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Water soluble metallization pastes for AlN
Water soluble metallization pastes for AlN a new ecological approach
Dr. Bernhard Mussler,
ANCeram GmbH &Co. KG
Esbachgraben 21
95463 Bindlach
Dr. Peter Otschik, Dr. Christel Kretzschmar, Dr. Winfried Schaffrath
Fraunhofer Institut für Keramische Technologien und Sinterwerkstoffe
Winterbergstr. 28
01277 Dresden
Abstract
The ecological disadvantages of the base materials which are currently in use for the production of
electronic ceramic packaging components can be avoided by new, environmentally save and
harmless products. This can be realized by the use of newly developed watersoluble metallization
pastes instead of standard pastes. The fundamental advantage lies in the cleaning of screens and
tools. Here water can be used instead of organic solvents like perchlorethylen or other CKW's.
The current waste waters, exhaust gases, and chemical residues can be minimized and can be
reintegrated in the manufacturing process or wasted without ecological impacts. This leads to a closed
loop where only environmentally save materials and processes will be used.
In this paper the first results of a new AgPd-paste for aluminum nitride substrates will be presented.
The properties and results of the conventional paste and the water soluble paste will be discussed and
the two process flows will be compared.
1. Introduction
The worldwide consciousness for environmental issues has been growing enormously over the past
two decades. Simultaneously industry had to face these new challenges. As a consequence the
development of new, ecologically save products and processes became top priority. Successful
changes to environmental friendly products e.g. watersoluble paints in the automobile industry have
been realized and became standard by now.
Unfortunately the progresses in the electronic industry are behind schedule. Especially in the area of
metallization pastes for thick film applications no real efforts for a change to environmentally saver
paste types can be observed. However, there have been attempts to eliminate hazardous and toxic
materials like Cd or Pb out of postfiring and soldering pastes and meanwhile fluorine-chlorinehydrocarbons have been substituted due to legal measures. The worldwide consumption of
chlorinated hydrocarbons (e.g. perchlorethylen for all kind of cleaning or degreasing purposes) and
hydrocarbons as ingredients in metallization pastes and as cleaning agents (bulk quantity) has been
minimized but is still tremendously high. One main problem is to find cost effective replacements with
equal cleaning efficiency.
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Water soluble metallization pastes for AlN
This controversy led to the idea to develop alternative watersoluble metallization pastes which are
able to replace the currently used standard pastes and can be cleaned with water. As a start one
standard AgPd-paste which was developed by the IKTS for postfiring applications on aluminum nitride
was chosen. Within a year a new paste formulation was found and the new watersoluble alternative
could be compared to the standard paste type.
This paper describes the outcome of the first screening test and the major results of an ecological
assessment.
2. Characterization of binders and pastes
For the realization of a watersoluble AgPd-paste for aluminium nitride substrates a completely new
binder system had to be developed. One crucial factor in the beginning was to find a suitable
watersoluble polymer. Furthermore, compatible organic solvents had to be found to realize a thick film
paste with good rheological properties. In Fig. 1 and Fig. 2 both standard system and watersoluble
system are compared. Fig. 1 shows the dependence of viscosity on shear rate for the binders, Fig. 2
for the paste. The binder viscosity increases with raising polymer concentration (different
polymer/solvent ratio). The viscosity-shear rate behavior is very similar to the regular binders.
With a watersoluble binder and a regular binder (both had the same viscosity) AgPd-pastes (FK
1205W and FK 1205) were prepared. The content and composition of the solids were identical for
both pastes.
The pastes were prepared in three steps:
• mixing of powders
• dispersion of solids in the binder (dissolver)
• milling of the pastes (three roll mill)
The overall viscosity-shear rate curves of both pastes reveal only minor differences. However, as can
be seen in Fig. 2 the viscosity of the watersoluble paste differs at shear rates < 0,1 s-1. The observed
higher values are an indication for an improved storage stability of the watersoluble paste. In the shear
rate range (1...10 s-1), which is important for the screening behavior of the paste, the slightly lower
viscosity of the water-soluble paste yields better results. For further details see chapter 3.
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Water soluble metallization pastes for AlN
Fig. 1: Viscosity versus shear rate of binders
Fig. 2: Viscosity versus shear rate of pastes
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Water soluble metallization pastes for AlN
All tools and equipment used for the preparation of the water-soluble paste could be cleaned with
water.
3. Screening results and microstructural aspects
The newly developed watersoluble AgPd-paste FK1205 W was tested and directly compared to the
standard paste FK1205. The paste was screened on ANCeram AlN180 discs with a diameter of 58
mm and a thickness of 2 mm. A special test layout ("AlN-medal") was designed for the paste
evaluation (see Fig. 3). Silk screens with 325 mesh prepared with a special water resistant emulsion
were used for both pastes. The screening parameters were identical in both cases.
Fig. 3: Front- and backside of AlN-medal with watersoluble AgPd-paste
Looking at individual letters of the medals using a light microscope at 20x magnification clear
differences between the two pastes could be seen. The watersoluble paste showed smoother and
more precise contours than the standard paste (Fig. 4 and 5). However, at higher magnifications (50x)
no further differences could be detected (Fig. 6 and 7).
Further SEM-examinations revealed again major differences between the two pastes. Comparing the
AlN/AgPd-interfaces a smooth and homogeneous paste structure and good adhesion to the ceramic
phase can be found in the case of the watersoluble paste whereas the standard paste shows a very
rugged texture, a marginal adhesion and many voids (Fig. 8 and 9).
