1 5 Oct 2015 Karthik Vijay kvijay@indium.com +44 7584 643 677

Dendritic Growth & Corrosion Under Low-Standoff Components in Automotive Electronics

A Flux Solution…

Indium Corporation

© Indium Corporation

What is Solder Paste?

No Clean Flux Technology

Activators - cleaning agent during reflow, Dissolves oxides off metal surfaces & promotes wetting

Rosin- Tacky and viscous, acts as an oxygen barrier

Gelling Agents – thickeners, Regulate viscosity and paste stability - rheological additives

Solvents - dissolve chemicals, activators, gelling agents, & resins to create a homogeneous paste

Reflow Profiles Involved

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Halogen-Free No-Clean Fluxes

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High SIR

Good Wetting Halogens may be

compensated with the use of large amount of hygroscopic organic acids Compromised SIR???

With more acids, rosin quantity that serves as oxygen barrier in the flux will have to be reduced to achieve adequate viscosity. Compromised Wetting???

Oxidation & Smaller Paste Deposits

SIR (Surface Insulation Resistance) J-STD-004B SIR, IPC-TM-650, 2.6.3.7

ECM – Electrochemical Migration

Automotive 1– More Stringent SIR Requirements

Automotive 2– More Stringent SIR Requirements

• Exposure – Steady State : 40 deg C / 90% RH; 504 hrs – Damp Cycling: 25 deg C / 95% RH; 50 deg C /

90% RH; 144 hrs

• Space between Traces : 0.3mm

• Bias : 50V DC

• Resistance Criteria : 100 MΩ Lower Limit (8 log 10)

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Low-Standoff Components with Poor Ventilation For Large low stand-off components such as Power MOSFETs, QFN, LGA, during SMT assembly, flux outgassing/drying is difficult due to poor venting

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Automotive Customer Requirements Reflowed Flux Residue Exposed to: - Dry heat test

- 120 deg C for 500 hrs; 10V - 150 deg C for 500 hrs; 10V - 120 deg C for 504 hrs; 30V

- Heated oil @ 85 deg C for 10 days

Automotive - High Heat Storage & SIR Electrical Reliability

Automotive - High Heat Storage Electrical Reliability for MOSFET Low-Standoff Component

PDSON Double-MOSFET device subject to Dry heat test - 120 deg C for 500

hrs; 10V - 150 deg C for 500

hrs; 10V - Dendritic growth

seen

Standard J-Std 004B SIR Test to test flux under Low-Standoff Component

Paste

1

2

3

4

5

- No SIR failures - But test does not simulate flux trapped beneath a

low-standoff component

J-Std 004B SIR Test with Glass Slide on Coupon to test flux under Low-Standoff Component

Paste

1

2

3

4

5

6

Barely Pass SIR

RCOOH encapsulated in solid rosin, incapable of engaging in corrosion reaction

Activator RCOOH in flux

RCOO- + H+ mobile in liquid environment

Solvent dried out

H2O H+ engage in corrosion reaction

Solvent retained

Flux Activator & Solvents during Reflow of Low-Standoff Components

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Flux Chemistry Solution

• Make wet flux residue non-corrosive and non-conductive by going very low on activators and flux activity.

• But this would result in – Poor wetting – Head-in-Pillow (HiP)

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How can the flux be designed so that - The wet flux residue is benign and does not cause

electrical failures - The flux activity is not compromised and achieve

good wetting and eliminate Head-in-Pillow

Flux Chemistry Design SIR Electrical Reliability Test

1 √ 2 √

3 √ 4 √

5 √ I √

X 1 2

3 4

5 I

X

X X

X √

SIR - Standard SIR + Glass Slide

Flux Chemistry Design Oxidation Barrier & Head-in-Pillow Test

Paste I

Paste 3

Flux Chemistry Design Oxidation Barrier & Head-in-Pillow Test

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1

2

3

4

5

I

x x

x

√ √

√

Flux Performance Summary

Paste SIR-Glass

SIR-Std HIP Wet (air) Wet (N2)

Paste 1 x √ √ √ √

Paste 2 x √ √ √ √

Paste 3 x √ x √ √

Paste 4 x √ x x √

Paste 5 x √ x √ √

Paste I √ √ √ √ √

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good acceptable unacceptable

Summary

• Good SIR performance with wet flux residue demands low activator concentration or low corrosivity in the flux residue.

• Good wetting, and good HIP need good flux oxidation barrier capability and good fluxing capacity.

• The dilemma between good soldering performance versus low corrosivity can be resolved with a breakthrough in flux technology

• This flux solution is expected to have better resistance for dewing tests as well

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Do it Right… Thank You