Using visualinspection in yourPCB test strategyBy Patrick
TongManaging DirectorTeradyne Hong Kong Ltd.spection consistency
and accuracy,increasing yields.
High-magnifica-tionopticsandautomatedinspec-tion provide more
reliable, consis-tentresultsthanhumanoperatorscan deliver,
especially on fine-pitchdevices, such as 0.508mm or 0.406mm QFPs.
On dense boards its es-pecially hard for human inspectorsto do a
good job. For the accurate,immediate feedback needed to con-trol
and improve the soldering pro-cess, AOIsystemsaresuperiortopeople.
The systems also offer sta-tistical process control (SPC) toolsthat
can extract key trends and de-fect
data.Thesystemsmaincomponentsincludehigh-magnificationopticalcameras(verticaland/orangled)mountedabovethecircuitboard(figure2).
AnLEDlightingdomeilluminates the board under inspec-tion from above
and from the sides.Using different programmed combi-nations of
camera angles and light-ing, AOI systems inspect for solderjoints,
solder shorts, and componentpresence and location. You can useAOI
systems in-line with automationequipment, which is often
integratedwith a repair station.Where electrical bed-of-nails
ac-cess is being lost, AOI systems caninspect for common faults,
such asNew applications for
smaller,denser,andfasterPCBsplaceincreasingperfor-mancedemandsonmanufacturingtechnology.Fine-pitchcomponentpackagingalsoincreasesthediffi-cultyinmanufacturingadvancedproducts,
even as it reduces electri-calaccessfortraditionalin-circuittest
(ICT). At the same time, indus-tryeconomicsdemandever-lowercosts
and cycle times.Considerthetypicalassemblyprocess SMT boards go
through (fig-ure 1). To ensure that you detect
faultsattheearliestpossibleopportunity,you would inspect or use ICT
at sev-eral points (table
1).Inthepast,mostassemblypro-cessesreliedonhumaninspectionfor
post-reflow inspection. However,the combination of fine-pitch
boardtechnologyandhighyieldrequire-ments means that human
inspectionis no longer acceptable as a qualitysolution. The
monotonous,
demand-WithadvancedcomponentpackagesanddensePCBshinderingtest, you
can use AOI systems to complement ICT and achieve near100-percent
fault coverage. AOI optimizes human inspection andimproves test
reliability.ing inspection task is a poor
matchtothecapabilitiesofhumansbutagood environment for automated
in-spection
systems.Automatedopticalinspection(AOI)offersseveralbenefitscom-pared
to human inspection. It
reducesthenumberofhumaninspectorsneededandtheassociatedcostsofhiring
and training. It also reduces
thenumberoflocationsthatyoumustexaminefromthousandstoafewsitesperboard.Thisgivesyoutheopportunity
to cut the number of hu-man inspectors down to one per line.Another
advantage of AOI sys-tems is their ability to improve
in-18Electronics EngineerSeptember 1998Production Test IFigure 1
Figure 1 Figure 1 Figure 1 Figure 1: The SMT board assembly process
requires multiple inspection andtest steps to ensure that
manufacturing defects are detected as early aspossible.Prototype
testBoard functional testSample testPost-paste inspection Pre- and
Post-reflow inspection Electrical process testSMT
boardassemblyElectricalprocesstestBoardfunctionaltestIntegrating/PackagingScreenprintPick/PlaceReflowThroughhole
loadWaveProductionramp-up / prepProto-type testSample testmissing
devices, shorts, and
opens.Theycanalsoinspectforfaultclassesthatcantbetestedelectri-cally,suchaslowsolder,billboar-ded/skeweddevices,andmissingbypass
capacitors.AOI vs. X-ray inspectionWhile X-ray inspection offers
manyof the same advantages as AOI whencompared to human inspection,
AOIandX-rayinspectionaresuitedtodifferent applications (table 2).
AOIsystems are a good choice for in-lineuse during manufacturing;
they de-tect the most common types of
sol-derfaultsunseenbyoperatorsormissed by electrical testing.Used
after screen printing, pick-and-place, and reflow, AOI
systemscandetecterrorsinpartoutline,stencilalignment,pastevolume,and
component placement on eitherside of the board. The systems
caninspectelectricallyinaccessibleparts,anddonotrequiretestfix-tures.
