Soldering flux and soldering paste

The invention provides a flux and a solder paste, which can reduce the cracking of flux residue caused by temperature change, can inhibit the solder paste from separating into solder powder and flux over time during storage, and can inhibit the reduction of solder strength even when an underfill is used. A flux containing a resin component, an active agent, and a solvent is used. The resin component contains: a copolymer (A) having a repeating unit (a1) derived from an olefin and a repeating unit (a2) derived from an acrylic acid in which a hydrogen atom bonded to a carbon atom at the alpha-position may be substituted by a substituent; and a rosin (B). The mixing ratio of the copolymer (A) to the rosin (B) is 1 or more in terms of the mass ratio represented by copolymer (A)/rosin (B). 本发明提供一种助焊剂及焊膏，其能够减少因温度变化导致的助焊剂残渣的破裂，能够抑制在保管时焊膏经时地分离成焊料粉末和助焊剂，并且在使用了底部填充剂的情况下也能够抑制焊接强度的降低。采用含有树脂成分、活性剂和溶剂的助焊剂。树脂成分含有共聚物(A)和松香(B)，所述共聚物(A)具有来自烯烃的重复单元(a1)和来自与α位的碳原子键合的氢原子可被取代基取代的丙烯酸的重复单元(a2)。共聚物(A)与松香(B)的混合比率以共聚物(A)/松香(B)所表示的质量比计为1