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.
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 xe2x80x9cVibration analysis of electronic equipmentxe2x80x9d, Second edition ed. New York: John Wiley and sons, 1988, pp. 443, electronic chassis assemblies that have high resonant frequencies can be effectively screened in vibration using the NAVMAT P-9492 xe2x80x9cNavy Manufactured Screening Program, 1979xe2x80x9d 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 xe2x80x9cAccelerated stress testing handbook: Guide for achieving quality productsxe2x80x9d 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 xe2x80x9cManagement and Technical Guidelines for the ESS Process.xe2x80x9d 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 xe2x80x9cA Comparison of Repetitive Shock and Electrodynamics Equipment for Vibration Stress Testingxe2x80x9d, Proceeding of the 5th Accelerated Stress Testing Workshop (AST 99), IEEE CPMT Society, Boston, Mass, 1999, pp. 319-328, by W. Tustin and al. in xe2x80x9cAcoustical Screeningxe2x80x94A Sound Solutionxe2x80x9d, Evaluation Engineering, Vol. 40, No. 8, pp. 58-63, 2001, by P. A. Rodger in xe2x80x9cVibration Tools for Accelerated Stress Testingxe2x80x9d, Proceeding of the 6th Accelerated Stress Testing Workshop (AST 2000), IEEE CPMT Society, Denver, Colo., 2000, pp.247-259, and by W. Tustin in xe2x80x9cRandom vibration for the developmental testing and for post-production screening of high-rel electronic productsxe2x80x9d, 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 xe2x80x9cAccelerated stress testing handbook: Guide for achieving quality productsxe2x80x9d 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 xe2x80x9cEnvironmental Stress Screening Guidelines for Assembliesxe2x80x9d, Institute of Environmental Sciences, March 1990, and to xe2x80x9cProduct Reliability Division Recommended Practice 001.1, Management and Technical Guidelines for the ESS Processxe2x80x9d Institute of Environmental Sciences and Technology, January 1999, pp. 57-64.
The use of modal analysis methods for determining dynamic vibration characteristics of a structure, including natural frequencies, mode shapes and damping factors is known. Such a method is disclosed in U.S. Patent Application published under no 2002/0183942 A1 to Lafleur et al. However, the application of known modal analysis methods to spectrum tailoring for specific structures such as PCBs to be subjected to ESS testing, in a view to maximize vibration power transfer to the critical elements of the PCB to be subjected to vibration according to a given testing environment temperature, has not been widely developed prior to the present invention.
It is therefore a main object of the present invention to provide methods for determining vibratory excitation spectrums tailored to physical characteristics of a structure having critical elements located thereon to be subjected to vibration testing, which spectrums are capable of maximizing vibration power transfer to the critical elements of the structure a given testing environment temperature.
It is also an object of the present invention to provide methods for determining vibratory excitation spectrums tailored to physical characteristics of structures such as printed circuit boards (PCBs) having critical elements located thereon including connectors, resistors, capacitors, inductances, Integrated Circuits (ICs) or Ball Grid Array components (BGA), to be subjected to vibration testing, under predetermined thermal conditions set forth by Environmental Stress Screening (ESS) procedures, to either detect or precipitate latent defects that may have otherwise caused failure of the PCB in the field.
According to the above mentioned main object, from a first broad aspect of the invention, there is provided a method for determining a vibratory excitation spectrum tailored to physical characteristics of a structure having critical elements located thereon to be subjected to vibration testing, said physical characteristics including a frequency response function corresponding to a testing environment temperature and defined in term of power spectral density amplitude over a global excitation frequency range for said vibratory excitation spectrum and characterized by a plurality of power spectral density amplitude peaks corresponding to a plurality of natural resonance frequencies each being associated with respective mode shape and damping factor, the method comprising the steps of: i) locating as part of said global excitation frequency range at least one anti-resonance frequency range extending between two said natural resonance frequencies considered as main natural resonance frequencies; ii) defining on the basis of the amplitude peaks corresponding to said main natural resonance frequencies at least two corresponding sets of amplitude peaks each including the amplitude peak associated with any other said natural resonance frequency near corresponding said main natural resonance frequency included in said corresponding set; and iii) defining from said at least two sets of amplitude peaks at least two spectral profile sections associated with corresponding frequency ranges as part of said global excitation frequency range, each said spectral profile section being expressed as power spectral density amplitude according to the mode shape and damping factor associated with corresponding said main and any other natural resonance frequencies and to the location of said critical elements, to form a vibratory excitation spectrum capable of maximizing vibration power transfer to the critical elements of the structure to be subjected to vibration at said testing environment temperature.
