Pressureless Sintering of Nano- Ag Paste with Low Porosity for High Power Die Attach

Transcription

1 Pressureless Sintering of Nano- Ag Paste with Low Porosity for High Power Die Attach Fen Chen, Sihai Chen, Guangyu Fan, Xue Yan, Chris LaBarbera, Lee Kresge, and Ning-Cheng Lee Indium Corporation 1

2 Introduction For high power devices, high-pb solder die attach materials run into limitation in service temperature, electrical & thermal conductivity. Ag-sintering pastes emerged as a solution. But, either high sintering pressure or polymeric binder is needed, thus suffering major constraints at adoption. A pressureless nano-ag sintering paste is developed without use of polymer binder. The performance will be presented and discussed. 2

3 Die Attach Materials & Parts 3 7 nano-ag sintering pastes were evaluated, including C1 (C sintered at 250C), C2 (C sintered at 280C), F1, F2, Y1, Y2, Y3, and Y4. Control Pb/5Sn/2.5Ag solder paste (T3, 89%). For all sintering pastes, a pressureless sintering process was used, with a customized sintering profile for each paste. Parts Silicon die: DBC: 3mmx3mm; surface finish - 750A Ti + 3kA Ni + 750A Ag 12.5mm x 12.5mm ceramic thickness mm Cu mm on both sides with Ni layer thickness of 3.5 µm, Au 200 nm

4 Die Attach Process & Testing Sintering paste printed onto DBC, then place die The sandwich was sent through heating profile under air without pressure. High-Pb solder paste reflowed under N2. Post sintering treatment (1) Thermal aging: 250C (2) Thermal Shock: -45 C/+240 C, liquid to liquid/10 min dwell Testing of Sintered samples (1) Shear Test: (2) Morphology Analysis: Fracture morphology OM & SEM (after shear test) Cross-sectional microstructure SEM & EDS (with & without shear test). 4

5 Ag Sintering Paste C before Sintering Viscosity: Brookfield - Cone & Plate, 25 C, 10 rpm, mpa s (cp): 24,900 Thixotropic index: Density: 4.5 g/cm 3 Metal load: 91% 5

6 Typical Properties of Sintered Materials Joint bond line thickness requirement: um Property Ag sintering Indalloy 151 Melting point ( o C) Density (g/cm 3 ) Electrical resistivity (µω.cm) 5 20 Thermal conductivity (W/mK) CTE (ppm/c) Extractable Ionic Content Cl - : 1 ppm Br - : 1 ppm NO -- 3 : 1 ppm SO 2-4 : 1 ppm Na + : 1 ppm K + : 1 ppm

7 Temperature (oc) Sintering Process The paste can be sintered under air with standard Pb-free reflow oven, the typical heat profile is as follows: mm x 10mm 3mm x 3mm Metallization of Parts: 200 Die with Ag or Au metallization DBC with Ag or Au metallization Time (min) 7

8 Shear Strength (MPa) High Temp Joint Shear Strength High temperature shear test shows that the service temperature of Ag sintering joint is 470C, while that of high Pb solder is about 230C. Relation between Shear Strength and Temperature Ag-sintered 92.5Pb5Sn2.5Ag Temperature ( C)

9 Shear Strength vs Thermal Aging C1 vs C2 The difference in the shear strength behavior probably can be attributed to a higher internal stress of C2 caused by a higher sintering temperature. Although the porosity of C1 is slightly higher than C2, the benefit of a lower internal stress appears to outweigh the higher porosity disadvantage. 280C 250C 9

10 Reliability Shear Strength of 250C Aged Joints 92.5Pb5Sn2.5Ag 10

11 11 C1 & C2 Continue on Sintering upon Aging

12 Ag Migrated to DBC Slowly for C1 & C2 & Maintained Good Bonding to Die. Belt structure 12

13 For C1 & C2, Ag migrated toward DBC but still bonded well to die Little Ag remain on die For C1 & C2, plenty Ag on both die and DBC side after aging 13

14 14 Ag Sintered Joint vs High Pb Joint (X-Ray Imaging Analysis)

15 Cross-sectional View of Thermally Aged Ag-Sintered Joints 0h, BLT=32um 144h, BLT=82um 336h, BLT=76um 840h, BLT=25um 1608h, BLT=70um 3200h, BLT=50um

