Download - Improving Final Product Quality
"Detecting Defects as Early as Possible to Reduce the Cost
of Rework and Repair“
Joe BelmonteProject Manager
Speedline TechnologiesAdvanced Process Development
Franklin, MassachusettsUSA
Agenda• Cost of a defect• Where are the defects?• Impact of solder paste printing defects on first pass yield• Preventing defect versus Reacting to defects• Solder paste inspection• 2D and 3D solder paste inspection• 2D and 3D inspection systems• Closed loop process control• Conclusions• Questions
Cost of a DefectSources of defect cost
• Repair• Rework• Defects and damage created by rework & repair (lead free)
• ESD• Handling• Excessive heat
• Scrap• Returns• Warranty• Delivery delays• Customer dissatisfaction• Lost business
To minimize these costs it is imperative to discoverdefects as early in the process as possible
Cost of a DefectDefects are becoming more difficult to find andcomponents are becoming more difficult to rework/repair
• Smaller• 0201• 01005• .4mm Chip Scale Packages (CSP)• Micro Ball Grid Arrays (BGA)
• More Complex• Column Grid Arrays• High Pin Count Quad Flat Packs (QFP)
• More Expensive• Complex Micro Processors• Custom Semi Conductors (ASIC)
The lead free process will add to the creation of defectsbecause the lead free materials do not solder as well
Cost of a DefectStudy Number 1 (USA Based Modem Manufacturer)
Defect Discover Process Step *Cost of DefectPrior to reflow soldering $0.44After reflow and prior to ICT $1.65In circuit test (ICT) $2.33Final product assembly $49.73In the field (after shipment) $550.00(1,250 times more costly than prior to reflow soldering)
*All cost in USA Dollars*Labor cost only. Does include component/materialreplacement cost
Cost of a DefectStudy Number 2 (Major AOI Equipment Supplier)
Defect Discover Process Step *Cost of DefectPrior to reflow soldering $0.50After reflow and prior to ICT $5.00In circuit test (ICT) $35.00In the field (after shipment) $350.00(700 times more costly than after reflow soldering)
*All cost in USA Dollars
Cost of a DefectStudy Number 3 (Second Major AOI
Equipment Supplier)
SMT AOI ICT FT Assy SystemTest
WIP
WIP
WIP
WIPW
IP
WIP
WIP
WIP
Repair Repair Repair Repair
$1 $6 $36 $216
Is Solder Paste Inspection (SPI) Worth the Cost Investment?
Consider the following example:• You place 4000 BGA’s per day either on 1000 boards (4
BGAs per board) or on 4000 boards (1 BGA per board)• Your manufacturing process runs 365 days per year• Your defect rate is 100ppm• Your BGAs costs $100 each• Your BGAs have 250 pins each• Your BGA rework costs $100 per BGA
You now have 36,500 BGA pin defects per year or 100BGA pin defects per day. Let’s assume the defectsoccur on ten boards a day. Your resulting rework andparts scrap costs would be $2,000/day or $730,000/year!
Is SPI Worth the Cost Investment?
$00%
$73K10%
$146K20%
$219K30%
$292K40%
$365K50%
$438K60%$511K70%$584K80%
$657K90%
$730K100%
Savings / Year%BGA defects prevented with
SPI (from example)
• If solder paste inspection prevented even a portion of these defects, significant cost savings can be realized.
• Washing a pasted board or cleaning a stencil makes more sense than reworking or scrapping a loaded board with expensive BGAs.
• Solder paste inspection may pay for itself.
Where are the Defects?• Opportunity for a Defect
• One for every solder joint• One for ever component• Total defect opportunities equals number of component leads
plus 1Example: 256 pin Quad Flat Pack (QFP)
256 solder joint opportunities1 component opportunity
257 total opportunities for a defect• Vast majority of defect opportunities (generally in excess of 75%)
are controlled by the solder paste printing process and soldering processes (reflow and wave soldering)
• One high volume manufacturer’s six month study indicates the average percentage of defect opportunities from the screen printing process, reflow process, and wave soldering process was84.08%
Where are the Defects?
Missing part
Offset part
Reversed partComponent
No solderInsufficient solder
Excessive solder
Bridging
Missing part
Offset part
Wrong part
Reversed part
Faulty partOut of spec
Handling damage
Pasteinspection
ComponentInspection (Optical, X-Ray, Some Electrical)
Functional test
-70 - 80%
20 - 30%
Impact of Solder Paste Printing Process Quality on First Pass Yield
Effects of Solder Joint DPMO (Defects per Million Opportunities) on Assembly Level Yields on a Printed Circuit Assembly with 3000 Solder JointsSolder Joint DPMO Printed Circuit Assembly Yield
5 98.5%10 97.0%25 92.5%50 85.0%100 70.0%
Types of DefectsSpecial & Common Causes
SPECIAL CAUSE!!
