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How to Shock and Waterproof Your Design –

Nathan Blattau, Michael Blattau, Craig Hillman

2

3

WHY SHOCKPROOF / WATERPROOF?

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

o Electronics must work (autonomous transportation, etc.)

4

WHAT IS MECHANICAL SHOCK?

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 10Gthat occurs less than 100,000times

EXAMPLE OF MECHANICAL SHOCK: JESD22-B110A

6

MECHANICAL SHOCK FAILURES

MECHANICAL SHOCK EVENTS

Tend to be overly focused on drop,

but excessive ‘shock’ can occur at

multiple points post-assembly

8

SHOCKPROOFING - PCB MOUNTING

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 (Rule of Thumb) 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

Damping

HOW TO MITIGATE SHOCK/DROP?

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: Component Level

o Component Selection

o Flexible terminations on ceramic capacitors

o Leaded parts

o Bonding

o Underfill/Edge-bonding/Staking

TDK

10

HOW TO MITIGATE SHOCK/DROP? (cont.)

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.

11

OPTION 1: REDUCING EXCITATION

Vibration Analysis for Electronic Equipment by David S. Steinberg

o Shock isolators

o Typically done at the chassis level

o Sometimes used on shock sensitive internal parts

o Example: Hard Drives

o Requirements need to be tailored to the expected vibration

and shock loads

o Isolators that reduce shock loads may amplify vibration loads

and vice versa

12

OPTION 1: REDUCING EXCITATION

o Shock isolators – Continued

o Natural Frequency or Isolator Resonant Frequency (𝑁𝑓)

o Shock Pulse Frequency

o 𝑓𝑝 =1

2𝑡𝑤ℎ𝑒𝑟𝑒 𝑡 𝑖𝑠 𝑡ℎ𝑒 𝑝𝑢𝑙𝑠𝑒 𝑑𝑢𝑟𝑎𝑡𝑖𝑜𝑛

o Different pulse shapes will have different amplification

responses (half–sine shown on slide 7)

Vibration Analysis for Electronic Equipment by David S. Steinberg

13

OPTION 1: REDUCING EXCITATIONo Shock isolators – Continued

o Example Problem (assumes no damping)o PCB Natural Frequency = 200 Hz

o Half Sine Shock Pulse 100G 5ms

o Pulse Frequency = 1

2𝑡𝑝= 100 Hz

o Since the natural frequency of the board is not 3X the pulse frequency an isolator will be required

o Selected Isolator Properties

o Natural frequency 20 Hz

o Frequency ratio (R) = (isolator frequency/pulse frequency) = 0.2

o Shock Amplification for Half Sine = 4 𝑓𝑠 (𝑡𝑝) = 4 20 (0.005) = 0.4

o Only works when the ratio < 1

o Shock Response of Isolators (𝐺𝑜) would be:

o 𝐺𝑜𝑢𝑡 = 𝐴 × 𝐺𝑖𝑛 = 0.4 × 100 = 40𝐺o For calculations of board deflection we can assume that the output pulse frequency is the

same as the natural frequency of the isolators

o Displacement needs to be considered (isolator geometry)

14

OPTION 1: REDUCING EXCITATION

o Shock isolators – Continued

o Expected Isolator Displacement

o 𝑑𝑠 =𝐺𝑜𝑢𝑡

(0.102)𝑓𝑛2 =

40

(0.102)(20)2= 0.98 𝑖𝑛𝑐ℎ𝑒𝑠

o We will need to check if the isolator is capable of this much deflection

without bottoming out (this would include load deflection concerns)

o For frequency ratios > 1 then the following chart will need to be used

www.rpmmech.com

15

OPTION 1: REDUCING EXCITATION

o Bumpers and Cushioning

o Elastomers applied externally to protect the enclosure and reduce

impact forces

o Determine if external bumpers would protect the chassis from a 72” drop

(6 feet)

o Calculate Kinetic Energy (KE) = Weight X Height = 1.5 lbs X 72 in

= 108 in-lbs

o Assume the device lands on one corner

o Assume corner cushions are spherical caps so the shape factor is 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 −𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠

𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟= 0.5

1.5 lbs(assume pads are 2” in diameter and 1” thick)

16

OPTION 1: REDUCING EXCITATION

o Bumpers and Cushioning – continued

o Assumptionso Urethane pads with a durometer 60A

o Linear compression modulus

o Kinetic energy (KE) is equal to spring energy (SE)

o 𝐾𝐸 =1

2𝑘δ2 = 108

o Using area and thickness the chart gives a spring rate (multiply stress by area and strain by thickness)

o k = 3927 lbs/in

o δ = 2 ×𝐾𝐸

𝑘=

2 ×108

3927= 0.23" 𝑜𝑓 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛

o Bumpers are thick enough to absorb the impact

Shape Factor

17

OPTION 2: COMPONENT SELECTION

o PCBs bend during shock which causes strain in the

attachment points between the PCB and component

o Leaded parts are typically more robust than leadless

o Thicker solder joints are also more reliable

Vibration Analysis for Electronic

Equipment by David S. Steinberg

18

OPTION 2: COMPONENT LEVEL

o Ceramic capacitors are susceptible to flex cracking

o Solutions

o Reduce capacitor length if possible

o Select a capacitor with a tougher dielectric

o C0G > X7R > Z5U

o Switch from surface mount to leaded version

o Select a capacitor with a flexible termination

o Industry has identified this issue and now makes

surface mount chips with flexible terminations

19

OPTION 2: BONDING

o Corner bonding, edge bonding, and underfilling sensitive

components

o A good adhesive can couple the PCB and component

together

o Care needs to be taken in selection of underfill

o Tg should be outside of the working temperatures of the

product

o CTE of underfill should be close to the CTE of solder (27 ppm)

20

OPTION 2: CORNER STAKING

o Example: Shock Analysis (BGA)

Do not mount sensitive parts near mounting holes

0.63% Probability of failure per event

21

OPTION 2: CORNER STAKING

o Example: Shock Analysis (BGA) with Staking

5.5E-6% Probability of failure per event

Corner Staking

6X decrease in ball strain

22

OPTION 3 – CHASSIS STRUCTURE

o 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)

23

OPTION 3 – CHASSIS STRUCTURE

o 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)

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

24

PCB AS A STRUCTURAL MEMBER

o Battery board connected to main board with standoff

(100G shock)

25

PCB AS A STRUCTURAL MEMBER (cont.)

o Battery board not connected main board

o 1000 µɛ lower strain

o 1 mm less deflection

26

OPTION 3 – CHASSIS STRUCTURE

o Stiffening the Chassis

o Plastic

o Material selection

o PEEK > PPS > NYLON

> ACETAL > PC >

ABS

o Fillers

o Glass fibers

o Mineral

o Design Features

o Ribs

o Sheet Metal

o Material selection

o Aluminum

o Steel

o Design Features

o Embossing

o Stiffeners

27

OPTION 3 – CHASSIS STRUCTURE

o Plastic

o Materialso ABS, ABS-PC Blends, and PC (polycarbonate) are the most commonly used

materials

o PC is stiffer than ABS but more expensive and more susceptible to environmental stress corrosion

o Higher working temperature than ABS

o Reduce cost by blending ABS with PC

o Fillers

o Glass fibers

o Increases material stiffness

o Increases brittleness

o Cost increase

o Harder to mold

o Surface finish

o Shrink issues during cooling

28

OPTION 3 – CHASSIS STRUCTURE

o Fillers - Continued

o Mineral Fill

o Increases material stiffness

o Inexpensive

o More brittle (increase in notch sensitivity)

