Brent,

I'm not sure if there is a 'standard' way, but here are a couple things to
consider.

If you have a source in mind - check with them for guidance.

If it is a standard ceramic (alumina) thick film, it will typically be on a
96% alumina body.  The thickness of the body (usually called 'substrates')
will vary depending upon the panel size the supplier wishes to deal with,
whether the substrate is green-scored, laser scribed, or laser cut (or
diamond sawn) after thick film processing.

The Pd/Ag metallization is usually screened onto the substrate and fired at
850-900°C.  Most Pd/Ag's have decent aged adhesion, and good wetting
characteristics.  Leaching can be a problem if the part is subjected to
multiple rework (reflows) operations, especially if exposed to 'an infinite'
source of solder - like a solder pot, or wave (as opposed to limited solder
with a stenciled deposit, etc).  

Pt/Pd/Au's and Pt/Pd/Ag's generally offer better aged adhesion and leach
resistance, but tend to wet with a little more difficulty than the Pd/Ag's.
Cost is also much greater.  Normally reserved for the high rel, or most
likely to be reworked type of product.

Wire bondable conductives are also screened on, in the same manner as the
solderable ones.  For gold wire bonding, one normally uses a gold conductor,
again fired at 850-900°C.   If aluminum wire bonding, one normally selects a
doped gold conductive - usually doped with a wee bit of Pt or Pd.  The gold
is normally not plated on.  There is an intermetallic region formed between
the Pd/Ag and the wirebondable gold - which if improperly designed can lead
to depletion of the gold at the interace.  Most non-garage shop operations
will know what they are doing so this should not be a problem.

Normally, the fired film thickness of the solderable and wire bondable
conductives will be on the order of 12-18um - although just because a
particular vendor's paste material is designed to fire outside this
'generic' window, does not automatically mean it is bad.  Check with your
supplier, and get their guidelines and paste suppliers (DuPont Electronic
Products, Ferro, EMCA-Remex, ESL .....).


Steven Creswick - CTS

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