How defect coverage as a variable
can be used to determine
test and inspection strategies
Stig Oresjo
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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I would like to acknowledge the following individuals:- Kenneth P. Parker- Kathy Hird- Bill FollisFor the original work on the PCOLA – SOQ
- Chris Jacobsen - James MahonFor work on the combined ICT, AOI, AXI coverage tool
Everybody working for Agilent
Acknowledgement
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Digital Functional Test:- Stuck at simulations- Often > 90% coverage claimed- Did that mean that 90% of defects detected? – NO
History of Coverage
In-circuit Test:- >95% coverage claimed in early 90’s- Problem 95% of defects detectable by ICT- Undetectable defects not considered.
A more realistic and complete coverage number is needed
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Defect DefinitionA DEFECT is:“At the end of the manufacturing process, an unacceptable deviation from a norm"
Keyword = unacceptable
“Unacceptable” means the user will take corrective action if aware of the deviation.
New distinction “at the end of the manufacturing line”important to differentiate from potential defect.
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Fault DefinitionA FAULT is:“A manifestation of a defect”
Faults are a subset of defectsAB
C
A B C0 0 00 1 01 0 01 1 0
FaultDefect
Example of defects that may or may not turn out as faults:- Missing bypass capacitor- Insufficient solder joint- Open connector pin that is not tested or used
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Example of defects and faultsResistor pack upside down and under IC
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Example of defect and maybe faultExtra Part (capacitor) on leads of IC
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Example of defect, probably not a faultMissing bypass capacitor
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Example of defects not a faultResistor pack insufficient solder
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A PROCESS INDICATOR is:“At the end of the manufacturing process, an acceptabledeviation from a norm"
A DEFECT is:“At the end of the manufacturing process, an unacceptable deviation from a norm"
Test/inspection job - to make 4 distinctions
A FAULT is:“A manifestation of a defect”
A POTENTIAL DEFECT is:“In the manufacturing process, a deviation from a norm, that may or may not be a defect at the end of the manufacturing process"
GOOD COMPONENTS, RIGHT PLACE and GOOD JOINTS
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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• Possible defects not included in set
Defect universe• Realistic set of possible defects• Structural information of the board (BOM) and placement• No assumption on how test and inspection is done
Defect universe
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Combined coverageDefect universe
ICT
AOI AXI
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PCOLA – Device propertiesIf a test/inspection passes what does that mean?
Device properties P - PresenceC - Correct componentO - Orientation (90,180, 270 degrees off)L - Live (basic function)A - Alignment
Perfect score is 100,000 (not achievable)
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SOQ – Pin PropertiesIf a test/inspection passes what does that mean?
Joint propertiesS - ShortO - OpenQ - Quality
Perfect score is 100,000 (not achievable)
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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PCOLA - SOQ - ScoringFor PCOLA - SOQA value from 0.0 – 1.0What does it mean if a test passes
1.0 = Full coverage0.5 = Partial coverage0.0 = No coverage
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PCOLA – Scoring Example
AOI inspection of a chip resistor No component markings – 10 kohm, ¼ WP = 1.0C = 0.5 (Don’t know the right value)O = Not applicableL = 0.0A = 1.0
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PCOLA – Scoring Example
ICT test of a chip resistor No component markings – 10 kohm, ¼ WP = 1.0C = 0.5 (Don’t know the right size - wattage)O = Not applicableL = 1.0A = 0.0
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PCOLA – Scoring Example
Combined AOI and ICT test of a chip resistor No component markings – 10 kohm, ¼ W
0.01.0-
0.51.0ICT
1.01.0A1.00.0L--O
0.50.5C1.01.0P
TotalAOI
Total = max of ICT and AOI score for each property
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Device property weightNot all properties are of equal importance Example: Presence more important than alignment
0.150.250.00.250.35Orientation irrelevant0.10.20.20.20.3Orientation relevantAwLwOwCwPw
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Device property weightNot all properties are of equal importance Example: Presence more important than alignment
0.150.250.00.250.35Orientation irrelevant0.10.20.20.20.3Orientation relevantAwLwOwCwPw
0.20.40.4QwOwSw
PCOLA Property weights
SOQ Property weights
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Total Coverage ScoreTwo numbers – Device & Connection score
Dt = (Ps*Pw+Cs*Cw+Os*Ow+Ls*Lw+As*Aw)*RNumber devices
Ct = (Ss*Sw+Os*Ow+Qs*Qw)*RNumber connections
R = Range = 100,000 = Maximum score
Device score = Sum of all device scores DtConnection score = Sum of all connection scores Ct
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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Example boardVery simple, only five components
2100 pFBypass CC221 uFBypass CC12ResistorR14GullwingICU28BGAICU1
PinsNoteTypeRef. Des.
