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.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Rencz, M.

Vass-Varnai, Andras

English

arXiv

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

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
Package Hermeticity Testing With Thermal Transient 
Measurements 
 
András Vass-Várnai
1,2
, Márta Rencz
1,2
 
vassv@eet.bme.hu, rencz@micred.com 
1
Budapest University of Technology and Economics (BUTE), Dept. of Electron Devices 
 Budapest, Hungary 
2
MicReD 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 (2,25×10
6
 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 transformed into a 
temperature transient curve.  
The resulting transient curve is a unit-step response 
function, it is characteristic to the structure inside the 
package and its environment. Applying the NID method 
(Network Identification with Deconvolution), from the 
transient curve a Cauer type thermal impedance model can 
be calculated, which is a ladder-like model of the heath flow 
path consisting of thermal resistances and thermal 
capacitances [6]. From the Cauer model structure functions 
are generated. The cumulative structure function provides a 
map of the cumulative thermal capacitances with respect to 
the thermal resistances measured from the location of the 
heating to the ambient [7].  The changes in the cumulative 
structure function indicate reaching new materials in the heat 
flow path, and their distance on the horizontal axis gives the 
partial thermal resistances between them, as it can be seen in 
Fig. 1. 
A.  Junction to ambient thermal measurements  
 In case of silicon devices exposed to the environment, we 
expected that the variation of the thermal resistance of the air 
as a function of its humidity content can be measured. The 
junction to ambient thermal resistance can be easily 
calculated if the measured temperature rise is divided by the 
applied power step: 
                            Rthja=∆Tj/P             (1) 
The results of the measurements have proven that the 
humidity content of the air has a very low effect on its 
thermal conductivity coefficient. Applying the same power-
step on the device at the two different RH levels, 11.3% and 
93.58%, the same temperature elevation was measured. In 
addition to this it takes a very long time, a few minutes or 
even more, depending on the structure of the device to reach 
the thermal equilibrium which is essential to obtain valid 
junction-to-ambient thermal resistance values. This method 
is not applicable for package hermeticity testing in this form. 
 
Fig. 1. Concept of the cumulative structure function. 
  
B.  Application of porous materials  
  
As the effect of the RH changes of the air on its thermal 
conductivity is too small to be measured with thermal 
transient measurements, a methodology had to be found to 
enhance it. The application of high surface porous materials 
is a possible solution. The thermal conductivity of porous 
materials is highly affected by their porosity. If we assume 
dry environment and 0% RH, the pores and cavities inside 
their structure contain only dry air. As the air has a low 
thermal conductivity the overall thermal resistance of the 
structure will be large. At low RH levels the water molecules 
from the gas phase start to adsorb on the wall of the pores 
and cavities. As the RH increases more and more layers of 
water adsorbs on the pore walls until at a certain RH level 
capillary condensation occurs [8]. As the water is a more 
than 20 times better heat conductor than the air, the overall 
thermal resistance of the layer decreases with the increasing 
humidity content. Naturally the moisture can penetrate into 
the pores from liquid phase as well, causing the same 
thermal effect. 
 Fig. 2. shows the heat flow path from the heated junction 
through the inner features of the SOT-93 package and the 
gypsum layer at different moisture contents into the heat-
sink. The measured structure shows the highest junction-to-
ambient thermal resistance (measured between the origin and 
the singularity point where the function goes to the infinite 
thermal capacitance values) when it is completely dry. The 
lowest junction-to-ambient thermal resistance value is shown 
when the structure has the highest moisture content, right 
after wetting it with a water droplet. The structure functions 
between the two states were measured one after the other 
with app. 2 minutes differences. As the humidity evaporates 
from the structure, the partial thermal resistance of the 
gypsum layer increases. The diverging point of the 
cumulative structure functions gives a good estimation for 
the junction-to-case thermal resistance of the package, in 
other words until that point the heat propagates inside the 
package, after that point the heat propagates through the 
gypsum layer. Assuming -2mV/K for the sensitivity of the 
silicon device, this point is very close to the 1.56 K/W value 
obtained from the datasheet of the transistor. 
Fig. 3. shows the same functions, but in a linear view 
along the capacitance axis.  In this view the small decrease 
of the thermal capacitance of the gypsum layer can be 
identified, as the humidity evaporates from the porous 
structure. This effect is very slight compared to the variation 
of the thermal resistances with the humidity content. 
 
 
 
 
 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. Cumulative structure functions indicating the decreasing moisture content of the gypsum layer 
 
Fig. 3. Linear view of the cumulative structure functions to visualize the change of the thermal capacitances  
 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
III.  CONCLUSIONS 
 
In this paper we have shown that the thermal transient 
testing is not an applicable method to evaluate the 
hermeticity of IC packages, as the change of the thermal 
conductivity of the air as a function of the relative humidity 
is negligible. However with the application of high surface 
porous materials the humidity changes of the environment 
can be indicated. We have observed significant increase of 
the thermal resistance of the gypsum layer as it lost its 
moisture content.  
The promising results of the model experiment encourages 
us to design and develop a microstructure which is capable 
of indicating the moisture content of its environment and 
therefore it can be used for package hermeticity testing as 
well. A porous alumina layer [9] will be used for adsorbing 
the moisture from its environment, and a simple p-n junction 
will be used as a heater and sensor.  
We plan to find other applicabilities of these results, as it 
is very important both in the food and the building industry 
to assess the drying of porous materials.   
 
ACKNOWLEDGMENT 
 
This work was supported by the PATENT IST-2002-
507255 Project and the NANOPACK FW7 No. 
216176/2007 IP Project of the EU. 
 
REFERENCES 
 
[1]  M. Rencz, V. Székely: “Determining partial thermal resistances in a 
heat flow path with the help of transient measurements”, Proc. of 
the 7th THERMINIC Workshop, Paris, France, Sept 24-27,2001, 
pp250-256 
[2] Stephen Brunauer, P. H. Emmett, Edward Teller,  “Adsorption of 
Gases in Multimolecular Layers”, J. Am. Chem. Soc., (1938), 60, 
309. doi:10.1021/ja01269a023 
[3]  W. N. Dos Santos, “Experimental investigation of the effect of 
moisture on thermal conductivity and specific heat of porous 
ceramic materials”, Journal of Materials Science 35 (2000), 3977 - 
3982  
[4]  www.micred.com/t3ster.html 
[5] T. Lu, C. Chen, “Uncertainty evaluation of humidity sensors 
calibrated by saturated salt solutions”, Measurement (2006), 
doi:10.1016/j.measurement.2006.09.012 
[6]  V. Székely et al. “New way of thermal transient testing” 
Proceedings of the XVth SEMI-THERM Symposium, March 9-
11,1999, San Diego, CA, USA, pp. 182-188   
[7]  M. Rencz, V. Székely, “Structure function evaluation of stacked 
dies”, Proceedings of the XXth SEMI-THERM Symposium, March 
9-11, 2004, San Jose, CA,USA, pp 50-54. 
[8] S.J. Gregg: The Surface Chemistry of Solids, Chapman & Hall 
LTD, London (1961), p. 59-70 
[9]  Tímárné Horváth Veronika, Juhász László, Vass-Várnai András, 
Perlaky Gergely  “Usage of porous Al2O3 layers for RH sensing” 
 
 


Package Hermeticity Testing with Thermal Transient Measurements

Abstract

Similar works

Full text

Available Versions

I-Revues

Crossref