They have a line-rate
through-puttheycanperformcompleteinspectionofaboardatthespeedoftheproductionline.Moreover,AOI
systems are easy to use: whenyou look at an optically
magnifiedProduction Test I18Electronics EngineerSeptember 1998area
of a PCB, the interpretationof the view is intuitive.X-ray
inspection systems arebetter suited to process develop-ment or
laboratory applications,inclu-dingdiagnosisforfunc-tional or field
failures. You wouldfindthesesystemsparticularlygood at detecting
corner cases,rare error types that include openson
BGAs.However,X-rayinspectionmay not be suitable for produc-tion
line use because the systemthroughputisslowerthantypi-cal
production linestypically,30 to 50 solder joints per second.If the
line rate is 20s or 30s forthe entire board, the X-ray
sys-temwillnotbeabletoinspect100 percent of the connections on
theboard with the required throughput.The inspection results are
also harderto interpretimagine an X-ray of
ahumanfacecomparedtoaphoto-graphofthesameface.Moreover,X-ray
systems cost about three timesas much as AOI systems.AOI
complements
ICTAOIandICTeachexpandoveralldefectcoveragebyfindingdefectsthat the
other technique cannot find.Less obvious but equally important,each
technique can also provide cov-erage on common faults
wheretheothertechniquedoesntworkforsomereason.Forexample,inthecase
of a wrong resistor, ICT can findthe error by testing for the
resistorsvalue, AOI cant detect that error be-cause the appearance
of both resis-tors is the same. On the other hand,AOI can detect a
missing device incases where electrical access is un-available for
ICT fixtures. By
com-biningbothtechniquesyoumayeliminatevirtuallyallprocessde-fects
before functional test, leadingto improved yields at final test
andfewer field returns.In addition to covering differentfault
spectrums, AOI and ICT
havedifferentstrengthsandweaknessesthatmakethemcomplementaryFigure2
Figure2 Figure2 Figure2 Figure2:ThePentaVisionfive-camerainspection
head uses structured light andmultiple camera angles.Table 1: Test
and inspection points in SMT manufacture Production ramp-up and
preparationAfter screen printingAfter pick-and-placeinstallation of
partsAfter reflow solderingof surface-mounted partsAfter loading of
through-hole partsAfter wave solderingFunctional test Design
problems Errors in the device program Manufacturability problems
Process setup problems afterchangeover Solder paste volume Solder
brick alignment Missing part Wrong or misaligned part Extra part
Solder quality Opens or shorts Cold solder joints Solder balls
Lifted leads/voids Missing parts Wrong or misoriented parts
Tombstoned parts Wrong part Missing part Reversed polarityShorts
Stress-induced componentfailuresOperational problems of powered-up
board Manual checkout X-ray In-circuit tester Flying prober Human
vision AOI Human vision AOI Human vision AOI X-ray In-circuit
tester Manufacturing process testerIn-circuit tester Manufacturing
process tester Hot mock-up Custom instruments Custom ATE
Manufacturing test platformProduction step Objective or Fault type
Test/Inspection approachesrather than competing. AOI has
theadvantage of not requiring a test fix-ture.UnlikeICT,
AOIcanbeusedonpartiallybuiltboardsandgivesyou direct feedback on
your solderprocess. One limitation of AOI is
thatrepeatabilityisdependentontheconsistency of the process. It
also re-quires visual access.In contrast, ICT applies power tothe
board and has the ability to testfor faults that arent physically
vis-ible. Repeatability in in-circuit test-ing is independent of
the
consistencyoftheassemblyprocess.Ontheotherhand,ICTrequireselectricalprobe
access and a unique fixture foreach board type.Choosing a test
strategyYourchoiceofateststrategyde-pends on the product mix (board
sizeandcomponentcount,highorlowvolume,highorlowchangeoverrate) and
the types of faults likely tobe encountered. To detect the
mostfaults in the shortest period of
time,youmaytargetanAOIsystematparticular areas where your
experi-ence shows that process defects areTable 2: Comparing X-ray
and AOI inspectionParameter AOI X-rayTarget applicationBest for
detectingThroughputCostProduction-line inspectionMost common
production faults:shorts, tombstoned, and missingcomponents100%
inspection at line rateOne-third the cost of X-rayProcess
developmentFailure analysisRare error types such as opensunder ball
grid arraysLower throughputThree times the cost of
AOImostlikelytooccur.Forexampleyoumaywanttoinspectallfine-pitch
devices below 0.508mm.If you also plan to perform func-tional test
to detect operational failures,use the same VXI manufacturing
testplatform for both in-circuit and func-tional testing, for
maximum flexibil-ity in the allocation of
resources.Afinalnoteaboutteststrategyconcernsthechoiceofsupplier.Its
important to be sure that the sup-plier of AOI equipment has the
re-sources and the infrastructure to pro-vide the continuing
development andworldwide support that a global
ma-nufacturerrequires.Chooseasup-plierwhocanbealong-termpart-ner as
your test strategy and productmix
evolve.YoumaysendyourcommentsonthisarticletoPatrick
Tongthroughe-mailat [email protected], or
fax:852-27351150.Production Test I18Electronics EngineerSeptember
1998ee
