Implementing Physics of Failure into the Design Process September 15, 2016 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Nathan Blattau, Ph.D. Senior Vice President of DfR Solutions, has been involved in the packaging and reliability of electronic equipment for more than ten years. His specialties include best practices in design for reliability, robustness of Pb-free, failure analysis, accelerated test plan development, finite element analysis, solder joint reliability, fracture, and fatigue mechanics of materials. 2 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Role of Modeling in Design Nobody can afford to repeatedly test and redesign to create reliable, cost o effective products Working with models allows an interdisciplinary design team to create a more o reliable design smarter, faster & cheaper! 3 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Simulation & Modeling o Performing thermal, mechanical & electrical simulations & extracting the results into a time-to-failure prediction 4 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Physics of Failure (PoF) PoF Definition: Use of science to capture an understanding of failure o mechanisms & evaluate useful life under actual operating conditions Using PoF, design, perform, and interpret the results of accelerated life o tests Starting at design stage o Continuing through lifecycle of the product o Start with standard industry specifications o Modify or exceed them o Tailor test strategies specifically for product design & materials, use o environment, and reliability needs 5 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Physics of Failure Definitions Failure of a physical device or structure attributed to o Gradual or rapid degradation of the material(s) in the device o In response to the stress or combination of stresses the device is exposed o to, such as: Thermal, Electrical, Chemical, Moisture, Vibration, Shock, Mechanical Loads . . . o Failures May Occur: o Prematurely o Gradually o Erratically o 6 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Using Physics of Failure During the Design Stage o Design the product for robustness and to meet the environmental requirements o Vibration o Mechanical Shock o Thermal Cycling 7 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Designing for Mechanical Loads o Unlike other materials, solder is a poor engineering material o Extreme dimensional variations o Presence of voids is normal and expected o Is constantly being subjected to inelastic deformations under thermal cycling and shock You would never use steel, titanium or aluminum under these o types of conditions, unless you want it to fail During vibration we need to prevent inelastic deformations o (plasticity) o This makes the field of electronics reliability unique 8 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Vibration Fatigue o Due to the high number of cycles we need to avoid inelastic deformations at all cost o Inelastic deformations (plasticity and creep) are low cycle fatigue (< 100,000 cycles) o During vibration cycles accumulate quickly Example, 100 Hz vibration – 100 cycles per second o Time to accumulate 100,000 cycles, 16.67 minutes o 9 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Designing for Vibration o Octave rule: the PCB natural frequencies should be at least 2X the chassis natural frequencies to prevent coupling o Recommended reading Steinberg’s Vibration Analysis of Electronic Equipment o If the chassis resonant frequency is close to the PCB then there can be significant amplification of the PCB deformations 10 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Natural Frequency o Do it by hand o Limited shapes o Simple support conditions o Use FEA to handle complex shapes and boundary conditions Vibration Analysis of Electronic Equipment David S. Steinberg 11 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Using Sherlock or FEA for Vibration During Design Phase o There are many factors that can be adjusted to modify the natural frequency response of the printed circuit board o Component placement o Boundary conditions (mount points) o PCB properties (thickness, laminate) 12 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Boundary Conditions Component Mass – Heatsinks Printed Circuit Board Properties Typically, the higher natural frequency the more robust the design 13 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Boundary Conditions o Chassis typically have lower natural frequencies than circuit boards o Usually looking for circuit boards having natural frequencies greater than 150 Hz o Natural frequency should not coincide with peaks 23 Hz is too low for most in the expect vibration applications, need to input make changes 14 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Boundary Conditions Add more support o Current, PCI-E type o Add an additional mount at high deflection area 15 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Modifying Boundaries in Sherlock 16 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Almost a 4X increase with one additional mount o Changing heatsink from Copper to Aluminum 17 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Heatsink from Copper to Aluminum o 82 to 93Hz 18 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Additional Mount Point and Increasing PCB Thickness 1.544 to 1.65 mm NF increases to 108.5 Hz 19 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Conduct Physics of Failure Assessment o Once an acceptable NF is achieved o During vibration the assembly is assumed to deform elastically so that strain on the PCB is proportional to the strain in the solder and leads o This allows PCB strain to be used to make fatigue predictions 20 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Vibration Fatigue Board bending Component Motion o HCF failures typically occur in the lead or solder joint 21 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Lifetime under vibration is divided into two regimes o Low cycle fatigue (LCF) o High cycle fatigue (HCF) o LCF is driven by inelastic strain (Coffin-Manson) c 2 N p f f -0.5 < c < -0.7; 1.4 < - 1 /c > 2 o HCF is driven by elastic strain (Basquin) f b E 2 N -0.05 < b < -0.12; 8 > - 1 /b > 20 e f 22 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com 22
Typical Vibration Levels Harmonic Steinberg D.S. Vibration analysis for electronic equipment. John Wiley & Sons, 2000. Random MIL-STD-810G Figure 514.6C-1 US Highway truck vibration exposure 1 hour is equivalent to 1000 miles Enter profile into Sherlock 23 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Physics of Failure Results – Random Vibration 24 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Random Vibration Results o U1 is failing because it is in areas of high bending (red) o Further design changes are necessary to get this board to survive the expected field environment o Additional mount points, stiffeners, etc.. 25 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Mechanical Shock Very similar to vibration o The higher the board stiffness o (Natural Frequency is directly related) the more robust with regards to mechanical shock Lower component mass, Increase board o thickness Due to today’s low profile o surface mount components, shock failures are primarily driven by board flexure BGAs don’t care about in -plane o shock, unless it causes the board to bend Shock tends to be an overstress o event (though, not for car doors) Failure distribution is ‘random’ o 26 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
In Plane Shock 27 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Design - Board Thickness Effects Board thickness 1.836 mm Board thickness 1.575 mm 0.68 mm displacement 0.97 mm displacement Board thickness 2.285 mm 0.41 mm displacement 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com 28
Using Sherlock During Design for Thermal Cycling Fatigue Examples o Package selection o Printed circuit board properties o Solder pad design o Plated through hole 29 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
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