Reliability Issues Concerning Use of Red Phosphorous as Flame Retardant
in Molding Compounds Used in COTS PEMs

Point of Contact: Ashok K. Sharma/NASA GSFC Code 562,
<Ashok.K.Sharma.1@gsfc.nasa.gov>

Flammability of microelectronic components, particularly those using
molding compounds as encapsulants, is a safety concern that can be
mitigated by the use of appropriate materials. In most common plastic
encapsulants, flame retardant capability is acquired through the use of
bromine-based aromatic compounds. These brominated flame retardants
(BFRs) are preferred because of their low cost and effectiveness,
especially in their ability of to ensure compliancy with industrial
flammability standards. Thus, the flame retardants with aromatically
bonded bromine have the highest market share, primarily because they
tend to be resistant to breakdown over time or temperature. When the
epoxy encapsulants are exposed to heat or open flames, the epoxy
compounds break down and in the process release free radicals such as OH
and H in the form of flammable gases. BFRs work by chemically
interfering with the gas phase of the combustion process. However,
bromine containing compounds have also been found to form dioxins during
the combustion process, and these active dioxins are considered to be
carcinogens by the Environmental Protection Agency (EPA). Therefore, a
strong trend is emerging for selecting bromine replacement materials
that is focused primarily on environmental, health, and reliability
concerns.

Phosphorous based flame retardants (PFRs) have been considered as
alternate replacement materials to BFRs. Phosphorous based flame
retardants impede the combustion process through multiple reaction
steps. These PFRs are primarily divided into organic and inorganic
compounds. Organic phosphorous based flame retardants currently used in
electronic applications consist of phosphates, phosphonates,
phosphinates, and phosphine oxides. However, the addition of organic
phosphorous into molding compounds tends to increase the water absorbing
property of the plastic encapsulants, increasing the probability of
popcorning and moisture related failure mechanisms.

Inorganic phosphorous compounds have been used, including red
phosphorous, to overcome the drawbacks of the organic compounds. Red
phosphorous has been used and has exhibited a very high flame retarding
capability. However, consistent concerns about long-term stability of
red phosphorous-based fire retardants with moisture have limited their
extensive use in microelectronic applications. When red phosphorus
reacts with water an electrolyte is produced. Improvements in
formulating and processing the red phosphorus-based compounds have not
eliminated this problem. Using red phosphorus as an alternative to
bromine compounds for flame retardancy is just one approach and not all
epoxy manufacturers use it, therefore this issue does not apply
uniformly across the industry.

Failures, in the form of electrical leakage and metal dendrite growth
between conductors have been linked with red phosphorus particles in
PEMs encapsulant. University of Maryland's (UMD) Computer Aided Life
Cycle Engineering (CALCE) center has published their study of this
issue. An overview and a link to the paper can be found at:

<http://www.calce.umd.edu/whats_new/2002/Phosphorus_020926.htm>

*The full paper is available to CALCE members (NASA civil servants and
NASA contractors contact <jfplante@pop500.gsfc.nasa.gov> to apply for
access)*.

Maxim has also published information on this topic which can be found
at:

<http://www.maxim-ic.com/TechSupport/QA/Reliability/SumitomoNote.pdf>

Dexter's discussion of this issue and their approach can be found at:

<http://www.loctite.com/int_henkel/loctite/binarydata/pdf/elec_GreenMoldComp.pdf>