resonant frequencies can be effectively screened in vibration using the NAVMAT P-9492
“Navy Manufactured Screening Program,
1979” p. 16. The vibration profile referenced in the NAVMAT P-9492 is a 6 g RMS random 20-2000 Hz Power Spectral Density (PSD) set of curves. However, this profile can adversely damage flexible products that have low resonant frequencies, as mentioned by Steinberg in the above-cited reference. As stated by W. Tustin, in “
Accelerated stress testing handbook: Guide for achieving quality products” edited by H. Anthony Chan and Paul J. Englert, New York: IEEE press,
2001, Chapter 10, pp. 155-181, it is desirable to carry out vibration testing experiments with other spectra to adjust the frequency content of the excitation. It is well known that a given defect will only be precipitated when sufficient fatigue damage is induced in the mechanical structure and this often occurs at the structures natural resonant frequencies. The vibration energy that does not correspond to the resonance frequencies of the unit under test is wasted, specially for simple mechanical systems such as PCBs. Therefore the most efficient ESS vibration applied to the unit under test should be based on the response of the product, and not on a predetermined spectrum. In “
Management and Technical Guidelines for the ESS Process.” Mount Prospect, Ill.
60056: Institute of Environmental Science and Technology, 1999, pp. 41-50, tailored input spectra an tailored spectral response methods are proposed, which include spectrum tailoring respectively at the PCB and assembly level.
The ESS process does not call for a specific type of vibration equipment. However, electro-dynamic and repetitive shock shakers are commonly used in typical ESS processes, and more recently acoustic vibrators as disclosed International PCT Patent Application published under no. WO 01/01103A1 to Lafleur et al. and naming the same assignee as the present patent specification. The physical operation modes of these equipment are quite different and have been the object of comparative works reported by several authors such as by E. K. Buratynski in “
A Comparison of Repetitive Shock and Electrodynamics Equipment for Vibration Stress Testing”, Proceeding of the
5
th
Accelerated Stress Testing Workshop (AST 99), IEEE CPMT Society, Boston, Mass, 1999, pp. 319-328, by W. Tustin and al. in “
Acoustical Screening—A Sound Solution
”, Evaluation Engineering, Vol. 40, No. 8, pp. 58-63, 2001, by P. A. Rodger in “Vibration Tools for Accelerated Stress Testing”, Proceeding of the 6
th
Accelerated Stress Testing Workshop (AST 2000), IEEE CPMT Society, Denver, Colo., 2000, pp.247-259, and by W. Tustin in “Random vibration for the developmental testing and for post-production screening of high-rel electronic products”, Proceeding of EtroniX 2001, Anaheim, Calif., 2001, pp.7. The electro-dynamic shaker is considered to be the most versatile type of vibration equipment. The electro-dynamic shaker allows to control random or sine vibration at the base of the unit under test in term of frequency and level typically from 2 to 2000 Hz, at high vibration level up to large payload. However, the relatively and high acquisition cost of this equipment constitute its principal disadvantage. On the other hand, the repetitive shock shaker creates vibration on the unit under test by impacting the vibration table with several air driven impact hammers. This mode of operation leads to a vibration without providing spectrum control, the spectrum being directed by vibration platform natural frequencies. As stated by C. Felkins in “
Accelerated stress testing handbook: Guide for achieving quality products
” edited by H. Anthony Chan and Paul J. Englert. New York: IEEE press, 2001, Chapter 9, pp. 137-154, if table resonance and product resonance overlap, then a way must be found to either damp the table resonance or alter the product fixtures. The repetitive shock excitation presents the advantage of allowing an easy 6 degrees of freedom. However it inputs more energy into high frequencies than low frequencies and it may cause some defect precipitation that are not related to early failure. Acoustic vibrators such as disclosed International PCT Patent Application published under no. WO 01/01103A1 can be used as economical system for testing structures such as PCB's. A main advantage of acoustic vibrator is to allow vibration control in amplitude and frequency over a wide bandwidth for flexible structures with non-contact and directional excitation. Typically, the acoustical excitation is able to achieve nominal vibratory response in random excitation of 20 g rms or higher in the 2 HZ to 2000 Hz frequency domain, as well a sine excitation at level reaching 100 g peak at PCB's resonance. While the electro-dynamic shaker is well adapted for heavier structures, acoustical vibrators are particularly suitable for flexible structures such as PCBs for cost and simplicity reasons, especially when combined with thermal stimulation. The acoustical chamber can be used as a thermal chamber, thus avoiding the use of specific thermal barriers, as required with electro-dynamic repetitive shock shakers. In WO 01/01103A1, Lafleur et al. discloses typical cycling temperature response curves as obtained using a thermal control subsystem provided with a set of temperature sensors, while imparting vibration to a PCB under test. In the presented example, a predetermined profile for performing thermal cycling stress screening of the PCB and as previously stored in the system computer memory is selected by a user. Characteristics of the selected cycling profile were determined according to well known criteria, including cycle characteristics (low temperature, high temperature, product thermal response rate, dwell times at temperature extremes), number of thermal cycles and PCB condition (powered, unpowered, monitored, unmonitored), with reference to “
Environmental Stress Screening Guidelines for Assemblies”
, Institute of Environmental Sciences, March 1990, and to
Lafleur François
Laville Frédéric
Thomas Marc
Boudreau Jean-Claude
Centre de Recherche Industrielle du Quebec
Kwok Helen
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