Embedded Mesh Technique for Increased Reliability between Substrate & Baseplate in IGBT Modules

Transcription

1 Power through Innovation Embedded Mesh Technique for Increased Reliability between Substrate & Baseplate in IGBT Modules G. Wilson (Indium Corp.) J. Booth (Dynex Semiconductor) 02/02/2017 IMAPS 12 th European Advanced Technology Workshop on Micropackaging and Thermal Management

2 Dynex Semiconductor

3 Bondline Control Inhomogeneous solder layer results in stress concentration at the thinner edge of the substrate Increased stress under cycling loading results in rapid delamination and cracking of the solder joint Joint thickness <200µm results in greater joint strain Source: K. Hayashi & G. Izuta Improvement of Fatigue Life of Solder Joints by Thickness Control of Solder with wire bump technique ECTC 2002

4 Traditional Bondline Technique Most common solution is to apply Al wirebonds to the baseplate to maintain 200µm bondline Coining is also used in copper baseplates however this study focuses on AlSiC type modules Source: K. Hayashi & G. Izuta Improvement of Fatigue Life of Solder Joints by Thickness Control of Solder with wire bump technique ECTC 2002

5 InForm Solution InFORMS are solder preforms with a braided metal mesh embedded in the solder preform. The mesh-to-solder ratios (height, area, and volume) are designed to maintain the required bondline and promote good solder wetting. - Solder joint collapse limited to mesh thickness - Mesh ensures uniform BLT across the entire area vs just 4 corners for wire bond

6 Novel Bondline Technique A novel technique to maintain bondline control is proposed whereby a metal mesh is embedded within the solder preform with no additional process steps

7 Test Set Up Test consisted of assembling six samples of each variant (no BLC, Al wirebond, metal mesh) Four of each variant was tested due to capacity limitations of the temperature cycling chamber 140x70mm AlSiC baseplates (37%Al) Cu-AlN-Cu active metal braze ceramic substrates 200µm SnSb5 Solder (Variant 1) 200µm SnSb5 Solder with 180µm Al wirebond (Variant 2) 225µm SnSb5 Solder with 200µm embedded metal mesh (Variant 3) - InForm

8 Co-Planarity Co-Planarity Variation Variation Embedded Metal Mesh Co-Planarity Variation = 52.5µm Max Deflection = 60µm Al Stitch Bond Co-Planarity Variation = 56.5µm Max Deflection = 70µm No BLC Co-Planarity Variation = 67.5µm Max Deflection = 90µm After assembly the samples underwent a laser surface profiling scan to determine the height variation across the substrate

9 Thermal Cycling Samples were thermal cycled (chamber to chamber) at -50 C to 150 C (based on actual customer requirements) Harsh test conditions promote creep in hold states and allows the samples to reach thermal equilibrium resulting in a greater ΔT when changing temperatures Samples were to be tested to failure Failure is defined as 50% delaminated area under the chip

10 SAM No BLC vs. InForm At 600 cycles, cracks observed in the samples without bondline control between solder and substrate Samples with embedded metal mesh shows no cracking/delamination No cracking/delamination seen for Al wire bonds

11 SAM Wirebond vs. InForm At 800 cycles cracks in the samples with Al wirebonds begin to show Samples with embedded metal mesh shows no cracking/delamination

12 SAM 2000 Cycles No Bondline Control Typical Fatigue InForm Some Mesh Fatigue

13 SAM 2000 Cycles Wirebond InForm

14 SEM 2000 cycles

15 Stress Modeling Samples with metal mesh exhibit longer lifetime after thermal cycling tests *Red lines represent movement of solder during tension state Metal Mesh Presence of the mesh not only maintains bondline uniformity but appears to also acts as a buffer to resist viscoplastic creep in the joint Presence of the mesh not only maintains bondline uniformity but appears to also acts as a buffer to resist viscoplastic creep in the joint

16 Summary Initial trials began to evaluate the metal mesh preform as a drop in replacement for the Al wirebond method Results indicate that the thermal cycling reliability of the metal mesh samples is superior to traditional bondline control method Samples with no bond line control show cracking at 600 cycles, samples with Al wire bonds show cracking at 800 cycles Theory suggests that the presence of the mesh acts to restrict the viscoplastic behaviour of the solder during thermal excursions (i.e. the joint becomes more creep resistant) Further evaluation (full simulation model and passive cycling tests) is on-going Samples with metal mesh provide true drop in replacement for bond line control

17 Summary Preform with metal mesh Increased thermal cycling reliability (-50/+150 C) No failures seen even at 2000 cycles No additional process steps True drop-in replacement Wetting and voiding similar to standard preform Preform with Al Wire bond For -50/+150C thermal cycling, cracks seen at 800 cycles Needs additional process steps Preform with no BLC For -50/+150C thermal cycling, cracks seen at 600 cycles

18 Indium Corporation

19 Aerospace Application IGBT Module for Aerospace Application Thermal cycling: -55/+150 C, 1500 passive cycles Failure defined as delamination > 50% of substrate/baseplate area 200μm-thick desired bondline thickness. Sn62/Pb36/2Ag alloy Substrate Al Nitride AMB /Ni Base plate AlSiC - Ni Plated Indium Corporation 0 TC No 2000 TC 3500 TC - Small delam following mesh pattern - Module still electrically functioning

20 Traction Application 0 TC 1500 TC - Soldered Sn/5Sb InForm - Fluxless Vacuum Process with Formic Acid - Low Voids <1% - Good Wetting - TC -40/+150C - 57mm x 49mm; 0.2mm overall thick 0.15mm thick embedded mesh

21 Traction Application Coplanarity Variation InForm vs Wirebond Trim InForm Coplanarity variation Wirebond Trim Coplanarity variation Less Coplanarity variation with InForm More Coplanarity variation with Wirebond Trim

22 Automotive - HEV Application Cu(Ni) Base Plate DBC/Ni Substrate InForm Sn/5Sb 225um InForm Sn/5Sb 250um 200um Mesh Soldered Sn/5Sb InForm Fluxless Vacuum Process with Formic Acid Low Voiding Good Wetting Indium Corporation

23 Automotive - HEV Application Tilt DBC/Ni Substrate to Cu/Ni base plate: Low Indium thickness Corporation variation

24 Summary

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