COMMON CAUSES!!
SPECIAL CAUSE!!
UCL
LCL
Identification of Special and Common Causes
Corrective Action :• SPC rule violations
"Special Causes - changes, anomalies, unusual events""Common Cause - Shift in mean, trend in mean, increased
variability "
• Implement contaminate plan, and monitor for repeatoccurrences.
-Stop defects from escaping to next process-Keep process in operation while permanent corrective action is implemented
• For repeat occurrences tie specific cause to corrective action using problem solving methods.
Never change operating parameters for Special Cause Variations
Defect Elimination Strategies
Proactive StrategyPreventing defects from occurring
Reactive StrategyFinding defects that have occurred(Solving the same problems every day)
In reducing defects and achieving “World Class Quality” there is
no substitute for good engineers doing good engineering work, training,
coaching, and process discipline!!!
How to Minimize DefectsPrevent Defects from Occurring (Proactive)
• Process design• Develop stable repeatable processes using statistical studies
such as formal design of experiments (DOE)• Identify and quantify all critical operating parameters (pressure,
speed, temperature, etc.)
• Implement Statistical Process Control (SPC) Monitor each processes critical output to insure your process is in “in control”• React to all “out of control” situations by stopping the process
and implementing corrective action (containment plan)• Use SPC data to drive permanent corrective action
How to Minimize DefectsPrevent Defects from Occurring (Proactive)
• Characterize all processes (reduce process variation)
• Understand the capability of each process• Conduct experimentation and studies to increase the
process capability (Cp and Cpk)
Find Defects that have Occurred (Reactive)• Implement effective inspection and test
processes• Design an effective test and inspection process• Select and implement the most effective equipment to
achieve test and inspection process goals
Cost of Defect Summary• Develop processes to minimize defects from ever
occurring (Proactive)• Monitor the process
• Find defects as early in the process as possible (Reactive)• Monitor the product
• Focus on solder paste printing• This process has the highest opportunities for defects• Even a small reduction in defects in this process will reduce all
the cost associated with correcting defects and improve the quality of the entire process and first pass yield of all products
Process Characterization MethodologyInvolves :• Management Commitment to Statistical Data Analysis and Problem Solving.
• Implement SPC to monitor key process parameters.
• Daily Quality Meetings.
• Root Cause Corrective Actions.
• Measurement Systems Analysis.
• Design of Experiments
• Continuous Process Improvements.
• SPC Audits.
Process Characterization Methodology
Results :
• Reducing Defect Rate (dpu), Cycle Time, Repair costs andincreasing Productivity.
• Increasing the time the process is in an ideal state.
• Decreasing the process variability.
• Increasing the time that the process characteristics mean remains at its target value or remains constant at acceptable level.
• Reducing the number of out of control conditions on SPC charts.
C
A
B
-6s -5s -4s -3s -2s -1s 1s 2s 3s 4s 5s 6s
LSL USLNominal
ConditionCapability 1 2
Cp = = 2.0 2.0AB
Cpk= = 2.0 1.5C0.5 B
Condition 1: DistributionAverage Centered onNormal Specification
Condition 2:Distribution AverageShifted 1.5s from theNominal Specification
LCLLCL UCLUCL
LSLLSLUSLUSL
Solder Paste Inspection
Why use Solder Paste Inspection Systems
• Many solder joint defects are caused by solder paste printing• Solder paste volume and registration for miniature
components such as 0201 and CSP is critical
• Components are becoming more difficult and expensive to rework
• The cost of in the printing process is the least expensive defect to correct
• This inspection process can be used real time as a process control and process monitoring tool
Determining Your Solder Paste Inspection Needs
What is the most challenging “Packaging Technology” you are dealing with? 0603s,0402s,0201s?, 15mil Pitch QFPs?, Etc. What is the smallest deposit size you are dealing with?
What kind of quality controls do you have in place today for your printing operation? Do you have any manual or automated solder paste inspection equipment currently? 2D or 3D ?
Do you feel your printing equipment, and processes are under control? Would you agree that the correct volume and registration of solder paste is paramount in creating the strongest, and most reliable solder joint geometry?
Determining Your Solder Paste Inspection Needs
Are your customers interested in having you inspect allcritical deposits of solder paste on their boards? Does 2D or3D matter to them?
Has your Pb-free production or experimentation experiencing an increase in solder paste related defects?
