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Enhance product performance under extreme environmental conditions
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Mercury supports multiple coating materials, from acrylic
to urethane to parylene; however, the standard product
is Mil-I-46058C and IPC-CC-830-compliant urethane.
Experiments have demonstrated that tin whiskers can tent
or project out of a thin layer of conformal coating but cannot
repenetrate it. While this affords some protection against
tin whisker growth, care must be paid to situations where
minute space between contacts (less than 0.010") allows
tin whiskers to grow even with a conformal coat in place.
This tight space can prevent conformal coat material from
fully encapsulating gull-wing leads, with the interior leg of
the lead being exposed and unprotected. Mercury 's MOTS
products minimize the use of fine pitch components and, when
required, tin-lead solder dip fine pitch lead-free devices.
Traditional mitigations may also not be applicable with
press-fit connectors. Conformal coating is often unable to
penetrate and cover exposed leads on these connectors due
to the design of the connector itself. To resolve this issue,
Mercury MOTS products apply underfill to these connectors
to provide the same benefits of conformal coating.
DFN/QFN PACKAGES
While convenient for many applications, DFN/QFN packages
present challenges when exposed long-term to highly rugged
environments. Primarily, the low height of a soldered package
is problematic. The very low-profile packages produce solder
that is correspondingly reduced in height. With less solder to
absorb movement from thermal expansion/contraction, the
stress from the movement is almost entirely expressed as
shear. The risk of long-term shearing to electronics integrity
is too high; therefore, Mercury MOTS-compliant designs
avoid these packages in favor of gull wing alternatives.
MANAGEMENT OF VENTED PACKAGES
Many large BGA flip-chip packages are vented – meaning
there is an open vent from the inside to the outside of
the package. This prevents issues when unwanted air or
contaminants expand within the package, causing damage
over time. Vented packages also carry challenges. To
remove wash-down moisture from inside vented packages
after conformal coating is applied, a significant and
measured baking cycle is required prior to coating.
As with BGA solder, die attach (when not wire bond) is preferred
to be accomplished with leaded solder – but mostly lead-free
solder is used in the package. Some device manufacturers
will produce a more rugged variant, using leaded solder
inside. In these instances, Mercury re-scans the device to
ensure no changes in height have occurred, as packages have
often either grown or shrunk during the process changes.
LEVELS OF MOTS
The standard MOTS offering includes all the protections
described above. MOTS+ is available for customers who need
to reach beyond Level 2B to Level 2C tin whisker mitigation
or require special test environments, special process
controls, specific underfills or other custom features. In
these cases, a customer-specific part number is generated
and the required tests and/or processes are applied
seamlessly to the product during manufacturing. The highest
level of MOTS, MOTS Ultra, solder bumps QFN packages to
avoid the joint fatigue issues for which they are prone.
Figure 6: Convert DFN/QFN packages to gull wing packages
Figure 5: Fine pitch leads on components
