Assessment of Impact of Flame Retardants on

Mold Compound Reliability

I have examined the available technical literature in regard to life
limiting mechanisms from introduction of flame retardant chemicals in
industry standard molding compounds for integrated circuits in common
plastic packages. In this regard, the epoxy resin is presumed to be the
standard (cresol) novolac epoxy and the package is 48 pin or less
surface mount (SOIC) or PDIP. Large pin count packages, or specialty
packages such as RF parts or packages with Interposers are excluded. I
attach the articles I have captured. Unlike the technical literature for
thermal and mechanical characteristics of mold compounds, which is
mostly in copyrighted books, review articles, and somewhat hard to
obtain journals, the published data I have used is mostly obtained from
IEEE technical journals and may therefore be circulated electronically
for easier review (for those interested).

Flame retardant is added to Epoxy Mold Compounds (EMCs) so that in the
event of a major fault condition where temperatures much in excess of
the EMC thermoset temperature (170°C for novolac epoxy based EMCs) are
engendered in the PEM package, that the molding compound will not flame
up and start a general system fire. Even with the flame retardant, the
plastic package will be damaged at these excessive temperatures and
constituents of the package will outgas into the ambient. There will be
permanent damage to the epoxy matrix. Flame retardant characteristics
are typically mandated by law.

Typical flame retardants used in industry are tetra-bromo-bisphenol-A
(TBBPA) (and its diglycide ethers) and/or Antimony Trioxide. Sometimes
these are used in combinations. Usually the flame retardants are
approximately 2% of the epoxy mold compound.

Flame retardants typically used in industry are chemically active
materials that have potential for corroding key elements of the package,
particularly the unpassivated bond pad and wire bond. As designed and
manufactured, the halogen based flame retardant chemicals are bound into
the epoxy matrix, are not mobile, and do not attack the sensitive
internal elements of the plastic package. However, there is concern that
at high temperatures some of these chemicals will become mobile and
attack the sensitive internal elements. This effect is known to be
exacerbated if significant moisture is also present in the epoxy matrix.
Older experimental data showed less bond degradation under dry bake
conditions with the Antimony Trioxide being the predominant flame
retardant chemical.

Substantial data is available in the open technical literature on
thermal characteristics of this effect, typically called flame retardant
precipitation, release, or "outgassing" (not really the best term, but
often used anyhow). It turns out that rather than this effect being
"turned on" at the glass transition temperature, experimental published
data follows the Arrhenius equation with activation energy dependent on
the details of the epoxy matrix (often the activation energy is between
0.8 and 1.0eV but other experimenters report activation energy in excess
of 2eV; the thermal characteristics are dependent on the detailed
content of the flame retardants and the epoxy binding characteristics)..

Premature gold aluminum (AuAl) bond failures have been found in plastic
encapsulated microcircuits due to this precipitation (outgassing) of
flame retardants commonly used in epoxy mold compounds. Failure analysis
has been performed of these "early" failures and shown that the physical
mechanism was Kirkendall voiding (intermetallics) in the Aluminum Gold
ball bond, similar to the "Purple Plague" effect observed in hybrids.
The rate of failure has been shown to be related to the outgassing rate
of active halogens in the epoxy mold compound. It is known that water
and halogens accelerate intermetallic formation and Kirkendall voiding
in Gold Aluminum bonds. In plastic parts, after the halogens have
precipitated out of the epoxy matrix and become mobile, both conditions
are present to have accelerated bond degradation occur.

Published data shows that use of plastic parts in high moisture
environments (and high temperatures) accelerates this failure mechanism.
However this is not relevant to long-lived space missions. Ground
storage prior to launch is presumed to be of short duration (less than a
year) and there is a long history of commercial parts being used in
semi-tropical environments (south Florida and Alabama) with very low (or
no) corrosion problems or bond degradation. Discussion is therefore
restricted to low humidity environments, so that dry bake is the correct
simulation.

In "The Microstructure of Ball Bond Corrosion Failures" by Ritz, Stacy,
Broadbent (IRPS 1987, p. 28), Aluminum Gold bonds in various epoxy mold
compounds were subjected to high temperature dry bakes between 175 and
200 degrees Celsius. Growth of the intermetallics is shown in
cross-sectioning studies after various hours of high temperature stress
(100-1000 hours). Increase in gold rich intermatallics is shown. These
are the same intermetallics associated with Kirkendall voiding. They are
brittle and will have sharply degraded bond strength after exposure to
high temperature operation.

R. J. Gale "Epoxy Degradation Induced Au-Al Intermetallic Void formation
in Plastic Encapsulated MOS memories (IRPS 1984 -- reports 0.8 eV
activation and approximately Arrhenius equation dependence of this
failure mechanism. Many different plastic packages were subjected to
high temperature bakes at 125, 150, 180 and 225 degrees Celsius. No
failures due to weak or failed bonds were found at less than 200 hours
(for 180 degrees Celsius bake). This paper has the largest sample size
tests I have seen.