Designing with Plastics and Composites: A Handbook

29

OPTION 3 – CHASSIS STRUCTURE

o Design Feature

o Ribs

o Increase tooling cost

o Sink mark issues

o Material cost increase

o Example Increase of Nf > 2X (weight increase by 10 grams)

| Plastics Engineering | October 2016 | 4spe.org |

plasticsengineering.org

30

OPTION 3 – CHASSIS STRUCTURE

o Sheet Metal

o Materials o Aluminum

o Recommend using 5052-H32 over 6061-T6

o AL 5052-H32 can handle tighter radii

o 1/3 the stiffness of steel

o Lighter than steel (2.5X)

o Broaching hardware will need to be installed after anodizing

o Steel

o Inexpensive

o Needs corrosion protection

o Stainless Steel

o Expensive

o Broaching hardware issues

o Inserts need to be made from harder material than the metal

31

OPTION 3 – CHASSIS STRUCTURE

o Sheet Metal

o Features

o Embosses

o Requires additional tooling (cost)

o Gussets

o Rivet stiffeners on flexible areas

32

OPTION 3 – CHASSIS STRUCTURE

o Embossing Example:Avoid PCB mounts in this area

33

WHAT IS WATERPROOFING?

o Driven by IEC 60529 (Ingress

Protection / IP)

o Does not take into consideration

condensation or “breathing”

34

WATERPROOFING

o Board Level

o Encapsulation (Potting)

o Chassis Level

o Sealed Plastic Housing

o Ultrasonic vs. Laser

o O-Ring Sealing

o Gaskets

o Moisture (Humidity) Issues

35

WATERPROOFING: ENCAPSULATION

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

36

ENCAPSULATION: POTTING MATERIALS

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, Tgwithin 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!)

37

WATERPROOFING: POTTING (ADVANTAGES)

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 coating

o Most coatings cannot withstand long-term contact with water

38

WATERPROOFING: POTTING (DISADVANTAGES)

o Not compatible with all technology

o Relays and switches need to be sealed

o Convective thermal solutions must be outside the potting

o Thermally conductive materials may be required

o More expensive

o Connectors need to be masked

o 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)

39

KEY DISADVANTAGE: POTTING CAN KILL

40

POTTING CAN KILL

o Concentrate contaminationo 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 jointso Driven by expansion/contraction

during change in temperature

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

41

POTTING MATERIALS

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’)

Polyurethane

Silicone

Silicone

Silicone

Silicone

Asphalt

Polyurethane

Asphalt

Silicone

Silicone

Silicone

Silicone

42

WATERPROOFING: SEALED PLASTIC HOUSINGS

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)

43

ULTRASONIC VS. LASER WELDING

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 Advantages

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

o Disadvantages

o 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)

44

ULTRASONIC VS. LASER WELDING (cont.)

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

45

WATERPROOFING: O-RINGS

o O-rings are one of the most common method of sealing

o Two types of sealing regimes

o Axial

o Radial

o Their usage in electronics has some limitations

o Significant force is required to compress an o-ringo Requires metal housing or thick plastic

o Size limitations

o Wall thicknesso More material required to accommodate the necessary groove

SEAL TYPES

46

RadialAxial/Face Seal

Groove Diameter

Bore Diameter

Use with Plastics

47

O-RINGS

o Radial Seals

o Good for use with plasticso Force is applied to the housing material and not on the fasteners

o Axial Seals (Face Seal)

o Use with metal (machined) housing

o O-ring Sizing

o Stretch should between 1% and 5%

o 𝑂 − 𝑟𝑖𝑛𝑔 𝐼. 𝐷. =𝐺𝑟𝑜𝑜𝑣𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟

% 𝑆𝑡𝑟𝑒𝑡𝑐ℎ+1

o 𝐶𝑟𝑜𝑠𝑠 𝑆𝑒𝑐𝑡𝑖𝑜𝑛 =𝐵𝑜𝑟𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 −𝐺𝑟𝑜𝑜𝑣𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟

