Avoiding Connector Failures in Challenging Environments ......Avoiding Connector Failures in...

©2015 DfR Solutions 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Avoiding Connector Failures in Challenging Environments - Wearables Randy Schueller, Ph.D.

June 26, 2015

©2015 DfR Solutions 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com 1

o As electronics continue to shrink and their performance capabilities grow, these electronics are becoming more and more integrated into our daily lives. The next step is the internet of everything and wearable electronics.

o Communication between devices and providing power through the use of connectors is critical; connector sales are a $50 billion/year industry.

o As critical as they are, separable connectors are often times the first item to fail in electronics. This problem is only expected to get worse as electronics are used in increasingly challenging environments.

o This Webinar will discuss contact physics, contact plating options, normal force requirements and general tradeoffs that frequently occur when designing or selecting a connector for an application. Physics of failure along with a number of connector failure examples will be presented as well.

Abstract


o Wearables (Connector requirements)

o Contact Physics

o Plating Materials

o Contact Plating Materials

o Plating options (fretting failure & importance of gold thickness)

o Normal force

o Base Materials

o Lubricant

o Connector Design Approach

o Connector Selection Best Practices

o Connector Failure Mechanisms

o Case Study

o Connector Specification

o Connector Qualification Testing

Connector Reliability Outline


o Almost anything outside the home/office

o Electronics exposed to:

o Moisture

o Pollution/contaminates

o Dust and debris

o Sweat or other body fluids

o High temperature

o Low temperature

o Mechanical shock and/or vibration

o Examples include wearables, automotive, smart meters, etc.

What is Challenging Environment?


Examples of Wearable Electronics


Google Glass on

Runway Models

Wearables are Becoming Fashion


o They are invading the workplace and home

o Tesco is using armbands that automatically track goods that workers are transporting to shelves

o Used for health and wellness programs

o There is an EEG headband that helps you understand your cognitive patterns, thereby giving you insights on when you are most creative and productive

o Bracelets to track location of employees, breathing, heartrate, etc.

o Study by Rackspace showed employees with wearables at work became 8.5% more productive and 3.5% more satisfied with their jobs

Wearables Today


Example – Samsung Simband


o Track GPS, continuous heart rate, sleep, distance walked,

calories burned, etc.

o Recent IPO (ticker FIT)

Fitbit


Example - Lifebeam


Other Sports Equipment


o Communication between devices and providing power

through the use of connectors is critical; connector sales are

a $50 billion/year industry

o As critical as they are, separable connectors are often

times the first item to fail in electronics

o This problem is only expected to get worse as electronics

are used in increasingly challenging environments

o Like all other aspects of electronic hardware, reliable

connectors are driven by design, materials, and use

environment

Connector Requirements


o Desktop Computer – 30 individual connectors

o Notebook Computer – 60 individual connectors

o Server/Storage – 80+ connectors

o There are thousands of opportunities for failure and it only

takes one to shut down the system.

o There are dozens of connector suppliers and cost cutting is

strongly pursued (this means quality often takes a back

seat if you are not diligent).

o Connector failure is often listed in the top 5 reasons for

system failure

Connector Importance


Connectors for Wearables – TE Connectivity


o Few Mating Cycles

o Highly reliable

Battery Connectors


o Common problems:

o Receptacle can get filled with lint

o Contacts can become dirty

o Contacts on cable can become worn or corroded

Power Connector – Example, Lightning Port on iPhone


o Plugged in cables have continuous voltage on some pins

o When placed in high moisture environment (by your sink,

or in your car) corrosion can take place

o Maximize Voltage/Distance

Lightning Cable

©2015 DfR Solutions 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Connector Physics


o Actual electrical connection is made through small asperities.

o Asperities are locations where metal to metal contact is made

across the contact interface.

o There may be as few as 3 such points.

o Only 1% of the apparent contact area is actually making contact

(cold welding can take place).

o These asperity locations typically provide a gas-tight seal

Connector Physics


o Actual current flow occurs through the small

asperities where metal-to-metal contact

takes place.

o Should these contact regions reduce in size

due to oxidation, contamination, reduced

normal force, etc. then the resistance

increases.

o Increasing resistance can cause local heating

that can increase oxidation and further

increase the resistance (thermal runaway)

o Eventually the resistance can increase to the

point of contact failure.

Constriction Resistance


o Various types of layers must be broken through to make

reliable contact.

Contact Challenges


o Increasing normal force provides more contact points and

decreased resistance.

o R vs Load curve will differ with each type of contact

material

Constriction Resistance VS Normal Force


o Absence of an oxide layer allows low voltage connection with low normal force (20-30 grams).

o Soft gold will wear off easily, hard gold offers more protection.

o Resistant to corrosive environment (when sufficiently thick)

o Avoid DIG (direct immersion gold on copper)!

o Ni base is required (to prevent Cu diffusion into Au).

o Ni should be free of pores (50u” min thickness)

o Gold should be sufficiently thick for the application

o 50u” for high reliability, high cycle situations.

o 30u” for high reliability, moderate cycles

o 10u” for moderate reliability, moderate cycles (or high reliability, few cycles)

o Flash gold (3-5u”) for use with single mate situations or those with low reliability requirements.

