Can lead-free and lead-containing PCBs be cleaned in a single process?
Figure 1a, b: Overview of representative customer bath sample data for two different cleaning agents in the same family.
an alkaline pH level, which facilitates based method was selected. To establish
the removal of flux residues from assem- the benchmarks and to help to observe
blies. Cross contamination on lead-free potential differences to cleaned assemblies
assemblies may occur as the lead from later on, lead-containing and lead-free test
lead-containing assemblies, which has been samples were used for initial analysis prior
dissolved during the cleaning cycle, could to cleaning. Measurements were taken at
now deposit on lead-free solder joints. This different spots of the test substrates (top,
deposition can occur either as elemental middle, bottom) to achieve a sufficient
lead, by redox reactions, or as ionic re- statistical distribution.
contamination, by the metal salts dissolved The measurement of the lead-free
in the cleaning agent. The amount of lead boards resulted in experimental values of
dissolved in the cleaning agent thereby has 0.0 to 0.3 mg/cm
2
. The critical limit of
a direct impact on the specific amount of 0.1%, as defined by WEEE and RoHS, was
Figure 2: Side by side comparison of lead-free (above)
lead deposited. After cleaning, assemblies not reached. The analysis of the lead-con-
and leaded (below) assemblies.
have to be properly rinsed to ensure the taining PCBs on the other hand showed
full removal of such depositions. lead readings between 3.0 to 5.0 mg/cm
2
.
For better understanding the risks of a Two additional samples (1st lead-free,
shows that lead is neither incorpo-
mixed cleaning process, random customer 2nd leaded) were analyzed by a specifi-
rated via a redox reaction nor via
samples were collected and analyzed to cally designed ‘RoHS’ test method. The
ionic contamination. An addition-
determine the levels of lead and other following spectra collected from these
al rinsing step did also not alter the
heavy metals. All samples were taken from measurements clearly show each sample’s
readings to any extent.
current customers using the same family of composition and the presence of lead in
Experiment 2
products. the second PCB. Figure 2 illustrates both
Cleaning Agent, 15% concentra-
The collected samples came from vari- product samples.
tion, 100 mg/l of lead salt, 5
ous manufacturing plants and reflected
minutes exposure time, room
the full bandwidth from low to high experimental laboratory analysis
temperature.
throughput (Figure 1a, b test series 1-11). In Using the analytical, XRF-based technique,
As can be seen in Figure 4,
spite of the huge differences in volume, it four cross-contamination experiments
an increase in lead salts did not
is interesting to point out that the quantity with identical test boards were conducted
change the quantity of detectable
of dissolved lead has no linear relationship in a laboratory environment. Different
lead.
with the throughput. Examining those solutions of a cleaning agent were prepared
customer samples, the maximum levels of with an altering amount of lead at differ-
Experiment 3
lead were observed to be always lower than ent temperatures. X-ray data were collected
Cleaning Agent, 15% concentra-
10 mg/l. on three distinct and repeatable areas,
tion, 50 mg/l of lead salt, 5 min-
The quantities of lead found in this designated as top, middle and bottom.
utes exposure time, approx 50°C.
analysis were used as benchmark values for
Based on the two initial tests,
Experiment 1
all further experiments.
even an increase in temperature
Cleaning Agent, 15% concen-
shows no rise of lead deposition.
Further x-ray analysis
tration, 50 mg/l of lead salt, 5
minutes exposure time, room
Experiment 4
To detect potential cross contamination of
temperature.
Cleaning Agent, 15% concentra-
lead-free boards, a nondestructive, quanti-
For this initial test, Figure 3
tion, 50 mg/l of lead salt, 5 min-
tative x-ray fluorescence (XRF) technology-
utes exposure time, approx. 65°C.
www.globalsmt.net Global SMT & Packaging – November 2008 – 29
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