2

1−%𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛

100

o Compression for static seals should be between 10% and 40%o Tolerances of materials will need taken into account

o From cross section O.D. can be determined

48

AXIAL SEAL

o ABS Plastic Housing with EPDM O-Ring

o Face sealing with plastic requires a lot of fasteners

49

RADIAL SEAL

o Example

Polycarbonate

PCB

Polycarbonate

SMA

50

O-RING MATERIALS

o Materials (most common)

o Buna-N (Nitrile)o Temperature Range -20°F to 225°Fo Do not use in chlorinated water (swimming pools)

o EPDMo Temperature Range -40°F to 212°Fo Good overall material

o Do not use around fuels and oils

o Siliconeo Temperature Range -40°F to 400°Fo Excellent chemical resistance

o Neopreneo Temperature Range -30°F to 212°F

51

WATERPROOFING: GASKETS

o Gasket Types

o Die Cut

o Molded

o LSR (Liquid Silicone Rubber)

o Injection

o Bulb seals

o Parameters Effecting Sealing

o Stress Relaxation

o Surface imperfections

o Sealing force

o Tolerance stack

52

STRESS RELAXATION

o Stress Relaxation/Creep

o Over time the sealing force will decrease (push-back)

o Denser materials typically exhibit have less creep than softer

materials

o Thinner gaskets typically creep less than thick ones

o A thick soft gasket will leak sooner than a thin stiff gasket

o However a thick soft gasket can handle surface imperfections better than

a stiff gasket

o Compression Set can be used as an indicator of creep

o Closed cell foams will also lose sealing force due to air diffusion

through the cell walls

53

COMPRESSION SET

Name Compress Set Name Compression Set

Nitrile (SBR) 4 Natural Rubber 4

Styrene-butadiene 3 EPDM 4

Butly (IIR) 3 XNBR 2

Neoprene 3 Silicone 3

Fluorocarbon 4 Polyacrylate 1

Urethane 2 Fluorosilicone 3

Ethylene 3 Hydogenated Nitrile 4

1 – Poor

2 – Good

3 – Better

4 – Best

54

SURFACE IMPERFECTIONS

o Texture on sealing faces

o Textured surfaces on either the gasket or surface requires

additional force (recommended 30% deflection may not

be adequate)

Ingress of salt water on touch screen gasket

• Closed cell foam

• Textured surface

• 30% compression

• Salt-Jacking

55

SEALING FORCE

o All foam gaskets loose their

sealing force over time

o Typical weather sealing doesn’t

require a lot of sealing force

o Water should not be allowed to

sit or pool up against a gasket

o O-rings use pressure difference

to help maintain a seal

https://www.rogerscorp.com/documents/2366/designtools/Sealing-

Design-Guide.pdf

HUMIDITY/CONDENSATION

56

57

SEALED PLASTIC ENCLOSURES

Polymer Hermetic

o All polymer materials allow

moisture into the housing

o Polycarbonate has a 2X higher

diffusion rate than acrylic

(PMMA)

o 14X higher than

polypropylene

PolymerPermeability coefficient (P0)

Rate (g · mm/m2 · day)

Acrylonitrile - Styrene 2.0 - 6.3

Acrylonitrile Butadiene Styrene (ABS) 2.0 - 6.3

Sabic Cycolac ABS 2.0 - 2.5

Styrene-Acrylonitrile (SAN) 3.2

Polystyrene (PS) 0.8 - 3.9

Polyamides (PA) “Nylon” 0.24 - 125

Polyetherimide (PEI) 2.3 - 3.0

High Density Polyethylene (HDPE) 0.1 - 0.24

Mid Density Polyethylene (MDPE) 0.4 - 0.6

Low Density Polyethylene (LDPE) 0.39 - 0.59

Polyethylene Naphthalate (PEN) 0.096 - 4.2

Polyvinyl fluoride (PVF) 0.83

Polytetrafluoroethylene (PTFE) 0.0045 - 0.30

Polyvinyl Chloride (PVC) 0.94 - 0.95

Polymethyl Methacrylate (PMMA) 1.7

Polyoxymethylene (POM) 5.9

Polypropylene (PP) 0.3

Polycarbonate (PC) 4.3

Polyethylene terephthalate (PET) 0.5

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

58

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

59

HUMIDITY IN AN ENCLOSURE

o Assume assembled in ESD controlled environment

o 40% RH @ 25°Co Relative humidity is the ratio of the vapor pressure to the saturation water vapor

