How to Shock and Waterproof Your Designs - DfR Solutions

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How to Shock and Waterproof Your Design –(Preview)Nathan Blattau, Michael Blattau, Craig Hillman

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

o Electronics must be everywhere (IoT, wearables, etc.)

o Electronics must work (autonomous transportation, etc.)

WHY SHOCKPROOF / WATERPROOF?

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Electronics Parts + Circuit Board

WHY SHOCKPROOF / WATERPROOF? (cont.)

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Batteries

(Dr. Challa)SSD/Memory

(A. Alagappan)

Cabling/Connectors

(Dr. Blattau)

Corrosion Protection

(M. Howard)

Fluid Flow

(C. South)

Enclosures

(M. Blattau)

Material Selection

(Dr. Hillman, S. Binfield)

Power Supplies

(Dr. Wunderlich)

System FMEA

(J. McLeish)


Come to the DfR

Conference!

Baltimore, MD

March 19, 2018

HOW TO SHOCK AND WATER PROOF? TEASER

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o Definition:

A sudden and irregular acceleration that induces a

mechanical displacement

o Better Definition (for electronic

systems):

A mechanical event

of less than 20ms with an

acceleration of at least 10G

that occurs less than 100,000

times

WHAT IS MECHANICAL SHOCK?

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EXAMPLE OF MECHANICAL SHOCK: JESD22-B110A


MECHANICAL SHOCK FAILURES

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Mechanical Shock Failure Modes


Tend to be overly focused on

drop, but excessive ‘shock’ can

occur at multiple points post-

assembly

MECHANICAL SHOCK EVENTS


o Option 1: Reduce excitation

o Shock isolators (primarily for large electronic assemblies)

o External cushioning (cell phone cases, bumpers)

o Ejection of mass (battery pops out)

o Option 2: Give your part flexibility

o Flexible terminations on ceramic capacitors

o Leaded parts

HOW TO MITIGATE SHOCK/DROP?

TDK


o Option Three: Strengthen your design (Stop the board

from bending!)

o Change your design

o Chassis structure

o Mount points, standoffs, thicker board, etc.

o Glue sensitive components

o Corner Staking, Edge Bonding, Underfill

HOW TO MITIGATE SHOCK/DROP? (cont.)

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o Option 1: Stop the chassis from deforming by increasing its stiffness

o If chassis doesn’t deform during the shock event, it is easier to simulate the effect the shock event will have on the circuit board

o Printed circuit boards are typically low mass and can withstand significant shock levels

o Avoid using the printed circuit board as a structural element in your design (the circuit boards are the passenger)

SHOCKPROOFING – CHASSIS STRUCTURE

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http://www.ruggedpcreview.com/3_notebooks_dell_e6420_xfr.html


o Battery board connected to main board with standoff

(100G shock)

PCB AS A STRUCTURAL MEMBER

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o Battery board not connected main board

o 1000 µɛ lower strain

o 1 mm less deflection

PCB AS A STRUCTURAL MEMBER (cont.)

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o Option 2: Design the chassis to deform (example: crumple zones)

o Deformation of the chassis will reduce the shock seen by the circuit board

o Deformation of the housing needs to be away from the circuit board

o If appropriate material, chassis will recover (no permanent damage)

SHOCKPROOFING – CHASSIS STRUCTURE

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o When a PCB is subjected to shock, it will deform and

then resonate at its natural frequency

o Can cause a shock amplification if shock pulse frequency and

the PCB frequency are close

o The resonant frequency of the board should be at

least 3X higher than the shock pulse frequency

o Example 10mS pulse

o 50Hz pulse frequency

o Board should be >150Hz

SHOCKPROOFING - PCB MOUNTING

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Damping


o Driven by IEC 60529 (Ingress Protection / IP)

o Does not take into consideration condensation or “breathing”

WHAT IS WATERPROOFING?

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o One common option for water-

proofing is encapsulate the

entire electronics in a polymer

o Encapsulation takes

many forms

(including the

iWatch!)

o Includes potting

and injection

molding

WATERPROOFING: ENCAPSULATION

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o Most common are silicone and epoxy

o Silicone tends to be soft (down to Shore A00), hydrophobic, high temperature resistant, Tg outside operating conditions, poor adhesion, expensive, high CTE

o Epoxy tends to be good adhesion, low cost, low CTE, hydrophilic, Tg within operating conditions

o Industry alternatives include urethanes and asphalt

o Adjustments to polymer chemistry and filler material creates wide range of possible options

o Asphalt primarily used in ballasts (thermoplastic; great for failure analysis!)