Excerpts of the results are included here:

"High temperature aging experiments were performed with Au wire bonded,
Al metallized 4K NMOS Static RAMs and epoxy molding compounds in two
types of DIPs. The initial study focused on the effect of the flame
retardant by using the epoxy with and without flame retardant in the
cavities of epoxy sealed ceramic DIPs. The apparent activation energy
for the degradation was determined by aging devices molded in DIPs using
the standard flame-retardant formulation at three temperatures. The Cl
and Br content of the epoxies was measured by neutron activation
analysis and water extraction. Electron and ion microprobe analysis and
showed that Cl and Br are present in the bond zone with the Au-As
intermetallics. The median life at 180 C was only a factor of three
higher without the flame retardant, even though the Br content was 600
times lower, suggesting that the Cl plays a major role. The apparent
activation energy for the bond degradation in the flame-retardant
molding compound is -0.8 eV, within experimental error of that for pure
Au-Al intermetallic formation \[4\] and that for the breakdown of the
brominated flame retardant determined by water extraction. The water
extraction results also indicate that the chlorinated impurities are
less thermally stable than the brominated flame retardant. The
accelerated failure of the bonds may be caused by the formation of
volatile Al halides, leaving the extensive voids at the Au-intermetallic
interface observed in cross-sections of the failed oond areas. These
results imply that the high temperaturre

performance of Au-Al bonds in epoxy molding compounds could be improved
by further

purification of the starting resins to remove the Cl."

The measured water extractable halogen ion content into the epoxy mold
compounds tested are shown here:

NEUTRON ACTIVATION ANALYSIS

OF EPOXY MOLDING COMPOUNDS

LOT Cl (ppmW) Br (ppmW)

A 256+27 9250±120

B 274±+ 24 15 + 2

C 287±20 9970±140

This data limits the applicability of the test results reported in this
paper as follows. Water extractable chlorine ions (as also discussed
within the paper) are much more active in causing corrosion than bromine
ions. The levels of bromine ions measured in this paper are upper bounds
to those measured in other experiments. However, the chlorine ion level
is much lower in devices tested later. It has been recognized in
industry that sodium, potassium and chlorine ion level within the EMC is
the major factor in corrosion failures of PEMs. Therefore the water
extractable chlorine ion level specified for modern EMC is 10-20 parts
per million (ppm), which is more than 10 times lower than the levels
measured in 1980-1990 made plastic parts. Therefore failure levels in
this paper should be considered very conservative upper bounds.
Arrhenius activation energy reported herein as 0.8eV is mainly due to
the chlorine and should not be used in evaluating modern plastic parts.
Failure data from this paper is shown here:

![](media/image1.wmf){width="5.947916666666667in" height="6.40625in"}

![](media/image2.wmf){width="5.947916666666667in" height="6.59375in"}

JPL D-8545 limits application of PEMs to junction temperature of +110°C.
However, flight parts may often be burned-in at +125°C for as long as
240 hours.

Therefore the data within this paper provides an upper bound of
approximately 0.01% in burn-in. This is very conservative since modern
plastic parts have much more tightly controlled chlorine ion level, so
that a more accurate upper bound due to flame retardant release would be
less than 0.001%.

Extrapolating to conditions more appropriate to mission operation would
be at an upper bound junction temperature of 100-110°C. Under these
conditions the upper bound failure rate due to brominated flame
retardant release would be 0.01% to 0.1% for 20,000 hours steady state
operation at these elevated temperatures (which corresponds to 3.3
years)

S. S. Ahmad, R. C. Blish II, T. J. Corbett, J. L. King, C. G. Shirley
"Effect of Bromine in Molding Compounds on Gold-Aluminum Bonds" IEEE
Transactions on Components, Hybrids, and Manufacturing Technology,
December 1986 p.379 (see attachment "Bromine AlAuBonds.pdf") did various
high temperature bakes and plotted the dependence of the time to failure
for lower (which is standard) brominated epoxy mold compounds and higher
brominated epoxy mold compounds. The lower brominated EMCs had
significant increases in bond resistance (but did not fail) between 50
and 120 hours at 200 degrees Celsius. The activation energy was 2.1 to
2.3 eV. The higher brominated epoxies had significant resistance
increases in the same time range, but with activation energy between 2.1
and 0.8 eV. At 2% by weight Br the activation energy was 1.6 eV. This
study used parts and test devices manufactured in the mid 1980's and
therefore had much higher levels of chlorine ions than are now
acceptable within industry. Therefore failure rate data requires
interpretation before use. However, data herein is sufficient to
eliminate concern about high temperature during standard burn-in of
flight parts.

M. Khan, H. Fatemi, J. Romero, E. Delania "Effect of High Thermal
Stability Mold Material on the Gold --Aluminum Bond Reliability in Epoxy
Encapsulated VLSI Devices" -- IRPS 1988, p.40 (see attached "Epoxy Mold
and Au Al Bonds.pdf") reported results on high temperature bake of both
standard (labeled Q) and specially made epoxy mold compounds which had
lower than average halogenated organic residues and therefore were
supposed to have long bond life at high temperature. I report here only
the results for the standard epoxy mold compound. (Indeed the special
mold compounds were better). The standard compound had a significant
failure rate (1%) at 225 degrees Celsius bake of 30 hours. The reported
activation energy was 0.72 eV. The extrapolated time to failure at 175
degrees Celsius was 292 hours. These tests were performed on special
test structures and not on complete devices.

