CMOS IMAGE SENSOR PACKAGING TECHNOLOGY FORAUTOMOTIVE APPLICATIONSTeoh Eng Kang1, Alastair Attard2, Jonathan Abela21UTAC Group, Corporate Advanced Packaging R&D22 Ang Mo Kio Industrial Park 2, Singapore 569506engkang [email protected] Group, Global Sales EuropeChemin du Bre 15, 1023 Crissier, Switzerlandalastair [email protected] / jon [email protected] the automotive industry makes further progress in the areas of advanced driver assistance systems(ADAS) and autonomous driving (AD) technologies, the demand for imaging cameras is steadily increasing tosupport applications such as lane detection, traffic sign detection, pedestrian/vehicle recognition and drivermonitoring, to name a few. This is, in turn, driving higher demand for image sensor packages that can meetstringent automotive reliability requirements. Whereas high reliability image sensor packages are typically basedon ceramic packages, these tend to have considerably higher costs and longer development cycles than laminatebased packages which are normally used in other market segments. In this paper, we present novel methods forpackaging image sensors on laminate substrates, enabling a reduction in cost, form factor and time-to-marketwhilst simultaneously meeting automotive reliability grades typically required for such devices. Examples ofpackage constructions and fundamental assembly requirements to achieve high assembly yield and increasedpackage reliability will be presented. The key challenges to qualify automotive image sensor packages and methodsto mitigate risk will also be discussed.Keywords: Imaging, Image Sensor, Packaging, BGA, Automotive, ADASIntroductionCMOS Image Sensors (CIS) have beenconstantly evolving from having increasedminiaturization and higher pixel counts towardstechnologies that enable better image quality andfunctionality. As with smartphones, manufacturers nolonger use pixel count as a key differentiator for theimaging cameras of their end products. Instead, themarket trend has moved towards having cameras withmore innovative features, such as 3D sensing, LEDflicker mitigation, larger viewing angles, highdynamic range (HDR), and faster frame rates, whilstat the same time having better reliability and a longerproduct lifetime, especially in harsh workingconditions. Two key markets developing similarrequirements are the automotive camera market, toaddress the evolving advanced driver assistancesystems (ADAS) and autonomous driving (AD)technologies, along with the industrial camera market,targeting advanced machine vision, factoryautomation, and surveillance cameras.In the automotive market, digital cameras withimage sensors were first adopted to cover blind spotsat the front and rear of the vehicle as parking aids.Recently, the focus of automotive imaging has shifted,and auto makers are now equipping their vehicleswith several imaging cameras per vehicle, with thekey targets in this case being to increase vehiclesafety and enhance driving comfort. Severaladvanced safety features, typically grouped under theterm ADAS, rely on imaging cameras for theiroperation, normally in conjunction with other sensortypes using sensor fusion solutions . Examples ofADAS using image sensors include: lane detectionfor active lane keeping; traffic sign detection toinform the driver of speed limits and road situations;pedestrian & vehicle detection to automatically stopthe car to avoid an accident; driver monitoring whichcan detect drowsiness/health status and automaticallybring the car to a stop in an emergency situation; andenhanced visibility at night time or in bad weatherconditions using IR imaging cameras   . Forincreased driving comfort, cameras allow for easierparking by providing up to a full 360 view aroundthe car, to assist the driver in observing the vehiclesurroundings. On some car models, the input from thecameras, as well as other sensors such as radars, allowthe car to park automatically in a space sufficientlybig for the car to fit in. Furthermore, other novelapplications such as e-mirrors are also gainingtraction, with the aim to replace physical side mirrors
