printed circuit cards and modeling system effects circuit cards... · 2018. 11. 16. · shear,...
TRANSCRIPT
© 2004 – 2010 9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Printed Circuit Cards and Modeling System Effects
Dr. Gil Sharon
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com2
Agenda
o Overconstrained boards
o Coating and underfill
o Mirrored components
o Questions will be addressed at the
end
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com3
What are the system level effects?
o Overconstrained boards
o Underfill and coating
o Component mirroring
Meifunas, M., et al. "Measurement and prediction of reliability for double-sided area array
assemblies." Electronic Components and Technology Conference, 2003. Proceedings. 53rd. IEEE, 2003.
Shi, Hong-Bin, and Toshitsugu Ueda. "Mitigation of thermal fatigue failure in fully underfilled
lead-free array-based package assemblies using partial underfills." Electronics Packaging
Technology Conference (EPTC), 2011 IEEE 13th. IEEE, 2011.
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com4
Why is this an issue?
o Increasing number of companies reporting early life failures during the
manufacturing process, testing or in the field
o Classic solder fatigue approaches do not seem to be capturing these risks
Real CTE mismatch
Shear, Tension
and compression
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com5
Overconstrained Boards
o Why does an overly constrained board cause failures?
o We usually consider the CTE mismatch between the board and the components
o The board CTE value is no longer valid if it is being affected by an external source
o The board CTE and the Aluminum CTE mismatch
o At higher temperatures: The Aluminum expands more than the board
o At colder temperatures: The Aluminum shrinks more than the board
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
A Little Bit of History
o The industry did not have components that were drastically affected by board bending
o Heat sinks on the component
o Small, supported by the leads
o Larger heat sinks connected to the board and even larger: directly to the case
o Weight supported by the case
o Localized effect
o Somewhere along the way we forgot how to design for this issue
Source:http://www.wisegeek.com/what-is-a-
motherboard-heatsink.htm
Early Failure
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Overconstrained Board: Case Study
o Solid State Drive in Aluminum Case
o 6 layer board: CTE 17.4ppm/°C
o Apply (-40)°C to 125°C temperature
range
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com8
Board Strain Results
o Without Aluminum case: o With Aluminum Case
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Effect of Housing: Sherlock and Outside FEA Tools
Mount Points
Sherlock
Solidworks
ABAQUS
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Overconstrained Board Mitigation
o Shoulder Bolts
o The fastener and shoulder prevent the board from being squeezed by the bolt
o The shoulder should be designed to have a small gap
o CTE in the Z direction
o Board: 30 to 40 ppm/°C
o Steel bolt: 10 to 17 ppm/°C
o Aluminum bolt: 24 ppm/°C
o Allow for more expansion in the board
o Consider a low-friction spacer material
o Tolerances are difficult
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com11
Overconstrained Board Mitigation
o Slotted Holes
o The slot allows more freedom for the housing to expando No friction between the bolt and board
o Maintains the Z direction constraint
o Wider than the shoulder
o Length of the slot
o The slot can be 0.1 mm larger than the shoulder and it will be sufficient in most cases
o Can usually use a larger hole diameter
o It is possible to decrease the board reliability with improper mounting conditions
o We can predict the effect of improper mounting
o It can be done experimentally too
o We don’t have an easy way to predict the failures due to bending
o Prevent this issue by using appropriate mounting conditions
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com12
Underfill and Conformal Coating
o Underfill is used to mitigate Shock
and Vibration failures
o Trade off between performance in
mechanical loads and temperature
cycling loads
o Underfill can be bad for TC
o Hard material and high CTE under
BGA and QFN components
Source: Chen, Deng Yun, and Michael Osterman. "Reliability of corner staked surface mount
packages." Accelerated Stress Testing & Reliability Conference (ASTR), 2016 IEEE. IEEE, 2016.
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Effect of Improper Conformal Coating
Test Vehicle after Thick conformal coating had been applied to the right side (coating visible due to UV light)
o Components
o 17x17mm, 256 IO BGA
o Test vehicle
o 2.05 mm (81mil) thick
o 8 layer PCB
o Populated with 60 BGAs
o ‘Thick’ coating was applied to components on one side of the board with a syringe
o Ensures that it wicked under the BGAs
o ‘Standard’ coating was then sprayed on the other half of the board
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Failures Due to Improper Conformal Coating
SAC305 Control sample (no coating) showing cracking along bottom pad
Failure of SAC305 BGA with acrylic conformal coating (-55 to 125°C)
o Results show a distinct difference in solder joint failure mechanism between Standard and Thick conformal coating processes
o Higher loads transfer to Pb-free solder resulting in severe plastic deformation
o Pb-free solder is less compliant than SnPb solder, which led to shorter fatigue life under similar test conditions
Standard
Thick
No Coat
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EFFECT OF CORNER STAKING
No Staking Staking – Namics UF
o Reduction in maximum lead strain
o Sherlock predicts effect of staking on lead strain in QFN packages
o Experimental data shows almost 40% improvements in fatigue life.
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Mirrored Components Interact With One Another
Meifunas, M., et al. "Measurement and
prediction of reliability for double-sided area
array assemblies." Electronic Components and
Technology Conference, 2003. Proceedings.
53rd. IEEE, 2003.
o Previous researchers have shown the negative effect of double sided
configuration on the thermo-mechanical fatigue life of area array components.
o A 2x to 3x decrease in reliability can occur in mirror image assemblies
compared to single sides configurations.
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Effect of Mirroring Components
o FEA modeling on 17x17mm BGA with 256 IO.
o Over-constraining of the PCB resulting in reduced package compliance and larger strain transfer to solder joints along with the DNP effect.
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Effect of Mirroring: BGA ball results
Single Sided Mirroredo Corners joints for mirrored BGA
package
o 25% increase in strain magnitude compared to single sided configuration at the start of the first high temperature dwell of 125°C.
o After 3 thermal cycles, peak difference between mirrored and control boards indicate 2x increase in strain energy density
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com19
EFFECT OF MIRRORING
o Elastic-plastic analysis simulation is performed to capture axial strain in each interconnect
o Interconnects with elevated strains are flagged as red/yellow/green
Mirrored BGA
Unmirrored BGA
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Sherlock Software
o Physics of Failure-based design reliability
analysis tool
o Predicts product failure early in design
process,
quickly and accurately
o Electronics-focused – used across all industries
o Mitigates thermal, mechanical, and
manufacturing risks
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com21
Summary (If you remember nothing else…)
o System level effects
o Overconstrained boards
o There is nothing you can do as a
board designer to prevent the case
from bending the board
o Underfill and coating
o Good underfill is good
o Conformal coat is not underfill
o Mirroring components
o Changes to mechanical system
o Mitigation
o Shoulder bolts
o Oversized holes
o Choose the right underfill
o Don’t let conformal coat get under
your components
o Know the effects of mirroring
o Design your board accordingly
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com22
Questions?
o Speaker Bio: Gil Sharon
o Research focus:
o Mechanical reliability of electronic systems and components
o Multidisciplinary reliability of complex electro mechanical systems
o Characterization and modeling of material behavior
o Physics of failure of electromechanical and MEMS system
o Mechanical performance of flip chip packages
o Doctoral research
o Solder reliability
o MEMS structures characterization
o Embedded components failure analysis
o Particle beam accelerator mechanical fatigue.
o Experience at Amkor technology
o Advanced product development group as senior engineer
o Analysis of chip-package interactions
o Ph.D, Mechanical Engineering (University of Maryland)
o Sales contact: [email protected]