Gold Plating


o Originally developed to improve lubication for Pd plated

contacts in the 1970s.

o Xerox demonstrated that flash gold was acceptable for

limited life products in very well controlled office

environments (< 5 cycles).

o Class II corrosion environment will result in pore corrosion

in less that 24 hours.

Flash Gold

4 - 10 u”

12 - 20 u”

30 – 40u”


o Pd – Good. However polymers can form on the

surface over time causing a film (it is usually plated

with thin gold to prevent this). Good wear resistance

when thin gold used as lubrication.

o Tin – Not as Good (see the Tin Commandments). Tin

whiskers also a concern.

Other Contact Plating Finishes


o Tin oxide is often described as “Ice on Mud”

Tin Oxide


o Thin tin oxide fractures,

allowing metal-metal contact

o Micromotion breaks the

contact and forms new

contact points

o Old contact points reoxidize

o This repeats with each

movement of the contact

o Tin-oxide debris builds up,

eventually making metal-to-

metal contact impossible

o High resistance and

intermittent contact are the

result

Fretting Failure Mechanism

Dark smudges


1. Contacts should be mechanically stable in the mated condition.

2. Tin-plated contacts need at least 100 grams normal contact force.

3. Tin-plated contacts need lubrication.

4. Tin plating is not recommended for continuous service at temperatures above 100 degrees C.

5. The electrical performance of contacts is not strongly affected by the choice of bright tight tin, matte tin, or tin-lead alloy platings.

6. Electroplated-tin coatings should be at least 100 microinches thick.

7. Mating tin-plated contacts to gold-plated contacts is not recommended.

8. Sliding or wiping action during contact engagement is recommended with tin-plated contacts.

9. Tin-plated contacts must not be used to make or break current.

10. Tin-plated contacts can be used under dry-circuit or low-level conditions.

Tin Commandments (some are more critical than others)


o Nickel is a hard material with a tough oxide. A

normal force of >200 g is recommended.

o Stainless steel has a hard chrome oxide on the

surface. Difficult to break through. Normal force >

300 g recommended (still expect high contact

resistance).

Nickel and Steel


o The contact force requirements are dependent on

o The contact finish material

o Gold; 20-30 grams

o Pd; 30-50 grams

o Tin; 100 grams

o Nickel; 200 grams

o Steel 300 grams

o The level of contamination (films, particulate, or corrosion)

Normal Force Requirements


o Mixed flowing gas exposure is a method to artificially

age contact materials.

o Normal force requirements are higher in contaminated

environments.

Normal Force


o All metal surfaces have a thin film of something on them.

o Gold has a thin layer of moisture and OH groups.

o Tin and Nickel have an oxide layer

o All materials may have particulate contamination

o Most connectors incorporate some amount of contact

wiping to help create a fresh location for asperities to

form.

o Ideal is 2 mils of wipe with some small reversal – so the

contact doesn’t rest on a pile of debris

Contact Wiping


o BeCu (beryllium copper)

o Best alloy for high spring force and resistance to stress relaxation.

o Most expensive (more rarely used these days).

o Phosphor Bronze (Cu, 3-10%Sn, 0-1% P)

o Good for most spring contact applications

o Moderate cost

o Temper of material makes a large difference

o Brass (CuZn)

o Poor choice for springs since low normal force and high stress

relaxation

o Lowest cost option

Base Contact Material


o All metals will creep (deform) under a constant load over

time

o Atoms diffuse from low to high stress regions to relieve the

stress (so creep occurs much faster at elevated temp)

o When a contact is deflected, the creep results in stress

relaxation (reduction of the normal force over time)

o Be-Cu; best

o Steel; best

o Phosphor Bronze; good

o Brass; poor

Stress Relaxation


o Insertion force can be a problem if it is too high (not

typically a problem with portable devices)

o Extraction force is a problem if too high or too low

o Too High; cables can be damaged when pulling them out

o Too Low; contacts can separate too easily

o Latches can be employed when there is concern for

unintended unmating

Insertion/ Extraction Force

Example: L-Com has USB

cables with a latch


o Loss of normal force

o Contacts jammed or bent (taken beyond yield strength)

o Stress relaxation of contacts over time at elevated temperature.

o Becomes a problem with lower cost contact materials.

o Contamination

o Particulates can become embedded under the contact when vibration occurs.

o Corrosion/oxidation occurs that prevents metal-metal contact (fretting is one example)

o Excessive Wear

o Noble metal is worn away exposing oxidizing metal

Common Causes of Increased Resistance


o I was once asked to help determine the

best contact lube to use on a battery

socket due to a high failure rate in the

field.

o Failure analysis was performed and

contact fretting was said to be the

problem.

