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Electrical Failures Electrical Failures
IPC / SMTA Cleaning Workshop
November 16, 2010
IPC / SMTA Cleaning Workshop
November 16, 2010
Content Content
• Technology Innovation
• Device Interactions
• Tin Whiskers
• Soil Effects
• Complexities
Rapid Technology InnovationRapid Technology Innovation
• More performance in smaller platforms
– High Speed Processors
– Lead-Free Soldering
– Reliability critical driver
ReliabilityReliability
• The capacity of a device to perform as
designed
– For specific interval under stated conditions
– Minimal failures
• The Goal: Performs as promised every time
Device InteractionsDevice Interactions
Device Interactions Device Interactions
Surface Mount
TechnologySemiconductor
Fabrication
Advanced
Packaging
-Photoresists
-Plating
chemicals
-Solder pastes
-Fluxes
-Coatings
-Wafer Bumping
-Flip Chip
-Package on Package
Mostly IPC
standardsMostly SEMI
standards
JEDEC
Standards
Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.
Increase in Hardware FailuresIncrease in Hardware Failures
• Gaseous contamination– Chemical effects
• Creep Corrosion
– Mechanical effects• Compressive Stress
• Tin Whiskers
• Optical signal interference
• Friction
– Electrical effects• Circuit impedance
• Dendritic growth
• Arcing
ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.
ReliabilityReliability
• Manufacturers are in a constant struggle to
– Maintain the reliability of their hardware
– Every Shrinking feature sizes
– Decreased distance signals travel
White residue flux migration
MiniaturizationMiniaturization
• Heat dissipation
– Air flow exposes circuits to gaseous contamination
• Non-hermetic sealed packages
– Moisture entrapment
• Decreased spacing
– Voltage differences increase the risk of ion migration
– Compressive stresses
• Corrosion
– Smaller component features reduces the distance corrosion needs to travel
ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.
Co-Planar Board FinishesCo-Planar Board Finishes
• Coplanar board finishes are especially susceptible
– Immersion silver plating (ImAg) over copper
– Organically Coated Copper (OSP)
• Silver and copper are highly reactive to sulfur
– Levels as low as 3 ppm is enough to induce corrosion
• Silver is more noble metal
than copper
– In the presence of atmospheric
water, forms an electrochemical
cell
Sulfur Bearing Gases Sulfur Bearing Gases
• Even in absence of moisture can
– Attack copper and silver
–Migrate as silver and copper sulfide
– Breeches exposes
underlying board
finishes
– Corrosion grows
and leads to
electrical opens
e-
Electron / Metal Atom Flow
e-
e-
e-
e-e-
e-
e-
e-
e-
e-
e- M
Anode
CopperCathode
SilverM
- +
Galvanic Cell Galvanic Cell
• Galvanic cell potential from copper and silver metal
reduction
– Copper, being more active metal, represents the anode when
electrochemical reactions with silver take place
– Copper corrodes faster than other based oxidizing
environment.
Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.
Tin WhiskersTin Whiskers
Tin Whiskers Tin Whiskers
• Lead-Free = High tin alloys
• Whiskers bridge the gap between conductors
• Cause shorts
• Carry enough current to stop the circuit from
functioning correctly
+_
Copper Trace
Lead-free Solder
Whisker
Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in
SAC 305. SMTA IWLPC, Toronto, CA.
Whiskers grow to relieve compressive stress in tin
Compressive Stress
Whiskers grow by atomic diffusion in a crystalline manner
Whisker GrowthWhisker Growth
Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in
SAC 305. SMTA IWLPC, Toronto, CA.
Greatest Incidence of whiskers on the edges of the traces
This is probably because
• Greatest concentration of flux residue
• Exposed copper / thin tin coating
Whisker Location Whisker Location
Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in
SAC 305. SMTA IWLPC, Toronto, CA.
Soil Effects
Assembly Contamination Assembly Contamination
• Stencil cleaning– Wet solder paste – Chip bonder adhesives
• Bare Board– Oxides – Ionic contaminants
• Flux residues – Organic acids – Rosin– Resin structures – Polymers – Functional additives
Flux Residues – A Common Enemy Flux Residues – A Common Enemy
Stencil
Printing
UnderfillSoldering FLUX
Conformal
Coating
Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.
Soil EffectsSoil Effects
• Electromigration: “Electrochemical Migration” (ECM)
– Surface Mount Technology:
– Movement of ions under a potential gradient–
• Closely associated with SIR (Surface Insulation Resistance)
• BETWEEN adjacent metal conductors
• Electromigration (EM)
– Semiconductor / Packaging:
– Movement of atoms caused by electrons
flowing through a metal
– WITHIN a metal conductor
Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.
Electrochemical Migration
No-clean, Halogen-free FluxesElectrochemical Migration
No-clean, Halogen-free Fluxes
Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.
Tin Salt Migration Tin Salt Migration
• Partially removed flux residue under component
– Dendritic growth
– Creep corrosion
Flux Residue Migration under
Conformal Coating
Flux Residue Migration under
Conformal Coating
Flux Left under Components Flux Left under Components
• Feature size reduction
– Increase failure risks
Ion Migration in an Electric Field Ion Migration in an Electric Field
• Propagated by
– The charge balance at the interface
– Current density entering and leaving the device
– Electrolyte from moisture causes ions to split and
form dendrites
AnodeCathode
High VoltageHigh Voltage
Source: FET1_Drain_Joint_Failure_18018797_3_jpg.htm
Shorting / Sparking / Failure Shorting / Sparking / Failure
Electrochemical Migration
Key FactorsElectrochemical Migration
Key Factors
• Contamination– Soils
– Gaseous Moisture (%RH)
• Adjacent metallic conductors
• Electric field (potential gradient)
• Ionizable metals
• Hydrophilic / mildly-hydrophilic continuous phase
Failure Region Failure Region
ConclusionsConclusions
• Reliability of hardware is more prone to risk
due to ever shrinking feature sizes
• Move to Lead-Free increases risk factors
– Co-Planar Board Surfaces
– Creep Corrosion
– Tin Whiskers
• Flux residues bridge conductions
– Electromigration risks
• Removal and mitigation of contamination
sources is increasingly important to assure
reliable devices that perform as promised
QuestionsQuestions