with cameras whose images are then displayed on amonitor inside the vehicle cabin, to improve cardesign and aerodynamics.This increase in automotive safety andcomfort features will require even more imagesensors to be integrated in the future car as the levelof automation increases  . Studies made by IHSMarkit  predict around 6 cameras per vehicle forLevel 3 AD (“Hands-off”) and 9 cameras per vehiclefor Level 5 (“Full Automation”), as outlined in Table1. In total, the automotive camera market size isexpected to grow at a CAGR of around 19% for theperiod 2017-2023, reaching an estimated total valueof USD 19 billion in 2023 .Exterior CameraInterior CameraTotal Camera ContentL3516L4819L5819STD 209E standards) will be required at theseprocesses to ensure that defects attributable toremovable or non-removable dust particles areminimized, thereby achieving a good device yield atfunctional test after assembly. Figure 1 shows atypical Class 10 cleanroom booth used to houseimage sensor package assembly equipment at UTACSingapore to guarantee a clean and high-yieldingassembly environment. Unique cleanroom setup andmaintenance procedures, dedicated machinesdesigned specifically for image sensor assembly,coupled with good practices by the operationpersonnel within the assembly line, are additionalfactors needed to achieve good assembly yield.UTAC Singapore has over 16 years of image sensorassembly experience and this has become one of thecore competencies of this assembly site, with verylow yield defects related to dust particles.Table 1 – Typical camera module content byautonomous driving levelThe desired levels of reliability of automotiveimaging cameras are considerably higher than what isdemanded for consumer applications, since thelifetime of a car is expected to be more than ten years,and the cost of recalls or repairs due to componentfailures can grow exponentially having significantnegative effects on an automaker’s bottom line (aswell as impacts to the Tier 1 and Tier 2 suppliers).Typically, the AEC-Q100 specification  is referredto when qualifying new automotive CIS packages,with this document defining the stress conditions thepackage must endure to be qualified for automotiveuse. The reliability requirements of such automotivecamera packages are very much dictated by theirpositioning within the car, the type of environmentalconditions they are exposed to, as well as thecriticality of their function, and this will greatlyinfluence the package structure, material selection,and assembly process of such image sensor packages.These considerations will be discussed in the nextsection, explaining how the right selections are madeto achieve an automotive grade reliable package.CIS Package Assembly RequirementsThe assembly of CIS packages is a demandingtask since this needs to fulfil various design- andapplication-related expectations linked to optical,electrical, thermal and package reliabilityconsiderations. Listed below are some of the keychallenges that need to be addressed to have anadequate CIS package assembly process.CleanlinessAn ultraclean manufacturing environment iscompulsory for image sensor package assembly,especially for assembly processes where the CIS dieis still exposed to the external environment. Acleanroom class of 100 or below (according to FEDFigure 1 – Schematic of Class 10 cleanroom booth setupused for CIS package assemblyAssembly Accuracy & Process CapabilityMost image sensor packages require veryprecise assembly processes, mainly to achieve thedesired optical and dimensional characteristics of thepackage. This is particularly true during die attach(bond line thickness, XY placement, rotationalorientation, die tilt, and epoxy bleeding clearance),wire bonding (ball and stitch bond location, and loopprofile consistency), and glass attach (bond linethickness, XY placement, rotational orientation, andglass tilt). State of the art equipment, together withdesign and process development considerations,enable UTAC to develop and assemble CIS packagesto the required specifications.Apart from requiring highly accurateprocesses, good process capability is also mandatoryto maintain stable processes which can deal with theexpected variations of high-volume production.Based on the AEC-Q100 standard, processcapabilities of at least 5 sigma (Cpk 1.67) arerequired to have sufficiently robust processes. Toreach this level of consistency, careful and thoroughprocess characterization and a deep understanding ofmachine consistency with good pro-active andpreventive maintenance procedures are required.
