System and method for empirically determining shrinkage stresses in a molded package and power module employing the same

A system for, and method of, empirically determining stress in a molded package and a power module embodying the system or the method. In one embodiment, the system includes: (1) a sensor, having a magnetic core exhibiting a known complex permeability in a control environment, that is embedded within the molded package and therefore subject to the stress and (2) a measurement circuit, coupled to the sensor, that applies a drive signal to the sensor, measures a response signal received from the sensor and uses the drive signal and the response signal to determine a complex permeability under stress of the core. The magnitude of the stress can then be determined from the core's complex permeability under stress.

TECHNICAL FIELD OF THE INVENTION 
The present invention is directed, in general, to power magnetics and, more 
specifically, to a system and method for empirically determining stresses 
(including shrinkage stresses) in a molded package (that may contain 
electronic components or magnetic devices) and a power module employing 
the system or the method. 
BACKGROUND OF THE INVENTION 
Magnetostriction is a phenomenon that occurs in magnetic materials such as 
ferrites, metals and alloys. Magnetization of the materials causes a 
change d1 in a dimension 1, creating a strain or magnetostriction .lambda. 
represented by .lambda.=d1/1. Magnetostriction is a material constant that 
can be either negative or positive. A magnetic material exists in one of 
three states or regimes. Above a Curie temperature, the material is in a 
paramagnetic regime and exhibits no magnetization. Below the Curie 
temperature, the material may be in either a ferromagnetic regime or a 
saturation magnetization regime. In the ferromagnetic regime, spontaneous 
magnetization occurs in small, randomly ordered molecular magnetic domains 
throughout the material. The overall magnetization of the material is, 
however, zero. A strong magnetic field, sufficient to align all the 
molecular magnetic domains, may be applied to the material to place it in 
a saturation magnetization regime. In this regime, the alignment of the 
molecular magnetic domains produces a maximum length change in the 
material and provides a value for the saturation magnetostriction of the 
material. 
Power modules are employed in many electronic devices to power the 
components therein. Power modules were initially available in through-hole 
packages consisting of a metal or plastic case, housing a printed wiring 
board (PWB) on which power module components were mounted. 
Electronic components are currently migrating towards surface-mount 
packaging in overwhelming proportions. Board-mounted power modules will 
inevitably follow, if only to assure assembly compatibility with this 
packaging technology. Surface-mount assembly operations, however, 
typically involve severe reflow temperatures and wash (or cleaning) cycles 
that may damage components in the power modules. As a result, power module 
circuits are encapsulated in a rigid epoxy molding compound via, in most 
cases, a transfer molding process. As a dense glass-filled epoxy with a 
high glass transition temperature and a high modulus, the molding compound 
is capable of withstanding the high temperatures found in surface-mount 
assembly operations. During encapsulation, the molding compound completely 
fills around all the components in the power module circuits, creating a 
solid package for the power module and providing a good thermal path for 
heat generating components. The molding compound thus protects the power 
module circuits from surface-mount assembly operations. 
The protection provided by the molding compound comes, however, at a cost. 
As the molding compound cools from a molding temperature to room 
temperature, it shrinks. Substantial thermal shrinkage stresses are thus 
imposed on the components in the power modules by the high modulus molding 
compound. 
Power modules typically use ferrite materials e.g., manganese zinc (MnZn)! 
as core materials in magnetic devices such as power transformers and 
energy storage inductors. As the molding compound shrinks and thermal 
shrinkage stresses are imposed on the ferrite materials, large strains are 
created, restricting the movements of the small, molecular magnetic 
domains during external magnetic field excitation. The required degree of 
alignment of the molecular magnetic domains cannot be achieved. 
Strain pinning between the domain walls occurs, increasing dissipation in 
the ferrite materials. The ferrite materials, therefore, cannot fully 
enter the saturation regime. A pronounced decrease in the magnetic 
properties results, with a corresponding degradation in performance of the 
magnetic devices (e.g., power transformers and energy storage inductors). 
As described in U.S. Ser. No. 08/604,637 filed on Feb. 21, 1996, 
"Encapsulated Package for Power Magnetic Devices and Method of Manufacture 
Therefor," magnetic devices containing ferrite materials, therefore, must 
be protected from the thermal shrinkage stresses of the molding compound 
to retain full functionality in surface-mount power modules or power 
modules in general. 
Since the transfer molding process is widely used in packaging integrated 
circuits, a determination of molding stresses in the packages during 
molding is necessary to the design of a long-life and robust package. 
Precise knowledge of thermal shrinkage stresses is also necessary in the 
development of protection schemes for the ferrite materials used in power 
modules. 
Knowledge of thermal shrinkage stresses in a molded package is typically 
obtained through analytical models and determined by conventional stress 
analysis or finite element analysis. This knowledge, however, is only 
theoretical. It would be advantageous therefore, to measure the shrinkage 
stresses during the three stages of molding (i.e., filling, packing and 
cooling). 
Accordingly, what is needed in the art is a method of empirically 
determining the stresses (including shrinkage stresses) present in a 
molded package. 
