Challenges of Megapixel Camera Module Assembly and Test

Challenges of Megapixel Camera Module Assembly and TestAsif Chowdhury 1 , Robert Darveaux 1 , Jay Tome 1 , Ron Schoonejongen 1 , Mitch Reifel 1 , Archie De Guzman 1 , Sung Soon Park 2 ,Yong Woo Kim 2 , Hyung Wook Kim 2 ,1 Amkor Technology, Inc.1900 S. Price Rd.Chandler, AZ 85236achow@amkor.com480-821-2408 x59492 Amkor Technology KoreaKoreaAbstractDriven by the market growth of mobile phones withcamera, production of image sensor devices has increaseddramatically in recent years. From 2004 an increasingnumber of these mobile phones contain megapixel cameras.The packaging of an image sensor in a camera modulepresents several unique engineering challenges. Megapixelcamera module assembly poses further challenges due tomore stringent particle control criteria. For particle control,special attention has to be given on facilities control,equipment and material selection, process flow, disciplineamong line personnel and continuous particle reductioneffort. Other material, process and equipment selectioncriteria for megapixel module are also discussed. Test formegapixel camera module also has its own challenges. Thispaper discusses these challenges and methods to meet them.IntroductionMegapixel image sensors are defined as image sensorswith more than a million individual sensors or pixels. Table1 shows different image sensor display formats. SXGA andUXGA are examples of megapixel sensors. Some examplesof megapixel camera modules are shown in Figure 1.Figure 1. Examples of megapixel camera moduleImage sensor devices are used in various applicationssuch as surveillance, digital still cameras, PC cameras,medical instruments, bar code readers, PDAs, Toys,automotive and mobile phones. Megapixel image sensorsare primarily used in digital still cameras but becomingwidely adopted in mobile phones as well. Today digital stillcameras primarily uses image sensors with 3 megapixel orgreater. For mobile phones the most common format todayDesignatorNamePixel ArrayNumber of PixelsCIFCommon Intermediate Format352 x 288101,376VGAVideo Graphics Array640 x 480307,200SVGASuper Video Graphics Array800 x 600480,000XGAExtended Graphics Array1024 x 768786,432SXGASuper Extended Graphics Array(or 1.3 Mega Pixel)1280 x 10241,310,720CUXGAUltra Extended Graphics Array(or 2 Mega Pixel)1600 x 12001,920,000Table 1. Common display formats0-7803-8906-9/05/$20.00 ©2005 IEEE 1390 2005 Electronic Components and Technology Conference

To keep device sizes relatively small, the pixel sizes aresmaller for megapixel sensors as shown in Figure 4.Currently most megapixel CMOS image sensors have pixelsizes ranging from 3.0 micron to 3.5 micron whilemegapixel CCD sensors have pixel sizes in theneighborhood of 2.8 microns. The maximum allowableparticle size on a die is typically equal to one pixel.Hence, for megapixel camera module assembly, particlecontrol strategies must improve.%9080706050403020100Particle on die Particle on glass OthersFigure 5. Defect pareto of a 1.3 megapixel camera module.There are numerous sources for particles in theassembly line. Particles can be generated due to poorfacilities control, from material selected for assembly, fromequipment, inadequate cleaning steps during process flow,too many people in the line, etc. Figure 6 shows someexamples of particle on image sensor die. Continuousparticle reduction must become a standard practice byassembly engineers. The next few sections discuss each ofthese in detail from the viewpoint of particle reductioneffort. They also cover additional criteria for each sectionwhich are required for megapixel camera module assembly.Facilities ControlTable 3 shows clean room requirement by assembly processstep. The portion of the assembly process where the die isexposed is most sensitive to particle. Until the module isencased with the lens system, assembly is typically done inclass 100 environment. Focus and barrel lock isrecommended to be done in class 1000 environment whileremainder of the assembly process can be done in class100K. The actual clean room particle count must bechecked on a regular interval and at strategic places withinthe clean room. For example particle detector should beplaced close to the die attach and wire bond process areawhere the die is exposed to the environment. This willprovide data on actual particle control level at those criticalprocess areas. Figure 7 shows a class 100 clean room withvarious particle check point strategically selected based onprocess location. Table 4 shows the actual particle count inthose locations.ProcessClean Room ClassWafer / Glass MountWafer De-tapeWafer / Glass SawDie AttachDie Attach CureWire Bond100Glass AttachGlass Attach CureMount AttachMount Attach CureBarrel InsertBackgrind 1,000FocusBarrel Lock10,000Image TestScreen PrintPick and PlaceReflow100,000Laser MarkSingulationFlex AttachTable 3. Typical facilities requirementFigure 6. Examples of particle defects on image sensor die.The environment inside all equipment should bemonitored and steps should be taken for particle reduction.It is possible that while the clean room particle count iswell under control, the particles inside some of theseequipment are outside the spec of class 100 clean room.Proper airflow inside the equipment is key in reducingparticles. Keeping covers of the equipment open or takingthem off completely can help reduce particle level insidethe machines. Figure 8 shows a picture of assemblyequipment in operation with the cover open.Assembly line layout is also another area where specialattention should be given. Smooth laminar airflow insidethe clean room is important in reducing particle count. Forexample adequate space between equipments, workbenches13922005 Electronic Components and Technology Conference

