**Assessment of Glass Transition Temperature Impact on Thermal and
Mechanical Properties of Mold Compounds used for PEMs**

**from Technical Literature**

I have reviewed readily available technical literature in regard to the
effect of Glass Transition Temperature on PEM Epoxy Mold Compounds
(EMCs) and similar novolac epoxy based resins (thermoset polymers). The
purpose of this effort is to summarize the technical literature for the
thermal and mechanical reliability of PEMs with EMC's at various glass
transition temperatures (Tg's). This review excludes EMCs not using
novolac epoxy (particularly bisphenol A and bismaleimide based resins
that are more popular in high pincount ball grid arrays, for example). I
also restrict this assessment to low pin count (48 pin or less) standard
plastic packaging, such as SOIC and PDIP. Special packages such as
multi-element packages with interposers or RF packages need a separate
analysis.

Previously stated concerns such as permanent changes in EMC
characteristics from exposure to temperatures above Tg, effect of Tg on
popcorning or moisture cracking sensitivity, damage anticipated from
mission use of PEMs above Tg are addressed.

Damage (particularly to gold aluminum ball bonds) from precipitation or
release of flame retardant additives to the EMC (such as
tetra\--bromo-bisphenol-A and antimony trioxide at high temperatures
(which turns out not to be related directly to Tg) is a real and
important effect that warrants a separate assessment.

I use review articles in:

"Thermal Characterization of Polymeric Materials" edited by Edith A.
Turi (copyright 1987, 1991 by Academic Press) - a truly encyclopedic
reference text with more than 2000 pages of text and many, many
thousands of references to original primary research and review
articles. I particularly use Chapter 6 --Thermosets by R. Bruce Prime,
which I refer to as Prime. There is also useful information in the
article by Bernard Wunderlich -- The Basis of Thermal Analysis.

"Plastic-Encapsulated Microelectronics" by Michael G. Pecht, Luu T.
Nguyen, Edward B. Hakim -- copyright 1995 by John Wiley & Sons, which I
refer to as Nguyen (he is the mechanical and mold expert from National
Semi).

"Plastic Packaging of Microelectronic Devices" by Lous T. Manzione --
copyright 1990 by AT&T Bell Laboratories , which I refer to as Manzione.

Novolac epoxy based encapsulants (Epoxy Mold Compounds or EMCs) are the
most commonly used electronic encapsulants for PEMs. They are of the
class of Thermoset Polymers. Thermal/mechanical characteristics of
thermoset polymers are dependent on the extent of cure (percentage of
the curing formation which is dependent on the time at the curing
temperature), the polymer resin selected, and the temperature of the
polymer.

Glass transition temperature is conventionally defined as the change
from the glassy regime of lower temperature coefficient of expansion and
more rigid regime of the polymer to the rubber or high temperature
coefficient of expansion and malleable regime of a polymer. Various
properties of the polymer suffer a step function change at the glass
transition tempreture including tensile modulus, heat capacity,
dielectric loss, and linear coefficient of thermal expansion (LCTE) (see
Manzione pp.38-40). Wunderlich pp.266-227 and pp.380-389 also discusses
the definition of glass transition temperature and other methods of
measurement (not TMA).

Thermal mechanical analysis (TMA) is a direct measure of the linear
coefficient of thermal expansion (LCTE) and its change with temperature
and is one of six thermal analytic methods frequently used to
characterize polymers. An important illustration of TMA measurement is
shown in Figure 35 (Prime p.1449). Since LCTE occurs over a range of
temperature values, DTMA or differentiation of the LCTE curve with
temperature yields a non-ambiguous measurement of glass transition
temperature, which is preferred. Otherwise the inflection point (which
is the defined Tg point) is difficult to determine. Manzione discusses
the details of measuring Tg using TMA on p.100 and shows a similar LCTE
curve in Figure 4-1 on p.101 for glass filled novolac epoxy (same type
used to encapsulated PEMs) (with the superior measurement using the
differential LCTE method).

Since a much higher LCTE is expected above the Tg (possibly three
times), thermal mismatch between plastic package elements is much worse
and should be avoided. It is important to note that TMA measurements of
Tg are dependent on the scan rate. Higher rates of transition or faster
thermal scans will lead to lower measurements of Tg. Industry standard
is to extrapolate measurements to a very low scan rate (see Wunderlich
pp.380-387). Also see Prime p.1401 for recommended scan rates. Repeated
TMA measurements will show an increased glass transition temperature
with each measurement (see Prime p.402-407) which corresponds to an
increased extent of cure. This is in agreement with many TMA
measurements performed at JPL. This is a permanent effect (see below).

