Strain gage testing: the delta effect of thermal cycle testing
through the card. along the intermetallic zone of a soldered
All PCB assemblies interconnection. Strain gage testing is an
have strain pathways analytical tool used to quantify strain and
or corridors that stress at a certain location on a PCB as-
are influenced by sembly. The benefits of strain gage testing
the location of large are measurable and can be quantifiable by
devices such as heat deriving an understanding of the forces be-
sinks, large compo- ing applied to a location and the duration
nents and attach- of these stresses.
ment locations as
well as connectors. case study summary
These large devices A high reliability electronic assembly has
impart stresses into two components that are located on the
!
the board which are same x-axis and are located .750-inch from
Graph C. Y-X Delta Data
then applied into ad- each other on the y-axis. One location is
jacent components named location A and has a maximum
and thus create the strain in the x-axis of -200 microstrains at
smaller and smaller component pack-
invisible forces that can fracture a compo- -36°C and -786 microstrains in the y-axis
ages which with their smaller attachment
nent lead attachment. Strain gages come at 103°C and +659 microstrains at -36°C.
volumes are susceptible to excessive
in many different styles and packaging for- In and of itself these numbers are not
stresses and strain pathways. The board
mats, from single gage style to multi-gage considered excessive for a .062-inch thick
thickness also plays a role as more and
style called a rosette. A multi-rosette style standard FR4 laminate PCB assembly.
more designs are moving away from the
has gages located at 0, 45 and 90 degrees Location B which is located .750-inch from
standard .062-inch thicknesses to .048-inch
(Figure 4) so as to capture the forces in both location A has a maximum strain of -583
and even .032-inch thicknesses. These
the x and y direction and their influence microstrains at -40°C in the x-axis and -851
reduced thicknesses now introduce more
on each other at 45 degrees. microstrains in the y-axis at 105°C and
board flexure as well as strain pathways
Where does microstrain originate? +192 microstrains at -36°C. Again, in and
because the board which was once a major
Where are the sources of this stress? This of itself these are not excessive microstrain
source of added stiffness is now a lot more
article looks at environmental accelerated levels when one considers Location B
pliable and thus susceptible to isolated
life tests. The use of thermal cycle testing has never failed thermal cycling for 2000
elevated stresses. Another concern is the
to replicate what an assembly will see in its cycles. This analysis would lead one to
new smaller component packages that are
lifetime with or without random vibra- believe that Location B would fail before
available for high density designs, e.g. QFN
tion testing is essentially the question for Location A because the highest recorded
style and their reduced solder attachment
determining if the applied strains being microstrain level was -851 microstrains in
volume and non compliant lead mate-
applied to components will meet the the y-axis. But in reality, Location A has
rial design, introduce a new problem for
design objective. There are a number of failed consistently between 750 to 1000
system designers which is a lower threshold
other sources that can damage or fracture cycles. The thermal temperature profile
for handling applied stresses during opera-
the solder attachment alloys used today in cycle is –45°C to +105°C. Looking at maxi-
tional use. These new high density designs
the manufacturing of electronic assemblies. mum forces does not tell the whole story.
can all be tested using accelerated life
Examples include: Again looking at the maximum and
cycling and strain gage testing to quantify
minimum forces seen during thermal cycle,
and validate these new layouts before being 1. Board depanelization
one would not jump to a conclusion that
fielded and thus can statistically lower the 2. In circuit / functional tests
Location A is more prone to failures than
probability of having field returns and war- 3. Accelerated life tests or envi-
Location B. The delta microstrain from
ranty issues later in the product life cycle. ronmental screens
negative to positive gives the first clue that
4. Vibrational test screens
Location A has a higher degree of move-
What is microstrain and its corre- 5. Handling and component
ment (difference from negative to positive)
sponding pathway insertion
than Location B in the y-axis. See below
Microstrain is a measurement unit that 6. Final assembly and integration
delta difference.
has no dimensions and is defined as 106 7. R e w o r k
The data does not reveal the problem
x (change in length) / (original length). A 8. Heat sink and connector inte-
until one can graphically overlay and ana-
microstrain is measured or recorded with gration
lyze the different data sets visually with all
a resistance bridge recorder that measures
This list can suggest that anywhere along variables being captured. The development
a change in resistance when a strain gage is
the manufacturing, testing and final assem- of a mathematical equation to capture the
subjected to an applied strain or stress that
bly integration one can introduce excessive delta effects of Y–X data graphically shows
shortens or lengthens the strain gage itself.
strain that results in a failure. This article the subtle differences and the applied
Strain gages are applied near or under
looks at a slow death whereby the fracture forces being exerted at the solder intercon-
components to gather the strain or forces
occurs over time – thermal cycles in either nect as a relationship between x and y at
that are being applied during operational
a test chamber or in the fielded state. CTE the different locations and temperatures
use or in accelerated thermal life testing.
is the culprit which takes multiple cycles realized at the solder joint. The graphs of
Strain gages are great for capturing the
before manifesting itself. We have seen it Delta Y-X represent the summation of the
strain being realized at specific locations as
occur in as few as 500 cycles or as long as X and Y data sets for the two locations in
well as defining strain pathways running
1500 cycles, but the end result is a fracture absolute magnitude of microstrains. This
14 – Global SMT & Packaging – February 2009 www.globalsmt.net
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