External Use
TM
Automotive Devices: Quad No-
Lead (QFN) Technology with
Inspectable Solder Connections
FTF-SDS-F0026
A P R . 2 0 1 4
Dwight Daniels | Package Engineer
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External Use 1
Agenda
• Wettable Lead Ends / Definition & Alternate Names
• QFN Packages / Intro to Features & Lead End Options
• Freescale Evaluations / Inspectable Solder Joint Study
• Solder Joint Life / How Good are They?
• Conclusion
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Purpose: Wettable Lead Ends A wettable lead end on a QFN is to promote wetting of solder to the ends of the QFN terminals during PCB assembly in order to form a solder fillet which is inspectable by AOI (Automated Optical Inspection).
PROMOTE WETTING OF SOLDER – ”wettable” does not guarantee wetting
SOLDER FILLET – presence and shape are dependent on many factors of the PCB assembly process
INSPECTABLE BY AOI – assumes expertise and equipment capability at the PCB assembly facility
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Wettable Lead Ends – A.K.A. … Nomenclature:
WETTABLE FLANK (WF) – the flank (end) of the lead/terminal is created to be wettable
INSPECTABLE JOINT (IJ) – the lead/terminal ends are created to be wettable to promote a visible, inspectable joint/fillet
SOLDERABLE LEAD END – the lead/terminal end is created to allow solder to readily wet to it
SIDE SOLDERABLE – the side of the package (I.O.W. lead ends) is created to promote solder wetting
SOLDER FILLET – the resulting formation/presence of the inspectable solder joint
In the case of the 1st 4 -- A rose by any other name is still a rose.
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Agenda
• Wettable Lead Ends / Definition & Alternate Names
• QFN Packages / Intro to Features & Lead End Options
• Freescale Evaluations / Inspectable Solder Joint Study
• Solder Joint Life / How Good are They?
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QFN Packages
(Quad Flat-pack No-leads) QFN / Package Structure Package Characteristics / Pro’s &Con’s Lead End Variants / Options & Creation
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Anatomy of a QFN Package
flip chip Chip-on-Lead configuration No pad (typically) Accommodate a larger die:package ratio
wirebond
Exposed pad Good thermal efficiency Added mechanical interface
Half-Etch Features Locking features, etc.
Terminal - Design Variations Standard – pull back e-Type - exposed lead end Wettable – plated lead end feature
Saw QFN
Small Footprint Due to no lead extension
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• Small body sizes
− <2x2mm up to ~ 12x12mm (sweet spot 3x3mm to 8x8mm)
− Relatively low lead count
• Thin package height
− ≤1mm max height is typical
• “Single sided” mold – no parting line
− Lead frame aligns with the seating plane of the package body
• Exposed flag is a standard feature (typically) − Good thermal efficiency
− Solderable feature to improve mechanical robustness & board life.
• Leads/terminals contained within the physical outline of the body − No bent/skewed leads – No Trim/Form/Excise required
− No lead structure which extends beyond the body
Small footprint
No lead compliance
Traditionally difficult to inspect solder joints after board assembly • Inspection requirements are market dependent
QFN Package Characteristics: General
This is why we’re here.
