Encapsulating Electronic Circuits
Epoxy and Polyurethane compounds are widely
used to encapsulate electronic circuits. Depending on the type of components
contained in the circuit and the performance requirements for the finished
components, it is critical to select the appropriate formulation. The type of
fillers, pigments, thermal conductivity, the electrical properties, the amount
of shrinkage during cure and adhesion to containers can all effect the output of
an encapsulated circuit.
The most common causes of change in circuit
output or failure are as follows:
-
Incorrect pigments in the formulation
Some pigments are more conductive than others.
Changes in output can occur when resins containing more conductive pigments
come in contact with high impedance devices.
-
Excessive amounts of impurities in the
filler or the liquid components of the formulation.
Conductive impurities in any of the components
of the formulation used can effect the output if the circuit especially those
containing high impedance devices.
-
Excessive shrinkage during cure.
Excessive shrinkage during the curing process
will result in pressure on the components. As a general rule, filled products
and slower curing materials shrink less. Some pressure sensitive components
such as discrete FETs may have to be cushioned with a softer material to
eliminate this problem.
-
The dielectric strength of the material is
too low.
Especially with high voltage devices, increased
leakage currents can cause a change in the output of the circuit. Dielectric
breakdown in certain areas could also cause changes in output.
-
The thermal expansion is too great.
Operation at elevated elevated temperatures
will cause the encapsulant to expand. This expansion can cause internal
pressures to be exerted on embedded components. Further to this, dielectric
heating can cause expansion resulting in pressure on internal components.
6. Components degraded by
excessive exotherm during cure.
Highly reactive materials, especially when
applied in larger masses, can generate heat in excess of 1500C
during cure. This heat can degrade the components being encapsulated.
The probable solution to the above problems is
to select a product with the correct filler content deigned to meet the
desired operating criteria. In some cases cushioning extremely pressure
sensitive devices may be necessary. Slowly curing systems with maximum filler
content are best suited to these applications.
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Disclaimer: The above information is general in nature and is based solely on experiences by Crosslink Technology Inc. The recommendations provided herein may not be applicable in all situations. They are provided to the recipient as part of our customer service and the user must determine the relevance of the information to his/her application, considering any limitations that may be applicable thereto. Crosslink technology Inc. does not accept any liability for direct or consequential damages resulting from the implementations of these recommendations or the use of this information.
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