Strain gage testing: the delta effect of thermal cycle testing
Strain gage testing: the delta
effect of thermal cycle testing
by Mark T. McMeen, STI Electronics, Inc., Madison, AL, USA
Strain gage testing has been around for defects are now common due to the density
Do you know what level of
years. It has been used on specific applica- and shrink factor of these types of packages
applied strain your printed
tions to understand the applied strain and and the reduction in interconnect material
circuit board assembly real- strain rates applied to printed circuit board volume.
izes in your fielded state?
assemblies from environmental forces dur- Strain gage testing is the method of
This article looks at one case
ing its working life. Today a renewed interest choice for quantitative analysis for evalu-
study of failed hardware and
is stirring among component manufacturers, ating the amount of strain that is being
the underlying failure mecha-
OEMs, and contract electronic manufactur- applied to these interconnects and package
nisms of temperature cycling.
ers due to the increased use of BGAs, micro types due to their location on a particular
All testing was performed
BGAs, and chip scale packages, as well as design. This type of test evaluation allows
small form factor packaged components. process and design engineers to have the
by the STI Electronics Ana-
The use of more lead-free alloys opens these ability to assess strain across the board
lytical Lab on customer-failed
finished assemblies up to fracturing due and its effect on a particular component
hardware.
to CTE (coefficient of thermal expansion) in a certain location. This is valuable
fatigue and strain forces. Lead-free solder information needed for designing a circuit
alloys are less ductile than 63/37 leaded sol- card assembly that needs to survive in an
der alloys. These types of packaging formats operating environment for five to ten years.
Keywords: Strain Gage, with their reduced rigid solder interconnects Strain pathways across the circuit card can
Microstrain Levels, Strain are now susceptible to applied strain at be so localized that even standard chip style
Rates, Strain Pathways,
these interfaces to the printed circuit card components can be fractured at their solder
Micro Fracturing
pads as well as to the component interface. interface. To this end, strain gage testing
Micro-fracturing of the solder interconnect allows the design engineer and process engi-
to the pad is the catastrophic failure. These neer the ability to assess component location
!
!
Figure 2. SEM micrograph of a fractured BGA
Figure 1. SEM micrograph of a fractured lead.
solder joint.
! !
This paper was originally presented
at PanPac 2009.
Figure 3. A strain gage located on a PCB assembly. Figure 4. A stacked rosette strain gage.
10 – Global SMT & Packaging – February 2009 www.globalsmt.net
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