JSC-66491
Revision: Baseline
July 2013
as an insulative barrier to Foreign Object Debris (FOD). However, spray or hand-brush application may
only be capable of achieving partial coverage for complex PWA designs, as the spray and hand-brush
processes only coat the visibly exposed surfaces, with some minor capillary flow under component
bodies. This leaves the most whisker prone areas of the assembly exposed, such as the backside of
component leads and leads located completely underneath component bodies (i.e.: BGA, flip chip, etc.).
b. Dip Coating
Dip coating, if performed correctly, will ensure the polymer flows under and around all package types
and terminations, producing a coating that wets all surfaces. Using a low viscosity material and a slow
submersion rate at a 30 – 45 degree angle will reduce the incidence of voiding (bubbles). Although
there is always some bridging of fine pitch component leads if a dip coat process is used, a slow and
steady withdrawal rate is recommended to minimize the effect.
c. Chemical Vapor Deposition
Of all the conformal coating processes, Paraxylene Resin (XY) is considered the best coating for
mitigation against whisker growth because it coats all exposed surfaces of the PWA with a stress-free,
pin-hole free, uniform thickness of material, including the backside of component leads and leads
located completely underneath component bodies (i.e.: BGA, flip chip, etc.).
3.2.4.4 EMBEDMENT / ENCAPSULATION
Use of embedment and encapsulation as mitigation is recommended for integrated circuit, fine-pitch, and
area array device packages, where conformal coating would have limited effectivity at coating the backside
of tightly spaced component leads (i.e.: IC, QFP, SOIC, etc.) or the terminations underneath the component
body (i.e.: BGA, flip-chip, etc.). The application process is similar to underfill, in that the embedment /
encapsulant material must flow underneath and completely fill the space between the component body and
the printed wiring board (PWB), then fully wet and cover all termination surfaces. This requires that the
materials be selected to closely match the coefficient of thermal expansion of the component and printed
wiring assembly (PWA).
a. Embedment or encapsulant material shall fully wet and cover all surfaces of parts and areas specified
by the approved engineering documentation.
1. The effectiveness of conformal coating as a whisker mitigation technique depends heavily on
the coating material selected, application method, process control, and workmanship.
2. The use of silicone (SR) or urethane (UR) conformal coating as whisker mitigation is based on
NASA and industry research indicating that a finish thickness of not less than 50 μm [0.002 in],
will increase environmental resistance, retard the development and propagation of whiskers,
and reduce the potential performance and reliability
issues of tin whiskers. This is
accomplished by using the conformal coating as a resilient barrier to trap and contain a
significant portion of whisker growths to within the coating layer, to reduce or prevent whiskers
from contacting and shorting adjacent conductors, to limit exposure to moisture, and to limit the
development of coronal discharge.
3. While current research has suggested that a paraxylene (XY) coating thickness of greater than
25 μm (0.001 in) will control whisker growth, a final coating thickness of 50 - 100 μm (0.002 –
0.004 in) is recommended to provide optimal protection. Minimum total cured thickness
specified for paraxylene (XY) may require application of multiple coatings.
Verify correct version before use at http://server-
mpo.arc.nasa.gov/Services/CDMSDocs/Centers/JSC/Home.tml. 13