Resolution of solder voids in pin-in-hole product
Figure 2. Oblique x-ray of solder voids. Figure 3. Micro-section of solder void, note excellent wetting angles.
was used to identify and screen out the PCB manufacturing site to PCBs made Micro-sections were taken of the solder
assemblies with solder void defects (Figure at other locations and confirm assembly joints that had been identified by x-ray
2). The frequency at which the solder voids parameters. The results of this comparison imaging as containing voids. Based upon
occurred within a DIMM connector PTH showed that the defect was unique to this a careful examination of these micro-sec-
array as well as within the overall assembly one particular PCB supplier. tions, it was determined that wetting of the
varied considerably. Defect rates ranged up A Dage XL6500 2D x-ray system was solder to the PTH barrel copper surface
to as high as 70%. Initially this defect was used to perform screening and quality was not related to the root cause of the
thought to have originated from con- monitoring of the finished product. solder void creation. Despite the fact that
tamination in the PTH and poor wetting most all of the solder voids were located
during solder joint formation. However, approach to problem directly in contact with the PTH barrel
this was ruled out as a potential root cause In the hopes of bringing the PTH solder copper surface the wetting angles were near
since the solder joints showed characteris- voiding to a quick resolution, a theory was perfect (Figures 3, 4 and 5), and the copper
tics typical of excellent wetting to the PTH initially postulated that either the solder surface was evenly coated with solder such
barrel copper even as they often exhibited voiding was related to poor wetting of the that there was no evidence of poor wetting.
evidence of blowholes (Figures 3-5). solder to the plated thru hole (PTH) barrel A review of technical reports on solder
copper surface or that the solder voids void formation suggested that solder void
assembly process were formed by the prolonged out-gassing formation could be influenced by the
At one particular CM site, the PCBs first of caprolactone and cyclopentanone result- PTH diameter.
3,4
Experimentation with a
go through a standard no-clean surface ing from an abnormally thick layer of cop- reduced PTH diameter was encouraging.
mount process. Pin-thru-hole components per oxide
2
. Based upon this theory, initial The results suggested that a reduced PTH
including the DIMM connectors are then efforts were narrowly focused on defining diameter had a significant impact on the
inserted into the printed circuit board and executing various design of experi- frequency of occurrence of solder voids
assemblies (PCBAs). The PCBAs are now ments (DOEs) that would bring about where the improvements to the Entek®
ready for wave solder. improvements to the Entek® 106A(X) 106A(X) application process did not. It was
A Vitronics Soltec 6622CC wave application process. then theorized that the solder voids were
solder machine is used with Sn/Pb 63/37, As these DOEs were executed and the thermally induced as are the moisture relat-
a spray fluxer, a preheat zone, and a results were found to indicate minimal ed solder voids. The reason the reduced di-
single solder fountain wave. The solder is improvement in the frequency at which ameter PTHs led to less void formation was
Shenmao SN63A solder bar and the wave solder voids occurred, it became clear the ratio of the thermal mass of the solder
solder flux is Alpha LS500A low solids no that a more robust approach was required to interstitial laminate was reduced. With
clean flux. with a broader focus on the entire printed the ratio reduced there was less thermal
The wave profile meets the limits circuit board (PCB) manufacturing process energy to force the out-gassing leading to
suggested by the flux vendor: 80-105˚C with consideration for other failure the solder voids. This led to experimenta-
for top side of PCBA and 115-140˚C for mechanisms. In response, a multi-faceted tion with shorter solder wave contact times
bottom side. approach was implemented based upon the that, in turn, led to a further reduction in
The CM reported insufficient PTH use of a variety of analytical techniques as the occurrence of solder voids. However, a
solder hole fill found during 2D x-ray well as an in-depth analysis of each step in root cause had still not been identified and
inspection solder quality check. Defect the PCB manufacturing process. the frequency of occurrence of solder voids
containment focused on the DIMM con- X-ray imaging was used to locate solder had still not been reduced to an acceptable
nector region of the PCBA, which is a joints containing voids. X-ray equipment level.
high-density PTH area. used by the contract manufacturers (CM) Since the data was suggesting that the
Several control builds were assembled included both 2-dimensional and 3-dimen- solder voids were thermally induced a new
to compare the product from the one sional techniques. theory was postulated that out-gassing of
www.globalsmt.net Global SMT & Packaging – February 2009 – 21
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