Measuring Hermeticity
This article reviews some of the science behind leak rate testing and the main method to determine leak rates.
Electronics are an important component in military
systems, and the key to their survival is to exclude water
vapor. Water vapor has been shown to cause corrosion of
key interconnects and leads to device failure. For typical size/
volume modules, the nomograph in Figure 1 applies, and
indicates that above 6,000 ppm water vapor condensation may
occur during normal thermal cycling. Therefore, the water vapor
content was set by military standard at 5,000 ppm
This rate is expressed mathematically by the Howell-Mann
equation:
0.5
0.5
 
 L⋅ t1  ⋅  Ma   −  L⋅ t2  ⋅  Ma  
−
 

 
Pe Ma 

 V⋅ Po   M   ⋅ e  V⋅ Po   M  
R( V) := L⋅ ⋅ 


 ⋅1 − e
Po  M 
0.5
Where the variables are defined as:
L : = 8.0 -10 -9 this is the equivalent lead rate in air or specification
Pe : = 4.94 this is the exposure pressure in atm absolute
Po : = 1 this is the normal environment atmospheric pressure
Ma : = 28.7 this is the molecular weight of air that leaks into package
M : = 4 this is the molecular weight of the tracer gas (helium)
t1 : = 90000000 this is the exposure time to Pe in seconds
t2 : = 1800 this is the dwell time in seconds, time from bomb to test
V : = 1,100 – 1000 this is the package volume in cubic centimeter
Figure 1. Dew-Point Nomograph that Correlates
Pressure, Temperature, and Water Content (As shown,
at 1 atm and 0°C the dew point is 6,000 ppm of water.)
To measure this level of hermeticity, the helium mass
spectrometer is used as a leak detector. The basis of the technique is that a volume of helium is resident inside the package.
In the leak test system, a 1 atmosphere pressure differential is
applied (vacuum outside the package). This provides a driving
force for the small inert helium molecule to pass through cracks
and pores and be detected by the spectrometer tube. In this
manner the actual number of helium molecules is counted and
reported as a leak rate.
This equation is based on standard exponential
decay rates. As the helium leaks out, there is less helium
inventory in the package, and the leak rate decays. The
higher the inventory, the better the probability a molecule
will find the crack site and leak out. The equation has two
main parts; the first exponential of the equation defines
this helium fraction forced into the package. The second
exponential is the decay of that helium volume leaking out
over a given time. The combination of the two defines the
final leak rate. This equation found wide use in the military
electronics industry and formed the basis for hermetic
sealing used today.
EWI is fully equipped to test hermetic seals.
Mechanical testing can also be used to fatigue welds in
shear or tensile modes to simulate environmental stress.
The EWI lab is fully instrumented and staffed to meet your
testing needs.
1250 Arthur E. Adams Drive, Columbus, Ohio 43221-3585, Phone: 614.688.5000, Fax: 614.688.5001, www.ewi.org