Compatibility of polymers and fluxes: Getting to the heart of the matter
for conformal coating are:
Incomplete coating: A frequent cause of
field failure of topographically com-
plex PCBs is ‘shadowing,’ or failure to
adequately coat the PCB, caused by the
presence of other components.
Partial cure: Failure to cure the coating
completely, leading to a tacky coating
that can attract dirt. It will also be a less
effective diffusion barrier that may flow
Figure 2.a. Process steps in a typical power semiconductor assembly process.
out and thin down under gravity.
Non curing in contact with flux
residues: Failure of the coating to cure
in contact with materials on the surface
of the assembly, leading to the possibil-
ity of moisture diffusing through the
protective coating and condensing to
form a liquid water film.
Electrical failures are the underlying con-
cern of the engineer in printed circuit board
assembly.
Figure 2.b. Replacing wire bonding with clip-attach processes can eliminate the costly cleaning step.
Test methods
combination of the two together. reflow may be caused by air-entrapment.
The first of the above failure modes is ad-
This can be caused by a poor flow-pat-
• Curing/adhesion: The IPC-CC-830B
dressable only by workmanship standards
tern of underfill or, in the case of fluxes
also covers adhesion, but under a
and final quality assurance. The test meth-
with higher residue levels, a simple
general comment about the need to test
ods for PCBA conformal coatings in contact
blockage to the flow of the underfill.
compatibility with many different types
with flux are:
Post-reflow voiding may be caused by
of materials, including no-clean flux
outgassing.
• Electrical, as measured by surface insula-
residues. The ASTM D3359-02 covers
tion resistance (SIR) testing
the use of x-cuts or lattice-cuts in a film
Delamination: In order to provide
on a ‘metallic substrate’ to which an
strong support for the relatively weak
• Curing and adhesion to surfaces
adhesive tape is applied.
solder joints in a flip-chip, the poly-
meric underfill must adhere strongly to
The closest test methods in existence for
both of these sets of properties are as fol-
Flip-chip semiconductor assembly
all interfaces.
lows:
Failure modes
Test methods
Underfill is the primary material that may
• SIR: IPC-CC-830B
4
: This covers QA-
Although it is known that many of the
contact flux residues from a no-clean reflow
type tests and also performance tests for
larger flip-chip underfill manufacturers
process, as shown in Figure 1. The major fail-
the three (A, B and C) categories of con-
have in-house test methods for adhesion to
ure modes for a capillary-flow underfill are:
formal coating materials. Table 3-1 from
fluxes, there is no universally accepted test
said document covers the test methods,
Voiding: Voiding in the underfill before method for determining the extent of void-
including IPC TM-650: 2.6, 3.3, and .4,
which are the test methods for flux and
conformal coatings, respectively. How-
ever, there is no SIR test method for the
Figure 3. The top junction of a simple P/N (single
junction) device with its metallization, as used in a
clip-bonding assembly process with no-clean solder
paste. Figure 4. The structure of the device shown in Figure 3.
www.globalsmt.net Global SMT & Packaging – September 2008 – 21
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