Richard Biddle at Texas Instruments provided a short paper-- "Plastic
Encapsulant Impact on Au/Al Ball Bond Intermetallic Life" (see
attachment "Bond_Intermetallic_Life.pdf"). In addition to making
references to the same technical literature herein cited, he reports a
back study of Biphenyl Epoxies used on advanced packages (these packages
should be worse for this phenomena). Wire pull failures (counting less
than 8.0 grams as a failure) occurred at 500 hours at 185 degrees
Celsius and 900 hours at 175 degrees. The approximate activation energy
was 0.91 eV. However a regression approach was used to get a prediction
of the lifetime at low temperatures. It was 2.1 years at 125 and 8.8
years at 105 degrees Celsius.

Richard also provided a paper he has submitted called "Reliability
Implications of Derating Leading Edge High Complexity Microcircuits"
(see attachment "Reliability Implications of Derating-Biddle.pdf"). In
that paper he has TI data on the Au/Al Intermetallic limited Life for
various encapsulants. The Biphenyl's (as expected) are worse with
lifetimes between 9.5 and 18.4 years at 105 degrees Celsius.
Multi-functional epoxies (which are cresol Novolac based) are much
better with lifetimes greater than 100 years at 135 degrees Celsius and
greater than 1400 years at 105 degrees Celsius.

Critical data is excerpted here:

  --------- ------------------ --------------- ------- ------- ------- -------- ------------
  Package   Mold Compound      Intermetallic                                    Activation
                               Life in Years                                    Energy

                               150°C           140°C   135°C   115°C   105°C    

  PQFP      Multi-Functional   \>7.1           \>25    \>190   \>783   \>3525   .1.9eV
            (novolac epoxy                                                      
            based)                                                              

  PBGA                         5.2             16.5    104     407     1426     1.74eV
  --------- ------------------ --------------- ------- ------- ------- -------- ------------

Also attached, for general interest, is the higher rate of degradation
seen in experiment done with high temperature exposures in humid
environments (Autoclave for example). See attachments "Gallo Mold Au Al
Bonds.pdf" and "High Temperature Reliability of PEMs.pdf". There is not
much data on dry bakes in these articles. What is presented does not
conflict with the data above.

Unfortunately, no discussion of flame retardants in EMCs would be
complete without the red phosphorus problem as reported in the GIDEP
system. Bromides (TBBP) and Antimony oxides are serious poisons. There
has been concern that during eventual disposition of electronic hardware
(including recycling) these poisons might be released to the general
environment. Much research has been conducted to develop "Green" molding
compounds without these chemicals. One such development was the use of
red phosphorus as a replacement. Sumitomo (one of two main encapsulant
manufacturers and the biggest) produced large quantities over a multi
year period of EME-U compounds with this formulation. Significant field
failures were seen in personal computer hard disk drives. A complete
description of the problem and root cause is contained in "The Story
Behind the Red Phosphorus Mold Compound Devices Failures" by Deng and
Pecht (2005 International Symposium on Electronic Materials and
Packaging (Tokyo) (attached). This line of compound was withdrawn from
production. Apparently all use of red phosphorus in molding compounds
was discontinued (military and commercial).

An important risk mitigation in this area is the availability of test
data (usually from the part manufacturer) of extended stress testing at
high temperature. Since the failure mode is catastrophic, the details of
the electrical measurements before and after stress are not important or
the bias conditions (unbiased high temperature bake is just as good as
high temperature life test). Let us suppose that 100 pieces have been
tested at +150°C for 1000 hours with no failures (I pick 100 pieces
merely as an illustration to make the numbers easier). Using an
activation energy of 1.0eV (again for simplicity) the statistically
demonstrated failure rate upper bound is (assumed usage condition is
100°C):

Failure rate is less than 4.6 per 100,000 hours at +150° (90%
confidence)

Or

Less than 4.6/39.55 or 1.2 per million hours at +100°C

Since we can reasonably combine all test data on devices using the same
mold compound, often a lot more than 100 piece high temperature data can
be obtained.

**Conclusion**

Use of +125°C package temperature during flight part burn-in represents
minimal risk from flame retardant release induced failures for standard
industrial (cresol novolac) epoxy mold compounds in typical plastic
packaged parts (SOIC and PDIP).

Mission operation (even for extended missions) at elevated package
temperatures at least to +100°C is low risk for package degradation due
to flame retardant release for standard plastic packages. Usage of PEMs
at continuous operation at these elevated temperatures has much greater
(and not necessarily insignificant) risk due to failures in the die than
corrosion related failures due to flame retardant release within the
epoxy matrix.

Concerns for unusual packages or unusual mission scenario will require
analysis. Validation of a reasonable risk level should be dependent on
analyses of large sample high temperature life test data (either high
temperature unpowered bake or high temperature life test) preferably at
least at +150°C.