QFN Singulation CSR (1063258)

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Singulation of QFN/MLP PackagesGideon Levinson [ADT - Advanced Dicing Technologies]QFN/MLP substrates behavemu ch differently fr omm o st kn o wn m at er ial sin the microelectronics industry, andtherefore present challenges for thedicing process. This paper will coverbasic substrate characteristics andvarious singulation topics including:s u b s t rat e g eo m et r y a n d m a te r i alcharacteristics, substrate design andparameters affecting the cut quality,quality specifications to which theindustry aspires, dicing on tape andtape-less mounting, optimizing thedicing process, and the constant demandmaintaining cut quality.

These factorscreate real challenges, not only in thedicing process, but also with the bladedesign, which will also be addressed.Substrate Geometry/MaterialCharacteristicscopper lead frame coated with PPFmaterials have different hardnessand brittleness characteristics thatin a high-volume production modeare challenging. The final die sizesmounting challenges, which will bediscussed later. Figure 1 illustrates thegeometry of a common QFN/MLP-typesubstrate.The lead frame in most cases is aThe copper material is relatively softFigure 1: Common QFN/MLP-type substrateand mainly ductile, which duringdicing, causes burrs and smearing.The polymer molding compound isreinforced by silica particles (Si 2O 3in the range of 30-70µm in size.

Themolding compound is relatively brittlecompared to the copper lead frame, andchipping on the molding part is anotherproblem during dicing. In general, thecombination of a ductile material witha brittle material in the same dicingprocess creates major challenges. Thegeneric plastic deformation graph inFigure 2 shows the difference betweena brittle material and a ductile softThe silica particles in the polymermolding compound are used as astabilizer to minimize stresses and tocontrol the flatness of the substrate.The size of the silica particles affectsthe chipping size; the larger the silicaparticles, the larger the chipping onthe molding. The silica grit pullout generates small craters that willcreate edge chipping after the dicingChip Scale Review Sept/Oct 2012 [ChipScaleReview.com]operation.Substrate Geometry Design AffectsCut QualityBoth the geometry of the copperlead frame and the outside geometryof the substrate have a major impacton blade loading, which affects cutquality.

In general, minimizing theamount of copper the dicing blade faceswhile dicing through the leads and atthe outer rim of the substrate is a keyfactor to be considered for substrateFigure 2: Plastic deformationdesign. Minimizing the copper wouldreduce the load on the dicing bladeand subsequently enable a higherThe copper amount and geometry isdecided by the conductivity and powerthrough the leads, so the design placesmore consideration on the functionalrequirements. The production-relateddifficulties, however, should alsobe considered.

Figure 3 shows themajor elements involved in a QFNtype substrate. The lead cross sectionat the dicing area needs to be as smallas possible. A small square lead crosssection is a better design than any otherirregular shape. A large lead crosssection can cause copper smearingbetween the leads, and this may causeelectrical shorts.Minimizing unsupported copperaround the QFN/MLP substrate isfairly crucial as it can easily break theblades and consequently affect theproductivity.

The best solution is toFigure 3: QFN/MLP glossarydesign the outer lead frame rim in a"snap off" form that can be manuallyremoved before dicing. If a segmentdesign is applied ( Figure 3area between the segments shouldbe minimized and it should includelarge stress release openings. It is alsodesirable that the molding be as closeas possible to the lead frame edge.Half-etched lead frame design hasbecome common in the industry. Such adesign is more favorable to dicing andcontributes to an improvement in thecut quality, as well as blade life.

TheChip Scale Review Sept/Oct 2012 [ChipScaleReview.com]29idea is to reduce the amount of copperthat the blade needs to remove fromthe substrate. In general, the larger themuch thicker - upto 4.0mm. ThickQFN substratesmade with muchthicker copper leadframes of >0.500mmintroduce singulationd i ff i c u l t i e s ; t h e yr e q u i r e d i ff e r e n tquality requirementsper application.However, theon the blade during the dicing process.Half-etched substrates can be dicedwith harder blades resulting in longerblade life while maintaining good cutquality.Effect of Lead Coating/Plating onCut QualityThe microelectronic industryis driven towards implementingenvironment friendly "green” productsand processes.

