printed circuit cards and modeling system effects circuit cards... · 2018. 11. 16. · shear,...
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© 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o The board CTE value is no longer valid if it is being affected by an external source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
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com7
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
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Effect of Housing: Sherlock and Outside FEA Tools
Mount Points
Sherlock
Solidworks
ABAQUS
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com10
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
9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com15
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: sales@dfrsolutions.com
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