changes in weight. The significance of an observed problem
depends on the application.
Compatibility problems that are readily and immediately obvious, while vexing, are not nearly as worrisome as
the problems that lead to field failure down the road. If, for
example, the adherence of a coating degrades after a change
in the cleaning process (hopefully, this happens during initial
testing rather than in production!), the problem might be
readily resolved by modifying the cleaning process. What if
there is loss of adherence after months or even years?
What is significant?
Weight gain of a given material of construction is sometimes
treated as a compatibility problem—and the problem may
or may not be considered significant. For example, weight
gain due to solvent adsorption may be reversible. It can
sometimes be dealt with by adding a bakeout; such bakeout
effectively enhances the drying step. Drying or bakeout must
be adequate. This may require testing, for example, by head-space gas chromatography. Since there is no such thing as zero
residue, it is up the manufacturer to determine the level of
bakeout that is adequate. Outgassing of solvent is very likely
to be unacceptable when the product will be used in a sealed
system. Outgassing is virtually certain to be unacceptable with
implantable medical devices, where the solvent would be considered to be a leachable residue.
Obvious compatibility problems are not
nearly as worrisome as the ones that
lead to field failure down the road.
BFK Solutions LLC
Pacific Palisades, Calif.
hemicals have the potential to interact with the prod-
uct, and not always in a desirable manner. Chemicals
are essential to the manufacturing process. We have to
strike a balance between the desirable function of the
process fluid or cleaning agent and the potential for damage to
the product being fabricated.
Sources of materials incompatibility
Because chemicals are ubiquitous in manufacturing and in the
surrounding environment, it is important to have an eagle eye
and an open mind. Cleaning agents are always a likely culprit,
but other process fluids can inadvertently damage the surface.
Co-processed product can result in galvanic interaction, particularly in aqueous processes. Airborne molecular contamination (AMC) can be a source not only of thin film contamination but also of surface changes.
Consider the impact of chemical reactivity. Even if the fluid
itself does not damage the product, breakdown products may
be problematic. One classic example is the breakdown of halogenated solvents to form acids that can damage both product
and processing equipment. Acetone is miscible with water. In
one instance, while no problems were reported at ambient
temperature, when acetone was used in a specially-designed
low-flashpoint system so that it could be heated, the acetone/
water blend reacted with magnesium.1
Often, cleaning agents are evaluated solely on the basis of
compatibility tables provided by the chemical vendor or are,
at most, tested by immersion of one material of construction at a time in the cleaning agent under consideration.
Compatibility tables can provide indications of what might
or might not be suitable.
Tables and single material ambient immersion might
be sufficient to exclude a chemical from consideration.
However, the same conditions that promote effective cleaning, such as temperature, time of exposure, and cleaning
force, can also exacerbate materials compatibility issues.
Therefore, proposed processes for critical cleaning must be
tested using dynamic compatibility studies prior to being
adopted for routine use. This involves evaluating the subassembly or product in the proposed process itself. In a companion article, we expand on this concept. 2
What does incompatibility look like?
Materials compatibility problems are diverse and ubiquitous.
In grossly obvious compatibility problems, the part may actually dissolve in the chemical in question. The surface may
change in color or texture. In other instances, there may be
dimensional changes, discoloration of the process fluid, or
What does the surface look like?
Fear of changing the surface means that process development and process change are often fraught with angst. This
may lead to dependence on a sub-optimal process. While
an over-emphasis on lab testing can devour scarce company
resources, judicious use of surface characterization can provide confidence and short-cut failure analysis times.
The right surface analysis can clarify if there is a residue
problem, a materials compatibility problem, or a combination
of the two. Therefore, particularly with high-value product, we
suggest both extractive and in-situ surface analysis as a benchmarking tool. Selecting the best methods of analysis requires
skill, logic, and experience. It is usually desirable to look at the
physical attributes of the surface. The right testing may also
involve elemental analysis and speciation. It may involve determining the pattern of residue relative to surface topography.