Do you think SPI would help you win more business? Produce higher quality modules?What are you solder paste printing process cycle time requirements?
Determining Your Solder Paste Inspection Needs
Capability Why It Is Needed
Speed / Throughput Keep up with line beat rates
100% Inspection Find systemic and random defects on all deposits on all boards
3D Volume can only be measured with 3D
Resolution / Accuracy Trends for smaller components, BGAs, and CSPs leads to smaller deposit sizes
Measurement Repeatability Eliminates false failures and allows process characterization
Ease of Programming Need programs ready at start up and for prototyping
System Uptime Can not have a process control tool go down
SPC Need to be able to view and alarm on trends in the process
2 Dimensional (2D) and
3 Dimensional (3D) Solder paste Inspection
Defect Identification
Paste to pad offset/ Misaligned print
Insufficient coverage/ Excess coverage
Bridge
Slump/ Large height variation
Volume high or low/ Height high or low
Smear
Solder Paste Problems and Solutions
SPI Measurement Possible Cause Action
Paste to Pad Offset Mis-aligned stencilBad stencil or boards
Adjust screen printerMeasure stencil and boards
Bridge Excess pasteDamaged apertures
Collect 3D dataInspect stencil
Smear Poor handlingPaste on back of stencil Snap-off height too high Clean stencil
Insufficient Coverage Dried paste on stencil apertures, Paste volume on printer too low Squeegee speed too fast
Clean stencilAdd fresh paste Adjust printer
Excess Coverage Poor aperture gasketing due to excessive squeegee pressure, debris on board, or damaged aperture
Adjust printerClean stencil & boardInspect stencil
Volume HighHeight High
Contamination at board/stencil interfaceWarped stencil
Clean stencil & boardInspect stencil
Slump Squeegee speed too fastPaste temp too highPaste has absorbed moisture
Adjust printer
Large Height Variation
Warped stencilSeparation control speed too fastSqueegee speed too fast
Inspect stencilAdjust printer
Volume LowHeight Low
Polymer blades scoop out pasteSqueegee speed too fast
Adjust printer
Paste to Pad Offset / Misaligned Print
SPI Measurement Possible Cause Action
Paste to Pad Offset Mis-aligned stencilBad stencil or boards
Adjust screen printerMeasure stencil and boards
• 2D • Offset defect• Potential area defect
• 3D• Potential volume defect• Potential height defect
Solder Bridge
SPI Measurement Possible Cause Action
Bridge Excess pasteDamaged apertures
Collect 3D dataInspect stencil
• 2D • Offset good• Potential area defect if bridge area is large
• 3D• Volume defect• Potential height defect
Solder Smear
SPI Measurement Possible Cause Action
Smear Poor handlingPaste on back of stencil Snap-off height too high Clean stencil
• 2D • Offset good• Potential area defect if smear area is large
• 3D• Potential volume defect• Height defect
Insufficient Paste Coverage/ Excess Paste Coverage
SPI Measurement Possible Cause Action
Insufficient Coverage
Dried paste on stencil apertures, Paste volume on printer too low Squeegee speed too fast
Clean stencilAdd fresh paste Adjust printer
Excess Coverage Poor aperture gasketing due to excessive squeegee pressure, debris on board, or damaged aperture
Adjust printerClean stencil & boardInspect stencil
• 2D • Offset good• Area defect
• 3D• Volume defect• Potential height defect
Volume High or Low / Height High or Low
SPI Measurement Possible Cause Action
Volume HighHeight High
Contamination at board/stencil interfaceWarped stencil
Clean stencil & boardInspect stencil
Volume LowHeight Low
Polymer blades scoop out pasteSqueegee speed too fast
Adjust printer
• 2D • Offset good• Area good
• 3D• Volume defect• Height defect
Slump / Large Height Variation
SPI Measurement Possible Cause Action
Slump Squeegee speed too fastPaste temp too highPaste has absorbed moisture
Adjust printer
Large Height Variation
Warped stencilSeparation control speed too fastSqueegee speed too fast
Inspect stencilAdjust printer
• 2D • Offset good• Potential area defect
• 3D• Volume defect• Height defect
Conclusions
• Both 2D and 3D provide valuable process information
• Some overlap exists between 2D defect calls and 3D defect calls
• 3D inspection provides volume measurements
2D and 3D Inspection Systems
2D Inspection (Existing Technology)• Can evaluate features in two dimensions, length and width (X and
Y axis)• Primarily designed to evaluate the solder paste coverage on the
printed circuit board pad• Uses vision “Grey Scale” to distinguish between printed circuit
board pad and printed solder paste • Before printing system “learns” pads on printed circuit board• User defines acceptable solder paste coverage
• Must consider aperture size versus printed circuit board pad size
• Can be incorporated into the solder paste printing equipment or as a separate in line or off line machine
• Must consider cycle time requirement in selecting equipment
Enhanced 2D with BridgeVision™(Newly Introduced Technology)
A printer-based 2D inspection system augmented with texture-based analysis of bridging and paste transfer.
