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 12th European Advanced Technology

Workshop on Micropackaging and Thermal

Management

Dynex Semiconductor

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

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

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

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

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

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

Co-Planarity Variation

After assembly the samples

underwent a laser surface

profiling scan to determine

the height variation across

the substrate

Co-Planarity Variation

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

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

SAM – Wirebond vs. InForm

Samples with embedded

metal mesh shows no

cracking/delamination

At 800 cycles cracks in the

samples with Al wirebonds

begin to show

SAM – 2000 Cycles

No

Bondline

Control

Typical

Fatigue

InForm

Some

Mesh

Fatigue

SAM – 2000 Cycles

Wirebond

InForm

SEM – 2000 cycles

Presence of the mesh not

only maintains bondline

uniformity but appears to

also acts as a buffer to resist

viscoplastic creep in the joint

*Red lines represent movement of solder during

‘tension’ state

Stress Modeling

Samples with metal mesh

exhibit longer lifetime after

thermal cycling tests

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

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

Summary

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

Indium Corporation

© Indium Corporation

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

Aerospace Application

0 TC

No Delam @ 2000 TC

3500 TC - Small delam following mesh

pattern - Module still electrically

functioning

Traction Application

- 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

0 TC 1500 TC

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

Automotive - HEV Application

© Indium Corporation

Soldered Sn/5Sb InForm® Fluxless Vacuum Process with Formic Acid Low Voiding Good Wetting

Cu(Ni) Base Plate DBC/Ni Substrate InForm® Sn/5Sb 225um InForm® Sn/5Sb 250um 200um Mesh

Automotive - HEV Application Tilt

© Indium Corporation

DBC/Ni Substrate to Cu/Ni base plate: Low thickness variation

Summary

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