According to the above mentioned main object, from a further broad aspect of the invention, there is provided a method for determining a set of vibratory excitation spectrums tailored to physical characteristics of a structure having critical elements located thereon to be subjected to vibration testing under a plurality of testing environment temperatures, the method comprising the step of: i) providing a plurality of frequency response functions representing said physical characteristics at said testing environment temperatures, each said function being defined in term of power spectral density amplitude over a global excitation frequency range for all said vibratory excitation spectrums and characterized by a plurality of power spectral density amplitude peaks corresponding to a plurality of natural resonance frequencies each being associated with respective mode shape and damping factor; ii) locating as part of said global excitation frequency range and for each said frequency response function at least one anti-resonance frequency range extending between two said natural resonance frequencies considered as main natural resonance frequencies; iii) defining on the basis of the amplitude peaks corresponding to said main natural resonance frequencies and associated with each said frequency response function at least two corresponding sets of amplitude peaks each including the amplitude peak associated with any other said natural resonance frequency near corresponding said main natural resonance frequency included in said corresponding set; and iv) defining from said at least two sets of amplitude peaks associated with each said frequency response function at least two spectral profile sections associated with corresponding frequency ranges as part of said global excitation frequency range, each said spectral profile section being expressed as power spectral density amplitude according to the mode shape and damping factor associated with corresponding said main and any other natural resonance frequencies and to the location of said critical elements, to form each said vibratory excitation spectrum capable of maximizing vibration power transfer to the critical elements of the structure to be subjected to vibration at each said testing environment temperature.
According to the above mentioned main object, from a further broad aspect of the invention, there is provided a method for determining a vibratory excitation spectrum tailored to physical characteristics of a structure having critical elements located thereon to be sequentially subjected to vibration testing under a plurality of testing environment temperatures, the method comprising the step of: i) providing a plurality of frequency response functions representing said physical characteristics at said testing environment temperatures, each said function being defined in term of power spectral density amplitude over a global excitation frequency range for said vibratory excitation spectrum and characterized by a plurality of power spectral density amplitude peaks corresponding to a plurality of natural resonance frequencies each being associated with respective mode shape and damping factor; ii) locating as part of said global excitation frequency range and for each said frequency response function at least one anti-resonance frequency range extending between two said natural resonance frequencies considered as main natural resonance frequencies; iii) defining on the basis of the amplitude peaks corresponding to said main natural resonance frequencies and associated with each said frequency response function at least two corresponding sets of amplitude peaks each including the amplitude peak associated with any other said natural resonance frequency near corresponding said main natural resonance frequency included in said corresponding set; iv) and defining from said at least two sets of amplitude peaks at least one spectral profile section associated with a corresponding frequency range as part of said global excitation frequency range, each said spectral profile section being expressed as power spectral density amplitude according to the mode shape and damping factor associated with corresponding said main and any other natural resonance frequencies and to the location of said critical elements, to form a vibratory excitation spectrum capable of maximizing vibration power transfer to the critical elements of the structure to be subjected to vibration at all said testing environment temperature.
According to the above mentioned main object, from another broad aspect of the invention, there is provided a method for vibratory testing a structure having critical elements located thereon at a plurality of sequential testing environment temperatures, comprising the steps of: i) determining a set of vibratory excitation spectrums tailored to physical characteristics of the structure from a plurality of frequency response functions representing said physical characteristics at said testing environment temperatures, each said function being defined in term of power spectral density amplitude over a global excitation frequency range for all said vibratory excitation spectrums; and ii) imparting vibration to said structure according to each said vibratory excitation spectrum sequentially at each corresponding said testing environment temperature to maximize vibration power transfer to the critical elements of the structure.
According to the above mentioned main object, from another broad aspect of the invention, there is provided a method for vibratory testing a structure having critical elements located thereon at a plurality of sequential testing environment temperatures, comprising the steps of: i) determining a vibratory excitation spectrum tailored to physical characteristics of the structure from a plurality of frequency response functions representing said physical characteristics at all said testing environment temperatures, each said function being defined in term of power spectral density amplitude over a global excitation frequency range for said vibratory excitation spectrum; and ii) sequentially imparting vibration to said structure according to said vibratory excitation spectrum at said sequential testing environment temperatures to maximize vibration power transfer to the critical elements of the structure.
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