Introduction to microsectioning printed circuit boards
sample removal
ture of about 180˚C
For thin laminates of 0.25 mm (0.010")
and a pressure of 290
or less, scissors or a hammer and chisel
lbs/in2 is used. Their
with the sample supported on a firm base
operating parameters
will very successfully remove the requisite
limit the use of the
sample. Thicker materials may be cut
method to samples
with a variety of tools, from dental burr,
which will not be af-
fly press or, commonly, a Dremel. Use
fected by temperature
of punches or presses to remove thicker
and pressure.
samples is not recommended unless carried
To encapsulate: A
out under very controlled conditions. They
measured quantity of
are not suitable for assemblies and particu-
the resin is poured into
larly lead-free, unless you want to pick the
the press barrel. The
components and joints off the floor. Some
sample, which will have
methods are listed below:
been degreased and
Punch/Press—may cause deformation
dried, is placed onto
to the sample if the punch tool is not
the resin, along with
correctly designed. Accurate fixturing is
an identification label.
Figure 3. Grinding a microsection section manually, best way to learn and
maintain those skills but watch the tips of your fingers.
essential. Method is dust free.
Push both the sample
and the label lightly
Dental burr—slow but accurate. Very little
into the resin. This
given a unique number, possibly with the
sample damage likely to be incurred. Very
will prevent the sample moving during the
last two digits being the year. The batch
fine dust produced.
heating and pressure cycle.
number and board number should also be
Abra file/Jigsaw—awkward, can cause vibra-
When using a manual press, raise
stated on the label. A simple logbook with
tion damage. Some dust.
the pressure again. After heating for 10
the microsection numbers in numerical
minutes turn off the heater and allow
order with date and sample description
Drill route—when fitted to an engraver the press/sample to air cool for a period,
completes the traceability. Any paper-
allows fast sample removal. Accurate then turn on the water cool. When the
work will carry the relevant microsection
fixturing not required. Variable sample size sample has been cooled sufficiently (10
number. Of course today everything can
removed. Does generate dust. minutes) release the pressure and remove
be logged on PC with all the images taken
the sample. With automatic presses, the
If sections are being prepared of assembled
before and after preparation to provide a
heating and cooling times and the pressure
boards, care must be taken with the meth-
full report on the investigation.
are pre-set.
od of sample removal to avoid damage. It
The encapsulated samples are ground
is often the case that chip components are
using silicon carbide papers either on a
Use of thermosetting resin
damaged during preparation of samples
rotary wheel or flat bed strip grinder—yes
These may be two part liquids, one of
or microsectioning and not caused in the
they still exist! A range of paper grades
which is the hardener, or liquid/powder
actual process of assembly.
should be used, starting with the coarse
mixes. Of the media evaluated, the liquid/
Whichever method of sample removal
one and progressing through to the finest
powder mixtures give the harder resin fin-
is adopted, ensure it is carefully evaluated
one. A continual flow of water of ad-
ish. Soft resins should be avoided as profile
and the shortcomings known. Never cut
equate flow rate to cover the paper and
edges will occur. The resin cures due to
too close to the sample and where possible
prevent clogging of the papers by ground
exothermic reaction at a temperature of
use control samples. It better to cut a larger
off debris shall be maintained. Unlike the
about 70˚C. The time it takes to cure is
sample then slowly grind/lap down the
dry method of grinding, the wet method
dependent on the temperature of the room
sample close to the point of interest before
allows pressure to be placed on the sample
in which it is left to cure. The sample itself
potting the section. It is faster, quicker and
to speed microsection preparation without
will affect the rate of cure and temperature
avoids uneven sections.
more and deeper deformation of the sam-
reached during polymerisation. Most sam-
ple. Deformation will be at its greatest with
sample encapsulation
ples will be unaffected by the temperature,
new papers and so the newness should be
but the drawback with these resins is the
Encapsulation of the sample prior to
removed before critical grinding.
critical mixing. Good or bad results will
microsectioning is necessary for two main
Three general rules during grinding:
be attained depending on how the resin
reasons:
is mixed and under what conditions it is
1. The pressure on the sample shall be as
1. To prevent the break-up of the plating allowed to polymerise. Excessive stirring
high as is comfortable, but care needs
under evaluation which can occur when and air temperature will result in aerated
to be taken with the amount of material
it is not supported, thus preventing an encapsulations, into which debris will col-
being removed. In the case of a 1 mm
accurate assessment of the sample. lect during grinding and polishing.
hole, it’s easy to stop before grinding
2. To make handling of the sample easier. Standard two-part epoxy is also very ef-
through the hole. With a 0.1 mm blind
The media used to encapsulate the fective and is a relatively low cost material
via the hole can be lost in 10 seconds.
sample is dependent on the finish of for many different types of materials. Care
2. The paper should be slightly worn.
the board from which the sample is needs to be taken when handling materi-
3. The time on each paper during grinding
taken and the sample makeup. als as some people can suffer from rashes
should be at least twice the time for re-
when contacting these materials. Wherever
moving the scratches left by the coarser
Hot pressing using acrylic thermoplastic possible wear gloves and refer to the health
paper. When microsectioning plated
This is a very quick and efficient method and safety data sheets.
holes the coarse paper should be used
of encapsulating samples. There are several
until almost halfway through the holes.
hot presses on the market, some manual
sample identification
Then progress on to the finer grades
and some automatic. Typically a tempera-
For traceability, each sample should be
until half the hole is exposed and the
www.globalsmt.net Global SMT & Packaging – November 2008 – 5
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