Solder materials flux residues

Although inhibitors are deliberately added to the silicone formulation to control cure rate, unwanted cure inhibition can be caused by other species that react to form strong complexes with the platinum catalyst. Most notable of these undesired inhibitors include organotin and other organometallic compounds, sulfur, polysulfides, polysulfones or other sulfur-containing materials, amines, urethanes or amine-containing materials, unsaturated hydrocarbons in plasticizers, and some solder flux residues. 

Conductive adhesive materials are very cost-competitive with solder materials, only require a relatively low-temperature cure, and leave no residues, and thus require no subsequent cleaning step. This is a significant advantage compared to lead-free solders that require a high-temperature reflow and, depending on the flux utilized, may also require a subsequent cleaning step. The print speeds of conductive adhesives are comparable to solder pastes. 

In no-clean flux applications, excess flux residue is of particular concern because it has been shown to impede the adhesion of underfill materials that facilitate improved solder joint reliability. 

Environmental contamination, fingerprints and skin oils, human breath, residual manufacturing and processing chemicals, improperly cured materials, solder fluxes, and surface moisture, such as humidity. 

Adhesion. The legend must be applied to a clean, dry surface and be properly cured to achieve optimum adhesion. This can be a challenge when legend is applied after HAST or other final finish that could leave a residue (such as HASL flux) on the surface. Legend adhesion must be evaluated in combination with the solder mask and its processing. Different legend materials have different adhesion to different solder masks. 

In the second experiment (Ref 26, 27), Sn-Pb and SAC results are for assemblies that used low residue or tacky flux. Thermal cycling was between —40 and 125 °C ( — 40 and 257 °F) with dwell times of 5 min. at the temperature extremes, and a 12 min. cycle duration. The underfill material was a standard material with a CTF of 35 ppm/°C and a Tg of 130 °C (266 °F). Failure modes were mixed, including solder fatigue and underfill delamination, with more of the latter in the SAC case than in the Sn-Pb case. 

Residues left after the assembly of a printed circuit board, particularly from flux, have been altered during the heating process, especially when oxygen is present. Printed circuit boards achieve a peak temperature of about 250°C locally and 215 225°C generally throughout the board during reflow soldering. The most commonly encountered residues are those that start as rosin (colophony), a component in many flux materials. 

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