Transcript HUMIDITY - Home - Hanse Environmental Inc

HUMIDITY
A Fresh Look
Definition
• HUMID'ITY, n. Moisture; dampness; a
moderate degree of wetness which is perceptible
to the eye or touch, occasioned by the
absorption of a fluid, or its adherence to the
surface of a body.
– Webster’s 1828 Dictionary
• Humidity is the amount of water vapor in the air
– USA Today
Relative Humidity
• Relative humidity is the amount of water
vapor in the air compared with the
amount of vapor needed to make the air
saturated at the air's current temperature.
– USA Today
• Weather for USA for June 1, 2003 showed
relative humidity ranging from 13 to 86%
RH
From Surviving Humidity – A
Pragmatic Approach
• Humidity has always been an enigmatic
problem for many engineers to grasp
because:
• Temperature – you can feel
• Vibration – you can both see and feel
• Humidity – may be invisible to the naked eye
Humidity Testing
• High Humidity
– Higher than is seen in real life situations
– Purposely apply stressful situation
– Temperatures higher than normal
– Easiest to get temperature to set point and
then raise humidity
– Often the final test after thermal only
Environments to Remember
• Use
• Transportation
• Storage
Types of Failures Found with High
Humidity
Environmental
parameters
High relative humidity
Principal effects Typical failure
resulting
Moisture absorption and
adsorption
Swelling
Loss of mechanical strength
Chemical reactions:
corrosion
electrolysis
Increased conductivity of
insulators
Physical breakdown,
insulation failure,
mechanical failure
From ETM750-2001-03-131
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decreased insulation resistance (increased leakage current) at the surface (surface insulation
resistance (SIR) and in the bulk of polymers and polymer composites such as printed circuit
substrates,
decreased surface dielectric strength and the possibility of flashover,
increased dielectric constant of polymers and polymer composites (printed circuit substrates),
swelling, weakening, or discoloration of organic and polymeric materials (e.g. solder mask, paper,
cardboard, pressure sensitive adhesive as used on labels,
dendrite growth in the presence of DC potential and water soluble contamination and exposed
closely spaced conductors, along interfacial voids associated with delaminated reinforcing fibers in
laminate structures, and beneath components with insufficient clearance to assure removal of
water-soluble contaminants such as those remaining from the printed circuit board fabrication or
printed circuit assembly operations,
oxidation and corrosion of metals,
localized condensation with adhesion or lubrication effects on moving parts,
degraded print quality of inkjet printers with water-based inks,
condensation on and obscuration of optical surfaces
absorption of moisture during transportation and storage by permeable packaging materials
(cardboard, paper, starch-based foams) under conditions of combined high temperature and high
humidity conditions with subsequent release and condensation of that moisture at lower
temperatures.
From Surviving Humidity – A
Pragmatic Approach
Dendrithic Growths
Dimensional changes
Delamination
Humidity Induced Problems
Corrosion
Mould Growth
Surface resistivity
From IEC 60068-2-28
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decreased insulation resistance (increased leakage current) at the surface (surface insulation
resistance (SIR) and in the bulk of polymers and polymer composites such as printed circuit
substrates,
decreased surface dielectric strength and the possibility of flashover,
increased dielectric constant of polymers and polymer composites (printed circuit substrates),
swelling, weakening, or discoloration of organic and polymeric materials (e.g. solder mask,
paper, cardboard, pressure sensitive adhesive as used on labels,
dendrite growth in the presence of DC potential and water soluble contamination and exposed
closely spaced conductors, along interfacial voids associated with delaminated reinforcing
fibers in laminate structures, and beneath components with insufficient clearance to assure
removal of water-soluble contaminants such as those remaining from the printed circuit board
fabrication or printed circuit assembly operations,
oxidation and corrosion of metals,
localized condensation with adhesion or lubrication effects on moving parts,
degraded print quality of inkjet printers with water-based inks,
condensation on and obscuration of optical surfaces
absorption of moisture during transportation and storage by permeable packaging materials
(cardboard, paper, starch-based foams) under conditions of combined high temperature and
high humidity conditions with subsequent release and condensation of that moisture at lower
temperatures.
From MIL-STD 180F
• Surface effects:
– Oxidation and/or galvanic corrosion of metals
– Increased chemical reactions
– Chemical or electrochemical breakdown of organic
and inorganic surface coatings
– Enhanced biological activity (i.e., fungal growth)
– Interaction of surface moisture with deposits from
external sources to produce a corrosive film
– Changes in friction coefficients, resulting in binding or
sticking
Cont.