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ICT CoverageSimple example
0.00.01.00.00.00.00.00.02100 pFBypassC20.01.01.00.01.00.00.51.021 uFBypassC10.01.01.00.01.0-0.51.02ResistorR10.00.01.00.00.00.00.00.04GullwingICU20.01.01.00.01.01.01.01.08BGAICU1QOSALOCPPinsNoteTypeRef. des
All PinsDevice
• No test for U2• C2 bypass capacitor and much smaller than C1• ICT can not test alignment and solder quality• Note: All pins treated the same in this example, not the case in reality
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AOI CoverageSimple example
• No test for U1 pins, hidden joints• U2 no reliable component marking• AOI can not test for Live
1.01.01.01.00.01.00.51.02100 pFBypassC21.01.01.01.00.01.00.51.021 uFBypassC11.01.01.01.00.0-0.51.02ResistorR10.51.01.01.00.01.00.01.04GullwingICU20.00.00.01.00.01.01.01.08BGAICU1QOSALOCPPinsNoteTypeRef. des
All PinsDevice
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AXI CoverageSimple example
• AXI good at joint coverage• AXI No test for Correct and Live• Orientation only for polarized capacitors
1.01.01.00.50.00.00.01.02100 pFBypassC21.01.01.00.50.01.00.01.021 uFBypassC11.01.01.00.50.0-0.01.02ResistorR11.01.01.00.50.00.00.01.04GullwingICU21.00.51.00.50.00.00.01.08BGAICU1QOSALOCPPinsNoteTypeRef. des
All PinsDevice
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Combined AOI, AXI, & ICT CoverageSimple example
• In this example perfect score for SOQ, not typical• U2 no ICT test and no component marking• Live for bypass capacitors will be 0 for combined AOI, AXI, ICT strategy
1.01.01.01.00.01.00.51.02100 pFBypassC21.01.01.01.01.01.00.51.021 uFBypassC11.01.01.01.01.0-0.51.02ResistorR11.01.01.01.00.01.00.01.04GullwingICU21.01.01.01.01.01.01.01.08BGAICU1QOSALOCPPinsNoteTypeRef. des
All PinsDevice
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Score for simple example
• Used scores and Property weights as shown previously• AOI, AXI, and ICT complements each other• Score of maximum 100,000 only in this simple example
100,00066,66791,11153,333Pin
81,50044,50040,50068,500Device
ALLICTAXIAOI
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Some issues to be aware of
- Manually possible – not recommended- Should be done automatically
- Small inconsistencies like U1 versus U1_a, • Reference designator inconsistencies
• Harvesting the program information
program needs to identify and provide easyway to modify one of them
- Normally done after ICT, not done after AOI or AXI. • Re-test after repair
If not done, scoring assumes correct repair action. - If higher escape rates than expected, repair
operation needs to be investigated.
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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Medium complexity – high volume• 1,000 devices, 8,000 solder joints
• Test strategy – AOI, ICT, limited AXI
50,89138,19925,4067,993Pin
65,89836,42928,79846,689DeviceALLICTAXIAOI
• Many BGAs and connectors (many hidden joints)• Mainly BGAs tested on AXI due to high volume
Device and Pin score for this example
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Medium complexity – high volume
• Surprise for user
50,89138,19925,4067,993Pin
65,89836,42928,79846,689DeviceALLICTAXIAOI
- Add ICT coverage• Actions:
- Add AXI coverage
- Low ICT coverage- Low Combined coverage
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High complexity – low volume• 2,500 devices, 11,000 solder joints
• No “Live” score due to only AOI & AXI
81,768081,76225,014Pin52,913027,89752,673Device
ALLICTAXIAOI
• Very low volume, no ICT test• Good AOI and AXI coverage
Device and Pin score for this example
• User happy with result, right trade-offs
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Topics
• Introduction and definitions
• Defect universe, coverage, & PCOLA - SOQ
• Scoring and Simple example
• Real board examples
• Conclusions
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Conclusions
• Currently not easy to get coverage numbers for combined test strategy. • Method presented to use PCOLA – SOQ method for ICT, AOI, AXI, and Combined coverage analysis
• ICT and AOI typically good device coverage, medium to low pin coverage.• AXI typically low to medium device coverage, high pin coverage.
• Real board analysis has shown that method works for combined coverage analysis, and trade-off decisions.
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