common mistakes by electronic design teams - dfr solutions slides for... · 2017-10-08 · common...

© 2004 – 2010 9000 Virginia Manor Rd Ste. 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Common Mistakes by Electronic Design Teams

Part 2

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

Who is DfR Solutions?

The Industry Leader in

Quality-Reliability-

Durability

of Electronics

50 Fastest Growing Companies in the Electronics Industry

- Inc Magazine

2012 Global Technology Award Winner

Best Design Verification Tool

- Printed Circuit Design


12 Years of Providing Solutions to the Electronics Marketplace

o DfR / DfM / DfT / DfS….. DfX

o Finite Element / Fluid Dynamics

o Physics of Failure Modeling

o FMEA / FTA

o 3rd Party Design Review

o Failure Analysis

o Root Cause Investigations

o Forensic Engineering

o Circuit Analysis

o Connector/Wiring Selection

o Analog/Power Design

o Material Characterization

o PCB / PCBA Onsite Audits

o Pottings and Coatings

o Software Risk Mitigation

End-to-End Quality and

Reliability Expertise


MECHANICAL

PART SELECTION


CABLES / CONNECTORS

o One of the most common, if not the most

common, failure location in Embedded Systems

o Be Aware: Recent rash of COTS cable and

connector failures due to red phosphorus flame

retardants

o Breaks down into phosphoric acid in hot humid

environments and then corrodes surrounding contacts

and PCB traces

o Failures occur only after 2 to 4 years in the field


DFR RULES FOR CONNECTORS

o Power and Ground should be separated

o Each mated pair should be shrouded

o Avoid blind insertion (increases risk of damage/mismating)

o Recommend alignment pins (limits off-axis mating)

o Connector should be keyed (prevents reversal of I/O pins)

o Use positive retention

o Connector wiring should have strain relief

o Specify plating material and thickness


MATERIAL AND PLATING GUIDELINES [GOLD]

o Material Specificationso Be-Cu or P-bronze base

o Ductile nickel underplate (50-150 min)

o Soft gold (Au) plating

o >10 min/Flash (single insertion); 15 min (1-200 insertions); 30 min (>200 insertions); 50 min (hi-rel)

o Porosity spec

o Connections and Environmentso Hi-speed digital or critical connections

o Low voltage (<5V) and/or low current (<10mA)

o Corrosive environment (gases such as H2S, SO2, Cl2)

o Risk of micromotion (>5-10 µm)

o Contact Specificationso 50-100 grams contact force

o Minimum of 2 contacts; 4 preferred

o Adequate contact wipe – 0.010” min.

o No mating with tin plating


MATERIAL AND PLATING GUIDELINES [TIN]

o Adequate contact wipe –

0.010” min.

o Do not use to make or break

current

o No mating with gold!

o Mechanical stability criticalo Lubrication is a potential

mitigation

o Minimum 100 gram normal force

o Minimum 100 micro inch thickness

o Not recommended for operating conditions above 100C


HOUSING CHALLENGES

o Because electronics are modular, but every housing tends to be unique, housing can create significant challenges for Embedded Systems

o Plastic Housing Concerns

o Boss Design

o Notch Sensitivity

o Environment

o Metal Housing Concerns

o Gaskets

o Thermal Pads

o Overly constrained PCBs


BOSS DESIGN

o Boss – A mounting feature that will receive a screwo Fundamental to attachment

o Biggest Mistake #1?o Inner diameter too large for self-tapping

screws

o Slippery plastics require smaller pilot holes

o Will eventually back out, but will take time

o In addition, account for draft when determining the boss outer diameter


BOSS DESIGN (OTHER COMMON ISSUES)

o Insufficient thread

engagement when using

thread-forming screws

o Brass Inserts

o Mineral content

o Reinforcing rib height issue

o Insert Jacking

BAD

GOOD

Insert is recessed and

will pull-out when

tightened


NOTCH SENSITIVITY

o A measure of material’s sensitivity to notches or geometric discontinuities

o Some plastics are more sensitive than otherso E.g., Acetal is a tough plastic yet very notch

sensitive

o Increasing filler content also increases notch sensitivity

o What does it all mean?o The lower the notch sensitivity, the rounder the

corners have to be to maintain impact strength

Crack initiated at sharp

internal edge due to

impact


ENVIRONMENT AND PLASTIC HOUSING

o Plastics are very sensitive to temperatureo The most widely used housing material, acrylonitrile butadiene

styrene (ABS), is only recommended up to 80C

o Higher temperature applications typically switch to polycarbonate (PC), which is good to 120C

o UV Exposureo Parts that will be exposed to sunlight require addition of stabilizers

o Environmental Stress Cracking (ESC)o Avoid tensile residual stresses; can be crack initiation points when

polymer is exposed to chemicals (and PC is far worse than ABS)

o ESC is a significant concern in outdoor applications (like Industrial IoT)