Device TraceabilityCIS packages are complex assembliesconsisting of the image sensor die itself as well as thematerials used to package the die. As with anymanufacturing process, it is challenging to maintainuniformity, especially during the development stageof a product. The CIS assembly process at UTACmakes use of device traceability to be able to uniquelyidentify packages: from the identification code andsupporting logging database, it is possible todetermine the assembly lot number, image sensorwafer lot ID and die location on the wafer, substrateID and module location on the strip, materials, andequipment used to assemble the package. Thisassembly data is particularly useful when correlatedto the package optical test data, to be able to identifypossible improvements in the assembly process andimplement them through further engineeringactivities.Quality & ReliabilityAll automotive grade image sensor packagesare subjected to package level qualification based onAEC-Q100 standard. However, apart from thepackage level qualification standards that need to bemet, the assembly site also needs to have beencertified for the production of automotive parts, toensure that the appropriate automotive quality ismaintained during manufacturing. The maincertifications which are typically required are theISO/TS 16949 (Quality Management System forAutomotive-related Products), and more recently theIATF 16949 (Quality Management System forAutomotive Industry), which has now superseded theISO/TS 16949. All UTAC sites are IATF 16949certified, thus allowing them to manufacturepackages for the automotive industry. Moreover,UTAC is also certifying its key automotive sites toISO 26262 standard, which is aimed at ensuring thefunctional safety of electronic systems in vehicles andis particularly relevant for electronics used for ADASand AD.Comparison of Different CIS Packaging SolutionsImage sensor packages are typicallycomprised of a substrate (ceramic, lead frame orlaminate), sensor die, wire bonds to electricallyconnect the die to the package, and a cover glass toprovide optical access to the environment. Adhesivesare used to bond the various components together,and some form of encapsulant is used to protect thedevice from the external environment. Differenttermination types (lands, bumps, balls or pins)provide the package with interconnects used toconnect it to the final product printed circuit board.The choice of substrate used for the sensor packagehas a significant influence on the package form factor,performance, reliability, and cost, as is explained inthe following sections and outlined in Table 2.CeramicLead frame LaminateMoistureresistanceHighHigh /ModerateModerateUnit costHighLowLowNRE costHighLowLowSubstratelead eModeratePerformanceHighModerateModerateTable 2 – Comparison of key CIS package attributes bysubstrate typeCeramic-based CIS PackagesEarlier generations of CIS packages mostlyused ceramic substrates due to the advantageousceramic material properties. Figure 2 shows thestructure of a commonly used Ceramic Leadless ChipCarrier (CLCC) package for image sensors, which isusing a ceramic substrate.Figure 2 – Schematic of a CLCC ceramic image sensorpackage structureCeramic substrates allow the CTE (coefficientof thermal expansion) mismatch to be minimized,thus reducing package warpage during thermalexcursions. This reduced warpage helps improve boththe reliability of the package as well as the opticalperformance. The good coplanarity to the CIS dieoffered by the ceramic substrate is another factorwhich helps to provide better optical performance.Ceramic packages also exhibit better moistureprotection and thermal performance due to s, which are also desired features for aCIS device. The multiple metal layer constructionpossible with ceramic substrates allows for rathercomplex and numerous interconnects to be handledwhilst remaining within a small package footprint,albeit this is typically sacrificed for a large packageheight. Nevertheless, the enhanced performance of aceramic package does come at a price: ceramicsubstrate suppliers are rather limited, ceramicsubstrates tend to be quite expensive, and incur longlead times and high NRE costs due to their pment cycles and time-to-market of new CISdevices. Notwithstanding these limitations, ceramicCIS packages are still a key package solution today,
especially where very high performance and harshenvironmental resistance are required.Lead frame-based CIS PackagesIn some cases, metal lead frames are selecteddue to their lower cost and good moisture protection(capable of achieving MSL1). However, lead framesoffer limited capability in terms of design flexibilityand interconnect density, especially when complexinterconnects or high pin counts are required. Thisstems mainly from the fact that lead frames offer onlysingle layer routing which is also limited in terms oftrace width and spacing due to the stamping oretching processes used to manufacture the leadframes. Furthermore, since lead frames are normallymade of copper, there is a very large CTE mismatchwhich can result in high package warpage, especiallyon larger devices, which can detrimentally affectpackage reliability and optical performance. Moldedinterconnect substrates (MIS) represent the nextevolution of lead frames by being able to offer multilayer and denser routing capabilities, thusovercoming some limitations of traditional leadframes, although there are still some restrictions interms of cost, yield, reliability and moisturesensitivity which need to be considered whenselecting such a substrate for a CIS package.Laminate-based CIS PackagesCompared to ceramic substrates, laminatesubstrates have undergone significant technologyadvancements, mainly driven by the aggressiveminiaturization and high cost sensitivity arising fromthe consumer device market, namely the smartphoneindustry. Laminate substrates can now be sourced ata low unit price, with short lead times, and entaillower development costs. They can also offer verydense routing, allowing high pin count and optimalform factors. Nevertheless, due to their constituentmaterial properties, laminate