Air shower#6#5#4Pass BoxAir lockMount attach#7Wafer sawDie attachWire Bond#1 #2 #3#Particle check pointsFigure 7. Example of strategic locations for particle check point inside class 100 clean roomOver 0.3 0.3 umum(Reference)Over 0.5 0.5 umumPoint 1 4 3Point 2 0 0Point 3 11 8Point 4 9 2Point 5 22 16Point 6 4 3Point 7 0 0Criteria(0.5um base)100 classTable 4. Actual particle count data at each of the particle check points shown in Figure 7.a) All moving parts should be away from the actualexposed die area so that particle generated doesnot land on die surface.b) Equipments with its own de-ionized airflowsystem will help reduce particle.c) Incoming air should come through a filtrationsystem ensure clean air coming in.d) Substrate strip indexing should be done on rollersinstead of on rails to reduce friction whichgenerates particle.Figure 8. Equipment shown with cover open to allowimproved airflow.and shelves must be allowed. A camera module assemblyfloor equipment layout will look sparse compared to that ofa standard packaging assembly floor. All workbenches andshelves should have adequate ventilation in the form ofperforation to allow airflow.Equipment SelectionEquipment material and design can inherently generateparticles during operation. All handling area and movingparts of the equipment should be made such that particlegeneration due to friction is reduced both from a design andmaterial selection view point. Examples are given below.In the last two years, a few of the well known assemblyequipment suppliers have come up with equipmentspecifically for image sensor assembly. These equipmentinherently generate lesser particle by.Megapixel module assembly may require equipmentwith tighter process control specifications. For example,megapixel modules require a tighter optical center to sensorcenter alignment. The current requirement is +/-50 micronsto +/-75 microns for VGA designs. The requirement formegapixel design is +/-25 micron to +/-50 microns, whichrequires advanced assembly equipment as well as tighterprocess control.13932005 Electronic Components and Technology Conference

Materials SelectionSeveral of the packaging materials used in cameramodules are standard. Among the standard materials aresilicon die, gold wire, laminate substrate, ceramic substrate,flex circuit, passive components, and connectors.However, there are several materials which are somewhatunique to camera modules when compared to other ICadhesives, mount, lens barrel, lens elements, and IR glassfilter. The detail material properties and selection criteriahave been discussed in the earlier paper published inSMTA [2]. A general discussion is provided below onsome of the unique material selection criteria especiallywith respect the issue of particle contamination.In case of adhesives, out-gassing properties areimportant in terms of potential cause of contamination.Adhesives with higher out-gassing may contaminate thesensor surface or the IR glass during the cure process.Table 5 shows comparisons of adhesives materials for lensbarrel, mount and lens attach in terms of their cureconditions and out gassing properties.For the mount and the lens barrel, the common plasticmaterials used are Liquid Crystal Polymer (LCP),Polyphenelyene Sulfide (PPS), Polyphenelyene Oxide(PPO) and Polycarbonate/Acrylonitrile Butadiene Styrene(PC/ABS). Part of the process is to screw the lens barrelinto the mount. For fixed focus modules the lens barrel isscrewed or unscrewed during the focus test depending onthe test methodology. Choosing the right materialcombination between the mount and the lens barrel willreduce particle generation during this barrel insertion andfocus test process. Another key criteria for selecting theright material for mount and lens barrel is its ability towithstand the adhesive cure temperature. Temperatureunder deflection under a certain load for each of thesematerials is shown in Table 6. It is seen that LCP and PPShave relatively higher temperature of deflection comparedto that of PPO and PC/ABS. For this reason, LCP and PPSare being widely used for mount and lens barrel. A test wasdone on mount and barrel each made of LCP and PPSmaterial. Simulated focus operation was performed on eachof these mounts and barrels and then the IR glass waschecked for particles. It was observed that LCP materialproduced no particle on the IR glass while PPS produced asmall percentage of particles. Table 7 and Figure 9summarize the results.Similar tests should be done before choosing lenselement material as well. Examples of lens elementcandidates with their processability and maximum servicetemperature are shown in Table 8. Among the plasticmaterials, Zeonex is being widely used for its goodworkability and relatively high service temperature.In the case of IR filter, particles can be generated fromthe IR coat material. Depending on the coat material andprocess, it may produce flakes during process conditionsand also during reliability test conditions. IR filter fromeach supplier should be tested under the conditionscorresponding to the actual process and reliability testconditions to ensure integrity of the coat material.Figure 9. Particle related performance for PPS (left picture)and LCP (right) as materials for mount and barrel.Assembly ProcessThe assembly process flow for a camera module isshown in Figure 10. The process is broken into 5 separateflows for wafer, substrate, mount, flex circuit, and module.Details of camera module process flow have been describedin the paper published in 2004 SMTA [2]. For fixed focusmegapixel camera module the process flow is no different.For auto focus modules the focus and lock is obviously notrequired.Careful consideration must be given to cleaning andinspection steps within the process flow. All jigs, fixtures,carrier frames, and trays must be cleaned prior to use in theclass 100 clean room environment. Use of ultrasonic wetcleaning process is common. Among the 5 separate flowsshown in figure 10, flows for wafer, substrate and mountare most critical from a particle related defect stand point.This is due the fact that the sensor die is exposed duringthese process steps. These sub-processes are also done inclass 100 clean room environment except for the mark andsingulation process.For the wafer flow, particles can be generated duringbackgrinding process from the backgrinding tape that isattached on the top surface of the die. Standardbackgrinding tape can be used if the wafer goes through awet cleaning process before die attach. The standard wetcleaning during the saw process is adequate. Si particlescan be a major source of particle contamination during thewafer saw process. The standard wet cleaning during thesaw process may not be adequate for removing Si dust.Two steps can be taken to reduce Si partcile contaminationfrom the saw process. First, the saw process must beoptimized to reduce Si dust accumulation on the wafer. Forexample it has been observed that a two pass saw showsless Si dust related contamination vs. standard one pass sawprocess. Secondly, a separate wet cleaning process afterwafer saw can further reduce any Si particle contamination.Sawn wafers should be inspected to ensure cleanlinessbefore die attach process.As mentioned in the material section, use of laminatesubstrate for camera module is increasing. These substratesmust be cleaned using a wet ultrasonic cleaning process13952005 Electronic Components and Technology Conference

