Dr. Hans-Peter Tranitz - RoHS Exemptions · Technology Project Manager at SiemensVDO since 2004 in Plant Regensburg for Press-fit Technology, Thermal Conductivity, Laser Marking,

Pressure-Induced Whisker Growth in Press-in Connections of PCB Through-Holes

Dr. Hans-Peter Tranitz

Project Manager–Advanced Technologies Continental Automotive GmbH

Dr. Hans-Peter Tranitz Project Manager–Advanced Technologies Continental Automotive GmbH

University career:

Diploma in Physics in 1998 at the Regensburg University, Germany

PhD in Physics end of 2001 at the Chemnitz Technical University in the field of Ultrashorttimespectroscopy on Semiconductor Nanostructures

Postdoctoral Fellow at the University of Cincinnati, Ohio in 2002 for 6 month, build-up a lab and train PhD students

Postdoctoral Fellow at the Regensburg University for 18 month in the field of spectroscopy of GaAs nanostructures, MBE growth and characterisation of highest mobility 2 dimensional electron gases

About 40 publications in peer reviewed scientific journals (1 Science, 9 Phys. Rev. Lett., 10 Phys. Rev. B, …)

Company career:

Technology Project Manager at SiemensVDO since 2004 in Plant Regensburg for Press-fit Technology, Thermal Conductivity, Laser Marking, Laser Welding of Plastics

Since 2007 involved with Tin Whiskers at Press-in Connections

Merge with Continental mid of 2007

Since 2009 Cluster Manager and Coordinator of two global teams and Senior Expert for Advanced Technologies Press-fit and Welding of Plastics at the Central Electronic Plant organization of Continental

Technology Project Manager of Whiskers at Press-fit Technology and Alternative Surfaces on Press-fit Zones

About the Author

Pressure-induced Whisker Growth in Press-in Connections of PCB Through-Holes

Dr. Hans-Peter Tranitz

Central Electronic Plants – Manufacturing Technology – Advanced Technologies

2 © Continental AG

Agenda

Introduction to Continental Automotive

Press-fit Technology: Why this connection is so reliable?

Whisker mitigation strategy within Continental

History of legislation and surface finishes on press-fit zones

Whiskers on Press-fit connections: Set-up of Investigation and Evaluation

Classification of whisker growth modes on different press-fit geometries

Alternative Surface finishes

Summary


3 © Continental AG

ContiTech

Air Spring Systems

Benecke-Kaliko Group

Conveyor Belt Group

Elastomer Coatings

Fluid Technology

Power Trans-mission Group

Vibration Control

Other Operations

Passenger and Light Truck Tires

OriginalEquipment

ReplacementBusinessEurope

ReplacementBusinessThe Americas

ReplacementBusinessAsia Pacific

Two-Wheel

Commercial Vehicle Tires

Truck Tires Europe

Truck TiresThe Americas

Truck Tires Asia Pacific

Industrial Tires

Chassis & Safety

ElectronicBrake Systems

HydraulicBrake Systems

Sensorics

Passive Safety& ADAS

Chassis Components

Powertrain

Engine Systems

Transmissions

Hybrid & Electric Vehicle

Sensors & Actuators

Fuel Supply

Interior

Instrumentation & Driver HMI

Infotainment & Connectivity

Body & Security

Commercial Vehicles & Aftermarket

Introduction to Continental AG: Divisions and Business Units

Continental Corporation

Automotive Group Rubber Group

Central Electronic Plants


4 © Continental AG

Automotive Group: Key Figures

in € millions 2009 2008

Chassis & Safety

Sales 4,373.6 5,134.0

EBIT -102.5 303.1

Adjusted EBIT* 353.4 512.0

Number of employees 27,148 26,680

Powertrain

Sales 3,399.2 4,040.0

EBIT -943.2 -1,046.2

Adjusted EBIT* -218.0 -188.9


Interior

Sales 4,362.7 5,856.7

EBIT -516.0 -462.6



Automotive Group

in € millions 2009 2008

Sales 12,042.4 14,900.0

EBIT -1,561.6 -1,205.8



* Before amortization on intangible assets from purchase price allocation,changes in the scope of consolidation, and special effects.