substrate warpage andmoisture sensitivity are not as good as their ceramicsubstrate counterparts. Package warpage can,however, be mitigated through careful packagedesign and with the help of thermo-mechanicalsoftware simulations. Factors such as substrate design(symmetrical construction, copper balancing, andcopper structuring for large copper areas), packagestructure (die thickness, glass size, glass thickness)and material property choice (low CTE and high T gmaterials) can be taken into consideration at designstage to create an optimal laminate-based CISpackage with minimal warpage, thus meetingstringent reliability requirements. Over the last fewyears, UTAC has worked intensively to successfullydevelop a robust and high-performing laminate-basedimage sensor package, termed iBGA (imaging BallGrid Array). The structure of an iBGA package isshown in Figure 3.Figure 3 – Schematic of the iBGA package structure,with cover glass inboard of the wire bondsSeveral assembly steps, each having finelytuned processes, together with meticulous inspectionand quality control procedures, are required tomanufacture an iBGA package to the required opticaland electrical performance, yield, cost target, andreliability level. The high-level assembly processflow used to manufacture an iBGA package is shownin Error! Reference source not found., highlighting the keyprocess steps. For the latest generation of imagesensors utilising advanced CMOS nodes and low-kdielectrics, a laser grooving process is performed toprevent passivation layer delamination and mitigateedge chipping during the wafer sawing process. TheIS wafer sawing process is performed with the waferfully submerged in water, to prevent deposition ofnon-removeable silicon debris onto the pixel area,and to help wash away removeable sawing debris,guaranteeing wafer cleanliness after sawing. The dieattach, wire bonding and glass attach processes,where the image sensor die is still exposed to theatmosphere, are all performed in a Class 10environment with intermediate ultrasonic drycleaning steps, again to ensure that no particles arepresent on the image sensor surface prior to glassattach process. One can also observe that for thispackage structure a dam and fill liquid encapsulationmethod is chosen over more traditional moldingsolutions. This not only helps to reduce the number ofassembly process steps, package cost and NREs (noneed for a dedicated mold chase), but softwaresimulations and reliability results have also shownthat a dam and fill encapsulation subjects the packageto lower overall stress, resulting in better reliabilityperformance.Figure 4 – iBGA package assembly process flowThe cover glass used in an automotive CISpackage is normally coated (with anti-reflective, IRand/or frequency sensitive filter coatings) and is a key
contributor to the bill of material cost of a CISpackage. Conventional CIS packages based onceramic cavity substrates need a large cover glassextending to the package edge, which increases theoverall package cost. Utilising a laminate-based CISpackage such as iBGA, whereby the glass is stackedon top of the CIS die using adhesive (as can be seenin Figure 3), the glass size is kept to a minimum,covering only the active pixel area and hence helpingto reduce the overall package cost. This packagestructure also helps to reduce the total package height.One disadvantage of this approach, however, is that adedicated space is required on the image sensor diesurface to allow for glass adhesive dispensing,without bleeding onto the wire bonding or activepixel areas. While some die designs can allow this,other denser sensor designs have active pixel areaswhich are very close to the wire bonding pads and noarea is available for glass glue dispensing. Toovercome this limitation, UTAC has developed a 2ndgeneration of iBGA packages, named iBGA2, whichcan support these denser designs (Figure 5).expensive ceramic packages to iBGA2 laminatepackage without new wafer tape-out), and also bringsmore freedom to glass size and package design choice.iBGA Package Qualification for AutomotiveAEC-Q100 Grade 2 ReliabilityAs discussed in the introduction, there iscurrently a significant growth in demand for imagesensor packages for automotive camera applications.Since most automotive cameras are located within thecabin or on the vehicle bodywork rather than near theengine or drivetrain, the environmental conditions arenot so harsh and thus an AEC-Q100 Grade 2reliability specification, representing an ambientoperating temperature range from -40 C to 105 C,is adequate for these devices. The automotive gradealso defines the stress tests which need to be appliedin order qualify the package, as shown in Table 3.Figure 5 – Schematic of the iBGA2 package structure,with cover glass extending over the wire bondsWith an iBGA2 structure, an ultra-low loopwire bond profile is used (with an average height of55µm), and the glass adhesive is dispensed over thewire bond pads. The cover glass, which can be aslarge as the sensor die, is attached very precisely inheight, such that the wire bonds are embedded in theglass adhesive bond line (having minimum thicknessof 80µm) without touching the glass (Figure 6).Table 3 – AEC-Q100 qualification stress testsFigure 6 – SEM images showing ultra-low wire loops(top left), wires embedded in glass attach adhesive (topright & bottom left), and cover glass attached over wirebonds (bottom right), in an iBGA 2 packageThis structure eliminates the need of dedicateddie area for glass adhesive dispense, allows denser diedesigns to be used (for example to migrate fromAs the number of cameras per vehicleincreases, cost reduction of the CIS package becomesmore appealing, yet the package must still be able tomeet these demanding automotive quality standards.UTAC has developed this laminate-based iBGApackage solution targeting a lower cost yet reliablealternative to ceramic packages for automotivecamera applications. A package cross-section of anassembled iBGA device can be seen in Figure 7,whilst the package top view at various phases of theassembly flow are shown in Figure 8 Error!Reference source not found.
