A Cookson Group Company
V 2 2010
LED Package-on-Board
Assembly: Key Issues
and Impact on Reliability
Ravi M. Bhatkal, Ph.D.
Alpha LED Technologies
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Outline
• Background
• Package-on-board assembly overview
• Key issues and impact on reliability
– Materials selection and design
– Manufacturing process and environment
• Recommendations
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Background
• In the lighting industry, surface mount technology (SMT) as applied to light source manufacturing is a relatively new process.
• SSL Assemblies need to meet high reliability requirements such as, for example Energy Star Category A and B50, L70 of 35,000 hours for outdoor lighting.
• As a result various issues in package-on-board assembly materials and processes need to be addressed along with their impact on reliability.
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Background
• LED soldering is different than soldering an electronic component due to the fact that an LED is a light emitting optical component.
• Assembly materials and process must not interfere with the design and lifetime requirements of the LED package and luminaire.
• The materials and process used must be designed to take performance and lifetime requirements into consideration.
• This paper outlines issues typically encountered in the LED package-on-board assembly process and optimization of the assembly process.
• Recommendations for material selection and manufacturing process are suggested that enable high reliability Luminaire fabrication.
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LED Package-on-Board Assembly
Component
Placement
Package-on-Board Assembly – Wave
Wave
Solder Inspection line Aqueous
Cleaner
Package-on-Board Assembly - SMT
Printer or
Dispenser
Component
Placement
Reflow Oven Inspection line
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Printing
Package-on-Board Assembly - SMT
Printer or
Dispenser
Component
Placement
Reflow Oven Inspection line
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What is Printing?
• Printing is the deposition of a known amount
of solder paste on a component pad.
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Factors That can Affect
the Printing Process: Stencil Design
Printer type
Board supports
Alignment
Gasketing
Solder Paste
Cleaning cycle
Printing environment
Printing parameters
Speed, pressure, cleaning
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Issues in Printing for LED Assembly
Issue Cause Recommendation
Bridging between anode,
cathode and thermal pad, or
solder ball formation
Excess solder paste deposit due to
improper stencil design and printing
parameters
Proper stencil thickness and
aperture design relative to the
board construction and
component pads
Package tilt upon assembly
causing off-axis light output
Unequal heights of solder joints under
anode, cathode and thermal pad, due
to unequal or inconsistent paste
deposits.
Proper stencil design relative to
the pad geometry of the package
and use of a solder paste that has
good release characteristics, and
correct print parameters
Insufficient joint thickness
negatively affecting joint
reliability in use
A lower joint thickness due to a
thinner stencil than recommended, or
insufficient deposit will cause
reduction in the amount of solder
deposited
Proper stencil thickness selection
and aperture design, as well as
selection of paste for proper
transfer
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Issues in Printing for LED Assembly
Issue Cause Recommendation
Contamination of lens
during or after assembly
Selection of a solder paste whose flux
has a low softening point relative to
the operating temperature of the
device, or has a high level of spread
during reflow, or excessive spatter,
thus resulting in migration of the
paste flux used and creating
contamination of the lens
Select a solder paste with with
appropriate chemistry
providing low residues, low spread
and low spatter,
Contamination of Board
Reflective Surface
Selection of a no-clean solder paste
with an opaque residue and high
spread or spatter
Select a solder paste with with
appropriate chemistry
providing clear residues, low
spread and low spatter,
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Recommendations - Printing
• Conduct process development early, ensuring
correct stencil thickness and aperture design,
accounting for component pad geometry, type
of board and board surface features such as
solder mask defined pads, paste selected and
print parameters.
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LED Package Pick
and Place
Package-on-Board Assembly - SMT
Printer or
Dispenser
Component
Placement
Reflow Oven Inspection line
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Issues in Pick and Place For LED Assembly
Issue Cause Recommendation
Damage to optical surface
(lens, encapsulation)
Incorrect handling of the LED
package during assembly, including
use of incorrect pick and place nozzle
Proper handling of LED which
avoids contact with lens or
encapsulation, and use of correct
nozzle
Incorrect placement of the
LED package, resulting in
misaligned or damaged
package
Misalignment of the LED package
with the solder paste deposits, or use
of excessive Z force, causing
displacement of solder paste deposits
Ensure proper alignment of solder
paste on pad and component on
solder paste, and use correct Z
force.
Misplacement of the LED
package – incorrect package
– board orientation
Misoriented package during pick-up Ensure that pick and place
equipment is capable of detecting
orientation and correcting if mis-
oriented
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Recommendations – Pick and Place
• Ensure use of appropriate pick and place
equipment, pick up nozzle and programming,
enabling correct pick up and placement with
correct alignment, orientation and pressure.
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Reflow Soldering
& Profiling
Package-on-Board Assembly - SMT
Printer or
Dispenser
Component
Placement
Reflow Oven Inspection line
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Anatomy of a Solder Reflow Profile
0
50
100
150
200
250
0 60 120 180 240 300 360 420
Time (sec.)