METAL HOUSING CONCERNS: GASKETS

o Gaskets

o Avoid textured gaskets

o Compression >30%

o Types of fluid exposure

o Example Case

o Units passed IP54 testing

o Saline fluid exposure results in salt

jacking and eventual failure of gasket

system


METAL HOUSING CONCERNS: THERMAL PADS

o Silicone thermal interface

material (TIM)(aka, pads) can be

a risk in sealed hot environments

o If overly compressed, silicone TIM will

degrade due to a process known as

reversion

o Outgasses are trapped and dissolve the

thermal pads, releasing silicone oils

o To avoid over compressing

silicone pads, consider cast-in-

place type interface materials


METAL HOUSING CONCERNS: CONSTRAINED PCBAs

o Metal enclosures can be rigid and directly influence solder joint

reliability on the PCBA

o Issue #1: Can cause buckling and changes in the coefficient of thermal

expansion (CTE) of the PCB

o Issue #2: Inability to flex will increase stress in the solder joints due to axial

loading

o Complicated issues, but avoid this problem!

o Allow for float around mounting holes

o Avoid extremely tight tolerances at

mount points

o Consider shoulder bolts and slotted holes

Ga

p

Shoulder

Fastener


GUARANTEEING RELIABILITY


PASS THE TEST? OR DESIGN FOR RELIABILITY (DFR)?

o Embedded System designers have historicallyo Not performed any reliability testing, or…

o Only performed testing specified by the customer, or…

o Performed a smattering of industry-standard tests

o These approaches increasingly introduce unacceptable levels of risko Electronics increasingly have a large number of ways to be unreliable

(difficult to test all of them)

o Some failure modes can’t be tested

o And testing is TOO LATE!

o Recommended formal Design for Reliability (DfR) process and reviewo Should include Rules of Thumb and Physics of Failure (PoF) to identify

wearout risks


RULES OF THUMB (CONSTANT TEMPERATURE)

o Lifetime becomes an issue when ambient temperatures exceed 40°C on a constant (24-hr) basiso E.g., standard life ratings (85°C/2000 hr) for

electrolytic capacitors would be insufficient for more than a 5-year life

o Many companies also limit solder joint temperature to a maximum of 75°C – 85°Co Some limit junction temperature of integrated

circuits to a similar range


RULES OF THUMB (TEMPERATURE CYCLING)

o In low-power, outdoor environments, interconnect (via and solder joints) fatigue is not much of an issue

o Diurnal cycling produces no more than ∆25C once per day

o Assumes lifetime is 10 years or less

o Greater concerns are in more severe environments (definitely Autonomous Transportation; maybe Industrial IoT)

o Diurnal heat sources with sufficient fluctuation (≥∆40C)

o Diurnal cycling ≥∆40C (e.g., solar loading)

o More than four (4) power cycles/day


RULES OF THUMB (VIBRATION)

o Design board natural frequency at least two (2) times greater than highest energy peaks in vibration spectrumo Good rule of thumb is >250Hz

o Could require more attachments, rail guides

o Issues occur when peaks in the power spectral density (PSD) curve exceeds around 0.01 G2/Hzo Lower threshold for higher

frequency peaks


RELIABILITY – NEXT STAGE

o Use rules of thumb to identify potential risks of limited

lifetime / wearout

o Focus on those components that could wearout

o Then use Physics of Failure (PoF) to calculate lifetime of

parts that could wearout

o What is your actual risk?

o How does that risk get mitigated with changes to materials, design, or

environment?

o Allows you to avoid arbitrary and obsolete design rules


WHAT WEARS OUT IN ELECTRONICS?

o Ceramic Capacitors (oxygen vacancy migration)

o Memory Devices (limited write cycles, read times)

o Electrolytic Capacitors (electrolyte evaporation, dielectric dissolution)

o Resistors (if improperly derated)

o Silver-Based Platings (if exposed to corrosive environments)

o Relays and other Electromechanical Components

o Light Emitting Diodes (LEDs) and Laser Diodes

o Connectors (if improperly specified and designed)

o Tin Whiskers

o Integrated Circuits (<45nm)

o Interconnects (vias and solder joints)


WEAROUT – CERAMIC CAPACITORS

o Relevant for most high capacitance (e.g., 0201/2.2uF or 0402/10uF), 6.3V X5R ceramic capacitors

Useful Lifetime (Yrs)


WEAROUT – SOLDER JOINTS

o Elimination of leaded deviceso Provides lower RC and higher package densitieso Reduces compliance

Cycles to failure

-40 to 125C

QFP: >10,000 BGA: 3,000 to 8,000

QFN: 1,000 to 3,000CSP / Flip Chip: <1,000


PHYSICS OF FAILURE (POF) ANALYSIS

o While potentially mind-numbing, PoF analysis

can be a game-changing advantage over

competitors and demonstrate thought-leadership

to customers

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CONCLUSION


CONCLUSION

o Stay agile!o Risk mitigations should not impede product development,

but instead work with it

o Focus on successo Improved processes and tools will allow you to compete

on delivery and performance, not necessarily cost

o The foundation of future growth in embedded systems rests on highly reliable systemso Develop the knowledge to meet market demand


QUESTIONS?

o Senior Sales Engineer: Mark Musitano

o Office Phone: (301) 640-5809

o Email: [email protected]

o Mechanical Engineer: Michael Blattau

o Office Phone: (301) 640-5818

o Email: [email protected]

mailto:[email protected]

mailto:[email protected]


DESIGN FOR RELIABILITY CONFERENCE

o Interested?

o Please visit www.dfrsolutions.com for signup information

http://www.dfrsolutions.com/

common mistakes by electronic design teams - dfr solutions slides for... · 2017-10-08 · common...

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