Texture-based system provides:• Most accurate analysis taught from stencil apertures reduces false positives• Programmable limits enable threshold values for application matching.• Minimal impact to cycle time as compared to competing solutions
Ease of use:• Teach function is modeled after the contrast based 2D system• Programmable limits are set on a % basis• What is inspected is selectable for each taught device
Yield improvement:• Identifies paste transfer and bridging defects early in the process line• Prevents unnecessary downstream processing and final yield defects
Current Inspection ArchitectureImage
Acquisition
Outputs and SPC
Paste Recognition (dual threshold) limited to pad area only
2-D2-D coverage of
pad only
Image Acquisition
Bridge Detection Outputs and SPC
Paste Recognition (texture based) over gap only
Bridge Detection Analysis (gap cover and span in gap)
Enhanced 2D with BridgeVisionTM
•Features–Option coupled with enhanced 2D inspection–Shift from contrast to texture based inspection (patent-pending)–Able to look at area and span features
•Capabilities–Minimal impact to 2D cycle time = fastest bridge detecting 2D in the market–0.016” (.4mm) pitch capability–Functions include Auto shutter speed adjustment to optimize image gathering –Programmable limits to alarm for both span (spike penetration) and area (% of gap covered)
Lighting Effects (continued)
User Defined Inputs
1) Maximum Amount of Paste to be Allowed in the Gap
2) Minimum Width to be Considered a Significant Bridge Feature
3) Maximum Span of a Significant Bridge Feature Across the Gap
Typical Parameters1) Maximum Paste Allowed in Gap = 55% of the total gap area
2) Minimum Bridge Width = 10 pixels wide (6.6 mil, 168 micron)
3) Maximum Span of Bridge = 70% of the width of the gap
0
10
20
30
40
50
60
70
80
90
1000 1 2 3 4 5 6 7 8 9 10
pg
Gap Projection and Sliding Average
Gap Projection
Sliding Average
(10 pixels)
Gap
Len
gth
in
Pixe
ls
Gap Span in Pixels
User Defined Span Limit (70%)
Weighted Projection
Gap ROI
Paste-Only TileRun-Time Tile
Area underthe curve
Outputs1) Surface area of gap covered by paste: Gap Cover (%)
2) Length of gap covered by bridge feature after sliding averaging: Span (%)
3) Flag: “ Excess Gap Cover” if paste coverage of the gap exceeds Maximum Paste Allowed in Gap
4) Flag: “ Bridge Feature Detected” if span exceeds Maximum Span of Bridge Allowed
5) SPC data: Span and Gap coverage (maximum, minimum, average) per device
Bridge Detection Impact on Cycle Time per Device
2D Bridge Detection Cycle Time (sec)25.93
X 31.63X X 34.5
Bridge Prevention Study
Agenda
• Purpose of study, Basic Idea• Board Inspection• Test Matrix, Observations and Results• Stencil Inspection• Test and Observations• Conclusions
Purpose
• Stencil Printing- Main source of end-of-line defects
• Investigation of a potential inspection technique to predict and prevent bridging
Idea
A 2A
Board Number
Threshold
Bridging Trend
Gap
Co v
er o
r Sp a
n(%
)
Printed Circuit Board Inspection
Test Matrix
FOLLOWING TYPES OF BOARDS AND SOLDER PASTES WERE USED:
BOARD TYPES
• MPM GOLD BOARDS (6 MIL GAP WIDTH)
• ALPHA BOARDS (8 MIL GAP WIDTH)
• 3 UP BOARDS (8 MIL GAP WIDTH)
SOLDER PASTE TYPES
• OMNIX 5000
• LR735
• OMNIX 6023
Conclusions• No trend permitting prediction of bridge defects could be
found regardless of paste and board combination.• There is a significant gap-to-gap variation• Board inspection is not sufficient for predicting bridging
• Based on these conclusions we decided to investigate the paste inspection on stencil to predict and prevent bridge defects
Stencil Inspection
Tests and Observations
Exp.3: Closing the loop on stencil wiping• Trigger wipe only when necessary: (maximum gap coverage>
60%)• Bridging appears to be under control with less stencil wipe
cycles than in Exp:2 ( 4 vs. 7 over 100 prints)
Gap Cover(Stencil) vs Bridge Span(Board)- No Stencil Wipe
0
20
40
60
80
100
120
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97
Board Number
Gap
Cov
er, B
ridge
Spa
n (%
)
Maximum Gap Cover (Stencil)Maximum Bridge Span (Board)
Gap Cover(Stencil) vs Bridge Span(Board)Wipe performed with 60% threshold on Maximum Gap Cover (Stencil)
0
20
40
60
80
100
120
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
Board Number
Gap
Cov
er, B
ridge
Spa
n (%
)
Maximum Bridge Span (Board)
Maximum Gap Coverage (Stencil)
Threshold for Stencil Wipe
No Stencil Wipe Closed loop ( maximum gap coverage>60%)
Conclusions• It is difficult to predict bridging by inspecting the boards• The stencil contamination shows a uniform trend• Bridging can be predicted and controlled by inspecting
stencil using our texture-based analysis of bridging and paste transfer.