Wafer FlowWafer MountBackgrindDe-tapeWafer MountWafer SawWafer CleanInspectionFlex Circuit FlowFlex Circuit FlowScreen PrintPick & PlaceReflowSingulateSubstrate FlowSubstrate CleanDie AttachCureDry/wet CleanWire BondDry/wet CleanInspectionMount AttachCleanInspectionCureBarrel InsertLaser MarkSingulateModule FlowImage TestFunctional TestFocusBarrel LockFlex AttachO/S TestPack / ShipFigure 10. Typical process flowMount FlowGlass MountGlass SawDry/wet CleanGlass AttachCureDry/wet CleanInspectionbefore die attach. After die attach process the units shouldbe cleaned again. Dry cleaning can be done with ionizedair-blow and vacuum pull on each sensor attached tosubstrate. This should be done with the substrate inverted(die down configuration) so that particles will fall off easily.Fig 11 shows a semi-auto dry cleaning equipment usedafter die attach process.Wet cleaning is usually done in an inert environmentusing ultrasonics. Similar cleaning processing should berepeated after the wirebond process as well. On the mountFigure 11. Semi-auto dry cleaning equipment usingdeionized airblow and vacuum.1396flow, the IR glass saw process should use a similar cleaningprocess as the wafer saw process. After the IR glass isattached to the mount, the mount sub-assembly is attachedto the substrate sub-assembly which has the sensor dieattached and wirebonded. Before the mount attach, themount should also be cleaned and inspected. Wet cleaningafter wirebond and before mount attach can significantlyreduce particle and improve yield.The remainder of the module flow and flex attach flow isdone in class 100K room. At this point the camera moduleis much less susceptible to particle defect as sensor die andthe IR filter are already encased.Cleaning frequency and methodology of equipment inclean room should be optimized. For example if adhesivecure ovens are not cleaned regularly, they can be a largesource of particle contamination.Process flow shown in Figure 10 has many inspection stepsto ensure module cleanliness. As process conditions areimproved (proven by higher assembly yield), some of theinspection steps can be eliminated or reduced to a samplingbasis.The listed processes all have specific requirements thatimpact the size and / or optical performance of the cameramodule. Some of these requirements exceed currentmanufacturer’s standard capability. Hence, advanceddevelopment with latest state-of-the-art equipment isrequired. Some of the critical process capabilityrequirements are shown in Table 9.Process Attribute Requirementwafer thickness 150µmBackgrindtotal thicknessvariation< 10µmdamage to microlenson diezerox,y placement +/- 25µmDieAttachdie tilt +/- 0.2 degreesoutgassingzeroWireBondmin die / mount space 300µmx,y placement +/- 50µmMountAttach overall optical tilt +/- 0.5 degreesoutgassingzeroFlexAttachMax lens temperature 85CTable 9. Typical process capability.Tilt control is critical for megapixel modules. Most ofthe digital still cameras and digital SLR cameras have roomfor self-alignment and tilt adjusting capability. Cameramodules in cell phones do not have the room to do this.Whereas VGA modules require about

educe tilt is by changing material from paste type to filmtype. Tape has better inherent tilt performance compared tothat of paste. Table 10 lists comparison data of a 8mm x8mm die tilt for film and paste die attach materials. Itshows a magnitude of tilt improvement for film adhesivematerial. However tilt performance using paste die attach iswell within the spec limit of

BeforeIR FilterIR FilterTape.Ejector Pin.AfterFigure 13. Routing area of laminate substrate canpotentially generate particles.IR FilterIR FilterTape.Ejector Pin.Figure 16a. Tape is stretched to ensure gap between IRfilters.Around 2.5milFigure 14. Ultrasonic cleaning equipment for laminatesubstrates.Figure 16b. Resultant gap of 2.5 mils between adjacent IRglass after tape stretch.Figure 15. Magazine handling was changed from vertical tohorizontal for improving particle on die.sheets are mounted on to saw tape and then sawn intoindividual sizes. Then the sawn IR filters are attached intothe mount or lens holder. It was observed that even thoughthe glass piece parts were completely sawn, they werestill touching the adjacent IR glass piece parts and weregenerating particles during the pick-up operation. Thecorrective action was to expand the tape after the sawoperation to ensure gap between adjacent IR glass filters.This is illustrated in Figures 16a and 16b. A second steptaken was to reverse the IR glass attach (on mount)sequence as shown in figure 17. The purpose is to ensurethat no particle falls on the IR glass that is already attached.1398Figure 17. IR glass attach sequence was reversed to reducethe potential of particle on die.To reduce particle caused by sticky tape, 2 steps weretaken as follows.2005 Electronic Components and Technology Conference