5 © Continental AG

Central Electronic Plants: Locations

Sibiu & Timisoara, RomaniaBudapest, Hungary

Brandys, Czech RepublicFrenstat, Czech Republic

Foix, France(Rambouillet, France)

Toulouse, FranceRubi, Spain

Cuautla, MexicoGuadalajara, MexicoNogales, Mexico

(Huntsville, USA)Seguin, USA

Changchun, ChinaTianjin, ChinaHuizhou, China

Cheongwon, S. KoreaIcheon, S. Korea

Manila, PhilippinesCalamba, Philipines

Penang, Malaysia

Bangalore, India

Bundoora, AustraliaGuarulhos, BrazilManaus, Brazil

Karben, GermanyIngolstadt, Germany

Nuremberg, GermanyRegensburg, Germany

Villingen, Germany

Kaluga, Russland


6 © Continental AG

CEP Manufacturing Technology: Responsibilities and Scope

Manufacturing Technology Roadmap

Technology and Material Evaluations and Generic Automotive Qualifications

Design For Manufacturing - Methods and Tools

Worldwide Technology Expert Network – TECH.net

Man

ufa

ctu

rin

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log

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Standardization of Processes and Equipment

Life Cycle Management (phase-in/phase-out) for Manufacturing Technologies

Locations: Nuremberg/Regensburg (Germany), Deer Park (USA), Manila (Philippines), Sibiu (Romania)


7 © Continental AG

Press-fit Technology: Introduction

ept T-com press®

Compliant (flexible) press-fit zone is inserted to a specially designed PCB through hole

With friendly approbation of the author


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8 © Continental AG

Mechanical Properties: Insertion and Retention forces (examples)

The remarkable reliability is based on the pronounced adhesion peak: evidence for material diffusion and cold welding

Reason for large mechanical and thermal stability of the connection/transition

Mandatory for common application with all different environmental conditions in the car

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9 © Continental AG

Press-fit Technology: Applications in Automotive Electronics

Almost all PCB (FR4) based Electronics contains press-fit technology

Chassis and Safety examples: ABS-electronics, Airbag Satellites and Control Units, Sensors, ElectronicParking Breaks and Electric Power Steering

Powertrain examples: Engine- and Transmission Control Units, Sensors

Interior examples: Body Controller, Fuse Boxes, Immobilizers, Door Control Units and many more

In summary billions of press-fit connections in all possible environments in the car and truck (0 dpb)

Press-fit into housing, connectors, component carriers, board interconnections, single standing pins, ….


10 © Continental AG

Continental Material Requirements – General Whisker Mitigation

Pb-free alloys selected by Continental Automotive show no risk for tin whisker growth.

SAC305 and SAC387 solders contain silver and copper alloying additions which reduce propensity for tin whisker nucleation and growth.

Solder:

Primary tin whisker growth mitigation methods are considered

Storage life limit is 6 months at ambient conditions, with whisker length < 50 microns.

Immersion tin thickness is controlled between 1.0 to 1.2 microns.

Printed Circuit Boards:

Qualification and testing for whisker prevention according to IEC 60068-2-82

Continental Automotive takes all necessary precaution to avoid returns caused by whisker growth.

The behavior of whiskers induced by external stress is different and content of the following presentation.

Summary:

no use of lead frames without barrier layer (like e.g. Nickel), alloy 42 is not recommended

no use of galvanic bright tin as finish layer

annealing of components to relax the stress

Components:

Supplier is strictly required to supply materials without

whiskers

These requirements are checked by basic

qualifications and reviewed by

Continental Supplier Quality Management



History and Legislation and Surface Finishes (on Press-fit Zones)

Surface finish

1st edition of ELV Annex II: lead-free

exemption for electronics

next revision of the exemption

Press-fit whisker lead to band / field returns in some special cases at different automotive electronic manufacturers

PCB: HAL (SnPb) imm. Sn imm. Sn alternatives?

Pin: SnPb SnPb pure Sn Sn, SnPb in some cases and alternatives?

3rd edition of Annex II: lead-free exemption for press-fit +

official interpretation by the Automotive Associations

2nd edition Annex II: no exemptions in

electronics after 2010

Increased effort for lead-free solutions

1995 2000 2005 2010 2015



Occurrence of short circuits

Whiskers potentially create short circuits or parasitary current paths. Fast growth of whiskers can be observed in press-fit connections due to high mechanical stress at pure tin surfaces.