• Changes in Material Properties
– Swelling of materials due to sorption effects
– Other changes in properties
• Loss of physical strength
• Electrical and thermal insulating characteristics
• Delamination of composite materials
• Change in elasticity or plasticity.
• Degradation of hygroscopic materials.
• Degradation of explosives and propellants by absorption.
• Degradation of optical element image transmission quality.
• Degradation of lubricants
Cont.
• Condensation and free water
– Electrical short circuits
– Fogging of optical surfaces
– Changes thermal transfer characteristics
Types of Failures Found with Low
Humidity
Environmental
parameters
Low relative humidity
Principal effects Typical failure
resulting
Desiccation
Embrittlement
Loss of mechanical strength
Shrinkage
Increase of abrasion
between moving contacts
Mechanical failure, cracking
From ETM750-2001-03-131
• embrittlement of paper,
• loss of lubricity of lamellar graphite
particles (at very low humidity),
• decreased available time to use or to
transfer water-based solutions and gels,
• increased buildup of electrostatic charge.
From IEC 60068-2-28
• embrittlement of paper,
• loss of lubricity of lamellar graphite
particles (at very low humidity),
• decreased available time to use or to
transfer water-based solutions and gels,
• increased buildup of electrostatic charge.
From Nortec (sensor mfr.)
• static electricity
• surface cracking
• weight loss
• shrinkage
• embrittlement
• cooling efficiency
• ESD risk
Normal Humidity Test Limits
According to IEC 60068-1, The standard range of atmospheric
conditions for carrying out measurements and tests is as follows:
Temperature
Relative Humidity
15 °C to 35 °C
25 % to 75 %
Air Pressure
86 kPa to 106 kPa
(860 mbar to 1 060
mbar)
Additional Notes on Humidity
Testing
• IEC – 60068-1
– A damp heat, steady-state test is often applied at the
end of the whole sequence of tests or, where not
included in the sequence, on separate specimens in
order to determine the long-term behaviour of the
component in a humid atmosphere.
• MIL-STD 810F
– Humidity testing may produce irreversible effects. If
these effects could unrealistically influence the results
of subsequent tests on the same item(s) perform
humidity testing following those tests.
Motorola Findings
• With all of this discussion, it must be pointed out
that humidity is considered a very weak stress in
general for electronic devices. It does have its
uses for specific component failure modes, and
verification of cleaning processes. It is typically
combined with high temperature and may also
be used in conjunction with altitude and
electrical power effects.
– From Motorola Web Site
More Findings
• Humidity, in an indirect inject chamber, has
been determined to have an ageing factor in
the range of 4:1 to 10:1. On the average,
TX14ALT can age a unit the equivalent of six
(6) years in four (4) days. This means that the
DUT has aged the equivalent of only 16 to 40
days from the humidity stimulant, while the
other stimulants have aged the DUT about
2,190 days. This is an ageing factor of 547:1
vs. 10:1 for humidity.
–
Motorola Advanced Life Testing
Other Considerations
• Humidity Sensor
– Wet bulb/Dry bulb
– Mirror
– Capacitance
• Airflow
• Temperature Sensor
• Chamber Size
Humidity Sensor Considerations
• Before the age of the transistor, horse hair
was used as a humidity sensor. A group
of horse hairs were attached [glued] to
two holders and as they stretched and
contracted with the humidity, the change
in their length was taken as a measure of
relative humidity.
– MadSci Network
Cont.
• Acting as a variable capacitor, the dielectric layer
is only a few microns in thickness for quick
equilibrium with the surrounding water vapour
pressure and the corresponding value is
displayed on the meter in Dewpoint
temperature. The pores of gold film and the
dielectric layer are precisely controlled by special
processes during manufacturer, so that the
reading is specific to water vapour regardless of
other gas which may be present.
– www.shawmoisture.com.my
Ways of Introducing Humidity
• Steam Generator
• Direct Injection
• Indirect Injection
Notes on Working Space
• It must be realized that %RH will vary
across the working space irrespective of
the design.
– Surviving Humidity – A Pragmatic Approach
• “Working space” is not necessarily the
entire open area of your chamber
– Temperature measurements
– Humidity measurements
Special Cases
• Ford Electronics (now Visteon) uses
turkey roasting bags in their nitrogen
chambers to keep humidity higher during
testing
• Siemens purposely drives their chamber
below freezing when humidity is high to
see if they can create ice in their circuit
cards
During HALT
• Can be used as an additional stress
• It is useful to at least know the humidity
level, so it is good to have a sensor