Potential Bridge Prevention Technique
STENCILBOARDGap Cover(Stencil) vs Bridge Span(Board)
Wipe performed with 60% threshold on Maximum Gap Cover (Stencil)
0
20
40
60
80
100
120
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97
Board Number
Gap
Cov
er, B
ridge
Spa
n (%
)Maximum Bridge Span (Board)
Maximum Gap Coverage (Stencil)
Threshold for Stencil Wipe
Gap Cover(Stencil) vs Bridge Span(Board)- No Stencil Wipe
0
20
40
60
80
100
120
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97
Board Number
Gap
Cov
er, B
ridge
Spa
n (%
)
Maximum Gap Cover (Stencil)Maximum Bridge Span (Board)
100
3D Inspection (Existing Technology)• Can evaluate features in three dimensions, length, width, and
height (X, Y, and Z axis)• Primarily designed to evaluate the solder paste coverage, height
and/or volume on the printed circuit board pad• Uses a laser to define height and/or volume of printed solder
paste • Before printing system “learns” pads on printed circuit board• User defines acceptable solder paste height and/or volume
• Must consider aperture size and stencil thickness• Can be incorporated into the solder paste printing equipment or as
a separate in line or off line machine• Must consider cycle time requirement in selecting equipment
Close Loop Printer Equipment Control
Closed Loop Control System(Developing Technology)
Closed loop printer control:• Optimizes volumes of solder deposits which ultimately results in more reliable
solder joints• Stabilizes the print process when perturbations occur
Stencil Printer Inspection System Placement Machine Reflow Oven
FeedbackControl
Close Loop Printer Control
5.54.53.5
20
10
0
Height
Freq
uenc
y
FSBS
5.54.53.5
20
10
0
Height
Freq
uenc
y
FSBS
Height distribution with control
BS = Back Squeegee StrokeFS = Front Squeegee Stroke
Height distribution without control
Cpk Analysis for OM5K
• Cpk analysis performed for each pad for BGA 36 Component
• Specification Limit for Cpk +/- 1.0 mil from Target
0.0
0.5
1.0
1.5
2.0
2.5
0 4 8 12 16 20 24 28 32 36Pad Number
Cpk
CL WC
Cpk improvement observed when close loop control is used
Close Loop Printer Control
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
0 10 20 30 40 50 60 70 80 90 100
Board Number
Ave
rage
Hei
ght (
mils
)
60
65
70
75
80
85
90
Tem
pera
ture
(F)
With Control No Control
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Cp Cpk Cp Cpk
Volume HeightC
p / C
pk In
dex
With Control No Control
Conclusions• Finding defects as early in the process as possible will provide
significant cost reductions and increased first pass yield• Calculate and understand the cost of a defect in your operation• Understand how your process is performing at all times• The primary effort must be in formal process design and
development and continuous process improvement to prevent defects from occurring.
• Focus on monitoring the process (SPC) not monitoring the product(Reactive versus Proactive Culture)
• React as quickly as possible to process “out of control” situations• Implement a containment plan to insure the defects do not escape
while you are developing permanent corrective action
Conclusions• A formal test and inspection process design should be evaluated
and implemented• Test and inspection equipment should only be selected after the
test and inspection process design is approved• There is excellent test and inspection equipment available from a
number of suppliers. Ask for accuracy and repeatability data.• Insure the equipment has the necessary precision and speed
• An effective continuous improvement program must be established to drive permanent corrective action from defect andSPC data.
• Follow the development of Closed-loop Process Control Systems that will be effective in monitoring and controlling a process
• Work with your equipment and materials suppliers to understand how to optimize the performance of their products
Thank You!!!!