Some 0-km and field returns identified at a body controller 2007

Whiskers create direct parasitary signal path at sensor exits (very low current flow)

Whisker length > 2 mm within 2-6 weeks after insertion in this case

Direct bridging of low signal electrical contacts



Comparative Study: Set-up of Investigation and Evaluation

Using series production PCBs of one product

PCB pretreatment: series production reflow process (1x lead-containing reflow profile)

Numerous types of series production press-fit pins with the same thickness of pure tin surface

300 pins of each type inspected: representative sample size

1000 h storage at 23°C and 40% rel. humidity (cabinet)

Whisker inspection with standardized method: see next slide

Categorization

representative test set-up

Category Length [µm] Description Risk Weight1 0 no whisker observed no risk 02 < 50 customer accepted whisker length no risk 03 50 - 100 low risk 14 101 - 200 electrical clearance may be affected medium risk 35 > 200 direct and indirect bridging high risk 9



Set-up of Investigation and Evaluation

Metallurgical microscope

Incident light geometry (bottom-up)

Adjustable light intensity and aperture of pin hole

Camera system and software

adjustabel x-y table

Tips of press-fit pins show in the direction of the objective

To be able to inspect the complete press-fit via, a magnification of at least 50x is required

Scroll focus through complete via

To the right: Micrograph of a press-fit pin in a press-fit via. A whisker can be observed in the bottom left corner.



Whisker Growth on different press-fit geometries (10 variants)

Results: • One cracking zone shows better performance (4) than the other pins• All press-fit zones show whiskers > 200 µm



Whisker Behavior of Cracking Zones (example)

Some fillet or similar mechanism is cracking fully or partially during insertion

cracking during insertion

pin removed

entrance area: large inelastic deformation

pin cracking in entrance area

surface stress is a well known mechanism

Cracking zones: Long whiskers grow in entrance area of the PCB



SEM (press-fit pin removed)Area of greater plastic deformation in the PCB

Expansion of the copper material in the area where the pin enters

Surface tension is a known mechanism for whisker formation

Observed whiskers originate in areas of great deformation where the press-fit pin enters.

The point of origin is the copper bushing of PCB

The tin layer of the pin peels off "upward or laterally" against the press-fit direction and adheres to the PCB in the upper area of the through-hole



Details in the Area of Deformation

Area of "stripped" cold welding

Tin/copper material interlocking due to the contact pressure of the press-fit pin

Top right: Expansion steps in copper, arise from pressing-in in the area of greater material stress. Here, the initially closed layer of the inter-metallic phase is probably also damaged.

Bottom right: In the area of deformation, seed crystals grow using the excess tin, so-called nodules and whiskers, to relieve the surface tension.



Whisker Behaviour of Eye of the Needle Zones

Eye of the needle pins pin removed

rim of via: tin abrasion of the pin

tin of pin almost lost for whisker growth

inset shows tin abrasion in detail of vertical cross-section

EON: Tin abrasion of the pin at edge of via (explanation for less whisker growth), tin of the PCB moved to the side and to the bottom of the PCB.

vertical cross-section horizontal cross-section

Tin agglomerations coming predominantly from the PCB occurs at the side pockets and at the bottom



Whisker Behavior of Spring Shapes

Additional bars (springs) connect the two legs of the press-fit zone

pin removed

deformation line along the insertion direction

tin agglomeration over the complete length of the bushing

whiskers are observed in SEM

Spring shapes: whiskers along the deformation lines within the PTH, almost no tin abrasion at the rim

horizontal cross-section

detail



Types of Press-fit Geometries: Summary and EvaluationCracking zones: some fillet or similar mechanism is cracking fully or partially during insertion

characteristics: high insertion and retention forces, relatively high deformations

large whiskers in entrance area with a tendency to grow out of the hole

These pin types have shown direct bridging in the past

SnPb finish was applied as solution for the most critical cases, worst pin removed from portfolio

Eye of the needle (EON): a needle-eye is stamped to the sheet metal

characteristics: relatively low insertion and retention forces, usually lower deformations

tin abrasion on the rim of plated through hole, lower whisker-rate, whiskers inside the through hole