Conventional QFN leadscomply with environmental standards,these materials are now being graduallyVisually, substrates with matte tinplating appear dull compared to thesubstrates. These types of substratesare also subject to potential damageduring conventional dicing due to thepossibility of leads melting as the mattetin melting temperature is lower at232ºC compared to the materials usedin conventional substrates (Figure 4To overcome this problem, the bladeand the process should be optimized forminimum load. This minimization couldbe realized by a softer blade, usinglower spindle speeds within the range: Lead meltingtrim design (Figure 3is smaller compared to the standardQFN market.blade type.Figure 5 illustrates the differentq u a l i t y c r i t e r i a . Ty p i c a l q u a l i t yof the lead pitch (may vary amongAdditionally, many users require 0.0200.030mm on the X, Y and Z burrdimensions.The above quality specificationis mainly used for QFN productsranging from 0.9mm to 1.2mm inthickness.

Thin QFN (typicallyquality specifications. Other QFNDicing on Tape and Tape-lessMountingThere are two different mountingmethods used in QFN singulation: tapemounting and tape-less mounting. Ingeneral, QFN/MLP substrates are not ascomplicates the singulation process.In today's singulation lines, end userstypically want relatively high feed ratesof up to 100mm/sec, which adds to thedifficulties associated with consistentmounting.Tape Mountingvarious thicknesses and adhesionforce.

Typical thickness may vary from0.100mm to 0.200mm depending on theand in some cases, chilling the waterto about 12ºC. The above parametersshould be optimized according to theapplication.Most end users have rathersimilar quality criteria. The qualityspecification is a function of the QFNsubstrate design and the end productrequirements. Many parameters affectthe quality of the singulation process.geometry (total thickness, copperthickness, device size, i.e., smallerdevices may shift during dicing due tosubstrate rim design, unsupported or30Chip Scale Review Sept/Oct 2012 [ChipScaleReview.com]Figure 5: QFN quality criteria- without losing devices that are more prone to shift duringforce.

The mounting method is relatively known and simple,and so is the dismounting; both methods are widely used inthe industry for a variety of applications.other applications on the same saw. The disadvantages of tapeTape-less MountingTape-less mounting has become a prevalent solution formass production. In integrated systems, the QFN substratesare loaded automatically from an input magazine onto adicing chuck. The jig is perforated with holes through whichvacuum is supplied to hold the substrate before dicing, andeach individual part after dicing. This singulation methodis the most common technology used today. The main ideaof this system is to use a unique vacuum chuck design thatcan provide vacuum clamp down for single devices after thesingulation process.

The singulated devices are then inspectedfor visual quality criteria after which an automatic handlerbelow the substrate into a trench in the rubber nest that holdsdown the diced devices by vacuum (Figure 6 Figure 6: Tape-less chuck concept Chip Scale Review Sept/Oct 2012 [ChipScaleReview.com]31The main limitation of the tapeless jig is its ability to hold devicesof each device to be held up to thejig solely by force of the vacuum. Inthose cases, a tape mounting is theonly practical alternative.

Still, thereare advantages of a tape-less mountingmechanical support for the side/rimbe used in a fully automated productionDisadvantages of a tape-less systemjig for each device size is required,to vacuum-related failures such as diecomplicated equipment compared tostand alone dicing systems.Optimizing the Dicing ProcessAny dicing process optimizationrequires a close look at the dicing sawprocess parameters and in parallel,optimizing the dicing blade. QFN/MLP applications differ from user touser. Most users have a variety of QFNsubstrates with different geometries,layout thicknesses, device sizes, numberof copper leads, lead designs, etc.

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