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QFN – Terminal Design Variants
“Exposed” Lead End (e-type) Increased terminal surface area Potentially wettable terminal end surface
“Standard” Lead End Plastic Mold Compound (EMC) fills in the half-etched* portion of the lead end
Wettable Lead End (WF or IJ*) Wettable terminal end surface Used for applications which require AOI (Automated Optical Inspection)
* Wettable Flank or Inspectable Joint -- or -- side solderable or …
*
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QFN – Terminal Design Variants
“Exposed” Lead End (e-type) Increased terminal surface area Potentially wettable terminal end surface
“Standard” Lead End Plastic Mold Compound (EMC) fills in the half-etched* portion of the lead end
Wettable Lead End (WF or IJ*) Wettable terminal end surface Used for applications which require AOI (Automated Optical Inspection)
* Wettable Flank or Inspectable Joint -- or -- side solderable or …
Plated Surface
Shown in Gray
*
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QFN – Wettable Lead End Variants
Wettable Lead End – methods of formation Mechanical Removal: created by removing material from the package during the assembly process, usually by mechanical saw. - Often referred to as Step Cut Etching: typically created by the half etch process during lead frame manufacturing - Often referred to as Dimple
Plating After Singulation: created after singulation in assembly by depositing wettable material on the lead end - Often performed by Electroless Sn plating
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Step Cut – 2-pass saw process Dimple – Leadframe ½-etch process
Wettable Lead End – Physical Comparison
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Step-Cut saw blade – 1st pass saw
Singulation saw blade – 2nd pass saw
Step Cut
2-pass saw process
Wettable Lead End – Step Cut Formation
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Mechanical (step cut)
Freescale’s preferred option
Design Flexibility – can be
applied to existing leadframe
designs
Created using existing QFN
assembly manufacturing
processes
IJ features are full lead width
IJ features can be greater than
half the lead frame thickness
Adds nominal cycle time to the
assembly process
Requires plating process in
assembly - even if using pre-
plated leadframes
Etched (dimple)
Created during the normal
leadframe manufacturing flow
Equally compatible with standard
Cu or pre-plated lead frames
Half-etched features are limited
to roughly half the leadframe
thickness
Features etched during
leadframe manufacturing can’t
be full lead width
Saw singulation of narrow, half-
etched features are difficult to
saw singulate cleanly
- Imposes a practical minimum
lead pitch for saw singulated
QFN package outlines
Consumes more of the surface
area of the lead/terminal
Post-Singulation (e-less)
Design Flexibility – can be
applied to existing leadframe
designs
Maximizes wettable height &
width for any given lead frame
thickness
Adds nominal cycle time to the
assembly process
Requires additive process in
assembly - even if using pre-
plated leadframes
Usually created using
electroless plating
- Not a standard QFN assembly
process
- E-less plating can be difficult to
maintain
- Can complicate handling in
assembly manufacturing.
Wettable Lead End Variants – Pros & Cons
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QFN & Wettable Lead Ends – Wrap-up
SMALL BODY, SMALL FOOTPRINT
GOOD THERMAL EFFICIENCY
VERSITILE MANUFACTURING CONFIGURATIONS
VARIOUS TERMINAL DESIGN OPTIONS – Including WF/IJ/SS options
STEP CUT – freescale PREFERRED OPTION FOR IJ/WF
– compatability with existing manufacturing – robust performance
More about this later
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Agenda
• Wettable Lead Ends / Definition & Alternate Names
• QFN Packages / Intro to Features & Lead End Options
• Freescale Evaluations / Inspectable Solder Joint Study
• Solder Joint Life / How Good are They?
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Inspectable Joint – Step Cut Evaluation & Inspection Results
Freescale Evaluation / Design & Set-up
Representative Solder Joints / Good vs. “Bad” or Ugly?
Inspection / Results and Comparisons
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2 package outlines: 5x5mm, 32ld QFN-IJ – 8 DOE legs (step cut) + control cells with e-type leads 7x7mm, 48ld QFN-IJ – 8 DOE legs (step cut) + control cells with e-type leads & punch dimple
2 PCB pad designs for each body size: Nominal lead size + 0.05mm (under the package) + 0.3mm (extension beyond the package) Nominal lead size + 0.05mm (under the package) + 0.6mm (extension beyond the package)
2 solder paste materials: Both – SAC305 (Sn96.5/Ag3/Cu0.5), 89% metal load, ROL0, “no clean”
Aging vs. No-Aging: 16hr, 155C aging bake prior to PCB assembly vs. No aging pre-bake
• Study Factors for Board Assembly:
• Factors were chosen based on former board mount studies of QFN-IJ
(wettable lead-end) packages.
Step Cut PCB Assembly Evaluation
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Board Assembly Study: overview of the numbers
768 study samples (step cut units): plus 288 control units 5x5mm, 32ld QFN-IJ – 8 DOE legs (step cut). 2 control cells with e-type leads 7x7mm, 48ld QFN-IJ – 8 DOE legs (step cut). 2+2 control cells with e-type & punch dimple
30720 step cut solder joints: plus 12288 control solder joints 15360 - 0.3mm pad extension 15360 - 0.6mm pad extension 12288 - control parts (all with 0.6mm pad extension)
16 DOE Study Populations: plus 6 control cells:
4000+ Temp Cycles (and climbing):
Step Cut PCB Assembly Evaluation
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PCB Assembly Process Factors
• Successful formation of inspectable solder joints is dependent on
many factors of the PCB assembly process.