SUMMARY OF THE INVENTION 
To address the above-discussed deficiencies of the prior art, the present 
invention provides a system for, and method of, empirically determining 
stress in a molded package and a power module embodying the system or the 
method. In one embodiment, the system includes: (1) a sensor, having a 
magnetic core exhibiting a known complex permeability in a control 
environment, that is embedded within the molded package and therefore 
subject to the stress and (2) a measurement circuit, coupled to the 
sensor, that applies a drive signal to the sensor, measures a response 
signal received from the sensor and uses the drive signal and the response 
signal to determine a complex permeability under stress of the core and a 
magnitude of the stress therefrom. 
The present invention therefore introduces the broad concept of embedding a 
sensor in a molded package to empirically determine the amount of stress 
present in the package. Feedback from stress measurements allows component 
designs (e.g., inductor parameters such as protection, gap, turns, etc.) 
to be adjusted to compensate for magnetostrictive effects. Once 
modifications are made during product development and design, regular 
stress measurements made during manufacturing will allow monitoring of the 
molded product. Excessive stresses leading to a failed component may thus 
be detected at the molding stage rather than at a later stage after other 
value-added operations have been performed. 
In one embodiment of the present invention, the sensor further has drive 
and sense windings located proximate the core, the drive winding receiving 
the drive signal and the sense winding generating the response signal. 
Alternatively, the sensor may have only one winding, in which case 
self-induction, as influenced by the permeability of the core, produces 
the response signal. 
In one embodiment of the present invention, the molded package contains a 
power module and the sensor is integrated into a power train of the power 
module. In a more specific embodiment, the sensor is selected from the 
group consisting of: (1) a transformer in the power module and (2) an 
inductor in the power module. Thus, the present invention may be separate 
from other circuitry embedded in the molded package or may employ a 
magnetic device preexisting in the circuitry to perform stress 
determination. 
In one embodiment of the present invention, the core is composed of a 
ferrite. In a more specific embodiment, the core is composed of manganese 
zinc (MnZn) ferrite. Those skilled in the art will understand, however, 
that the present invention is operable with all materials that are subject 
to magnetostriction. 
In one embodiment of the present invention, the measurement circuit is 
located in the molded package. Of course, the measurement circuit can be 
located outside of the molded package. 
In one embodiment of the present invention, the sensor and measurement 
circuit are operable during a molding of the molded package. This allows 
stress to be measured throughout the molding process, providing valuable 
insight into optimal production techniques. However, such capability is 
not necessary to the broad scope of the present invention. 
The foregoing has outlined, rather broadly, preferred and alternative 
features of the present invention so that those skilled in the art may 
better understand the detailed description of the invention that follows. 
Additional features of the invention will be described hereinafter that 
form the subject of the claims of the invention. Those skilled in the art 
should appreciate that they can readily use the disclosed conception and 
specific embodiment as a basis for designing or modifying other structures 
for carrying out the same purposes of the present invention. Those skilled 
in the art should also realize that such equivalent constructions do not 
depart from the spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION 
Referring initially to FIG. 1, illustrated is a cross-sectional view of a 
stress determination circuit 100 for empirically determining shrinkage 
stress constructed according to the principles of the present invention. 
The stress determination circuit 100 includes a sensor 110 embedded within 
a molded package 170 by a molding compound 180. The stress determination 
circuit 100 further includes a measurement circuit 190, coupled to the 
sensor 110 that determines a complex permeability (having real and 
imaginary permeability components) under stress of the sensor 110. The 
sensor 110, in the illustrated embodiment, consists of a toroidal ferrite 
core 120, having a primary (drive) winding 130 and a secondary (sense) 
winding 140 through its center hole. 
In the illustrated embodiment, the toroidal ferrite core 120 is composed of 
MnZn ferrite having an initial real permeability in the range of 1000 to 
3000. Although the illustrated embodiment uses a MnZn toroidal ferrite 
core 120, those skilled in the art should realize that the use of any 
magnetostrictive material to measure shrinkage stress falls within the 
broad scope of the present invention. 
Turning now to FIG. 2, illustrated is a graphical representation of a 
stress dependence of a complex permeability of the toroidal ferrite core 
120 of FIG. 1. Complex permeability is a combination of two components: a 
real permeability .mu.r' and an imaginary permeability .mu.r". The real 
permeability .mu.r' decreases monotonically as compression stresses are 
applied to the toroidal ferrite core 120. In contrast, the imaginary 
permeability .mu.r" increases as compression stresses are applied. The 
compression stresses on the toroidal ferrite core 120 may therefore be 
determined by measuring the real and imaginary permeability .mu.r', 
.mu.r". 
With continuing reference to FIG. 1, the stress determination circuit 100 
operates as follows. Before molding, the toroidal ferrite core 120 is 
calibrated by measuring its unstressed complex permeability in a control 
environment (which is preferably relatively stress-free and most 
preferably free air). 
In one embodiment of the present invention, the measurement circuit 190 
applies a drive signal to the toroidal ferrite core 120, measures a 
response signal from the toroidal ferrite core 120, and uses the drive 
signal and the measured response signal to determine the complex 
permeability of the toroidal ferrite core 120. 
