Clean “no-clean,” or use a water wash solder paste?
of an aqueous solution, water soluble flux residue is vulnerable too. However, depending on the rigour of the cleaning process, water insoluble and non- saponifiable contamination may remain. There are many glycol ether cleaning
solvents on the market. These, too, are generally good solvents for rosin. However, they have less propensity to other flux contamination, especially more polar (low molecular weight) carboxylic acids. When applied as a semi-aqueous process, in which it is mixed with water and/ or a water rinse follows, the range of contaminants removed increases. Cleaning with pure water (without
saponifier) will remove water soluble contamination only, unless there is significant physical force and/or high temperature, in which case a physical “scrubbing” action will operate. This can be effective, but it may risk PCB integrity
Figure 1. Removed amount of flux from the solder paste. Data are the means of the parameter levels.
Feasibility of cleaning a “no-clean” paste Experiment Although there are many reasons not to clean a “no-clean” solder paste, there is increased interest in doing so. “No-clean” solder paste exploits a flux chemistry that is designed to encapsulate activators that remain on the board after soldering. It is not meant to be cleaned after soldering; therefore, the residues are harder to remove from a printed circuit assembly. The residue that remains after soldering contains activators, gelling agents and resins. The amount depends on the composition of the solder paste and the process conditions (e.g. reflow temperatures) to which the assembly was exposed. The first experiment was a study
to identify the possibility of cleaning a “no-clean” solder paste and to define the impact of the different parameters on cleaning performance. A full factorial design of experiment was run with parameters and levels shown in Table 1. The experiment was done using a small
lab scale washing device. Solder paste was printed on copper samples (stencil 107 x 76 x 0.2 mm with three round holes or 6.5 mm diameter aperture). The samples were reflowed in a
convection oven using a typical three minute tin-lead reflow profile with a peak temperature of 215˚C. The reflowed samples were washed using different saponifier concentrations, temperatures and times. The amount of residues removed was weighed using a four figure
Figure 2. Board cleanliness as a function of cleaning time and concentration saponifier.
balance. The average amount of solder
paste applied on the samples was 0.070 gram. After soldering, 51 percent of the flux remained. The other 49 percent evaporated as a result of the reflow process.
Data analysis All factors in the experiment—temperature, concentration and cleaning time— significantly impacted on the cleaning result. DI-water only was unable to clean a “no-clean” solder paste. No-clean solder paste contains non-polar water insoluble residues. These can only be cleaned by the additives in water such as saponifiers. The concentration of the cleaning
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agent and the cleaning time had the highest impact. Figure 2 shows the relation between both factors.
Further cleaning tests These data formed the basis of two cleaning processes for a test board that was soldered with three different no-clean solder pastes: • Spray in air • Ultrasonic
The test boards were soldered, cleaned and tested for cleanliness by means of visual inspection and using an ionic contamination measuring device. The maximum levels of flux residues
Global SMT & Packaging – Celebrating 10 Years – December 2010 – 11
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