Thermoset polymers are characterized by the degree of cure. Cure is the
densification or hardening of the polymer during exposure to the
thermoset temperature (it also occurs at lower temperatures but much
slower). Cross linking within the polymer increases with increased
extent of cure. Few industrial polymers can go to 100% cure (takes too
long -- see Manzione p.240), but high degrees of cure are usually the
target. Glass transition temperature increases with higher degrees of
cure. A typical curve of this relation is shown in Prime p.1558 (figure
92). DiBenneto's equation models this data and is frequently used in
industry. More complex curve fittings are also used and the best fitting
is in Hale (1991) - see Prime equation 55 on p.1561 and reference on
p.1752. Other Tg versus cure curves are shown in figure 93 (Prime
p.1562) and Figure 94 on Prime p.1566. The dependence of degree of cure
on time and temperature is very complex and is modeled in different ways
(Prime pp.1567-1658). Another illustration of the conversion versus cure
time is Figure 2.20 (see Nguyen p.92). A good illustrative curve of
glass transition temperature versus time at cure temperature is Figure
98 on Prime p.1575. The effect of the cure temperature on the extent of
cure and glass transition temperature is shown in Figure 3.20 (Nguyen p.
145). Also see Figure 6.15 (Nguyen p.265). Although the cure temperature
does affect the rate of reaction and cure, it has small effect on the
glass transition temperature curve versus reaction percentage (extent of
cure).

The important aspect of all of these curves is that glass transition
temperature is a monotonically increasing function of degree of cure and
also time at the cure temperature. Therefore, the degree of cure or the
time at the cure temperature uniquely identifies the glass transition
temperature for a particular polymer.

Therefore, exposure of a polymer to a stable and fixed high temperature
such as the cure temperature increases the glass transition temperature.
Since increase in the glass transition temperature is associated with
improved reliability, high temperature that is slowly achieved and
stabilized, increases reliability.

However, rapidly changing polymer temperatures above the glass
transition temperature should be expected to significantly increase the
dimensional change of the polymer (effect on LTCE). Since other elements
in the plastic package (silicon die, die attach, flying wire bonds,
leadframe die paddle) have different and often lower LCTE than the EMC,
this leads to high stress and potential damage within the plastic
package.

Operation of a part above the glass transition temperature is expected
to lower the bond shear strength (due to stress from the LTCE mismatch)
but lower die stress will result from decreasing modules of elasticity
(Nguyen p.35)

Bisphenol A has a low glass transition temperature (100 to 120°C)
(Nguyen p.56).

Filler (typically crushed silica) is added using proprietary processes
to lower the LCTE to reduce stress against the internal elements in the
package (so-called low stress encapsulants) (Nguyen p.56). Fillers do
not significantly change the glass transition temperature for novolac
epoxy based PEM encapsulants (Nguyen p.66).

Internal package stresses may be computed using formula 2.1 (Nguyen
p.80-83). Glass transition temperature measurement using LTCE (TMA) is
described in detail and reference to appropriate ASTM standards in
Nguyen pp.81-82.

An extended description of the manufacture of plastic packages (e.g.
transfer molding) is contained in Manzione pp.187-243.

Popcorning is an important concern since absorbed moisture may cause a
package crack during the soldering process (with attendant substantial
degradation in long term reliability of the PEM). The significant
dependencies for popcorning are amount of moisture present in the EMC,
die paddle size (area) and thickness of the EMC below the die paddle
(see Nguyen pp.181-191). Specially formulated low glass transition
temperature EMCs actually absorb less moisture (see Nguyen p. 184-186)
and have been explored as a potential anti-popcorn mold compound
(without commercial success to date). In these experiments the lesser
moisture absorption in the low glass transition temperature EMCs is more
significant than other effects in reducing popcorn sensitivity. Also see
Figure 6.13 (Nguyen p.263).

Various studies have been conducted to delineate the dependencies of
moisture cracking sensitivity on mold compound characteristics. Figures
4.14 and 4.15 (see Nguyen pp.188-189) show typical results. Moisture at
the die mold interface during the soldering process and resin thickness
under the die paddle have the most significance.