Tem
pe
ratu
re (
C)
Ramp Soak Reflow Cool
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(1) Ramp
• Temperature rise rate to attain solvent (carrier) evaporation
Time Temperature
(1) Initial Ramp
Solvent Evaporation
Large Mass PCBs
Low Mass PCBs
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(2) Preheat/Soak
• Temperature range at which the flux begins to remove oxides
and temperature deltas are allowed to stabilize/soak
• The ‘Soak Zone’ has to be long enough for components to come
to temperature, to avoid thermal shock (cracking)
– Longer for higher mass PCBs
Time
(2) Preheat
Temperature
Soak Zone Large Mass PCBs
Low Mass PCBs
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(3) Reflow
• Temp spike to melt the alloy and allow the solder to wet and form the
proper fillet
– Melting point (Tmelt) of solder
• Typically >217°C for SAC solder paste
– Max Temperature achieved must be below the max temperature for
all components, including the board
– Time Above Liquidus (TAL) will largely determine the inter-metallic
growth, usually 30~60 seconds
Time Temperature
Tmelt
TAL
Tmax
Large Mass PCBs Low Mass PCBs
TAL
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(4) Cooling
• Solidification of the solder joint and cooling of the laminate
– Faster cooling results in finer grain size with higher fatigue
resistance
• Cooling Zone Affects:
– Joint Strength
– Joint Appearance
Time Temperature
(4) Cooling
Large Mass PCBs
Low Mass PCBs
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Issues in Solder Reflow for LED Assembly
Profile Zone Too Little Time Too Much Time
Ramp (fast ramp)
Solder Spattering
Tombstones (if no soak)
High delta T’s entering spike zone
(if no soak)
Component pop corning/cracking
(slow ramp)
Hot slump, potential MCSB’s
Fluxes spent before metal
reached liquids
Tombstones (if no soak)
Soak (fast, cool, or no soak)
High delta T’s entering spike
Little protection against
tombstones
Potentially heavier flux residues
* Recommended soak parameters
should be more like a slowed
ramp (0.5 C/sec) than a flat
plateau.
(long or hot soak)
Charred flux residues, difficult to
probe at test
Spent flux resulting in poor wetting
Random solder balls
Mid-chip solder balls
Solder bridging (will not pull back)
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Issues in Solder Reflow for LED Assembly
Profile Zone Too Little Time Too Much Time
Spike (short TAL or low peak T)
Grainy-looking joints
Poor fusion
Low fillet wetting height
Minimized spread
Intermittent opens (center BGA)
(Long TAL or high peak T)
Dewetting
Charred residues
Board warpage and delamination
Component damage
Cool (Rapid cooling)
Thermal shock to components
possible (follow mfg guidelines)
Typically, the faster cooling
provides a finer grain structure,
which makes for better cosmetics
Intermittent opens (corners
PBGA)
(Slow cooling or ambient)
Extremely poor cosmetics:
-rough surface
-fillet cracking
-fillet lifting on PTH
Displaced components/disturbed
joints (when exit temp is above
solidus)
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Reflow Attribute Balance
Attribute Fast or Cool Slow or Hot
MCSB Better Worse
Tombstones Worse Better
Voiding Worse Better
RSB Better Worse
Spattering Worse Better
Cosmetics Better Worse
NOTE: This speaks in general terms, does not refer to extremes, and can
vary with products formulated for specific market segments
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Recommendations - Reflow
• Consider the component and material stack
while designing the reflow profile
– Leadframe based LED or ceramic submount
LED
– FR4 or MCPCB and surface finish
– Thermal mass of the board
– Solder alloy and flux (paste used)
• Conduct process development early to
determine voiding, shear strength and thermal
cycling performance
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Materials Selection
Issues
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Impact of Materials Stack on Reliability
• Reliability/lifetime is dependent on:
– The material stack selected (submount
/package, solder, board material and dielectric,
and respective geometries),
– The manufacturing process conditions and
– The use environment (temperature swings and
harshness of conditions)
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Materials Stack Impact e.g. Al MCPCB
• Key materials stack variables – ΔCTE between the package/submount and the board
material
• E.g. ΔCTE between AlN submount and Al MCPCB is 20
– Geometry of the materials stack
• Length scale of the package/submount
– Larger package, higher stress
• Thickness of solder joint
• Thickness of dielectric
– Moduli of materials
• Moduli of submount, solder and dielectric
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Process and Environment Impact
• Key process and use environment variables – Process temperature and speed of temperature
change
• Higher temperature coupled with higher ΔCTE will induce more residual stress in assembly
• Faster temperature changes will induce more residual stress
– Temperature swings in use
• Large temperature swings coupled with high ΔCTE between the package/submount and the board material will cause high stress and creep fatigue in use
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Lifetime of Solder Joints
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Material Stack Impact on Reliability
Al2O3 CTE =7
AlN CTE = 5
Al CTE = 25
Cu CTE = 16
FR4 CTE = 17-18
Differential thermal
expansion
Stress relief via creep in solder and
strain in dielectric
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Implications
• A combination of relevant material stack design,
thermal cycling tests, coupled with advanced
modeling, can provide a mapping function
between accelerated testing and estimated
lifetime in use.
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Implications
• Data shows that:
– The creep resistance of the solder is a significant
factor in minimizing failures in solder joints due to
strains incurred in thermal cycling.
– The relationship between the modulus of the dielectric
to the modulus of the solder over the temperature
range in the thermal cycle can be an effective way to
manipulate the strain away from the solder joint in
thermal cycling, hence reducing failures due to solder
joint fatigue.
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Materials Selection Impact on Reliability
Recommendations
• For a given use cycle (temperature swing and dwell time):
– Use a more creep resistant solder material to increase reliability of the solder joints subjected to such cycling.
– An MCPCB with a copper substrate would put less strain on the solder joint resulting in less damage to solder joints.
– Use a lower modulus dielectric to help absorb some of the strain from differential thermal expansion.
– Use an optimized combination of solder and dielectric materials in order to provide the required reliability for a given application.
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Conclusions
• A combination of materials, manufacturing
process and environmental use conditions
determines reliability of LED package-on-
board assemblies and luminaires
• It is possible to optimize design, materials
selection and manufacturing process to
provide reliability required for a given end use
environment condition.
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Thank You
For more information, contact:
Ravi Bhatkal
+1-908-791-3013