pressure

o Saturation Pressure (25°C ) = 31.7 hPa (Pws)o 1 kPa = 10 hPa

o Vapor pressure of water at given temperature

o Partial Pressure = 12.7 hPa (Pw)o Vapor Pressure X RH/100

o Ideal gas behavior AH (absolute humidity) = 𝐶 ×𝑃𝑤

𝑇

o C = 2.16679gKJ

(for water)

o T = Temperature in K

o Pw = Vapor pressure in Pa (100Pa = 1 hPa)

o 𝐴𝐻 = 2.16679 ×1270

298.15= 9.23

𝑔

𝑚3

o Water content is the same as absolute humidity

o RH at operation temperature (50°C)

o Saturation pressure (50°C) = 123.44 hPa

o Back Solving for Relative Humidity yields RH = 11.15% Lide, David R., ed. (2004). CRC Handbook

of Chemistry and Physics

60

SEALED PLASTIC ENCLOSURES

o Example: PC enclosure 150 X 150 X 50 (mm)

o 𝑃0 = 4.3 g∙𝑚𝑚

𝑚2∙day

o Wall thickness = 2 mm (𝑡)o Assume that the bottom isn’t exposed to air

o Surface Area = 150 X 150 + 4 X 150 X 50 = 52500mm2 = 0.0525 m2

o Temperature/RH% Outside = 30°C / 65%

o Temperature/RH% Inside Box = 50°C / 11.15%

o Pressure Difference (atm)

o ∆𝑝 = 𝑝𝑖 − 𝑝𝑜 = .1218𝑅𝐻𝑖

100− .0419

𝑅𝐻𝑜

100

o ∆𝑝 = 0.0136 atm – 0.0272 atm = −0.01365 atm

o Mass Transfer Rate = 4.3 / 2 * 0.0525 * 0.01365 = 0.0015 grams per day

o 𝑀𝑇𝑅 =𝑃0

𝑡× 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 𝑚2 × ∆𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑎𝑡𝑚)

o Transfer should stop when ∆𝑝 = 0 (RH inside the box is 34.4% )

61

WATER CONTENT

o Rate of moisture

ingress will decrease as

the system nears

equilibrium

o At around 22% RH

there isn’t enough

pressure to drive

moisture into the box0

2

4

6

8

10

12

14

16

18

20

22

24

0 10 20 30 40 50 60 70 80 90 100

%RH

Time (Years)

Change in %RH

62

DEWPOINTo Td (dewpoint)

o Pw = Pws @ 50°C X 22/100 = 123.44 X 0.22 = 27.16 hPa

o 12.344 kPa = 123.44 hPa

o Pw needs to be in hPa for this equation

o Td = 240.7263/(7.591386/10log(27.16/6.116441)-1) = 22.44°C

o Water will condensate inside the enclosure if the power turns off and

the temperature drops to 22.4°C

www.vaisala.com

63

SOLUTIONS

o Is it a concern?

o Vent the enclosure with a barrier material

o Add desiccant to the enclosure

o Mass Transfer Rate will remain the same as the

desiccant absorbs moisture in the box

o Increase the wall thickness (rate is inversely proportional

to thickness)

o Move air inside the enclosure

64

CONCLUSION

o Condensation concerns inside waterproof (submersible)

products will require internal PCBAs to be conformally

coated

o Ex: Pool vacuum robots

o Foam gaskets should only be used in a weather seal role

o They should not be used if impingement or standing

water is possible

o Potential to pass initial test but fail after stress

relaxation

o

QUESTIONS?

65

66

Drop Shock Testing

o Component Level

(assembled state)

o Product level (free

fall drop testing)

o Shipping

o Product packed

in it’s shipping

configuration

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

CONCLUSION

o Due to today’s low profile surface mount components, shock failures are primarily driven by board flexure

o BGAs don’t care about in-plane shock

o Every attempt should be made to limit board flexure

o Specific failure modes are

o Pad cratering (A,G)

o Intermetallic fracture (B, F)

o Component cracking

o Shock tends to be an overstress event (though, not for car doors)

o Failure distribution is ‘random’