In summary the first analysis of the newly developed AgPd-paste was very convincing in all regards.
Compared to the standard paste equivalent or in most cases even better screening results could be
detected. Paste handling, screening- and brazing-parameters, etc. were identical. Thus, no changes
at all were necessary.
The cleaning of the screens and other equipment can be done with water. But further investigations in
respect to the long term stability of the screen emulsion and the corrosion stability of the screen
against water will be necessary.
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Water soluble metallization pastes for AlN
Fig. 4: Watersoluble AgPd-paste (20x)
Fig. 5: Regular AgPd-Paste (20x)
Fig. 6: Watersoluble AgPd-paste (50x)
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Water soluble metallization pastes for AlN
Fig. 7: Regular AgPd-paste (50x)
Fig. 8: Interface AgPd to AlN (watersoluble paste) [bar = 10 µm]
Fig. 9: Interface AgPd to AlN (regular paste) [bar = 10 µm]
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Water soluble metallization pastes for AlN
4. Ecological comparison
There are many arguments about the advantages or disadvantages of watersoluble metallization
pastes. Besides handling, screening behavior and physical parameters there remains the most
important question about the ecological impact. Is it worthwhile to switch to an alternative watersoluble
paste system ?
To evaluate the environmental efficiency a comparison in form of an ecological screening was made
by the Fraunhofer Institute IVV - a very competent address for ecological assessments in Germany.
The desired outcome of this screening was to get an objective judgement about the advantages or
disadvantages of the newly developed watersoluble AgPd-paste (type FK1205W) compared to the
standard paste (type FK1205)
The analysis was limited to an ecological evaluation of the following process steps:
• paste preparation (without assessment of the metal powders)
• screening process
• cleaning of tools and equipment
• waste and recycling of paste residues and cleaning agents
AgPd-Paste with standard
binder formulation
AgPd-paste with water-soluble
binder formualtion
(FK1205), 1 kg
(FK1205 W), 1 kg
1600 g Pregan Universal 4
none
CO2 (direct combustion)
11,99 kg
0,0007 kg
CO2 (combustion process)
0,03 kg
0,08 kg
Emission of solvents
Terpineol 133,2 g
Other emissions into the
atmosphere
Dibutylphthalat 44,4 g
Emissions via waste water
Dibutylphthalat 0,61 g
Butylcarbitol 1,68 g
Critical waste
1,95 g
none
Primary energy
consumption
None
12 kJ
Butylcarbitol 155,6 g
Table 1: Environmental data of both paste types
The necessary information and specific data were supplied by different raw material vendors, a silkscreen supplier, a hybrid manufacturer, and a recycling company.
Identical materials and process steps which yield the same contributions in either case were not
considered in this evaluation. More details are listed in table 1.
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Water soluble metallization pastes for AlN
The main results of the environmental efficiency analysis can be summarized as follows:
• total energy consumption and global warming potential of the water- soluble paste is an order of
magnitude lower than the standard paste (quantitative data available, see Fig. 10)
• the watersoluble paste shows slightly better values in regard of critical wastes (quantitative data
available, see Fig. 10)
• the toxicological potential of the water soluble paste is much lower (qualitative judgement)
Fig. 10: Ecological comparison of the three critical environmental parameters
Considering all relevant data about emissions, critical wastes, and energy consumption an estimation
of the contribution to the global greenhouse effect is possible. The manufacturing process of the
standard paste exceeds that of the watersoluble paste by a factor of 150 x. The main negative
contribution has its origin in the CO2-emission caused by the combustion of the cleaning agent
Pregan Universal 4. This mixture of solvents is mainly used for cleaning screens and tools. These
emissions can be totally avoided by the use of water.
Essential differences between the two processes in respect to the ecological potential result from the
emissions into the atmosphere (solvents used for cleaning purposes and exhaust gases from the firing
process) and the waste waters (residues of organic binder or paste during paste manufacturing and
cleaning processes). Emissions of heavy metals and chemicals like dibutylphthalate, terpineol and the
components of the cleaning solvent Pregan Universal 4 are most relevant. In the new process there
exists only one chemical (butylcarbitol) with a possible negative toxicological potential. Unfortunately,
a quantification of the individual contributions of these components cannot be completed at the
moment but a quantitative estimation will follow as soon as the toxicological data of these materials
are available.
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Water soluble metallization pastes for AlN
Although a fully quantitative ecological assessment cannot be completed at the moment due to
missing toxicological data, a good qualitative comparison between the two paste types and processes
based on the known environmental contributions could be made. The results speak for themselves.
5. Summary and Conclusions
With a newly developed AgPd-paste for postfiring applications on aluminum nitride substrates it could
be proven that an alternative watersoluble metallization paste with comparable or even better
screening results than the standard paste can be realized. A supplemental ecological assessment
which took into account all material and process related differences between the two paste types
revealed substantial advantages of the new watersoluble paste. One important aspect is that all
cleaning can be done in water. Thus, the use of organic solvents can be eliminated completely.
In a consecutive project phase it is intended to develop a complete paste system for aluminum nitride
substrates including insulator and resistor pastes. As a further logical consequence an extension to
other paste systems which are used in the electronic industry should be taken into consideration. By
using water for all cleaning purposes hazardous and toxic solvents can be substituted completely.
Furthermore a more efficient material recycling can be realized. Besides the ecological aspects water
would generate an additional cost reduction.
This project was supported by the German Federal Ministry BMBF (01-ZV9421/2, 01-ZV943B/8)
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