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

[email protected]

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?


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?


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


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


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


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


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


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


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!!!!

Improving Final Product Quality

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