By Mike Konrad, President and CEO, Aqueous Technologies Corporation
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Trident batch cleaning system.
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Within the
cleaning sectors of the electronics assembly industry lays a strange
mixture of science, tribal knowledge, contradictions and urban legends.
As the electronics assembly industry again embraces cleaning as a
mainstream process, it may be prudent to review the "conventional
wisdom" as it relates to contamination removal from circuit assemblies.
First,
let us begin with some basic historical facts. Cleaning is not a new
process. In fact, cleaning has been a staple of the electronics assembly
process from the invention of the electronic circuit board. In the
cleaning industry, we divide our world into two sections; pre-1989 and
post-1989.
Before 1989, virtually all circuit assemblies were
cleaned after reflow. Flux and other contamination were removed from the
assembly prior to use. We stuffed the board full of components, cut and
clinched the leads, soldered the components to the board, then removed
the flux. Much of that changed in 1989.
CFCs Banned from Cleaning
Chemicals
containing CFCs were determined to be harmful to the Earth's ozone
layer and, thus, were restricted or eliminated from production. The most
common cleaning solvents used to clean assemblies contained CFCs that
presented a problem for the electronics industry. While some
manufacturers converted to environmentally responsible water-based
cleaning technology, another option soon presented itself. So-called
"no-clean" flux was introduced. No-clean flux was designed to not be
cleaned. The majority of the electronics industry embraced no-clean
technology except for military, medical and a handful of other
high-reliability manufacturers. Virtually overnight, the cleaning
industry shrank to a fraction of its pre-1989 size. Cleaning for the
majority of manufacturers was dead.
Today, buyers of
cleaning/defluxing systems fall into two groups. The first group is
comprised of people who were in the industry before 1989. In many cases,
their knowledge of cleaning machines and processes is obsolete. The
second group is comprised of "younger" people who have had no direct
experience with cleaning and who frequently lack any reference point.
Those in either group may find this information helpful.
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Cleanliness testing system.
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So
many things have changed since cleaning was considered to be a
mainstream process. Today's assemblies, due in part to miniaturization,
have a much lower "contamination tolerance" threshold. While some
assemblies function reliably with a specific volume of contamination,
others rapidly fail. Clock speeds, component densities, assembly
geographies, solder alloys, environmental influences and other factors
determine how much contamination an assembly can handle without failure.
Urban Legends Debunked
Water may be used to remove water-soluble (OA) flux but not rosin.
False. Water, mixed with a low concentration of a water-based defluxing
chemical, removes all flux types (water soluble, rosin, no-clean).
Solvents work better than water.
False. While solvent technology works well, a water-based process works
better — in most cases. Some in this industry may remember the pre-1989
days when an assembly that had to be "extra" clean — i.e. for conformal
coating purposes — was cleaned using a solvent, then followed up with a
DI water cleaning process to improve the cleanliness of the assembly's
surface. Water-based cleaning and, more importantly, water-based
rinsing, typically produces superior cleanliness results compared to
solvent processes. This fact was reiterated in IPC's "Phase II" test
results — a comparison of solvent vs. water-based cleaning.
Conveyor vs. Batch Cleaning
Inline (conveyorized) cleaning machines are better than batch cleaning machines.
True and False. The fact is that both batch and inline technologies
clean equally well. No one configuration cleans better or worse than the
other. One should choose batch or inline based on the volume of
assemblies that need to be cleaned. Batch cleaning systems are capable
of cleaning volumes ranging from low to high. Inline cleaning machines
normally are associated only with high volumes. When all cost centers
are accounted for, the operational cost of a batch process is
approximately 10-20 percent of the cost of an inline cleaning process.
As the cleaning volume increases, the cost differential lowers. The cost
per assembly on an inline cleaner begins to lower when the cleaning
volume approaches several thousand assemblies per day. Volumes less than
that generally are better suited for batch-format cleaning processes.
Dishwashers/Glassware washers work well.
False. While many batch-format cleaning machines may resemble
dishwashers, dishwashers typically are not compatible with the defluxing
chemicals nor are they capable of acceptable under-component
penetration. Dishwashers are for dishes.
We do not need a powerful machine. The defluxing chemical lowers the surface tension, allowing adequate impingement.
False. While defluxing chemicals do lower the wash solution's surface
tension and allow for improved under-component penetration, the real
issue is the rinse cycle. Today's defluxing results fall into two
categories: excellent and disastrous. The worst thing one can do is to
partially clean an assembly. Wash solution allowed to remain on an
assembly, due to inadequate rinsing, is far worse than flux being
allowed to remain on an assembly. Rinse is the most critical part of a
successful defluxing process. Rinse water, unlike chemical-enriched wash
solution, has a higher surface tension. That means the rinse water must
be forced into tight spaces to effectively displace the lower surface
tension wash solution. This is accomplished by forcing water through
multiple nozzles that diffuse the solution into smaller water particles.
The smaller the water particle, the better its ability to get under
tight spaces.
IPC standards state how clean is clean.
True and False. While IPC TM650 does state "how clean is clean," we
cannot recommend that anyone actually embrace that value. IPC allows for
the use of a resistivity of solvent extract (R.O.S.E) test to determine
the cleanliness of an assembly. These testers have been in production
for more than 30 years. In fact, the cleanliness standard of >10µG
NaCl/in.2 was developed in the 1970s. Today's assemblies
contain advanced geographies not imaginable in the 1970s. There are some
that suggest that we abandon the use of R.O.S.E. testers. That would be
like throwing the baby out with the bath water. Instead, one should
calculate the cost of failure. If an assembly fails, will that lead to
property damage, loss of life, loss of reputation? If so, one should
consider selecting a lower pass/fail value. In short, the more critical
the reliability, the closer to 0.0 one should strive to achieve.
