Clean “no-clean,” or use a water wash solder paste?
Figure 5. Graping on 0603 components. Figure 4. Ionic residues levels: All are well below the maximum of 40 mg/cm².
“In particular, reliability concerns result from highly active organic acid and/or halide/ halogenated flux residues trapped under low stand- off components.”
Currently, there are insufficient
process control and quality assurance methods for detecting flux residue trapped in this way5
. If a water wash flux is used, all residues
must be removed from the assembly. If not, they are a potential risk (e.g. dendrite growth). A more serious issue arises with the recent trend of cleaning a “no-clean”
solder paste using DI-water with a (small) concentration of the cleaning agent. As with water wash paste, if the flux residues are not completely removed from the assembly, they also are a potential risk for failure, since the protective properties of the rosins have been damaged by the cleaning attempt. Miniaturization is one of the major
trends in electronic assembly. Components become smaller and smaller. As a result, pick-and-place machines and printers need to be more accurate and solder paste powders may require a powder type 4 or 5 instead of type 3. These smaller powder sizes require a review of the solder paste flux system. The fine powder has more metal surface and therefore may require more flux or a modified activation system. More flux in the paste means more flux residues after soldering underneath small components. Another phenomenon that occurs
with fine-pitch components is solder graping—poor coalescence. Solder graping refers to poor solder wetting: where the solder paste has partially melted, but has not coalesced or flowed completely. Solder powder (oxidation, contamination of the metal) may impact graping, but the flux system also requires a stronger activator or materials that give it more thermal stability.
Figure 6. Entrapped solder spheres in flux residues on top of the solder of the SOIC lead.
Figure 7. After cleaning the “no-clean” flux, residues are gone, including the entrapped solder spheres.
Figure 8. Component dimensions and stand-off.
www.globalsmt.net Global SMT & Packaging – Celebrating 10 Years – December 2010 – 13
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