ENCAPSULATION: POTTING MATERIALS

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o Removes some constraints on housing design

o No O-ring? No welding? No venting?

o Lower cost?o Some potting solutions are 50¢ (or less)

o No tooling required

o Kill two (three? four?) birds with one stone?o Potting can provide water protection, shock protection, compliance

with creepage/clearance requirements, and security from reverse engineering

o Better than conformal coatingo Most coatings cannot withstand long-term contact with water

WATERPROOFING: POTTING (ADVANTAGES)

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o Not compatible with all technologyo Relays and switches need to be sealed

o Convective thermal solutions must be outside the potting

o Thermally conductive materials may be requiredo More expensive

o Connectors need to be maskedo Even more expensive (sometimes more than the potting material!)

o Housing must act as the mold

o Potentially low throughput due to pouring/agitation/curing

o Failure analysis can be challenging (depends on the material)

WATERPROOFING: POTTING (DISADVANTAGES)

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KEY DISADVANTAGE: POTTING CAN KILL

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o Concentrate contamination

o During dispense, liquid potting can gather and concentrate on-board contamination

o High concentration levels can occur under low standoff components and where flow terminates

o Break components/solder joints

o Driven by expansion/contraction during change in temperature

o Dependent on potting properties, support, parts, and volume

POTTING CAN KILL

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o Mechanical properties of potting materials are typically

overlooked

o Low CTE / Low modulus is preferred, but hard to obtain

o Sometimes a blend works (‘cushion coat’)

POTTING MATERIALS

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Polyurethane

Asphalt

Silicone

Silicone

Silicone

Silicone Polyurethane

Silicone

Silicone

Silicone

Silicone

Asphalt


o Another method for waterproofing is welding a plastic enclosure

o Depending on functionality, may still require additional options (feedthroughs, sealed buttons, infrared windows, etc.)

o Welding process can either be ultrasonic or laser

o Either process has limitations in regards to housing design and materials

o End result can be IPX8 or IPX9 (depending on need for entry/exit points)

WATERPROOFING: SEALED PLASTIC HOUSINGS

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o Ultrasonic is the more common plastic welding operation

o Friction induced by micromotion (15 to 40kHz) generates heat, causing the plastic(s) to melt and seal

o Advantageso Fast, clean (no solvents, etc.), low tooling costs

o Disadvantageso Limited to plastics that melt (thermoplastics) –

still many to choose (ABS, PC, Nylon, PVC, PPS, etc)

o Joint should contain a step and an energy director

o Limited size (too much absorption with bigger enclosures)

o High frequency vibration can damage certain electronics (wire bonds, crystals)

ULTRASONIC VS. LASER WELDING

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o Laser welding has one key limitation

o One material has to be transparent, other

material must be absorbent to the laser

o Laser has another key limitation

o Expensive (high tooling costs)

o An improvement on ultrasonic in

specific applications

o Need for high throughput, no particulates,

very tight alignment, large size,

certain materials

ULTRASONIC VS. LASER WELDING (cont.)

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Polymer Hermetic

o All polymer materials

allow moisture into the

housing

o Big question: How fast?

o Polycarbonate has a 2X

diffusion rate than acrylic

o 30X higher than

polypropylene

SEALED PLASTIC ENCLOSURES

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ResinWater Vapor Transmission

Rate (grams/m²)

Ethylene tetrafluoroethylene (ETFE) 1.6

Perfluoroalkoxy alkanes (PFA) 2

Polybutylene succinate (PBS) 2.2

Ethylene ChloroTriFluoroEthylene (ECTFE) 3.2

Polypropylene (PP) 3.9

Polytetrafluoroethylene (PTFE/Teflon) 4.0

Polypropylene Copolymer (PPCO) 4.4

High Density Polyethylene (HDPE) 5.4

Polyethylenimine (PEI) 5.8

Fluorinated ethylene propylene (FEP) 6.2

Polyethylene terephthalate (PET) 18

Low Density Polyethylene (LDPE) 19

Polyvinylidene Difluoride (PVDF) 30

Cyclic Olefin Copolymer (COC) 40

PMMA/Acrylic/Plexiglas 55

Polyphenyl ether (PPE) 70

Polyvinyl chloride (PVC) 15 - 80

Acrylonitrile butadiene styrene (ABS) ~100

Polycarbonate (PC) 115

Nylon 6 ~230

Polymethylpentene (PMP) 775

Polystyrene (PS) 1220 - 6102


o Fast diffusion, for most polymers, is still slow (days)

o Can result in high humidity in the box with low temperature outside the box (condensation)

o Cyclic power cycling can make things worse

HUMIDITY INSIDE vs. HUMIDITY OUTSIDE

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Container and Ambient Relative Humidity

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 50 100 150 200 250 300 350 400 450

Hours

% R

H

INT. %RH

EXT %RH


o Come to the DfR Conference to learn more!

o In Baltimore on March 19th

o Only $99 for a full day of talks, discussions with

experts, meeting up with peers

o https://www.dfrsolutions.com/2018-design-for-

reliability-conference

CONCLUSION

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o Due to today’s low profile surface mount components, shock failures are primarily driven by board flexureo BGAs don’t care about in-plane shock

o Specific failure modes areo Pad cratering (A,G)

o Intermetallic fracture (B, F)

o Component cracking

o Shock tends to be an overstress event (though, not for cardoors)o Failure distribution is ‘random’

Mechanical Shock Failures


o Component Level

(assembled state)

o Product level

(free fall drop

testing)

o Shipping

o Product packed in

it’s shipping

configuration

Drop Shock Testing

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Don’t mix the specifications, the shock levels are very different

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How to Shock and Waterproof Your Designs - DfR Solutions

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