and processing speed will be necessary to keep the test timeshort, especially for very high megapixelResolutionPixel ArrayTotal numberof pixelsTest TimeFactorVGA 640x480 307200 11.3 megapixel 1280x1024 1310720 42.0 megapixel 1600x1200 1920000 6Table 12. Test time factors for 1.3 and 2.0 megapixelcompared to VGA camera modules.modules. Secondly, due to the smaller pixel size and highernumber of pixels, the test pass-fail criteria is more stringent.This issue goes back to the particle control criteria. The testyield can be improved by tighter control of particle.There is no industry standard for focus and test method.Details of the basics of focus and test methodology hasbeen described in the paper published in 2004 SMTAconference [2].ReliabilityThere are no industry standards for reliability testrequirements for camera module in general. The passingcriteria is also defined by each image sensor manufacturer.These are discussed in detail in the SMTA conferencepaper [2].ItemsSubstrateMaterail DescriptionLaminate PCBFlexible Circuit PropritearyDie attachadhesiveAblstik 2035SCGlass attach &mount attach Ablstik 8387BadhesiveUV adhesive Loctite 190024 (3106)Mount and lensbarrelLCPIR glass PropritearyWire1.0 MIL(25.40UM) GOLD WIRESolder paste F10 SN63-90M4 (TYPE 4)Capacitor 4 piecesFluxConnector PropritearyCHEMICAL, RMA 615 PEN FLUXTable 13. Material description for 1.3 megapixel cameramodule.A reliability test was done to qualify a 1.3 megapixelcamera module. The camera module shown in Figure 3 wasmanufactured (but one die was used instead of 2 die) usingthe process shown in Figure 10. The module was a 32 pin8mm x 8mm package. Materials for the 1.3 megapixelcamera module are listed in Table 13. Table 141400summarizes the passing criteria and test results showingthat all samples passed the reliability requirements.ReliabilityitemTemp CycleHigh Temp. &HumidityStorageLow Temp. &HumidityStorageCondition-40℃ -> 85℃( each30 min, 24 cycles)-40 ℃ / 96 Hr(natural dry, for 3hours)1.5m drop, 1 X 6SampleSize45 eaResultPASS80 ℃ / 80% / 72 Hr 45 ea PASS45 ea PASSDrop Testplane5 ea PASSVibration20 ~ 2000 Hz, 3 axis(X,Y,Z)5 ea PASSFPC & PCBAdhesiveStrengthPeeling Test: 0.8 Kg 5 ea PASSHolderAdhesive3 Kg 5 ea PASSStrengthLens Torque 0.4 Kg 5 ea PASSTable 14. Reliability test conditions, passing criteria andresults.Conclusions1) Cross-section of a typical megapixel camera modulewith 3 element lens was shown. The design can varydepending on types of modules required by end market.2) The need for improved particle control for megapixelcamera module was identified.3) It is critical to monitor and control actual cleanliness ofclass 100 clean room. The layout of the clean room isalso important. Inside of equipment must also be keptclean and methods for doing this were described.4) Equipment design and selection is key for improvedparticle control.5) Material selection criteria to reduce particle generationwere discussed.6) Need for wet and dry cleaning and inspection duringthe assembly process was described.7) Engineering discipline is a key element to maintain andcontrol low particle in the clean room.8) Continuous particle reduction effort must be an integralpart of the assembly process.9) Example of a methodology of particle sourceidentification, and process modification to reduce thesedefects was shown.10) Optical selection for megapixel module was brieflydiscussed.11) Test for megapixel module may follow similarmethodology as that of VGA module but test time can2005 Electronic Components and Technology Conference

e longer. Pass/fail criteria is also more stringent formegapixel image sensors.12) There is no industry standard for reliabilityrequirements for megapixel camera module.13) Reliability results of a 1.3 megapixel camera modulewere provided.AcknowlegementThe authors would like to acknowledge all the membersof the Amkor Technology Korea and Amkor TechnologyTaiwan teams for contributing to this paper.References[1] Shellcase, IC Packaging Technology Expo, January,2003.[2] Robert Darveaux, et al , “Camera Module PackagingTechnology”, Proceedings of 2004 SMTA InternationalConference, pp. 27-38.14012005 Electronic Components and Technology Conference