16 Porosity (%) Porosity of Ag-Sintered Joint C1 Lower at Center of Joints Porosity of Ag joint decrease toward the center of joints, attributable to venting routing factor, where outgassing moving from inside toward outside. Relation between Joint Location Under Die & Porosity hr 144hr 336hr 840hr 1608hr 3200hr 0 Distance to Die Center 16

17 Porosity Under Die (%) Porosity of Ag-Sintered Joint C1 Higher for Thinner Bondline Porosity of Ag joint decrease with bondline thickness Relation between Bondline Thickness & Porosity Upon sintering, Ag shrinks in all dimension. 2. Die and DBC does not shrink with sintering, thus cause tension on Ag when Ag shrank due to sintering 3. The thinner the bondline, the higher the tension on bulk Ag, thus the less shrinkage achievable y = x R² = Ag Die Bondline Thickness (microns) DCB 17

18 Porosity (%) Porosity Did Not Reduce with Thermal Aging Due to Stress Equilobrium Porosity of Ag joint remain stable with thermal aging, due to stress equilibrium between tension and shrinkage. Equilibrium established at end of sintering. Relation between 250C Aging Time and Porosity y = x R² = C Aging Time (hrs)

19 Reliability of Hig Pb Joint (92.5Pb5Sn2.5Ag) Mechanism of spalling is as follows: - Sn react with Ni, form Ni3Sn4 - Cu diffuse thru Ni, react with Sn forming NiCuSn IMC - NiCuSn IMC spall off formed a floating belt - Diffusion of the Ag on the Si surface into solder - Sn react with Ni, form NiSn IMC - Voids form due to the diffusion of Sn - Cracks form starting from the void and extend C/840hr 250C/3200hr

20 Ag Migration Severe at Thin Bondline Si/2.7Ni/94.1Ag Ag/1.8Cu/2.1Ni Ag/2Cu/2.4Ni (thin bondline) Ag/2.2Cu/4.8Ni/2Au Tendency for Ag to migrate to the DBC to form a dense layer is driven by the tendency for Ag to alloy with Ni, Cu, and Au. The supply of Ni and Cu is plenty on the DBC side, while that on the die side is very limited due to the thin layer of Ni on die. Thin bondline may aggravate impact of Ag migration.

21 Ag Migration Moderate at Thick Bondline Si99.1Ag Ag Ag (thick bondline) Ag/1.33Cu/0.84Ni Tendency for Ag to migrate to the DBC to form a dense layer is driven by the tendency for Ag to alloy with Ni, Cu, and Au. The supply of Ni and Cu is plenty on the DBC side, while that on the die side is very limited due to the thin layer of Ni on die. Thick bondline should be more forgiving on Ag migration. Au sintering paste is expected to migrate as well driven by alloy formation.

22 F1 after Thermal Shock 100 cycles (-45/240C) Au and Ni were detected by EDS in the middle of the sintered Ag area on ENIG DBC and the die backside, suggest the sintered joints partially peeled off from the substrate. root cause of thermal shock failure. 22

23 Thermal Shock Test Too Severe? A blank DBC coupon x was also subject to thermal shock. It was found that the Cu layer delaminated from the ceramic core in less than 100 cycles. This result suggested that this thermal shock test condition was too harsh for a reliability study of the die on the DBC. Smaller DBC reduce the challenge level. 23

24 Conclusion (1) A novel nano-ag sintering paste C has been developed for a pressureless sintering process under air. Both C1 and C2 exhibited a microstructure much more stable than the control 92.5Pb5Sn2.5Ag, which suffered both IMC spalling, voiding and cracking after thermal aging. Ag migrated toward the DBC to form a dense layer of AgCuNi(Au). Ag migration attributed to tendency of Ag to form alloy with Au, Ni, and Cu at the DBC side, and may be affected by the chemistry of nano-ag paste. 24

25 Conclusion (2) Porosity of sintered joints was lower toward center of die, due to the venting route factor. Porosity was also lower with a higher bondline thickness, due to a reduced tension in the joint. Thermal aging time has negligible effect on porosity, presumably due to balanced stress between tension and sintering shrinkage. The maximum service temperature of Ag sintered joint is about 470 C, versus 230 C for high-pb joints. A liquid to liquid thermal shock test from -45 C to 240 C was attempted, and was considered too harsh for the die/dbc system employed in this study. 25

26 26 Thank You