Compatibility of polymers and fluxes: Getting to the heart of the matter
Compatibility of polymers
and fluxes: Getting to the
heart of the matter
by Andy Mackie, Ph.D., and Christopher Nash, Indium Corporation, Utica, NY, USA
introduction
development, mostly due to the fears of
Few things strike more dread in
Fluxes are the liquid or semi-liquid materials
engineers about process failures.
the hearts of technical service
that are used to remove metal oxides from
Figure 1 shows the basis of the processes
personnel than the words “Is
solder and solderable substrates before
that will be discussed in this paper for stan-
your flux compatible with mate-
reflow of the solder onto the substrate to
dard SMT, power semiconductor die-attach
rial XYZ?” This issue is espe-
form an electrically and thermally conduc-
and semiconductor (flip-chip) assembly.
tive, and mechanically strong solder joint
1
.
cially difficult if XYZ polymerizes
Throughout this paper we will be using the
printed circuit Board assembly
(cures). The customer’s expec-
term ‘polymers’ as a blanket term for those
(pcBa): conformal coating
tation is that there will be an
materials that are polymeric, and also those
Failure modes
unequivocal yes-or-no answer, yet that will become polymers after a final cur-
In standard SMT and wave-soldering
there is a myriad of complexities ing stage.
processes, polymers may be applied to the
behind the question that materi-
No-clean processes
2
have been standard
finished electronic assembly as conformal
als suppliers and users alike are
in SMT and wave soldering for years. How-
coatings after the final soldering step. A
sometimes unaware. This ques-
ever, in the semiconductor assembly and
conformal coating is typically a low viscosity
tion may come from engineers in
test (SAT) industry, the movement toward
hydrophobic (water-repellant) polymeric
any of the fields of SMT (surface
no-clean has yet to occur.
material applied to the PCB by spraying,
mount technology) to power semi-
There are obvious cost savings in terms
curtain-coating or dipping and then cured
of capital equipment, materials and waste
conductor die-attach to flip-chip
by a variety of methods, including mild heat-
treatment for companies that choose no-
ing and exposure to ultraviolet light. The
underfill.
clean processes. The question naturally
coating provides a physical barrier against
This paper explains customer
arises: why don’t semiconductor manu-
liquids, particularly aqueous solutions, and
fears that lie behind the compat- facturers and outsourced SAT companies
a diffusion barrier against water vapor
3
. In
ibility question, gives a better shift to no-clean? In most instances, the
this instance, the coating is used to provide
concept of what may be in the
answer lies in the fact that the reflowed flux
protection against harsh environments
customer’s mind when they ask
residues are in contact with polymers. As
where the surface of the assembly becomes
if a material is ‘compatible,’ and
we will see, this apparently simple interface
contaminated with water or even salt-spray,
also explains potential failure
between flux residues and polymers intro-
leading to possible electrical shorting
modes, main control parameters
duces significant complications into process
between adjacent points. The failure modes
and what test methods are
currently available to test
‘compatibility.’
Keywords: Fluxes, Polymers,
Conformal Coating, Flip-Chip
Assembly, Power Semiconductor
Die-Attach
Figure 1. Underfill is the primary material that may contact flux residues from a no-clean reflow process.
20 – Global SMT & Packaging – September 2008 www.globalsmt.net
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