PCB PAD DESIGN Pad length/extension (beyond package outline), feature clearance, consistent PCB quality of finish
SOLDER PASTE MATERIAL flux system, alloy, quality/uniformity, compatibility (with overall process)
STENCIL DESIGN & MATERIAL material, thickness, aperature, release characteristics
PCB ASSEMBLY PROCESS OPTIMIZATION placement/alignment accuracy, dispense volume, reflow profile & conditions
REFLOW CONDITIONS oven type, # of zones, temperature control, oven atmosphere, board layout
Note: PCB assembly for this study was performed at an independent, 3rd party
contract board assembly provider – no pre-assembly optimization was performed.
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PCB Assembly Process Factors – Package dimensions
Package Vehicle 1:
Body size: 5x5mm
Body Thickness: 0.85mm
Leads: 0.4 x 0.25mm @ 0.5mm pitch
Exposed Pad: 3.6 x 3.6mm
Package Vehicle 2:
Body size: 7x7mm
Body Thickness: 0.85mm
Leads: 0.4 x 0.25mm @ 0.5mm pitch
Exposed Pad : 5.05 x 5.05mm
Note: In this study the exposed pads of all packages were soldered to the PCB
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Pad & Stencil Design Details:
2 PCB pad designs for each body size: Nominal lead size + 0.05mm (under the package) + 0.3mm (extension beyond the package) Nominal lead size + 0.05mm (under the package) + 0.6mm (extension beyond the package)
PCB Pads
Under Package Exterior to Nominal Package Outline
0.275 mm wide
0.35mm + [0.55 / 0.25] mm – based on pad design
for 0.6 / 0.3 pads
0.25 mm wide Solder Paste
Package Lead e.g. 0.4 x 0.25mm
Step Cut PCB Assembly Evaluation
Pictorial representations are NOT to scale
0.6 mm pad extension
0.3 mm pad extension 0.45mm +
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0.3mm pad design 0.6mm pad design
PCB Assembly – Solder Fillet Formation
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0.3mm pad design 0.6mm pad design
PCB Assembly – Solder Fillet Formation
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PCB Assembly – Solder Fillet Formation (cross-sections)
0.3mm pad
0.6mm pad
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PCB Assembly – Solder Fillet Formation (cross-sections)
20
0µm
20
0µm
20
0µm
20
0µm
2
00
µm
0.3mm pad
0.6mm pad
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PCB Assembly – Solder Fillet Formation
comparison: Step Cut & Dimple - punch
Dimple
Step Cut
solder joint formation solder joint cross-sections
20
0µm
2
00
µm
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Solder fillet formation by cell –AND– Manual vs. AOI Inspection results
Step Cut PCB Assembly Evaluation
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Solder fillet formation vs. DOE Factors – & – Manual vs. AOI Inspection
results
Pad design is a
dominant factor
for consistent
fillet formation
------------
This result is
consistent with
past Freescale
fillet formation
studies
There is general
alignment between
manual & automated
inspection methods
- specifically, complete
agreement that pad
design is the key factor
------------
Presence of a fillet
and interpretation
of the shape of the
fillet are somewhat
subjective
Step Cut PCB Assembly Evaluation
Note: Manual inspection & AOI confirm no missing fillets on all control lots. All control lots were assembled on pads with 0.6mm pad extension.
Control cell 5-A-6-n exhibited relatively high occurrence of convex fillets
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PCB Assembly – AOI Failure Mode
Short pads are more susceptible to variation in fillet formation due to
placement &/or solder printing process tolerance.
This solder joint
is formed and reliable,
but AOI is all but
certain to interpret
the joint as “missing.”
AOI failure
AOI pass
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Poor alignment on a short pad is more likely to form a solder fillet
that AOI will reject.
PCB Assembly – AOI Failure Mode
pin 25
Note: Mitigation strategy for poor fillet formation is process optimization and
increased (longer) pad length with accompanying paste print increase.
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Comments on Common Solder Joint Inspection Methods:
• AOI (Automated Optical Inspection)
− Faster and more repeatable than human eye or X-Ray
− Desirable for high reliability applications, such as automotive
− Relatively complex set-up and programming for interpretation of lighting
• X-Ray
− Relatively slow
− Preferred for identifying voids and defects located under the package
Voids do not correlate well with early solder joint life failures
Defects (such as electrical shorts) under the package are not common
• Human Eye Inspection
− SLOW! And subject to fatigue, distraction, boredom, hunger, restlessness, etc.
