Clean “no-clean,” or use a water wash solder paste?
be trapped in flux residues and can cause bridging in a worst-case scenario. Cleaning these boards will remove all
flux residues including solder balls unless they are connected to the solder melt (Figures 6 and 7). For fine-pitch circuitry, there is a
disproportionately high level of oxide (pads and component finish) with less flux (smaller volumes of solder paste). Miniaturization introduces some
challenges for cleaning processes. Distances between the pads reduce dramatically from 3.5 mm for 2010 to 0.1 mm for 01005 components. The risk of bridging, electro-migration, etc. increases and the stand-off height of components reduces. This requires a cleaning chemistry with a low surface tension and sufficient capillary force to penetrate below these small components. After removing the soldered chip
Figure 9. Mean effects of parameters on wetting. The smaller the wetting angle, the better.
components, it was clear that the entire volume underneath each component smaller than 0603 was completely filled with solder paste flux residues, blocking the ingress of the cleaning agent. To verify the cleanability of small
components with low stand-off an assembly was cleaned using the lab device from the previous design of experiment (DoE). Using the same cleaning agent (20% concentration) at a temperature of 50˚C, the boards were cleaned with varied cleaning times. Components were removed to visually inspect the presence of flux residues.
Reflow profiles and impact on residues The reflow profile will impact the solder and cleaning performance of the solder paste. The heating profile also will affect wetting, the amount of flux residue and the hardness/clean-ability of the residue. A Taguchi DoE was performed to
Figure 10. Taguchi parameters monitored in the reflow profile. Graping should not be considered a
defect if only the outer solder spheres are joint to melting and remain part of the
Wetting Angle (˚)
SAC 305 SN100C SnPb
No-clean 19.2 17.9 9.5
Water wash 16.9 14.8 9.2
molten bulk of solder, and they do not violate the minimum electrical clearance. Non-melted solder spheres also can
Remaining Flux (%)
No-clean 23.2 18.8 21.2
Water wash 55.7 50.8 59.4
Table 5. Wetting angle for the different solder pastes on copper coupons soldered under best reflow conditions with nitrogen atmosphere.
define the best wetting conditions for the solder paste and the amount of residue remaining on the assembly after soldering. The factors in the experiment cover
the three critical heating phases in the reflow process: preheat, soak and peak temperatures. The fourth factor is the atmosphere (air vs. nitrogen). A thermogravimetric analyzer (TGA)
was used to reflow the solder paste that was printed on the copper coupons. A solder paste deposit of 1.5 mm diameter was printed with a 100 µm stencil. During soldering the weight loss was monitored and the amount of residue could be defined. Under a microscope, the wetting
14 – Global SMT & Packaging – Celebrating 10 Years – December 2010
www.globalsmt.net
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