Eletromechanical Migration

ELECTROCHEMICAL MIGRATION ON HASL PLATED FR-4 PRINTED
CIRCUIT BOARDS
Elissa Bumiller
Naval Surface Warfare Center, Carderock Division
West Bethesda, MD, USA
Michael Pecht
CALCE Electronic Products and Systems Center
College Park, MD, USA
Craig Hillman
University of Maryland
CALCE Electronic Products and Systems Center
College Park, MD, USA

ABSTRACT
As the current trends towardminiaturization take hold,
proper cleanliness levels become more difficult to achieve.
In addition, effective cleaning requirements may not be in
place. The smaller spacing between conductors also yields a
larger electric field, which in conjunction with insufficient
cleaning can lead to dendritic growth. This paper
investigates the effects of chloride contamination and
electric field on themigration behavior of hot air solder
level (HASL) plated FR-4 printed circuit boards. It was
found that chloride contamination level and electric field
influence which failure mechanism is occurring,
electrochemical migration or electrolytic corrosion. This
study concludes that current industry specifications for
cleanliness may be inadequate for the next generation of
electronicassemblies.
Key words: electrochemical migration, dendritic growth,
electric field, contamination
INTRODUCTION
Electrochemical migration can be a vexing problem in
electronic assemblies. It may cause electrical shorts and/or
intermittent operation of equipment. In most cases, with
proper cleaning measures in place, dendritic growth can be
eliminated. However, as spacings on circuit boards andcomponent lead pitch decrease, effective cleaning becomes
more difficult. The reduction in spacing also increases the
electric field, which is a primary driver of dendritic growth.
Dendritic growth is defined as “ growth of conductive
the
metal filaments on or through a printed circuit board (PCB)
through an electrolytic solution under the influence of a DC
voltage bias” [1]. This definitionencompasses the
phenomenon known as conductive filament formation
(CFF) in addition to electrochemical migration (ECM). For
the purposes of this investigation, ECM will refer
specifically to migration over the surface and CFF will refer
specifically to migration below the surface.

The definition given by IPC requires two components
external to the PCB itself: an electrolytic solution anda DC
voltage bias. The electrolytic solution is provided from
humidity present in the operating environment. Water
adsorbs to the surface of the circuit board, eventually
connecting two oppositely biased conductors. The amount
of water necessary to provide a sufficient path is
approximately 20 monolayers thick [2]. The presence of
contaminants, especially chlorine, in the water serves toexacerbate the situation.
Halide ions act as catalysts in localized corrosion, with
chloride ions having the strongest effect of the halides [3].
However, the minimum levels necessary to induce corrosion
are uncertain, with several sources specifying a range of
recommended contamination limits, from 2 µg/in 2 to 10
µg/in 2 [4 – 8]. Bromide ions, which are also known to be a
problem, havereported ranges from 15 µg/in 2 to 20 µg/in 2
[4 – 8]. The combination of chloride and bromide may also
prove harmful, but this effect has not been covered in the
literature.
In addition to halide contamination, the role of a DC voltage
bias is of interest. The voltage bias allows for calculation of
the electric field, which in turn is directly proportional to the
force applied on acharged particle (e.g., a metallic ion)
within the field. The force applied on a charged particle may
affect the time it takes an ion to move through the
electrolytic solution. The electric field can be calculated
from the applied voltage and the geometry of the structure
under consideration,
E = V/d
where E is the electric field, V is the applied voltage, and d
is the distance separating two...