Solder flux

A soldering/brazing flux comprising at least one ionic liquid, which is a Deep Eutectic Solvent (DES), and at least one additive which improves flux functionality, wherein the at least one additive is a rheology modifier and is selected from one or more of:

• (i) ethylene glycol, glycerol and mixtures thereof;
• (ii) a hydrophilic polymer;
• and
• (iii) a polyol selected from xylitol, erythritol, arabitol, ribitol, sorbitol and mannitol;
• wherein the DES comprises:
• (a) a mixture of two or more compounds of formula (I),
• (R+)n(Xn−) (I)
• or a hydrate thereof, wherein
• n is 1, 2 or 3,
• R+ is a primary, secondary, tertiary, quaternary or unsubstituted ammonium cation or a quaternary phosphonium cation,
• Xn− is a monovalent, bivalent or trivalent anion;
• and
• (b) one or more compounds of formula (IIIa) and/or one or more compounds of formula (IIIb),
• wherein:
• R8 represents H, C1-10 alkyl optionally substituted by one or more F atoms, phenyl optionally substituted by one or more substituents selected from halogen, C1-10 alkyl and C1-10 alkoxy or N(R9)R10,
• R8a represents H or C1-10 alkyl optionally substituted by one or more F atoms,
• R9 and R10 independently represent H or C1-10 alkyl optionally substituted by one or more F atoms,
• Y represents C2-10 alkylene or C4-8 cycloalkylene optionally
• (i) substituted by one or more substituents selected from F, OH, SH, N(R11)R12 and C1-10 alkyl optionally substituted by one or more substituents selected from F and OH, and/or
• (ii) interrupted by one or more groups selected from O, S and NR13, and R11 to R13 independently represent H or C1-10 alkyl optionally substituted by one or more substituents selected from F and OH.