THB(85 C/85% RH)HTSL(125 C)Figure 7 – iBGA package cross-sectionPASSN/APASSPASSN/APASSTable 4 – Summary of reliability results oniBGA/iBGA2 packages as per AEC-Q100-G2 standardConclusionThe increase in demand of automotivecameras for ADAS and autonomous drivingapplications has created the need for novel packagingconcepts for image sensors, which have a lower costand can still guarantee excellent optical and electricalperformance, as well as high levels of reliability.UTAC has developed its iBGA packages to addressthis need and demonstrated that the iBGA packagesare capable of meeting AEC-Q100-G2 requirements.With its cost competitive positioning and very goodperformance characteristics, the iBGA package istherefore being adopted in the automotive segmentand is also expected to become an interestingpackaging solution for other emerging applicationareas, such as machine vision and advanced imagingfor smart factory applications.Figure 8 – iBGA package top view after: IS die attach(top left); wire bonding & glass attach (top right);encapsulation, laser marking, ball attach & singulation(bottom left); and after SAM delamination check postreliability stress testing (1000TC / 1000hr THB / 1000hrHTSL), with no delamination observed (bottom right).Using a robust bill of materials and thoroughprocess characterisation, the iBGA/iBGA2 packagescan meet 5 sigma processes (Cpk 1.67) and passAEC-Q100-G2 qualification (see Table 4). To enablethis, several key challenges had to be addressed. Thecorrect substrate design and materials have beenchosen to attain the desired moisture resistance leveland minimize package warpage. The materials wereselected with a focus on adequate adhesive strengthto prevent any package delamination, and appropriateproperties such as modulus, CTE and Tg to mitigatepackage stress. The overall package structure andstack-up was optimized to minimize the packagewarpage and prevent package delamination and glasscracks from occurring, especially during temperaturecycling. All these factors were investigated at designstage with the help of software simulations, and hencethe correct package design considerations could benarrowed down prior to substrate tape-out and actualsample assembly builds for process and reliabilityvalidation, thus mitigating risk and reducing overalldevelopment time.PreconditioningMSL3 ReflowTC(-55 C/ 125 C)TimeZeroPostPrecon1000cyc /1000hrsPASSPASSPASSPASSN/APASSReferences “Sensor fusion for autonomous sistant-systemsadas/sensor-fusion/. [Accessed 2019]. Nicolas Roux, “Automotive In-cabin SensingSolutions,” STMicroelectronics, 2018. “Audi Driver Assistance Systems,” AUDI AG,2017. [Online]. Available: con7180/driver-assistance-systems-7184. “From assisted to automated driving,” InfineonTechnologies AG, [Online]. ies/adas-to-ad/. [Accessed 2019]. “SAE J3016 Levels of Driving Automation,”2018. [Online]. 9D-standard-for-selfdriving-vehicles. “Automation: From Driver Assistance Systemsto Automated Driving,” VDA Verband derAutomobilindustrie e.V, Berlin, 2015. Akhilesh Kona, “(R)Evolution of AutomotiveElectronics,” Semicon Europa, Munich, 2017.
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CMOS IMAGE SENSOR PACKAGING TECHNOLOGY FOR AUTOMOTIVE APPLICATIONS Teoh Eng Kang1, Alastair Attard2, Jonathan Abela2 1UTAC Group, Corporate Advanced Packaging R&D 22 Ang Mo Kio Industrial Park 2, Singapore 569506 [email protected] 2UTAC Group, Global Sales Europe Chemin du Bre 15, 1023 Crissier, Switzerland