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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
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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
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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?
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Examples of Wearable Electronics
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Google Glass on
Runway Models
Wearables are Becoming Fashion
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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
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Example – Samsung Simband
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o Track GPS, continuous heart rate, sleep, distance walked,
calories burned, etc.
o Recent IPO (ticker FIT)
Fitbit
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Example - Lifebeam
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Other Sports Equipment
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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
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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
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Connectors for Wearables – TE Connectivity
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o Few Mating Cycles
o Highly reliable
Battery Connectors
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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
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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
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Connector Physics
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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
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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
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o Various types of layers must be broken through to make
reliable contact.
Contact Challenges
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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
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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
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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”
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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
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o Tin oxide is often described as “Ice on Mud”
Tin Oxide
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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
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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)
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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
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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
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o Mixed flowing gas exposure is a method to artificially
age contact materials.
o Normal force requirements are higher in contaminated
environments.
Normal Force
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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Connector Specification—Electrical Section
Electrical
Requirements
(IR, DWV,
Impedance, etc.)
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Connector Specification—Mechanical Section
Minimum durability
cycling requirements
Insertion Force,
Withdrawal Force,
Retention Force
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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
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o Vibration Testing
o Mixed Flowing Gas
Connector Qualification Testing
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
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o Thermal Cycling
o Dust Testing
Connector Qualification 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
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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
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o Connector Specific Quality Audit
Supplier Management
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Detailed Audit of Each Area
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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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