Cleanliness
with OA Flux
The selection of a water soluble (OA) flux is usually driven by a desire
for a very high level of cleanliness on the final assembly. Many medical
companies choose an OA flux because it allows them to completely avoid
addressing the issue of possible entrapment of contaminants in the residue
of a no-clean flux.
The function of
a flux is to remove oxides and other nonmetallic impurities from soldering
surfaces to increase reception to the solder process. After soldering,
the OA flux residues must be removed promptly because the residues are
both conductive and corrosive.
The cleaning process plays a critical role in the manufacturing yield
and product reliability of electronic assemblies produced using OA flux.
While the degree of cleanliness required may vary with product type
and performance needs, the cleaning process must be thorough and cost-effective
without damaging the components.
Mixed
Mode Assemblies
In mixed mode assemblies with one or more SMT operations and a wave
solder step for the through-hole components, the board and the parts
are cleaned at least two times. With two fusion operations, the board
is cleaned three times. Components that cannot be water cleaned must
be hand soldered in an “after wave” step.
Most assemblers elect to use a no-clean flux for any hand soldered components
to prevent inconsistencies from trying to spot clean the board. Most
manufacturing engineers agree that the presence of small amounts of
a no-clean flux is preferable to the potential for residual OA flux
from an incomplete cleaning operation.
Ever since the elimination of the use of solvents such as chloro-fluoro
carbons (CFCs), cleaning has become a process with a narrow process
window and a limited ability to correct for poor handling or assembly
practices. The cleaning process must address the type of flux, the final
cleanliness levels, and the technology of the assembly.
Surface
Mount Assemblies
In general, the cleaning of surface mount assemblies is more difficult
than through-hole components. SMT parts have small spaces between the
board and the components that may entrap flux. When chip resistors and
capacitors are attached with adhesive, the adhesive itself fills most
of the space between the chips and the boards, leaving little room for
flux entrapment between the adhesive perimeter and the pad metalization.
This positive benefit is completely eliminated if the adhesive is not
properly cured. Rapid curing of the adhesive or curing at too high a
temperature may result in small fissures that entrap flux molecules
during the wave soldering operation. The entrapped flux is almost impossible
to remove and leaches out during exposure to condensation cycles. Chip
adhesives must not be cured with fusion profiles.
Components with fine pitches (lead spacing below .020") have lower
standoff heights coupled with a large area underneath the part. This
increases the potential for flux entrapment by making it more difficult
for cleaning solutions to penetrate beneath the part to remove the flux.
Fortunately, the ball grid arrays (BGA) are increasingly replacing very
high pin count fine pitch devices. BGAs are easier to clean because
of their higher standoff. The exception is the micro BGA (µBGA),
which has a standoff height of only 3.0 mils (.003").
Standard processes do not remove flux from beneath µBGAs. Although
higher water temperature helps, increased nozzle pressure does not.
Micro BGAs typically require higher water purity levels and the addition
of a material to reduce surface tension.
Deionization
Flux manufacturers
recommend saponifiers for some difficult-to-clean assemblies or cases
requiring exceptional cleanliness. However, most OA fluxes are adequately
cleaned using deionized (DI) water.
In deionization, water is passed over a resin bed to remove ionic contaminates
through an ion exchange system. In this process, the dissolved minerals
(ions) are removed from the water by cation and anion resins in an exchange
process. The cation resin replaces all positively charged ions, such
as calcium and sodium, with hydrogen ions (H+). The anion resin removes
all negatively charged ions and replaces them with hydroxyls (OH¯).
When H+ and OH¯ combine, they form purified water.
The cation resins and anion resins may be used in one tank or in separate
tanks, which is more common. The tanks are replaced when the resistivity
of the DI water falls below a prescribed resistance level (about 2 megohm-centimeters).
Washing and rinsing
with an aqueous media does not raise any concerns for potential component
damage by static charges, since both the aqueous media used and the
humidity in the process chambers counteract the buildup of static electricity.
A measurement device called a static meter determines whether anti-static
measures need to be employed. It is a recommended practice to electrically
ground the cleaning and drying equipment, including the conveyor belt
of the in-line cleaner.
Aqueous
Cleaning Systems
Batch and in-line
systems are the most common types of aqueous cleaning systems in use.
A batch aqueous system is very much like a household dishwasher: the
assemblies are loaded vertically like dishes; detergents or saponifiers
may be added; and cleaning and drying are accomplished in different
timer-controlled cycles.
In-line systems are similar to batch systems except that the board passes
through different modules rather than different cleaning cycles. In-line
systems are used for high volume production. High cleanliness levels
of assembled boards can be achieved using an OA flux if the proper process
is followed:
