KRYPTON-85 TEST SEQUENCE (77kb)
The Krypton-85 Leak Testing method is a highly sensitive technique used to
measure fine and gross leak rates in high reliability devices. It is the preferred method to characterize small leak rates in critical military
components.
The technique was developed in the 1950’s and is still in use today for evaluating military hybrids, commercial sensors, medical implants,
silicon based MEMS packaging applications and many others.
The advantages of Krypton-85 over other methods include extremely fast test times, lower overall testing cost, lower leak rate detection
(<10-12 atm cc/sec Air limits), minimal absorption to glasses, leak site(s) identification and the ability to test in ambient
conditions.
Description of Test Procedure:
The device is placed into a specialized test chamber and pressurized with a gas mixture of air and
small amounts of Krypton-85 gas. The mixture will then enter the package cavity if a leak is present. The Krypton-85 leak rate equation determines
the pressure and duration of the test based on a reject leak rate value. The client only needs to determine the reject leak rate (Qs)
value for the package under test.
The Krypton-85 equation is defined as:
Qs = R
(S)(K)(P)(T)(t)
Where:
Qs = reject leak rate (atm cc/ sec Kr)
R = detector reject level (cpm)
S = specific activity (uCi/atm cc)
K = counting efficiency (cpm/uCi)
P = (Pe2 – Pi2)
Pe2 = external bomb pressure (atm)
Pi2 = internal pressure of part (atm)
T = bomb time (hours)
t = 3600 (sec/hr)
After the pressurization cycle, the packages are typically screened using an X-ray scintillation crystal. The crystal is capable of
detecting up to 15,000 cpm/uCi of residual Krypton-85. This type of ultra sensitive detection capability allows for the measurement of
extremely small quantities of Krypton-85 gas that may have entered the package.
Various crystal geometries are available but two types are the most common. A flat crystal and/or a well type crystal are
commonly utilized depending on the package size and quantity being tested.
The net X-ray count rate measured is proportional to the total Krypton-85 content within the package. The count rate measured is the
actual detection of the disintegration rate of Krypton-85 molecules. Each Krypton-85 molecule emits a 0.51 MeV Gamma ray. The total
number of molecules that entered the package can also be calculated and hence the total leak rate of the package. In addition, the
detection process is performed at ambient conditions and thus the packages are not exposed to vacuum. Testing at ambient conditions
mitigates or eliminates the problems encountered with helium based leak testing.
Gross leaks can also be measured using a small directional counter (Geiger Müller tube) that detects the 0.69 MeV Beta particles. This
system can be used to pin point the exit site of the gas from the package.
Test Methods:
° Mil-Std 750 method 1071 condition G1, G2
° Mil-Std 883 method 1014 condition B1, B2
Specifications:
° Radioisotope: Krypton-85 gas
° Pressurization range. 30-120 psia
° Minimum detection limit: <1X10-12 atm cc/sec Air
° Accuracy: +/- 10% of value.
° Measurement method: Batch Testing or Read and Record
