| Mr. Tech Dweeb Tech Tip | |
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DI water purity in the U.S. is measured with resistivity in megohm-centimeters, while Europe uses conductivity, which is the inverse. |
Volume 2, Number 2, April 2002
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.
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.
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.
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).
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The exact resistivity level chosen depends on how the DI water is to be used.
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.
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:
If the process has been validated with SIR testing or ion chromatography, cleanliness testing can be used to provide a valid indicator of process control and to ensure customer requirements are met.
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