− Better at judging if a solder joint is formed when the fillet is not optimal
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PCB Assembly Evaluation Results - Wrap Up
Board assembly of 1000+ units, in 22 distinct populations, was performed using common industry methods & pre-selected BOM, for the purpose of studying inspectability & reliability of QFN packages with step cut IJ feature.
GOOD PROCESS YIELD – especially considering the process wasn’t optimized >99% process yield – “hard” failures (opens/shorts due to print errors)
ALIGNMENT BETWEEN HUMAN-EYE INSPECTION & AOI agreement that we achieved 100% solder fillet formation on long (0.6mm) pads agreement that all discrepent or marginal fillets occurred on short (0.3mm) pads poor agreement on which solder joints (fillets) were considered visual fails
SUCCESSFUL AUTOMATED OPTICAL INSPECTION capable of performing relatively fast inspection requires expertise to set-up and program properly likely to reject reliable solder joints if board assembly process exhibits significant variation
SOLDER WETTING & WETTED HEIGHT excellent wetting to the lead ends wetted solder height consistently >140um increased variability in solder fillet formation/shape on shorter (0.3mm) pads
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Agenda
• Wettable Lead Ends / Definition & Alternate Names
• QFN Packages / Intro to Features & Lead End Options
• Freescale Evaluations / Inspectable Solder Joint Study
• Solder Joint Life / How Good are They?
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Solder Joint Life Joint Life / Study Results
Feature vs. Feature / Where does the weakness lie
Fillet formation / How this factor influences outcome
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Solder Joint Test Study Conditions
Solder Joint Life Testing (SJRT) performed at internal Freescale lab.
DAISY CHAIN CONFIGURATION – ALL UNITS/BOARDS
AIR-to-AIR TEMPERATURE CYCLE Temperature range = -40C / 125C approx 1 hour cycles – 15minute dwells & 15minute transitions
INSITU MONITORING real-time event detection/recording.
ALL BOARDS TEMP CYCLED SIMULTANEOUSLY All SJRT printed circuit boards are cycling in the same chamber
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Note: e-type control cells show similar SJRT performance (all control cells = 0.6pads & no bake).
3 of 4 e-type control cells have 0 failures at 4333 cycles.
1 control cell (e-5-A-3-n) has 9 failures – 1st fail at 2796 cycles.
5x5mm 32ld:
Step Cut Solder Joint Reliability Testing – Results to date
7x7mm 48ld:
7x7mm Step cut DOE - SJRT 4333 cycles
CELL paste pad ext aging Cell 1st Fail # Failed % Failed
Step Cut 71 A 0.6 No 71 3593 4 13%
Step Cut 72 A 0.6 Yes 72 2304 3 9%
Step Cut 73 A 0.3 No 73 3233 8 25%
Step Cut 74 A 0.3 Yes 74 1589 13 41%
Step Cut 75 B 0.6 No 75 0 0%
Step Cut 76 B 0.6 Yes 76 3438 1 3%
Step Cut 77 B 0.3 No 77 3668 4 13%
Step Cut 78 B 0.3 Yes 78 4152 2 6%
5x5mm Step cut DOE - SJRT 4333 cycles
CELL paste pad ext aging Cell 1st Fail # Failed % Failed
Step Cut 51 A 0.6 No 51 2638 10 31%
Step Cut 52 A 0.6 Yes 52 2798 7 22%
Step Cut 53 A 0.3 No 53 1540 7 24%
Step Cut 54 A 0.3 Yes 54 1474 12 38%
Step Cut 55 B 0.6 No 55 3553 2 6%
Step Cut 56 B 0.6 Yes 56 3157 1 3%
Step Cut 57 B 0.3 No 57 3459 1 3%
Step Cut 58 B 0.3 Yes 58 0 0%
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Solder Joint Reliability Test results – as of 4333 cycles
5-A-6-n
4145
2638
3222
3419
3435
3524
3697
3728
3960
4002
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
32 . . .
5-A-6-b
4087
2798
2830
3528
3557
3758
3816
5-A-3-n
1540
2521
3118
3892
3946
4037
4051
5-A-3-b
4200
4255
1474
2633
3170
3231
3235
3629
3855
3873
3993
4058
5-B-6-n
3553
3939
5-B-6-b
3157
5-B-3-n
3459 *
5-B-3-b
*The single solder joint from cell 57, labeled as “missing” based on human-eye
inspection, survived at least 3459 cycles during board life testing (-40/125C).
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Board Life Charts – 5x5mm study cells (w/enough fails to chart)
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Board Life Charts – 5x5mm study cells (w/enough fails to chart)
Outlier early life failure – probable manufacturing “artifact” due to non-optimized PCB assembly process
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Solder Joint Reliability Test results – as of 4333 cycles
7-A-6-n
4217
3593
3746
4208
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
32 . . .
7-A-6-b
2304
3027
3218
7-A-3-n
4136
4228
4251
3233
3420
3577
3752
4001
7-A-3-b
1589
2283
3074
3225
3233
3416
3476
3658
3722
3735
3832
3849
4046
7-B-6-n 7-B-6-b
3438
7-B-3-n
4223
7-B-3-b
4152
4266
3668
3812
3898
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Board Life Charts – 7x7mm study cells (w/enough fails to chart)
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Board Life Charts – 7x7mm study cells (w/enough fails to chart)
Outlier early life failure – probable manufacturing “artifact” due to non-optimized PCB assembly process
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All PCB Assembly cells
Solder: A vs. B
64 11
Pad: 0.6 vs. 0.3
28 47
Aging: y vs. n
39 36
Body size: 5x5 vs. 7x7
40 35
Solder Joint Reliability vs. DOE Factors (based on failures after 4.3K cycles)
Solder paste system shows
the largest impact on solder
joint life
------------
Pad size may be a factor,
but likely due to board
assembly variation
rather than actual weakness
related to the presence or
“quality” of the fillet.
Step Cut PCB Assembly Evaluation
Note: e-type control cells show similar SJRT performance and response to DOE factors.
3 of 4 e-type control cells have 0 failures at 4333 cycles.
1 control cell (e-5-A-3-n) has 9 failures – 1st fail at 2796 cycles.
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Agenda
• Wettable Lead Ends / Definition & Alternate Names
• QFN Packages / Intro to Features & Lead End Options
• Freescale Evaluations / Inspectable Solder Joint Study
• Solder Joint Life / How Good are They?
• CONCLUSION
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Conclusion Freescale IJ Offering / Preferences & Availability
Practicality / In the hands of the customers
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Freescale Inspectable Joint (IJ) Features
The step cut (mechanical removal by saw) is Freescale’s primary
solution for QFN-IJ (QFN packages with inspectable joint lead-end features)
DEMONSTRATED MANUFACTURABILITY Repeatable results for our customers as well as for Freescale
ROBUST SOLDER JOINT RELIABILITY SJR >2000+ cycles (-40/125C) – assumes a well designed/optimized PCB assembly process
SUPERIOR AVAILABILITY/COMPATABILITY Adaptable to the widest array of QFN outlines and manufacturing lines
The dimple (etched) IJ feature is offered by Freescale as an
alternative QFN-IJ solution for use cases which cannot use the step
cut due to specific application requirements.
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Consistent formation of inspectable solder joints requires a well
designed and optimized board assembly process.
WF / IJ FEATURE TYPE: Doesn’t really matter Step-cut or half-etched or e-type – all offer excellent solder joint life, Step-cut or half-etched or e-type – all have good potential for formation of inspectable joints Wettability of the lead end does not guarantee wetting to the lead end or an inspectable joint
DESIGN: Matters One of the more significant factors in producing good, consistent inspectable solder joints PCB pad design, feature clearances, finish quality, PCB construction, etc.
PROCESS : Matters Many factors of the board assembly process influence successful formation of inspectable solder joints Material choices consistently prove to be a significant factor in successful formation of inspectable solder joints as well as solder joint life. Choose a good quality, reliable solder paste material system & supplier
AOI: Customer’s choice Capability & repeatability are highly dependent on set up, programming, lighting, etc. Likely to reject reliable solder joints if PCB design and assembly process aren’t optimized
Practical Considerations – What Matters
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THANK YOU THANK YOU
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© 2014 Freescale Semiconductor, Inc. | External Use
www.Freescale.com