Leadless Devices4 QFN Challenges that will Make you Scream!
BEST Inc. [email protected]
Bottom Terminated ComponentsThe 4 “Biggies” Void Reduction Cleaning Inspection Rework
1. Void ReductionAchieve Reliable Assembly with Minimal Voiding
Large area in thermal ground, lots of volume of flux, lots of outgassing
Voiding impedes thermal transfer, reducing the efficiency and lifetime of the package
No single approach has proven to be the solution Important to reduce overall % voids and size of largest void Try combinations of proven approaches to achieve the best
possible process
Solder paste is ~50% flux by volume
• Large paste deposit contains volatiles• During reflow volatiles must escape to accomplish
complete coalescence• Low standoff leaves no exit for excess volatiles
Strategies to Reduce Voiding
Thermal pad and via design Stencil design Reflow profile optimization
Thermal Pad Design Considerations
Strategy: break up thermal pad to discourage large voids and facilitate venting
“Window paning” creates pathways to vent volatiles
Reduced pockets of volatiles cause smaller voids and less overall voiding
Downside: Vent channels result in less solder joint continuity
Split Thermal Pads
Split thermal pads reduce the size of voids, but do not significantly reduce the total voiding area
For thermal transfer, large voids are more disruptive -> smaller voids allow for more transfer
Solder mask defined pads offer further improvement
Dr. Lee et al. study this in “The Effect of Thermal Pad Patterning on QFN Voiding”
Dr. Lee et al. study this in “The Effect of Thermal Pad Patterning on QFN Voiding”
Voiding in QFN Unfilled Via-in-pad
This is why you must “seal” via-in-pad !
Voiding + Via in Pad
• Voiding even with Pad window Paning! • Excessive voids can also cause co-planarity issues and open QFN joints• IPC & JEDEC stipulates at least 50% coverage criteria for the thermal pad.
Stencil Design
Instead of printing one large square at the center of QFNs: With greater paste volume, voiding increases, so aperture
designs that limit paste volume will reduce voiding Break up this square into smaller apertures Spaces between printed areas leave paths for volatiles to
escape
“Influence of Reflow Profile and Pb-free Solder Paste in Minimizing Voids for QFN Assembly” by T. Jensen, E. Briggs et al.
Effects of Window Paning
W Coleman; “Printing and Assembly Challenges for QFN Devices”; SMT Magazine, 4/11
Voiding vs. Stencil Thickness
Songninluck, et al; “QUAD FLAT NO LEAD (QFN) PACKAGE PROCESSING IN HIGH THERMAL MASS ASSEMBLY”; SMTAI-10
Voiding vs. Window Pane
Songninluck, et al; “QUAD FLAT NO LEAD (QFN) PACKAGE PROCESSING IN HIGH THERMAL MASS ASSEMBLY”; SMTAI-10
“Focal Dot”8mil spacing
Reflow Profiling There is no “magic” reflow profile to
minimize voiding For each assembly, the best profile will
balance several requirements: Thermal requirements of assembly Optimal profile for the chosen solder paste Requirements of other components (ie BGAs)
There will be an optimal profile, it may or may not minimize voiding
Profiling must be considered but not a solution
Low clearance Large center pad
High paste/flux volume Difficult cleaning flow path
2. Cleaning Underneath BTCs
BTC: No-Clean Residue Issues
No-Clean: Solvents can’t escape => residue not safe (even though full time/temp exposure)
Flux volatilization is a critical factor with regard to the complexity of the flux residue so that it becomes benign
The fluxes are not benign until they have seen enough heat to volatilize
the solvents create the insulative residue this can only be done with enough heat and
removal of the flux carrier
Courtesy of Foresite Inc
BTC No-Clean Residue Issue In a no-clean process, when the flux
is activated and fully volatilizes, it leaves a benign residue with no impact on field performance.
But when this solvent is not allowed to release: Flux residue is conductive and
moisture absorbing. Product in the field for 9 months to still
have gooey conductive flux causing performance problems.
BTC: No-Clean Residue Issues
T Munson; “ELECTRICAL LEAKAGE FAILURES DUE TO ENTRAPPED FLUX BELOW A QFN PACKAGE”; SMTAI-06
BTC Cleaning Need to have enough standoff
distance (min’m 2.5mils) Need to have pathways for the
cleaning solutions to get enough impingement energy to the “soils”
Need to have enough kinetic energy to break soils free
Measure using ion chromatography
Courtesy Zestron
BTC Cleaning Study
AOI: Pretty worthless Manual: Depends on wettable
surface Endoscope (edge viewing optical
microscope) reasonable for edge joints
X-Ray: good for voiding, bridges, wetting only method for central pad
3. Inspection
IPC Inspection Criteria
Pullback vs. No-pullback
V Solberg; “ PCB Design Principles for QFN and Other Bottom Termination Components”; APEX-11
Metallization-easier inspection
Shadowing; Solder Balls
Multiple pads different AOI Algorithms
Bastin & Krastev; “QFN PROCESS CONTROL VIA 2D AND 3D CT X-RAY INSPECTION TECHNIQUES “; SMTAI-11
More Defects; CT Scan
IPC 7711 3.11
4. ReworkHigh Yield, Simple and Fast
Rework TechniqueIPC 7711 3.11
Rework TechniqueIPC 7711 3.11
Step #1
Align and apply stencil to device. Roll solder paste through apertures.
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)IPC 7711 5.8.1.2.
Step #2
Reflow per solder mfr’s profile recommendations
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Step #3
Remove stencil and clean. Surface now is “bumped”
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Inspection of “bumped” device
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Step #4
Prep land area on PCB where device is to be placed
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Step #5
Align and adhere board stencil on PCB. Roll solder paste into apertures and clean off residue.
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Step #6
Place “bumped” QFN into stencil apertures. Feel the “bumps” fit into the apertures
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Step #7
Reflow as per solder paste manufacturers’ profile recommendations. Perform visual inspection. Perform X-ray inspection.
Part Bumping Using a PolyimidePeel-n-Stick Stencil(s)
Print Quality
Print Reliability
BEST [email protected]