Resolution of solder voids in pin-in-hole product
Figure 4. Micro-section of solder void.
a solvent from the dielectric underlying carbon and oxygen Figure 5: SEM micro-section of solder voids. Note excellent wetting angles.
the copper PTH wall or the out-gassing of were all that were
some form of contamination in the PTH found.
these various factors.
entrapped during the wave solder process It was known that one plant site was
Out-gassing is one of the most com-
was leading to the formation of solder producing PCBs which had an unaccept-
mon causes of solder void formation.
voids. Standard moisture induced solder able level of voiding after wave soldering
Evaluation of out-gassing as a root cause
voids was eliminated as a source early on while a second plant site from the same
considered two different factors in the
due to the fact that a standard bake prior company was producing PCBs which had
design of the experiments: 1. The sources
to wave solder made little if any impact an acceptable level of solder voids after
of the gas. 2. The time required for gas
on the frequency of solder voids in the wave soldering. In order to glean as much
generation. According to the results of the
product.
5
Further work then focused on insight as possible into the root cause of
failure analysis, nodules underlying the
attempting to identify the source of the the solder voids a detailed process com-
plated copper were found at the core of the
contaminant that was out-gassing. To parison was conducted between the two
solder void (Figure 7). This suggests that the
do this, various parallel approaches were plant sites for each step of the manufactur-
seed of the nodules were attached to the
employed. ing process with a focus on attempting to
hole walls at or prior to the permanganate
Solder joints that were determined, via identify differences that could account for
desmear process. If this were the case, the
2-dimensional x-ray inspection, to contain foreign contaminants being introduced
nodules might release gases during wave
solder voids were examined more carefully that would later out-gas. This involved a
soldering. Therefore, the permanganate
in an attempt to identify foreign contami- detailed review on paper as well as review
desmear process is one of the areas of
nants that were present within the solder of some of the processes that were suspect.
the PCB manufacturing process which
void itself. Initial approaches were based In addition, micro-sections were reviewed
required further investigation.
upon micro-sectioning into the solder void further for clues that could narrow down
Moisture or solvents are vaporized
and then analyzing the internal surfaces of the focus of the investigation based upon
when heated, and gas is generated. An ex-
the solder void for contaminants. However, the elimination of processes beyond a
periment was conducted to verify whether
it was found that the micro-sectioning certain point.
a more robust bake prior to the wave sol-
process introduced too many foreign con-
der operation can help eliminate the solder
taminants which made the identification experimental design
voids. In addition, the factors affecting the
of a foreign contaminant from the PCB The goal of the experiments designed in
rate at which the solder fills the PTH were
manufacturing or card assembly process this study was to evaluate the proposed
also considered. The organic solderability
very difficult if not impossible. Therefore, hypotheses in order to determine the root
preservative (OSP) pretreatment might
subsequent approaches employed micro- cause of the PTH solder voids. The previ-
affect the roughness of the PTH barrel
sectioning to grind very near to the outer ous section referred to a series of detailed
copper surfaces and hence affect the rate at
surface of the solder void without breaking examinations of PCBs with solder voids.
which the solder fills the PTH during the
through to the inside of the void. This was From these examinations, three potential
wave soldering process.
followed up with the use of a focused ion root causes were hypothesized including
Based on the above considerations,
beam (FIB) to carefully cut into the void out-gassing, poor wetting and incomplete
boards were fabricated and wave soldered
while hopefully disturbing the inside of escape of flux residues. For each of the
in order to check the importance of the
the void as little as possible. SEM-EDS, hypothesized root causes, there were a
various factors. The test board had a layer
Micro-FTIR, and TOF-SIMS analysis number of possible factors that could
count of 6, and an array of 29 mil PTHs
were then employed to identify potential affect the soldering process resulting in
with the pitch size of 79 mils.
contaminants within the solder void (Figure PTH solder voids. A set of experiments was

6). Other than the expected metallurgy, designed to differentiate the significance of
22 – Global SMT & Packaging – February 2009 www.globalsmt.net
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