In a message dated 15/04/99  08:45:11PM,  [log in to unmask] writes:

> Does anybody know of a dry film adhesive that is conductive?  If so what
>  conductivity?  What about interfacial conductivity, as in where the
>  adhesive is attached to another material is the conductivity lower than
>  that of either the bulk adhesive or what it is attached to?
>
>  Thx

There are several suppliers of dry film adhesives which can be either
electrically  (which is what I guess you are asking) as well as thermally
conductive, or thermally conductive and electrically insulating. You didn't
specify the type of application you had in mind, eg ensuring electrical
continuity for shielding, or device assembly etc. This will effect your
choice of supplier as they tend to specialise.

Here are some general comments which  I hope will be helpful. Be aware that
the use of adhesives is very application specific so simplified
generalisations of the type you are about to read should be read with that in
mind.

Compared to metals electrically conductive adhesives are orders of magnitude
lower. For actual numbers you will have to verify for each supplier and
product as there are several trade offs to be made according to your actual
application.

You are right to question the effect of the surfaces you are going to bond
to. What you are interested in is the overall conductivity of the assembly
and conduction through the bondlines is a significant part of this. Different
metals will give different characteristics and because there is a distinct
interface in adhesive bonding (unlike solder joints) characterisitics can
change in relatively short periods. Generally metals which tend to oxidise
(usually light white metals like aluminium are avoided), the heavy passive
metals which do not oxidise, (eg gold) are good for conductivity. By the well
known Law, the opposite is true for adhesive properties.

An extreme example would be the use of aluminium (aluminum in US). This will
go high resistance (open circuit) in a matter of hours electrically whilst
still giving excellent thermal performance and adhesion. This is due to the
formation of aluminium oxide (aka alumina) at the bondline.

With the correct choice of materials and service conditions conductive films
are reliable and have a long history in hi rel applications going back many
decades.

One of the longest established suppliers of highly electrically conductive
films is Ablestik in California and they are probably as good a start point
as any.

Good luck

Mike Fenner

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