Package Hermeticity Testing with Thermal Transient Measurements
📝 Abstract
The rapid incursion of new technologies such as MEMS and smart sensor device manufacturing requires new tailor-made packaging designs. In many applications these devices are exposed to humid environments. Since the penetration of moisture into the package may result in internal corrosion or shift of the operating parameters, the reliability testing of hermetically sealed packages has become a crucial question in the semiconductor industry.
💡 Analysis
The rapid incursion of new technologies such as MEMS and smart sensor device manufacturing requires new tailor-made packaging designs. In many applications these devices are exposed to humid environments. Since the penetration of moisture into the package may result in internal corrosion or shift of the operating parameters, the reliability testing of hermetically sealed packages has become a crucial question in the semiconductor industry.
📄 Content
9-11 April 2008 ©EDA Publishing/DTIP 2008
ISBN: 978-2-35500-006-5 Package Hermeticity Testing With Thermal Transient Measurements
András Vass-Várnai1,2, Márta Rencz1,2
vassv@eet.bme.hu, rencz@micred.com
1Budapest University of Technology and Economics (BUTE), Dept. of Electron Devices
Budapest, Hungary
2MicReD Ltd
Budapest, Hungary
The rapid incursion of new technologies such as MEMS and smart sensor device manufacturing requires new tailor- made packaging designs. In many applications these devices are exposed to humid environments. Since the penetration of moisture into the package may result in internal corrosion or shift of the operating parameters, the reliability testing of hermetically sealed packages has become a crucial question in the semiconductor industry. Thermal transient testing, a well known technique for thermal characterization of IC packages [1] can be a suitable method for detecting hermeticity failures in packaged semiconductor and MEMS devices. In the paper this measuring technique is evaluated. Experiments were done on different measurement setups at different environment temperature and RH levels. Based on the results, a new method for package hermeticity testing is proposed.
I.
INTRODUCTION
It is a well known fact that the relative humidity (RH) of
the ambient atmosphere has a very low effect on its thermal
properties such as thermal conductivity and thermal
capacitance, especially at room temperatures. At elevated
temperatures this effect gains higher importance, but it is
still hard to detect the small changes of the mentioned
parameters with thermal transient measurements.
A possible solution could be based on the fact that water
has a good thermal conductivity coefficient (0.58W/m·K),
compared to the air (0.0257 W/m·K) and a relatively high
specific
heat
of
evaporation
(2,25×106
J/kg).
An
encapsulated porous adsorbent material in the package under
test may serve as a sensing layer. The humidity content of
the porous layer, the rate of adsorption and desorption is
highly dependent on the relative humidity inside the
package. As the RH increases the number of the adsorbed
molecules should increase and vice versa [2]. As the water
fills the pores the overall thermal conductivity of the layer
increases [3]. In case of high surface adsorbents the decrease
of the thermal resistance can be significant.
II.
EXPERIMENTAL
A.
Measurement on open packages
BC 300 transistors mounted on TO-39 type packages were
used to evaluate the effects of the moisture content of the air
at different RH levels. The metal caps were removed from
the packages in order to expose the transistors to the
environment.
Thermal
transient
measurements
were
performed with the T3Ster equipment [4] to measure the
thermal resistance of the structure between the junction and
the ambient at two different RH levels. Precise RH
environments were set over two different saturated salt
solutions [5]. LiCl and KNO3 solutions were used resulting
11.3% and 93.58% RH at 25°C. Cooling curves were taken
up, 0.1W power step was applied.
B.
Application of porous materals
In order to prove the applicability of high surface porous
materials for package hermeticity testing, model experiments
were carried out. A 0.1 mm wide gypsum layer was placed
between a BD245C power NPN transistor in a SOT-93
package and an aluminum heat-sink. This type of transistor
is designed to have a relatively small junction to case
thermal resistance, 1.56 K/W. Prior to the measurements the
gypsum was dried for 48 hours.
The transistor was used as a heat source in this
measurement setup. The heat was forced through the gypsum
layer into the heat-sink. Thermal transient measurements
were made with the T3Ster equipment to measure the
thermal resistance of the gypsum at different moisture
contents. The first measurement was made on the completely
dry material, after that it was wetted with a small drop of
water. The same thermal resistance measurements were
performed one after the other on the wet system.
III.
RESULTS AND DISCUSSION
The goal of both experimental setups was to identify the changes of the humidity content of an environment with thermal transient measurements. This method is suitable and widely used in the industry for the thermal characterization 9-11 April 2008 ©EDA Publishing/DTIP 2008
ISBN: 978-2-35500-006-5 of semiconductor packages. The semiconductor device inside the package is heated up (or cooled down) by a power step. The temperature change is measured through corresponding temperature sensitive electrical parameters, in our case the emitter-base voltage (UEB) of the transistors. The measurement of the UEB change begins in 1µs after the power step and it is continuous until the voltage reaches a steady state. Knowing the temperature sensitivity of the device, the voltage function can be easily transform
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