|
Proper processing is important to achieve
optimum system performance after an encapsulating system is selected.
Processing an encapsulating system involves three steps:
Preparation
of the casting mix
Casting
Gelation
and Cure
Casting mix preparation
Before removing encapsulating system
components from the container, stir materials thoroughly. This is
particularly important for filled systems because filler may show a
tendency to settle during storage. In addition, resins with a high
filler content should be heated in an oven at 60C to 80C (140F to 178F)
in their original containers before mixing.
This procedure will facilitate stirring and pouring. Note:
Preheating hardeners is not recommended because of higher vapor
pressure.
To combine resin and hardener, mix encapsulating system
components with a low viscosity of less than 3,000 cP at room
temperature either manually or in an open mixer.
Resin systems with a higher viscosity should be reheated to 40C to 50C
(I 04F to 122F) before the room-temperature hardener is added. To
improve system uniformity and dielectrical properties, degas the blended
casting mix under a vacuum of more than 28 inches of mercury before
pouring.
Casting
To cast resin systems with a
long work life, pour mixed components by hand or from a batch mixer into
an open mold at atmospheric pressure. However, if the encapsulating
system has a short work life, or for long production runs, use of
continuous-type mixing and metering equipment is preferable,
In many cases, it is advantageous to preheat electronic assemblies to a
temperature 20C to 30C higher than the processing temperature of the mix
to facilitate part wetting and help remove trapped air.
To prevent foaming under a vacuum, the pressure in the casting
chamber
should be higher than the vacuum used to prepare individual
system components.
Gelatin and
Cure
Curing is typically conducted according
to the schedule recommended in the product data sheet. Room-ternperature
systems cure at about 25C (77F). Depending on the reactivity of the
casting mix and the ambient shop temperature, cure. tirne is usually
from 12 to 72 hours. The cure cycle of an encapsulating system can be
accelerated by postcuring at elevated temperatures.
Heat-cure systems are typically cured at temperatures above 90C
(194F). However, some encapsulating systems can be cured at temperatures
slightly above 80C (176F). To minimize internal stress, cure
encapsulants in two stages, gelation followed by postcuring. This
ensures good high temperature and thermal cycling performance.
Note: The exothermic heat liberated during the cure process,
especially for room -temperature-cure systems, can produce internal
casting temperatures that are significantly higher than the cure
temperature of the encapsulating system.
For further details on system handling and
mixing contact a technical representative at 1-800-323-5158
|
