There are special criteria for improving reliability for selected types of
(1) Power Semiconductors
Avoid the use of hot-case (electrically) packages where practical. Hot- case
components require well thought out isolation techniques and additional assembly
steps that are susceptible to error.
Hot-case components are subject to assembly errors that can cause secondary
failures due to their high-energy-handling capability. This may be a low-risk
problem during the initial assembly of a power supply but becomes a
very-high-risk problem whenever any rework is required on the component or its
An "isolated junction" package requires two additional tests to be done by
the component manufacturer to guarantee the specified isolation between the
semiconductor and the case.
a. High-voltage isolation test
800 V (minimum) between the terminals and the package with 100% screen (all
b. Thermal resistance
Tight sample or 100% if critical application.
(2) Isolation and Thermal Conditions
Since JANTX or JANTXV semiconductors typically do not utilize "isolated
junctions," it becomes necessary for the power supply manufacturer to provide a
reliable method of mounting with low thermal resistance. Various materials and
techniques have been used, each with its particular problems. Thinner materials
(i.e., mica and mylar film) are subject to "cut through" by burrs and sharp
edges which are inevitable, while high thermal conductivity ceramic insulators
require thermal grease and, because of their brittleness, are easily
The use of thermal grease is difficult to control, attracts contaminants and
affects reliability and maintainability; therefore, its use should be avoided on
all future power supplies. Thermally conductive plastic impregnated fiberglass
isolators are now available which approach the thermal conductivity of beryllia
plus grease and reduce the susceptibility of shorting due to burrs because of
their thickness (15 to 20 mils). Careful consideration of the isolator
compressive force is imperative, however, and drawings should specify the torque
to be applied as a function of screw pitch and mounting hardware used.
measure the thermal resistance of an isolated case package, it is necessary to
measure the thermal time constant of the total package after thermal stability
of the package is reached, typically 2 to 5 seconds. This should be followed by
a correlation evaluation for measuring thermal resistance on the slope of the
defined thermal time constant curve (0.5 seconds to 1.0 seconds, dependent on
package size) to facilitate production testing.
(3) Use Certain Components Cautiously
The bond that holds a semiconductor die to the package is usually solder. The
die-bond is made directly to the interface in typical construction of high
current power devices.
Thermal cycling, often caused by power cycling, tends to bring out inherent
weakness in die-bonds resulting in stress-induced cracking of hard soldered
units or void formations in the bonding layers of soft soldered units.
There are several die-bonding systems for components of differing
metallurgical properties that can be used, including epoxy for stress/strain
relief. All application factors must be carefully considered when choosing a
particular die-bonding system.
b. Material interfaces
Packages made of plastics, porous ceramics and transfer-molded bodies
(primarily axial lead) should not be allowed in military applications.
(4) Selection and Control of Vendors Are Necessary
The selection and control of component manufacturers are necessary in order
to consistently obtain high reliability components. All standard components with
the same component number are not equal in quality, reliability, packaging
techniques and materials. For example, from manufacturer to
a. The seal strength of the terminal-to-glass interface of a stud rectifier
can vary by as much as five-to-one.
b. The tensile strength of axial lead glass-body packages varies.
Metallurgical bonds do get weak and can break if the process is not carefully
c. The thermal resistance can vary by as much as three-to-one.
d. The external crimp connection technique may vary. Crimp-plus-weld is
preferred. If the component has only crimp connections, then a pulsed
high-current electrical verification test should be done.
The power supply manufacturer should understand the differences and optimize
component selection without compromising reliability.