A preferred embodiment of the present invention uses a conventional 
impedance measurement technique to determine the complex permeability of 
the toroidal ferrite core 120. The measurement circuit 190 produces, using 
conventional processes, a drive voltage of a known magnitude and phase. 
The drive voltage is then applied to the primary (drive) winding 130. The 
measurement circuit 190 then measures, using conventional processes, a 
magnitude and phase of a resulting sensed current generated by the 
secondary (sense) winding 140 of the sensor 110. The measurement circuit 
190 can thus compare the drive voltage to the sensed current to determine 
the complex permeability of the toroidal ferrite core 120. 
In a preferred embodiment, the measurement circuit 190 may use analog to 
digital converters to convert the magnitude and phase of both the drive 
and the sensed signals to digital signals. The complex permeability of the 
toroidal ferrite core 120 may then be computed. Of course, the measurement 
circuit 190 may also be performed by analog circuitry. 
Alternatively, the complex permeability of the toroidal ferrite core 120 
may be measured using conventional B-H loop measurements. B-H loop 
measurements are familiar to those skilled in the art, and, as a result, 
will not be described. 
The measurement circuit 190 may, in one embodiment of the present 
invention, be located in the molded package 170. Of course, the 
measurement circuit 190 may also be located outside of the molded package 
170. 
During the three molding steps of filling, packing and cooling, the sensor 
110 may be measured again to determine empirically the shrinkage stresses 
imposed on the toroidal ferrite core 120. Again, conventional methods for 
determining the complex permeability (e.g., impedance measurement, B-H 
loop measurement) may be used. 
In a preferred embodiment, the measurement circuit 190 again applies the 
drive voltage to the primary (drive) winding 130. The molecular magnetic 
domains within the toroidal ferrite core 120, however, are restricted by 
the shrinkage stresses and therefore cannot achieve the required 
alignment. The measurement circuit 190 may then measure the magnitude and 
phase of the sensed current through the toroidal ferrite core 120 at the 
secondary (sense) winding 140. The drive voltage may then be compared to 
the sensed current to determine the complex permeability under stress of 
the toroidal ferrite core 120. The magnitude of the molding stresses 
imposed by the molding compound 180 in the vicinity of the sensor 110 may 
thus be derived. Those skilled in the art should realize, of course, that 
the operability of the sensor 110 and measurement circuit 190 during the 
molding of the molded package 170 is not necessary to the broad scope of 
the present invention. 
Turning now to FIG. 3, illustrated is a schematic diagram of a power module 
300 employing a sensor constructed according to the principles of the 
present invention. The power module 300 includes a power train having a 
conversion stage (not separately referenced) including a power switching 
device 320 for receiving input electrical power V.sub.IN and producing 
therefrom switched electrical power. The power module 300 further includes 
a filter stage (not separately referenced, but including an output 
inductor 330 and output capacitor 340) for filtering the switched 
electrical power to produce output electrical power (represented as a 
voltage V.sub.OUT). 
The power module 300 further includes a power transformer 350, coupled to 
the conversion stage, having a ferrite core, a primary (drive) winding 355 
and a secondary (sense) winding 360. The power module 300 still further 
includes a rectifier (including rectifying diodes 370, 380) coupled 
between the conversion stage and the filter stage. The power module 300 is 
embedded in a molded package by a molding compound (not shown). 
The power module 300 further includes a stress determination circuit for 
empirically determining stress in the molded package. The stress 
determination circuit includes a sensor and a measurement circuit. The 
sensor may be integrated into the power train of the power module 300. In 
the illustrated embodiment of the present invention, the power transformer 
350 may perform the function of the sensor. The power transformer 350 
contains a ferrite core that is adversely affected by molding stresses 
exhibited by the molding compound. Prior to molding, a complex 
permeability of the power transformer 350 (consisting, as described above, 
of real and imaginary permeability components) may be characterized in 
free air. After molding, the complex permeability of the power transformer 
350 may again be measured. As the molding compound thermally shrinks, 
imposing stresses on the ferrite core, the real permeability monotonically 
decreases. A method of indirectly measuring the stresses acting on the 
ferrite core of the power transformer 350 is thereby provided. Of course, 
the output inductor 330, with its ferrite core and the addition of a 
second (drive or sense) winding, may also perform the function of the 
sensor. Again, the power transformer 350 and power module 300 are 
submitted for illustrative purposes only and the use of the sensor in 
other devices and applications are well within the broad scope of the 
present invention. 
For a better understanding of power electronics including power supplies 
and conversion technologies, see "Principles of Power Electronics," by J. 
G. Kassakian, M. F. Schlecht and G. C. Verghese, Addison-Wesley (1991). 
For a better understanding of magnetic devices and construction techniques 
therefor, see "Handbook of Transformer Applications," by William Flanagan, 
McGraw Hill Book Co. (1986). The aforementioned references are 
incorporated herein by reference. 
Although the present invention has been described in detail, those skilled 
in the art should understand that they can make various changes, 
substitutions and alterations herein without departing from the spirit and 
scope of the invention in its broadest form.