Case Study Example


o Contact fretting made sense because the battery socket

plating had recently been changed from gold to tin (for

cost savings).

o I wasn’t totally satisfied because I saw a small uptick in

failure data prior to the contact material change.

o I then learned from the battery supplier that some years

back procurement changed the battery order from

CR2032L to CR20032D (for a cost savings).

o This meant the battery cover changed from nickel to

stainless steel (our engineering documents only stated

CR2032).

Failure Analysis


Battery Comparison; Ni vs SS (Contact Resistance)

Contact normal force in a socket

ranges from 100 – 200 grams.

Data clearly shows that the SS

version is more sensitive to

normal force – which implies

that the oxide barrier is thicker

and tougher to break through.

Stainless

Steel version

Nickel plated

version


o A gold plated contact made great contact with the nickel

coated battery.

o A gold contact was barely acceptable with a stainless

steel battery.

o A tin plated contact on SS was a disaster.

o We were able to convince the organization to go back to

gold plated contact with a nickel battery.

o Failures went away for next generations of systems.

Battery Socket Conclusion


o Can decrease sliding friction by 80%.

o Can protect surface from oxidation/corrosion.

o Can reduce wear of gold and fretting with tin.

However,

o Process can be messy and uncontrolled

o Can react with connector plastic body

o Not recommended for most applications

Lubricants


Tin Whiskers Tin whiskers are concern when tin plating is used on the connector pins or the shell.

Failures can occur from:

o Direct Contact

o Causes an electrical short (arcing)

o Requires growth of sufficient length and in the correct orientation

o Electromagnetic (EM) Radiation

o Emits or receives EM signal and noise at higher frequencies

o Deterioration of signal for frequencies above 6 GHz independent of whisker length

o Debris

o Whisker breaks off and shorts two leads (primarily during handling)

42

Courtesy of P. Bush, SUNY Buffalo

Observation of tin whisker debris as

reported to NASA from Sanmina-SC

DfR Solutions

42


Tin Whisker Drivers

Sn whiskers occur on tin primarily due to compressive stress which can be caused by:

o Stress during plating of Sn

o Intermetallic formation with Cu

o Mechanical or CTE mismatch stress


o D-Sub Connectors with bright

tin shells have been known to

grow whiskers that can short

our pins (if connector is

unmated).

Bright Tin Whisker Examples

Whiskers also found to

grow in screw holes.

Ref: L. Flasche & T. Munsun,

Foresite, Inc. 9/09.

Ref: Emerson


o Flex Circuits with Connector Mating

o Pressure from contacts with the soft polymer substrate

creates force over a large area of tin.

o Don’t use Sn plating in mated flex with a spacing less

than 200 micrometers.

o Use gold plating under such conditions.

Contact Pressure on Flex Cables


o Forbid bright tin (connector shells and mechanical

parts).

o Ensure nickel underplate is used, OR tin is heat

annealed after plating.

o Use gold plating on pressure contacts to flex circuits.

o Enforce whisker testing on fine pitch components (get

data from suppliers).

o Forbid tin on iron or brass (mechanical parts) since

these whiskers can grow very long.

Main Actions to Reduce Tin Whisker Risk


o Use Test and Design (Toyota Approach) as opposed to

Design and Test

o Understand what highly reliable and cost effective

connectors are available – design these in if possible.

o Pin in socket

o Card edge connector

o Board to board

o Flex Connector

Connector Design Approach


o Ensure appropriate insertion/withdrawal force for the application.

o Use locking mechanism when appropriate

o Use a key for proper mating

o Have lead-in for blind mating

o Use appropriate materials for contact finish and base metal

o Address tin whiskers when necessary

o Ensure some amount of wipe

o Ensure mechanical robustness of the connector to board attach (SMT can be risky).

Some Connector Best Practices


o Check for fretting

o Check for excessively worn gold

o Signs of Corrosion

o Check for particles/fibers that are interrupting the

connection.

o Make sure all contacts are free moving and not stuck.

o Make sure normal force is sufficient

o Check for cracking or damage to connector body

Connector Failure Analysis Approach

Ro

ot C

au

se

Root Cause :

1.3. Observing crack on cross section of the defective sample :

The crack occurred in bottom slot, not tail side.

Resulted in reduced contact normal force

Case Study Example – Importance of connector body

This simple crack resulted in a $40 mm issue for DIMM connectors.

