Flexible Circuits,Flexible printed circuit boards, Flex PCB, FPC Manufacturer,Rigid-Flex PCB

RayMing Flexible Circuits

A few additional tests that can be performed on incoming printed circuit board product include solderability and thermal stress analyses.

 A few additional tests that
can be performed on incoming
printed circuit board product
include solderability and thermal
stress analyses.
 
A few additional tests that can be performed
on incoming printed circuit board product in-
clude solderability and thermal stress analyses.
Solderability testing is just as it sounds and
is performed to ensure that the metallic surfaces
on a printed circuit board will wet with molten
solder under standard test conditions. The test
involves a commonly available soldering flux
and a molten solder pot. A simple confirmation
of whether the board can pass a solderability
test, such as one called out in IPC-J-STD-003:
Solderability Tests for Printed Boards [3] , is a good
place to start.
Thermal stress testing is also just as it
sounds. A printed circuit board specimen is ex-
posed to heat cycles via a molten solder pot to
determine whether the board (and all its con-
nections) can withstand the potential solder re-
flow cycles that the board might see during the
assembly process yet to come. After exposure of
the test sample to the solder reflow cycles, cross-
sectional samples are prepared, as described
above when we discussed microsection analy-
sis, for another evaluation of the internal board
structures—this one after a stress event. For a
guideline, IPC-TM-650, method 2.6.8: Thermal
Stress, Plated-Through Holes [4] is a commonly
utilized test for thermal stress analysis.
So far, the testing mentioned in this column
has dealt mostly with the “structures” associ-
ated with printed circuit boards—traces, plated
through-holes, via holes, etc. Although these
aspects of printed circuit boards are clearly im-
portant and relevant, there is another aspect of
the board’s construction that is equally as im-
portant, yet commonly overlooked. That aspect
relates to the material which is used to physi-
cally construct the board, and its “structures”—
the laminate. When designing and developing
a printed circuit board, a laminate material is
called out on the board’s drawing. The material
is chosen for various reasons, such as glass tran-
sition temperature, thermal expansion capa-
bilities, and delamination (or more specifically,
lack-of-delamination) properties, just to name
a few. To investigate these types of properties,
one must call upon the use of thermal analysis
test equipment. Of specific interest would be a
differential scanning calorimeter (DSC) and a
thermomechanical analyzer (TMA).
A DSC measures heat flow in or out of a test
specimen as a function of time or temperature.
Determining the glass transition temperature
(Tg) of a board’s laminate material is a com-
monly monitored material parameter, as the
Tg value is an important factor when selecting
the laminate material for the board’s construc-
tion, due to the reflow profile that will be used
during the board’s assembly process. At the
same time, degree of cure—a property which
sheds light on whether a printed circuit board
was cured properly—is also commonly investi-
gated as an incompletely cured board can lead
to board population issues during the assem-
bly process. As a guideline, one can use IPC-
TM-650, method 2.4.25: Glass Transition Tem-
perature and Cure Factor by DSC [5] if interested
in these properties.
A TMA measures distance change of a test
specimen as a function of time or temperature.
Although TMA can also be used to determine
Tg, like DSC, it’s more generally used to mea-
sure thermal expansion-related properties. Like
the DSC testing described above, board design-
ers also use thermal expansion properties of the
board when designing the board’s layout and
when choosing the materials for the board’s
construction. A commonly used test method
for measuring these properties is IPC-TM-650,
method 2.4.24: Glass Transition Temperature
and Z-Axis Thermal Expansion by TMA [6] . In
addition to measuring the specific thermal ex-
pansion values, obtaining information about
the board’s ability to withstand delamination
of its layers at elevated temperature can also be
investigated via TMA. There is a group of test
methods for this type of delamination testing,
with IPC-TM-650, method 2.4.24.1: Time to
Delamination (TMA Method) [7] the most com-
monly used.
Clearly, a variety of steps that can be taken
to help ensure you’re getting what you paid
for…and doing any testing is honestly better
than doing none at all. PCB
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