We report the results of a new pre-solder bumping technology of injection molded solder (IMS) for fine pitch organic substrates. Pure molten solder is injected through a reusable film mask (mask IMS) or directly injected without a mask (mask-less IMS) on the pads of an organic substrate to overcome the limitation of current pre-solder bumping technologies such as solder paste stencil printing and micro-ball mounting.
In the case of mask IMS, targeted solder height over the solder resist (SR) is designed into the mask which has desirable thickness and hole sizes. Three different solder bump heights such as 30, 50, and 70 microns over SR were demonstrated for commercial organic substrates which have a pitch of 150 μm for 5,000 area array pads. To show the extendibility of the mask IMS bumping method to very fine pitch applications, 100 μm pitch bumping of 10,000 pads and 80 μm pitch bumping of 15,000 pads were demonstrated.
In mask-less IMS, the pure molten solder is directly filled into the opening volume of the SR. After the injection of molten solder, solidification of the solder under low oxygen leads to solder protrusions above the SR surface because 100 % pure solder is filled into the whole SR opening volume. For a 150 μm pitch commercial substrate, we demonstrated minimum bump heights of 15 μm over the 20 μm thick SR. Since there is no need to align mask and substrate, the maskless IMS method lowers process costs and makes the process more reliable. By manipulating the opening in the SR, it is possible to enable variations in the height of the solder bumps.
Flux or formic acid is not needed during solder injection of both described processes, but a low oxygen environment must be maintained.
In this paper, we will discuss laboratory scale processes and bump inspection data, along with the discussion of manufacturing strategies for IMS solder bumping technology for fine pitch organic substrates.