DIMM Connector


Contact Insertion Tool was Cracking the Body

51

Ro

ot C

au

se

Root Cause :

e) Confirming the insert fixture：

The thickness of insert fixture﹕ Spec.: 1.20+/-0.005mm check: 1.20mm; radius R of inserting

fixture﹕ Spec.: 0.5+/-0.02 check: 0.96mm

The dimension of insert fixture is out of Spec. , refer to below about angle R﹕

Insert

fixture

Contact

Housing

There is no interference

between insert fixture

and housing

There is interference

between insert fixture

and housing

Insert

fixture

Contact

Housing

There is interference between insert fixture and housing in abnormal state, then the housing will bring crack.


o It is being added as a flame retardant in some power

cords and in some connector bodies

o It can react with water to create phosphoric acid

o This can lead to attack of metals and dendritic growth

Red Phosphorous in Connector Body


o 1996: Polymer supplier introduces red phosphorus-based molding compound

o Labeled as “green” molding compound or “bromide-free” molding compound (phosphorus content not always clearly stated)

o Marketed as either environmentally friendly or improved resistance to Kirkendall voiding

o 1998: Large-scale ramp up

o Use by numerous semiconductor device manufacturers and contract packagers

o Late 1999 / early 2000

o First field failures reported

o Late 2001

o First public acknowledgement of potential issues

o 2002

o Red phosphorus-based molding compounds pulled from the market

o 2006

o Production of power cords with red phosphorus

Timeline of Red Phosphorus


Connector Specification—Electrical Section

Electrical

Requirements

(IR, DWV,

Impedance, etc.)


Connector Specification—Mechanical Section

Minimum durability

cycling requirements

Insertion Force,

Withdrawal Force,

Retention Force


o High Temp Life

o Cyclic Temp and Humidity

Connector Qualification Testing

5.4.3 EIA-364-17

Temperature life

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability

(preconditioning)

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.4

Reseating

5.4.5 EIA-364-32

Thermal shock

5.4.6 EIA-364-31

Cyclic temp &

humidity

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability

(preconditioning)

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.4

Reseating


o Vibration Testing

o Mixed Flowing Gas


5.4.7 EIA-364-17

Temperature life preconditioning

5.4.8 EIA-364-28

Vibration

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability

(preconditioning)

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability (preconditioning)

5.4.7 EIA-364-17

Temperature life

(preconditioning)

5.4.9 EIA-364-65

Mixed flowing gas

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.10

Thermal

Disturbance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.4

Reseating

5.4.1 EIA-364-23

Low-level contact

resistance


o Thermal Cycling

o Dust Testing


5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability

(preconditioning)

5.4.7 EIA-364-17

Temperature life

(preconditioning)

5.4.11

Thermal Cycling

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.4

Reseating

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.2 EIA-364-09

Durability

(preconditioning)

5.4.12 EIA-364-91

Dust

5.4.10 Thermal disturbance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.1 EIA-364-23

Low-level contact

resistance

5.4.4

Reseating


SUMMA

RY

Pass /

Fail Comments Low-level contact

resistance PASS Durability PASS Low-level contact

resistance PASS Temperature life PASS Reseating PASS Low-level contact

resistance PASS

DETAILED DATA

Data

Point

Sample

#

Contac

t # LLCR

Durabilty

P/F Durability Comments LLCR Temperature Life Comments Reseating Comments LLCR General Comments

1 NO.1 NO.1 18.30 PASS 18.50 18.70

2 NO.1 NO.2 18.50 PASS 18.80 18.90

3 NO.1 NO.3 18.60 PASS 18.70 19.50

4 NO.1 NO.4 17.50 PASS 19.90 19.80

5 NO.1 NO.5 17.60 PASS 18.60 18.50

6 NO.1 NO.6 17.90 PASS 18.20 18.60

7 NO.1 NO.7 18.60 PASS 18.80 19.50

8 NO.1 NO.8 18.20 PASS 18.90 19.70

9 NO.1 NO.9 18.60 PASS 18.50 18.80

10 NO.1 NO.10 18.40 PASS 18.60 18.30

11 NO.2 NO.1 18.50 PASS 18.80 19.80

12 NO.2 NO.2 18.30 PASS 18.20 19.90

13 NO.2 NO.3 18.50 PASS 18.60 19.40

14 NO.2 NO.4 17.60 PASS 18.50 18.20

15 NO.2 NO.5 17.20 PASS 18.70 18.60

16 NO.2 NO.6 17.30 PASS 18.60 19.20

Qualification Report Example


o Connector Specific Quality Audit

Supplier Management


Detailed Audit of Each Area


o Connectors are one of the most critical components in the system

o Wearable devices may have demanding connector requirements

o Many insertion cycles

o Outdoor environments

o Sweat and particulate contamination

o When possible select connectors with long history of success

o Use of reputable suppliers pays off in the long run

o Be aware of the primary failure mechanisms and spec plating materials to avoid them

o Ensure adequate reliability testing has been performed

Summary

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