Operation of a Vacuum Reflow Oven with Preliminary Void Reduction Data
Fred Dimock
Mgr, Process Technology IntInternational
Mike Meilunas Universal Instruments - Advanced Process Lab
Arvind Srinivasan Karthikeyan Auburn University
Vacuum Reflow
The science of using nothing
to remove empty spaces from
something
Outline
Voids
Why inline Vacuum Reflow?
The Inline Vacuum Reflow Oven.
The Vacuum Reflow Cycle?
The Vacuum Trial The test vehicle
Preliminary results from an extended trial at the
Universal Advanced Process Lab
Things to Consider
Voiding is of particular concern forThermal management of QFN and other devices
All about keeping the chip cool by eliminating voids in
Thermal Pads because empty spaces don’t transfer heat
Void issues
• Voids form for many reasons including:▫Paste solvent volatilization▫Paste solids volatilization▫Oxide volatilization▫Outgassing (PCB, Component)
Void formation and growth
• Voids grow by merging
• Then escape the solder if they contact an exposed surface.
Why Vacuum reflow?
“Low void” pastes have shown that void reduction is possible,
but void levels are still higher than desired.
Typical reflow results in ~ 40% voids in thermal pads
Initially there was hope that the solder paste guys could
find a way to reliably lower or eliminate voids with flux
and thermal profile modifications.
Why Vacuum reflow?
“Low void” pastesMichael Meilunas, “Solder Joint Void Analysis: Effects of Paste and Reflow Parameters”
AREA Consortium
Low Void Paste
Hot Profile
Average Cumulative Void % Standard Paste
Cold Profile - Air
Atmosphere Paste Profile MFL100
Standard NC Cold 33%
Air Cold 19%
Low Void Hot 13%
very Hot 13%
Cold 18%
Nitrogen Low Void Hot 13%
very Hot 10%
Vapor phase reflow + vacuum: expensive, slow, messy
Large voids are more likely to contact exposed solder surfaces and escape
Why Vacuum reflow?
Convection reflow + vacuum:
no vapor, faster throughput, minimum mess
Vacuum reflow has been shown to be effective at reducing solder voids
Voids grow as surrounding pressure decreases (Pascal’s Law)
Large voids have an opportunity to combine and become larger voids
Vacuum Chamber is placed
between the last heated zone and cooling section
with automated board transfer in and out of the chamber
Inline vacuum reflow oven
Four Basic Vacuum Process Steps
Not including transport
Pump Down
Vacuum level
Hold time
Equalization
Rate at which vacuum is applied Torr /sec
The amount of vacuum Torr
The time at the chosen vacuum level Seconds
The rate at which the chamber is Torr /sec
returned to room atmosphere
Pump Down Rate at which vacuum is applied
Too fast
Voids explode when leaving the joint
causing solder balls and splatter
We experienced parts moving and flipping
with a 100 Torr/ sec pump down.
0
100
200
300
400
500
600
700
800
30 to 60 Torr / sec
at hard vacuum levels
Vacuum levelThe amount of vacuum
0
100
200
300
400
500
600
700
800
Levels as low as 1 and high as 250 Torr
(and higher) are possible
Experienced significant void reduction
even at 250 Torr
0
100
200
300
400
500
600
700
800
Hold timeThe time at the chosen vacuum level
Hold times as short as 1 sec
and long as a few minutes are possible.
At 1 sec there is little time for voids to
move to the surface
Times greater than 40 seconds have
been proven to not be needed
Equalization Rate at which the chamber is
returned to atmospheric pressure
0
100
200
300
400
500
600
700
800
Currently no known limitations
Experiments done at ~ 85 Torr/sec
Trials at the Universal Instruments Advanced Process Lab
Mike Meilunas Universal Instruments
Arvind Srinivasan Karthikeyan Auburn University
• Two Layer Test Board• 78x104x1mm
• Immersion Sn and ENIG finishes• MLF: 4, 8 and 12mm bodies• WLCSP: 0.4mm pitch• LGA: 0.4mm pitch • SMR: 2512 and 1206• BGA: 1.0mm pitch • D2PAK
Trials at the Universal Instruments Advanced Process Lab
Air & Nitrogen
Numerous pastes
SAC 305
Innolot
Tin Lead
Three paste suppliers
Two board finishes
Two pad designs some with vias
Varied Vacuum level
Hold time
We ran
over 200
boards
Assembly Process
• Screen print solder paste• 100 micron thick stencils
• Paste inspection• Koh-Young 8030-3 & Optical
• Placement & Inspection• Reflow soldering
• BTU Pyramax 125N (control – no vacuum)• BTU Pyramax Vacuum Oven
• KIC SPS Profiler• Post-Reflow Inspection
• Visual• X-ray analysis (Dage XD7600)
Atmosphere Nitrogen
Solder paste SAC 305
Board finish ENIG
Profile RTS - 240 peak
Pad Design Window pane
Pump down rate Fixed
Refill rate Fixed
Multiple
Vacuum levels
Hold Times
Preliminary results from 3 MLFs 100, 52, & 16
MLF 16 no vac vs vac
MLF 100 no vac vs vac
Hold times % Voids @ 60 Torr
Torr Sec MLF100 MLF52 MLF16760 0 38.54 27.81 20.37
60 10 9.3 4.2 3.060 20 8.0 3.4 1.760 30 6.1 2.6 2.560 40 6.2 2.5 2.2
20 5 7.320 10 3.65 2.49 1.1820 20 3.35 1.24 1.1420 30 1.93 0.72 1.0420 40 1.99 1.13 0.66
> 10%< 10%< 3%
Hold times % Voids @ 20 Torr
Conclusions:
Vacuum level % Voids @ 20 Sec hold time
Torr Sec MLF100 MLF52 MLF16760 0 38.54 27.81 20.37
500 20 26.08120 20 6.7 4.1 2.445 20 4.2 2.1 1.320 20 3.35 1.24 1.146 20 1.86 1.86 0.75
> 10%< 10%< 3%
500 30 26.8250 30 9.79 5.62 5.2185 30 7.4 3.1 2.520 30 1.93 0.72 1.046 30 1.5 0.66 0.96
Vacuum level % Voids @ 30 Sec hold time
Conclusions:
Conclusions:
1. Nothing removes most of the empty spaces
from something
3. Vacuum levels as soft as 245 Torr and short as
5 seconds remove some of the empty spaces.
4. Hard vacuum of 20 Torr with a hold time of 30 seconds
results in void levels of about 2% in MFL 100s
2. It is easier to remove voids from small components
than large ones.
Things to consider
Voids located at critical points (“hot spots”)
may be the real issue
Void Size
Large voids are most likely the primary concern
Small voids may not be troublesome
Cumulative voiding may only have a secondary affect
QFN Void Distribution Analysis: QFN100
At 120 Torr and
lower vacuum
levels, the odds of
getting a void larger
than 1% is small
Fred Dimock
Mgr, Process Technology IntInternational
Thank you!
Mike Meilunas Universal Instruments - Advanced Process Lab
Arvind Srinivasan Karthikeyan Auburn University
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