How to Shock and - DfR Solutions DfR Conference Presentati… · How to Shock and Waterproof Your...
Transcript of How to Shock and - DfR Solutions DfR Conference Presentati… · How to Shock and Waterproof Your...
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WHY SHOCKPROOF / WATERPROOF?
o Electronics must be everywhere (IoT, wearables, etc.)
o Electronics must work (autonomous transportation, etc.)
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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
MECHANICAL SHOCK EVENTS
Tend to be overly focused on drop,
but excessive ‘shock’ can occur at
multiple points post-assembly
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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
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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.
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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
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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
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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)
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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
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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)
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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
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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
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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
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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)
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OPTION 2: CORNER STAKING
o Example: Shock Analysis (BGA)
Do not mount sensitive parts near mounting holes
0.63% Probability of failure per event
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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
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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)
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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
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PCB AS A STRUCTURAL MEMBER (cont.)
o Battery board not connected main board
o 1000 µɛ lower strain
o 1 mm less deflection
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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
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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
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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
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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
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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
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OPTION 3 – CHASSIS STRUCTURE
o Sheet Metal
o Features
o Embosses
o Requires additional tooling (cost)
o Gussets
o Rivet stiffeners on flexible areas
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WHAT IS WATERPROOFING?
o Driven by IEC 60529 (Ingress
Protection / IP)
o Does not take into consideration
condensation or “breathing”
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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
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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
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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!)
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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
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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)
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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
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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
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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)
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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)
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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
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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
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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
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AXIAL SEAL
o ABS Plastic Housing with EPDM O-Ring
o Face sealing with plastic requires a lot of fasteners
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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% )
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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
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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
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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
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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’