Wire Bonding Quality Assurance and Testing Methods Visual and mechanical testing methods of wire bonds • Light- and scanning electron microscopical examinations • Pull test • Shear test β β β β Dr.Dr.---Ing.Ing. M. SchneiderSchneider---RamelowRamelow phone: +49 30 / 46 40 3-172, fax: -162 [email protected] Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (IZM) Gustav-Meyer-Allee 25, D-13355 Berlin sheet 1 Wire Bonding Quality Assurance and Testing Methods Wire bonding and quality assurance: General requirements Global formulation: • low cost technology • easy to realize and flexible during application • strong mechanical stability • thermal stability (in regard to further processing steps) and • good long term stability • low contact resistance • as much as possible great welding area between wire and metallization • no physical and chemical interactions with other materials Test methods: • visual inspektion MIL-STD 883 method 2010, 2017 • pull test MIL-STD 883 methode 2011 (destructive), methode 2023 (non destructive) • shear test ASTM F 1269-89 sheet 2 Wire Bonding Quality Assurance and Testing Methods Bonding methods Wire Quality assurance Bonding metallization ! principle ! diameter ! quality test methods ! material, pretreatment ! parameters ! elongation, pull-strenght ! standards ! hardness, property ! bonding material ! hardness ! parameter control ! layer thickness/structure ! process kinetics ! surface topology ! product/application ! contamination etc. ! behavior during bonding requirements etc. etc. etc. Influences due to the bonding process Bonding machine Bonding conditions Bonding substrate Design ! bonding procedure ! personal know-how ! material, assembly ! layout ! bonding speed ! personal motivation ! vibration behavior ! functionality ! adjustment, accuracy ! production sequence ! layer adhesiveness ! simulation ! pattern recognition ! manuf. Infrastructure ! work holder ! compabibility to package system ! maintenance etc. and device standards etc. etc. etc. sheet 3 Wire Bonding Quality Assurance and Testing Methods Bond pads on chip: • Multiple imprints of test probes or deep very imprints • Strong adverse effects of surface quality • Influence on bondability sheet 4 Wire Bonding Quality Assurance and Testing Methods Visual inspection MIL-STD 883 method 2010, 2017 to proofproof:: selective parametersparameters:: - geometry of bond contacts - wire deformation - loop geometry - snap off behaviour - placement of bond contacts - position and welded area - predetermined quantity of loops - adhesion of metallization - short circuits - missing bonds or loops RemarkRemark:: Visual inspection of bondpads before wire bondingbonding!! sheet 5 Wire Bonding Quality Assurance and Testing Methods Ball and wedge bond lift offs of wafer metallizations while TS-Au-B/W-Bonding Wafer metallization lift offs while TS-Au-W/W-Bonding sheet 6 Wire Bonding Quality Assurance and Testing Methods SEM-View (BSE): PCB metallization lift off and undersurface of a lift off bond (wedge) sheet 7 Wire Bonding Quality Assurance and Testing Methods Quality Tests (Visual Methods) loop high Ball Bond loop high Wedge Bond loop length loop length influence to horizontal and neck and heel vertical deformation SEM – Figure of a typical Ball Bond loop size and sweeping position of bonds on the pad Geometrical Parameters during Wire Bond SEM – Figure of a typical Wedge Bond sheet 8 Wire Bonding Quality Assurance and Testing Methods Ball bond failures sheet 9 Wire Bonding Quality Assurance and Testing Methods Visual inspection of wire bonds sheet 10 Wire Bonding Quality Assurance and Testing Methods Visual inspection of wire bonds sheet 11 Wire Bonding Quality Assurance and Testing Methods Wedges on chip Drahtbonds auf dem Chip -• Starke strong Verdrängung squeezing outder Chipmetallisierung of chip metallization while bonding durch die Drahtbonds -• Möglicherweise possible evidence Hinweis of aauf too zu softenweiche surface metallization Oberfläche sheet 12 Wire Bonding Quality Assurance and Testing Methods A W wire standard cutting edge D A B P W A B P W 100µm 125µm 125µm 45µm 50µm 75µm 75µm 38µm 1Mil .004in .005in .005in .0018in .002in .003in .003in .0015in B 125µm 150µm 200µm 75µm 90µm 125µm 140µm 70µm 2Mil .005in .006in .008in .003in .0036in .005in .0056in .0028in 180µm 200µm 250µm 125µm 120µm 170µm 150µm 112µm 3Mil .0072in .008in .01in .005in .0048in .0064in .006in .0045in D 200µm 280µm 280µm 130µm 150µm 220µm 190µm 130µm 4Mil .008in .0112in .0112in .0052in .006in .0088in .0076in .0052in wire Heavy wire PP D A B P W A B P W 250µm 300µm 350µm 150µm 175µm 250µm 250µm 150µm 5Mil .01in .012 .014in .006in .007 .01in .01in .006in A pad width W bond width 250µm 500µm 400µm 200µm 230µm 320µm 250µm 180µm 6Mil .01in .02in .016in .008in .0092in .0128in .01in .0072in B pad length P pitch 400µm 800µm 600µm 320µm 350µm 500µm 350µm 300µm 10Mil .016in .032in .024in .0128in .013in .02in .013in .012in D wire diameter 20Mil 800µm 1300µm1150µm 650µm 660µm 950µm 700µm 600µm .032in .052in .046in .026in .0264in .038in .028in .024in sheet 13 Wire Bonding Quality Assurance and Testing Methods Overbonded wedges (too much deformation), right: Detail sheet 14 Wire Bonding Quality Assurance and Testing Methods Results of a tilted glued or soldered chip on bonding process: • Surfaces of chip pads aren´t horizontal • Bonding tool is touched down tilted, too • Wedge is deformed non- uniform • Pad metallization or chip could be damaged sheet 15 Wire Bonding Quality Assurance and Testing Methods Optical inspection of die tilting tilted chip IZM: Hillmann, Großer, Ghaharemani sheet 16 Wire Bonding Quality Assurance and Testing Methods Wire Sweep Definition Normal Lead bond line finger α Swept lead Integrated circuit chip L Sweep % = α/L x 100% sheet 17 Wire Bonding Quality Assurance and Testing Methods MechaniMechanicalcal bbond ond contact inspection ((teststeststests)) to proof: selected tests: - mechanical stiffness of loops - pull test - ball bond strength - shear test - heavy wire wedge bond strength - shear test - simultaneous proofing of many bond contacts - centrifugal test - fatigue behaviour of loops - vibration test - crack initiation and growth - mechanical shock test - sweep off behaviour - air jet test sheet 18 Wire Bonding Quality Assurance and Testing Methods Principle of pull test β β β β Quelle: Dage Firmenschrift sheet 19 Wire Bonding Quality Assurance and Testing Methods Failure modes: 1, 5 Bond lift off from pad metallization (lift off) 2, 4 Wire break at bond (heelcrack or neck break when ball/wedge bonding) FR 3 wire break 1. Bond 3 2. Bond 1 2 4 5 sheet 20 Wire Bonding Quality Assurance and Testing Methods LoadLoad/Force/Force distribution while bond pull test load at chip: F Θ Φ cos ( 2 - ) 2 1/2 Φ F1 = F F = F d sin (Θ + Θ ) F1 = F2 = 1 + 1 2 2 sin Θ 2 2 h load at substrate: true for Φ = 0, both bond contacts at Θ Φ Θ Θ Θ cos ( 1 + ) one level and 1 = 2 = F2 = F F1 Θ Θ sin ( 1 + 2) h Θ 1 F2 chip H Θ 2 support d1 d2 d sheet 21 Wire Bonding Quality Assurance and Testing Methods Quality criteria for pull testing β β if β β Conditions Characteristics laboratory fabrication pull force (based on nondeformed wire) - average value >50 % >50 % (based on average value) - standard deviation <15 % <25 % (based on standards) - proportion of values < cN 0 % 0 % lift offs - pull lift off 0 % <10 % - bond lift off 0 % 0 % sheet 22 Wire Bonding Quality Assurance and Testing Methods Minimum pull forces for destructive pull test (MIL STD 883, Methode 2011): 1000 1 2 Wires: 3 3 1 Au (preseal) 2 Al (preseal) 100 Au (postseal) 3 Al (postseal) 10 Minimum Bond Pull Limit [cN] 25 250 2500 wire diameter [µm] sheet 23 Wire Bonding Quality Assurance and Testing Methods Pull Force increasing quantity increasing quantity of pull lift offs of heelcracks optimum increasing parameters sheet 24 Wire Bonding Quality Assurance and Testing Methods Principle of shear test 1 Substrate/Pad 2 Interface Ball/Substrate 3 Au-Ball 4 Shear Tool S Shear Heigth 5 Shear Level F Shear Force Source: Dage and „DVS-Merkblatt Drahtbonden“ S sheet 25 Wire Bonding Quality Assurance and Testing Methods Standard report of pull test: Standard AlSi1 wire (30 µm) errorFehlerraterate pullPullkraft force 20,0 100 cN % 16,0 6cN [%] 6cN [cN] < pull lift offs[%] lift pull bond lift [%] bond offs s pull force US power power US [scale] bond 1./2. average value pull [cN] force 80 104/110 10,5 1,6 16,7 10,0 5,0 12,0 109/115 12,0 1,4 6,7 5,0 1,7 114/120 14,5 0,9 0,0 0,0 0,0 60 124/130 15,0 0,8 0,0 0,0 0,0 8,0 134/140 14,5 0,9 0,0 0,0 0,0 144/150 11,5 1,0 11,5 0,0 0,0 40 154/160 8,5 1,2 20,0 0,0 0,0 4,0 20 averagePullkraft MW pull [cN] force [cN] standardStandardabweichung deviation s [cN] [cN] pullPullkraft force < <6 cN6 cN[%] [%] number of tests: n = 30 0 104/ pullPullabheber lift offs [%] [%] 109/ 110 114/ Bondabheber [%] 115 124/ bond lift offs [%] US time: 30 ms 120 134/ 130 144/ 140 154/ bond force: 32 cN 150 160 tensile wire strength: 19 cN US power 1./2. bond [[scale scalescale]] sheet 26 Wire Bonding Quality Assurance and Testing Methods Failure modes while shear test schear tool metallization ball shear heigth substrate (chip) Ball lift offMetallization lift off Ball shear Metallization crackingChip cratering sheet 27 Wire Bonding Quality Assurance and Testing Methods Quality criteria for ball shear testing (TS bondingbonding)) Conditions Characteristics laboratory fabrication shear force (based on minimum shear force value) - average value > 140 % > 120 % (based on average