Conductive Paints and Pastes for SEM Sample Preparation
When it comes to important samples, don't settle for anything less then
the best!
A few general comments about the SPI Supplies® brand of conductive paints
and pastes:
All of paints and pastes listed on this page have been designed for specific microscopy
laboratory applications, but we also know that all of them can be used to make any surface
conductive. We offer the following comments to help those trying to decide which of these
various products would be the most appropriate for their own particular application.
Remember that the ultimate characteristics of the paints and pastes spring from the inherent
properties of the respective metals, those being either silver
or
platinum.
Applications in microscopy:
The SPI Supplies Brand of silver paints are used widely throughout the world
in the leading laboratories where scanning electron microscopy (SEM) and
scanning probe microscopy (SPM) is performed. The high purity of the silver
solids and suspending medium make the SPI Supplies Brand the preferred brand
wherever these kinds of microscopy are performed.
Conductivity:
All of these paint products are electrically conductive. The carbon paint, however, while
it is sufficiently conductive for its intended use for the grounding specimens for scanning
electron microscopy, may not be sufficiently conductive for other applications, such as
plating. One would have to try it in order to find out if it would work in their own
specific plating application. The silver and platinum paint products are much more conductive
than the carbon paint products, but also more expensive.
Cost:
The silver and platinum paints, especially the platinum product,
are expensive, and their cost may be prohibitive for certain applications.
Texture:
For SEM and most other applications, surface texture of the dried paint layer is not important.
But for plating applications, it can be very important.
All of the paint products are designed for brush application, using the brush applicator in
the bottle cap. Any reasonable thickness of plating may "see" the texture of the brush strokes
in the underlying paint. With "brush plating", we simply do not know what will happen in terms
of the final surface texture. For all non-SEM applications, the buyer will have to do their
own testing to be sure of suitability of use and only they will be able to determine whether a
particular paint will or will not work in their particular case.
Compatibility:
With the exception of our new SPI SafeShip™ Nonflammable Silver Paint, all of our paint products
are solvent-thinned systems and when the application is the surface of a plastic, there is always the
possibility that the thinner could attack, swell, or partially dissolve the surface. Since the
SafeShip™ Nonflammable Silver Paint contains some water in its diluent system, we would expect
that this product would be less aggressive on many plastic surfaces.
Adhesion:
In general, the adhesion of the paint to a surface is dependent on a variety of factors
including surface preparation before the application of the paint. For the typical SEM
application, one does not do any substrate preparation. But for a plating application,
and if the substrate is plastic, there are some tradeoffs. If the solvent attacks the
surface sufficiently to get really good adhesion, the substrate will be damaged (roughened)
and this damage will be visible through a final plating. If the solvent does not "wet" the
surface, and if it is a non-solvent for the plastic, then the adhesion is likely to be poor,
and so will be the plated layer. For the attachment of fine wire electrodes, any of the paints
should work acceptably.
Drying rates:
All of the products on this page were designed to be air drying at room temperature. For the
silver paint products, the SPI Supplies
Flash-Dry™ Silver Paint has a liquid carrier system with the highest vapor pressure
and therefore evaporates (dries) the fastest. On the contrary, because of the aqueous fraction
in the diluent, the SPI SafeShip™ Nonflammable Silver Paint
will have the longest drying time.
Cryogenic Applications:
We are frequently asked about the use of SPI Conductive Silver Paint to
make electrical connections at cryogenic temperatures. A better choice would be
SPI Silver Paste Plus™.
We know that some customers have used either product in cryogenic applications,
but we do not have detailed information on what they were doing.
Typically, in the typical use, the paint or paste may be used to attach a
conductor to a pad, but the actual contact is between the conductor and the
pad, so the conductivity of the paint or paste is less important than its
mechanical properties, which are not the best. A better result may be
obtained by using the firing procedure, the bond is converted to metallic
silver, which should provide a high level of conductivity even at cryogenic
temperatures. However, in more cases than not, the greater concern seems to
be more about the effect of thermal stresses on the quality of the bond
(as opposed to questions of conductivity).
Where shipping rates for small quantities of flammable materials become prohibitive,
the SPI SafeShip™ Nonflammable Silver Paint
can probably be used because of its inherently higher viscosity.
Temperature range of use:
At temperatures about 500°C, silver starts to sublime, and this is undesirable since
silver contamination is hard to clean out of a vacuum system. This is then the kind of
situation where one must turn to the
SPI Supplies Platinum Paint.
Small conductive "dots":
The smallest silver "spots" or "dots" can be made with
SPI Silver Kwik-Stik™ while larger
dots can be made with the SPI Supplies brand Silver Streaker™.
Non-conductive paint:
Not everyone wants a paint that is conductive. Some would prefer an insulating tape,
and for that reason, we have formulated the SPI-Tac™ Liquid Adhesive.
Shipping costs:
This paragraph is meant only for those placing orders from countries where
one can not ship 
designated flammable products.
It does not apply to shipments to domestic USA destinations, Canada, most western European
countries, and certain countries in Asia like Japan, Taiwan, and Singapore. It is meant to
those destinations where anything flammable, even in the smallest of quantity, must be
shipped either by sea freight or by air freight, and at far higher shipping costs than
if the shipment was made via one of our preferred courier services, UPS or FedEx. It is
for these countries that we have specifically formulated the innovative and so far as
we know, unique product, SPI SafeShip™ Nonflammable Silver Paint,
which has been formulated with enough water to ensure that its flash point is sufficiently high that it is
not considered to be a flammable material.
Carbon
Platinum
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Tuesday May 13, 2008
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