Optimizing batch cleaning for removing lead-free flux residues on PCAs
must be developed to lower the dynamic Two of the three hard residue lead-free joints, attack solder mask or remove part
energy required for physical removal and/or solder pastes were successfully cleaned. For markings. In a few cases the label adhesive
increase the static rate of dissolution. assemblers who plan to remove solder paste failed. Additionally, the cleaning material
Raising the temperature, improves the flux residues, the selection of the solder performed well at removing hard and soft
static rate, approximately doubling the rate paste from a cleanability perspective should lead-free flux residues.
of saponification for every 10°C increase in be considered. The high process cleaning rate achieved
wash temperature. Increasing the tempera- Time effects: The research findings on the boards processed at 200°F and 18%
ture also improves solvency, the ability to indicate that time is important but not as raised a curiosity among the research team.
dissolve more residue in a given volume of important as temperature effects. Board The team decided to lower the cleaning ma-
solvent, which directly improves the static processed for an additional 25 minutes for a terial concentration from 18% to 9% and to
rate of cleaning. The data findings indicate total of 40 minutes wash time at 150°F were 5%. Reduced cleaning material concentra-
that batch wash temperatures 165ºF give a the same as boards cleaned for 15 minutes. tions lower the dynamic surface tension.
better cleaning result; even in tight spaces. The test ran at 175°F for 25 minute wash Boards processed at 9% cleaning material
Softening the flux residues could play time cleaned well. Additional research is concentration provided excellent cleaning
an important role. The reflowed flux resi- needed to quantify time effects in relation on all flux residue types except one lead free
due remaining under the parts is a mixture to temperature effects. hard residue. Boards processed at 5% clean-
of high molecular weight compounds Pre-heating boards before cleaning: ing material concentration also provided
collectively called resins or rosins. Most One set of boards was preheated before the excellent results with a slight cleaning drop
resins and rosins soften with temperature. cleaning process. The data indicates that off from boards processed at 9% cleaning
These compounds usually have a softening cleaning was less effective. This data point material concentration.
temperature and a melting point that can indicates that the pre-heat cycle hardened
vary by more than 50°F. The temperature the residue making it more difficult to conclusions
range between the softening point and the remove under the Z-axis. The purpose of this designed experiment
melt temperature is the softening range. Cleaning material effects: The research is to report optimized cleaning process
It turns out that rosin, the most common findings indicate the benefit of cleaning parameters for removing lead-free flux
flux material, softens at a temperature of under Z-axis components by increasing wash residues on populated circuit assemblies
165°F and melts at a temperature of around temperature and wash time. A concern with using innovative cleaning fluid and batch
212°F
5
. Resins generally used in fluxes have this approach is the circuit assembly mate- cleaning equipment designs. Quantitative
a similar to slightly higher softening range. rial compatibility effects. Aqueous clean- experiments were run on both inline and
By heating the part above the softening ing materials processed at elevated wash batch dishwasher cleaning machines using a
point of the flux matrix, the residue is temperatures and wash times commonly best in class cleaning material.
softened and is rendered more susceptible dull solder joints, remove part markings, at- Establishing process equivalence
to lower energy erosion, thus increasing the tack anodized aluminum coatings and oxide between inline cleaners and batch cleaners
dynamic cleaning rate. yellow and soft metals. assures an equal result in both cleaning pro-
From the five lead-free solder pastes in The building blocks for engineering elec- cesses. This is highly desirable if a company
this study, temperature affects significantly tronic assembly cleaning materials consist is manufacturing in multiple assembly loca-
improved removal of residues under the of: tions or with different contract manufactur-
Z-axis. One of the lead-free pastes had the 1. Solvency: Materials that dissolve ers. This leap in batch process performance
opposite effect when increasing wash tem- flux resin and polymer structures, requires rethinking the cleaning rate
perature. The data indicates that LFHR3 thus placing the soil into solution. fundamentals.
cleaning under the Z-axis dropped off when 2. Builders: Materials that rapidly Results indicate that wash temperature
temperature rose. soften resin and polymer structures in the wash fluid improves cleaning perfor-
Soil selection: When cleaning high- allowing dissolution in the solvent mance on the more difficult to clean geom-
density surface mount assemblies and under matrix. etries and fluxes comparable to near that
Z-axis components the data findings indi- 3. Wetting: Lowering surface tension of today’s best inline processes. Although
cate that the selection of the solder paste is by reducing the wash droplet size. the time, temperature and cleaning agent
an important consideration to ensure that 4. Minor ingredients: Materials that concentrations are different, the results
every opportunity is taken to enhance the destabilize foam and inhibit attack were equal if the batch higher temperature
ability to clean. Lead-free soft-residue solder to metal alloys. process parameters developed in this study
pastes provided a wide processing window, were used.
When elevating wash temperature and
especially when the wash temperature was No material effects were noted on the
wash time an inherent limitation with most
increased. eutectic tin/lead and lead-free solders used.
aqueous cleaning materials is attack to the
Hard residue no-clean solder pastes The cleaning agent selected was formulated
board material subset. Many aqueous mate-
are more difficult to clean. Some paste with corrosion inhibiting agents built-in
rials darken solder joints when exposed to
formulations’ use polymers, which cross- to the solution to allow longer and hotter
elevated wash temperatures. Part markings
link at reflow. The hard film is designed to cleaning cycles. All solder connections
are more susceptible to removal at longer
encapsulate ionic and non-ionic salts from tested remained un-oxidized, bright and
wash times and temperatures. Anodized
the reflow process. Since the design of the shinny. The boards and the components
coatings tend to fail at elevated wash times
solder paste is to not clean the residue, the show no signs of discoloration or damage in
and temperatures.
ability to remove the residue under Z-axis the higher heat cleaning cycles.
The cleaning fluid design used for this
components, where impingement effects are It is clear that high wash temperatures
experiment optimizes the four design build-
reduced, becomes increasingly complex. can result in shorter batch cleaning cycles
ing blocks. The boards processed at elevated
The cleaning material used in this study because of the improved cleaning rate. This
temperatures and times did not dull solder
is designed to remove lead-free flux residues. would result in less power consumption,
18 – Global SMT & Packaging – September 2008 www.globalsmt.net
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