SnPb avoids whiskers, Sn surface has got a very low potential for bridging between pins

Spring shape: additional springs between the two legs of the pin

characteristics: medium to high insertion and retention forces, medium deformations

whisker growth along the deformation lines inside the through holes

SnPb avoids whiskers, Sn surface has got a low potential for bridging between pins

Additional remarks

There may be observed a mixture of some of these mechanisms e.g. at cracking zones

In case of incorrect galvanic treatment (pollution, bright surface, …) also EON pins may show many and long whiskers

In case pure tin finish is used, additional design rules need to be respected



Whiskers on Cross-SectionsBlack flaws are observed on cross-sections in the intersection of pin and PCB’s copper bushing

SEM pictures show that these are whiskers that grow within days perpendicular to section plane

This demonstrates the three necessary pre-conditions for tin whisker growth:

1) bare tin exposed 2) tension at the surface 3) free space (at least in the range of whisker diameter)



Development of Alternative Surface Finishes on Pin and PCBPure Tin Containing Alternatives

Analyses of alternative finishes:

Same PCB and pre-treatment (see slide 16)

Same PCB-supplier for alternative PCB finishes

300 Pins for each variant

Using Pin Type Nr.1 (most whiskers)

First four variants of Pin finish using imm. Sn PCB

Last two variants of PCB finish using Pin 1 as a reference (Sn 1 µm galv.1)

Results

No significant dependence on tin thickness in the specified range (0,3-1,5 µm)

PCB finish chem. Ag shows less whiskers than PCB finish OSP pure Ag surface seems to have a mitigation effect

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Pure Sn finish (over Ni)



Sn finishes with temperature storage of the pins

Used press-fit zone: Pin type 9 (variants 1-3), Pin type 3 (4)

Imm. Tin PCB

Results

temperature pretreatment improves whisker frequency and risk for bridging

50 µm requirement of whisker length is exceeded within this investigation

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Sn finish with temperature



SnXy finishes

Used press-fit zone: Pin Type 1 (variants 1- 3 + 7); PinType 9 ( variants 4 – 6)

Imm. TIn PCB

Results

SnPb shows no whisker

All SnAg variants show whiskers, sometimes more than pure tin finish

SnBi shows whiskers in all categories, tendency: less whiskers with high Bi content

For SnAg and SnBi: lower press-in/push-out performance than SnPb

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SnXy - finish (over Ni)



Tin free alternatives

Results

Tin free finishes show less whisker

OctaDecaThiol as protection for oxidation and as lubricant

Ag-flash shows 2nd best whisker result

Worse press-in/press-out performance

Not usable for all pin types and all automotive applications

Indium substitutes Sn/SnPb: widely reduced whiskers reduced length (max 125µm)

Very good press-in/press-out performance: lower insertion forces and pronounced adhesion peak

Used press-fit zone: Pin Type 1 (variants 1-4), Pintype 9 (5)

Imm. Tin PCB

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Sn ‐ free



Summary

The press-in connection containing free tin on pin and/or PCB results in whiskers larger than 50 µm on a large variety of press-fit zones.

Some cracking zones with pure tin surface are candidates for whiskers growing out of the through hole and resulting in a risk for direct bridging.

Some plating alternatives reduce the risk of direct bridging but only SnPb seems to fulfill the requirement of whiskers < 50 µm.

For automotive industry, the cold welding is necessary in harsh environmental reliability conditions.

From today‘s technical point of view, SnPb is the only surface that efficiently mitigates whiskers and ensures a good cold welding.

A very good long-term alternative that is whisker- and Pb-free, works for a large variety of press-fit geometries, and fulfills all requirements for automotive electronics is required.

Promising candidates Ag and In are under investigation



Acknowledgement

Many thanks to my former diploma student

Sebastian Dunker

Many thanks

to our main development partners:

Schempp & Decker

FCI Connectors

EPT

Interplex

Trainalytics

and for many useful discussions with

Atotech

Tyco Electronics

Fraunhofer IWM (Institute for Material Mechanics)

Thank you for your attention!

Documents

Dr. Hans-Peter Tranitz - RoHS Exemptions · Technology Project Manager at SiemensVDO since 2004 in Plant Regensburg for Press-fit Technology, Thermal Conductivity, Laser Marking,