Solder bump formation on a circuit board using a transfer sheet

A method of forming solder bumps on portions to be soldered on a first side of a circuit board including the following steps (a)-(c):

• (a) disposing a solder transfer sheet having a solder layer which is adhered to at least one side of a supporting substrate and which has a size sufficient to cover at least two adjoining portions to be soldered so that the solder layer of the transfer sheet opposes the first side of the circuit board, wherein the solder layer is selected from a layer in the form of a continuous solder coating and a layer in which solder particles distributed densely without gaps to form a monoparticle layer are adhered to the supporting substrate by an adhesive layer,
• (b) heating the circuit board on which the transfer sheet is disposed which was obtained in step (a) under pressure to a temperature lower than the solidus temperature of the solder alloy constituting the solder layer of the transfer sheet to selectively produce solid phase diffusion bonding between the portions to be soldered of the circuit board and the solder layer of the transfer sheet, and
• (c) after step (b), peeling the transfer sheet from the circuit board, thereby obtaining a circuit board having the solder layer adhered to the portions to be soldered.