Micro-solder precoat technology by Precoat by Powder Sheet method

Micro-solder precoat technology is expected especially for fine pitch assemblies. We have developed PPS, “Precoat by Powder Sheet”, method which can be applicable to fine pitch patterns, e.g. less than 100 μm pitch patterns. PPS sheet has mono layer of solder powder typically with a diameter of less than 10 μm. First, PPS sheet is pressed to the electrodes of the work with heat, which temperature is usually lower than the melting temperature of the solder powder. The diffusion of elements between solder powder and electrode will make metallic connection. The strength of the connection will be much stronger than the tackiness between the tacky layer and solder powder on the PPS sheet. Thus, solder powder pressed on the electrodes will be selectively transferred after the PPS sheet is removed from the work. This method has several advantages, for example, uniformity of solder bump height, availability of any solder compositions, no-need of precise arrangement between PPS sheet and the work. Especially in case of Ni-Au pads, Au will be easily dissolved in solder powder pressed on Ni-Au pad. The Au dissolved solder will show lower melting temperature than that of the solder powder originally coated on the PPS sheet. When the PPS transfer temperature is set to be higher than the melting temperature of Au dissolved solder but lower than that of the solder powder, selective transfer will be observed only between the Ni-Au pads and the solder powder due to the PPS transfer mechanism.