David,

A 'no flow underfill', or flux underfill is a polymer that is applied to the
substrate (PWB or ceramic) prior to placement of the flip chip - hence, no
(post reflow) capillary flow is performed/required.

Prior to reflow the polymer acts as to retain the device in position.
During the preheat portion of the reflow profile, the polymer becomes active
(like a flux).  As a consequence of reflow, the activity of the material is
ideally fully expended, leaving a full polymer underfill.

They also come in two versions.  One that is essentially fully polyimerized
as a consequence of reflow, and another that is only 20-30% polymerized
thereby allowing rework of the flip chip before a 'final' cure.

It can stream-line the process significantly compared to a post-applied
underfill, but has some drawbacks.

To date, these materials consist of unfilled resins = big thermal
coefficient of expansion. Large devices on high TCE substrates (PWB's, etc)
with large temperature excursions are not likely to be the ideal
application.  Small die (<0.100" square are much more forgiving - naturally,
as are low I/O counts with large, high-lead bumps.

We use LTCC systems having a TCE of 5.8PPM (compared to alumina at 7-8PPM,
or generic FR-4 at 18-20PPM).  Testing continues.

Three established suppliers come to mind -

Kester - Speak with Torey Tomaso about their 9101 systems.

Alpha - I forget their part number.

Emerson & Cuming (Amicon) - E1300 series (1330, 1350) materials

In any case, check out the websites.

Good luck

Steven Creswick
Mgr. Advanced Process Development
CTS RF Integrated Modules

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