Low-residue fluxes

Nomenclature of these fluxes moved from low solids flux to low residue flux to noclean flux. The thinking was that the amount of residue left by these fluxes after soldering was so minimal that it did not need to be removed. This is only true if the residues are noncorrosive. 

Lower cost manufacturing can be achieved with low residue fluxes if the cleaning step can be eliminated. This assumes that the incoming components and boards are clean, and that operators handling the boards are careful not to introduce contamination. This requires a flux with noncorrosive residues that will not impede or contaminate the electrical bed-of-nails test probes. 

No visible residue fromcleanable or any activated fluxes is allowed. Production operations may not be required to remove cleanable residues if qualification testing is performed that demonstrates no need for cleaning the assembly. No-clean or low-residue flux chemistries may be allowed during the soldering process. However, it is critical that the cleanliness of the elements such as the bare boards and components be specified, controlled, and closely monitored otherwise, the contaminant build-up can far exceed allowable limits for the end item s functional performance. 

Low-solids fluxes (LSFs) and low-residue fluxes (LRFs) were introduced to allow the viability of no-clean processes. In Germany, Hartmann [1] added adipic and formic acids into a nitrogen gas stream with the intent of eliminating the use of liquid flux altogether. Each acid performs a speciflc function. Adipic acid [HOOC-(CH2)4-COOH, mp= 153°C] removes oxides on the base metal. The powder is dissolved with a 1-2% solution in isopropanol alcohol. Formic acid (HCOOH) is introduced over the wave to remove any remaining tin oxide. At temperatures above 150°C, formic acid reacts with oxides as follows ... 

Faiz et al. (1996) have applied micro-PIXE analysis to study solute distributions in a single crystal sample of YiBa2Cu307 5 high temperature superconductor (YBCO) of dimensions 1.3 mm x 1.5 mm x 75 pm. It contained a small secondary crystal overgrowth of dimensions 340 x 340 x 100 pm3. The interface region between the smaller crystal and the base crystal was covered with a material which appeared to be residual flux. The instrument employed a 2.5 MeV focused proton beam of about 4 pm resolution, which could scan an area of 500 x 500 pm2 on the sample surface. The microbeam current was kept low (typically about 30 pA) to avoid any damage to the sample. 

As for the solid residue (char), it should be observed that the high yields reported at low heat fluxes also include some unreacted material. 

Failure to thermosiphon. At low heat fluxes, reboiler liquid flow is sometimes difficult to initiate. If the flow is not started, the reboiler may vaporize some of the liquid, leaving a residue too heavy to lift or too heavy to boil. The problem is common at startups, especially when... 

The saiqples were metallized with aluminum and copper by using Knudsen effusion cells under UHV. The residual pressure during the metallization was -10" Tort. Very low metal fluxes (- atoms... 

At no load, there is no field current, and voltage is due to the residual flux of the stator core. The voltage rises rapidly over the range of low currents, but the resistive drop soon becomes dominant. 

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. 

The Corrosion Test is being developed to serve as a quantitative test for flux residues. Figure 10 shows the corrosion data for a low solids flux-treated coupon. Based on the data collected for a number of different fluxes, the following corrosion factors are suggested for a 7-day test at 85 °C (185 °F) and 85% RH with the following criteria for flux classification ... 

Eutectic low-residue solder paste flux removal... 

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