Scott: The first question you have to answer is what is the highest contact
resistance the design will tolerate. Most straight carbon polymer thick film
(PTF)materials can only go down to about 20-30 ohms/sq.

Processing shouldn't be a problem. We have built literally hundreds of
thousands of boards and millions of keypads. The PTF is terminated over HASL
on both the top and bottom sides of the board. Then processed thru either
single side reflow, double side reflow and/or wavesolder depending on the
type of product we are building.

No-clean flux is used for SMT and water soluble flux for wavesolder.
Cleaning is done with inline conveyorized equipment, semi-aqueous aliphatic
hydrocarbon cleaning chemistry for the no-clean and a saponifier for the
water soluble flux.

We do not mask thru any of these processes.

I would recommend you contact the PTF manufacture and use the correct
material(polymer type) for your processing applications. All PTF's are not
created equal.

The only other advice I would give you is to keep the termination area to
the PTF small. We normally use a 12 mil trace that is overlapped by the PTF.
It is also important that the bare board supplier apply the PTF over a clean
HASL termination followed by a complete cure. A poorly cleaned HASL surface
or incomplete cure will result in possible interfacial failure.

                                                                Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076

-----Original Message-----
From: Scott Lefebvre [mailto:[log in to unmask]]
Sent: Thursday, January 20, 2005 11:37 AM
To: [log in to unmask]
Subject: [TN] Carbon Keypad


Currently we use gold for all of our keypads.  I want to reduce the cost of
a product and thought about switching to a carbon keypad.  I am not familiar
with the process and wanted to know a few things.  We mostly use a
water-soluble process and our entire product is washed.  Some boards are a
mixed technology using SnPb currently.

?  Are there any Reflow solder concerns I should be aware of?
?  Are there any Wave solder concerns I should be aware of, like masking
pads during wave solder?
?  Are there any other concerns period that I should be addressing.

Thank you,

Scott






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