Introduction to microsectioning printed circuit boards
Bob Willis
Introduction to
microsectioning printed
circuit boards
Producing microsection is a key part of quality from the author on CD or as wall charts; inner pad configurations occur. It is
control in PCB manufacture, its also a very these are also available from the IPC for good practice to arrange for the first
useful skill in any failure analysis laboratory. training and reference. and last holes drilled to be adjacent to
Your author is still producing his own micro- The majority of microsections are taken each other; this allows evaluation of
sections—there is no substitute for practice, one from test coupons, test boards or scrap both in one section. This is particularly
day he will crack it! boards. Test coupons are manufactured true and relatively easy where a test site
due to the prohibitive cost of loading spare is included in the design.
introduction test boards. There are drawbacks with test 4. Consider the colour of the resin; if you
Microsections are an essential tool in the coupons; their proximity to the edge of the are going to take photographs, a white
quality evaluation and control of PCB board and differences in make, e.g. number or yellow coloring agent added to the
manufacture. It is very important that of layers at different hole positions, must resin can improve the colour separation
the people who produce and assess these be taken into consideration. If a specific is- on solder mask images. This is also true
microsections are well trained and super- sue comes to light it, is fairly simple to add when conducting sections of conformal
vised by a person who is well versed in the test coupons and, with care, they can be coating over a solder joint.
methods of PCB manufacture. Regardless representative of the real board. It is easier 5. Decide whether the sample requires
of some of the standards in the industry, to change a test coupon than an interna- overplating and with what material;
there are areas where cosmetic defects tional standard. overplating reduces the distortion of the
should be considered to be indicators for There are other variations which should plating for measurement purposes, and
improvements rather than rejection of be evaluated on each design. Once the it is normally confined to tin/lead and
products. Visual standards are available variables are known, test coupon results other thin or soft coatings.
may be used with more confidence. One of
the skills reviewed during the ITRI innova- overplating
tion workshops is microsectioning. Details This acts as a support to the plated deposit
of the workshops are available from www. under evaluation, preventing the outer
askbobwillis.com/faworkshops.pdf. edge of the deposit from deformation and
Before taking samples for microsection- profiling during polishing.
ing, the following should be considered: The overplated metal should not chemi-
cally react with the plated deposit or be
1. What are we looking for? This may seem
softer. Overplating should be carried out
an absurd consideration, but the param-
where possible prior to the pattern being
eter being checked will determine which
etched. The samples shall be degreased and
part of the board/test coupon should be
chemically cleaned to allow the surface to
sectioned. Always try x-ray first, as some
wet, the water break test prior to overplat-
Figure 1. X-ray showing poor uneven plating on
failures can be determined, it’s quicker
ing. Without the pre-clean, the overplate
0.2mm through via which lead to an open circuit.
and easier than sectioning, and you still
will peel away and possibly give misleading
have a board to examine latter.
results.
2. If plating is to be assessed, avoid areas
In most cases, overplating is unneces-
in the proximity of jigging fixtures.
sary if adequate care is taken in the prepa-
Choose holes as near as possible to the
rations and assessment of the microsection.
board centre. Section holes in ‘x’ and ‘y’
Gold can be used as it is nice and hard;
axis; this allows a possible defect to be
copper is simple and readily available. It is
determined.
Figure 2. Section holes in “x” and “y” axis; this
softer than gold but harder than tin/lead.
3. Drilling and plating evaluation: choose
allows a possible defect to be determined.
an area where a range of hole sizes and
4 – Global SMT & Packaging – November 2008
www.globalsmt.net
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