Injection molded solder - a new fine pitch substrate bumping method

Abstract

Injection molded soldering (IMS) technology has been developed for solder bumping of fine-pitch organic substrates. Pure molten solder is injected through a flexible film mask that is aligned to the recessed pad openings to form solder bumps on the substrate. The new substrate bumping method is a simple one pass operation for various size pads, with the capability of forming high solder volume on fine pitch substrate. Targeted solder volume over solder resist is designed into the flexible mask which has the desirable thickness and hole size. Using an optimized combination of pressure and temperature, molten solder is directly injected into the aligned holes and fills the solder resist openings. In this step, by controlling the process in low oxygen, flux or formic acid is not needed. After the solder has been metallurgically joined to and solidified on the pads, the flexible mask is separated from the substrate. The flexible film mask follows the surface topography of the organic substrate and enables intimate contact between the mask and the substrate to avoid solder bridging during the molten solder injection process. The holes in the mask have a tapered angle which helps facilitate separation of the mask from the solidified bumps and significantly increase the life time usage of the mask. This IMS substrate bumping technology has been evaluated using commercially available 150 um pitch organic substrates, along with the polyimide patterned copper substrates at 100 and 80 um pitches, respectively. The size of the substrates is 42.5 x 42.5 mm. Three different bump heights of 30, 50, and 70 um over solder resist layer have been demonstrated on the commercial organic substrate which has a minimum pitch of 150 um and a total of 4, 503 area array pads distributed in 11.1 x 15.4 mm area. To show the extendibility of IMS bumping method to very fine pitch application, 43 um height bumps on 100 um pitch and 35 um height bumps on 80 um pitch have been demonstrated in a 10 x 10 mm C4 array patterned on the copper substrates. Extendibility of IMS to 50 um pitch holes in a flexible film was also demonstrated. In this paper, we will discuss the laboratory scale processes and bump inspection data, along with a discussion of manufacturing strategies for IMS solder bumping technology for fine pitch organic substrates.