Double solder bumps on substrates for low temperature flip chip bonding
Multiple injections of molten solder are employed to form double solder bumps having outer layers that melt at lower temperatures than the inner portions thereof. During a flip chip assembly process, the reflow temperature is above the melting temperature of the outer layers and below the melting temperature of the inner portions of the solder bumps. As the inner portions of the solder bumps do not collapse during reflow, a flip chip assembly can be made at relatively low temperatures and have a high stand-off height. A structure having double solder bumps facilitates flip chip assembly.
1. A method comprising:
obtaining a structure including:
a substrate including a plurality of recesses;
a plurality of electrically conductive contact pads, each contact pad being positioned in one of the plurality of recesses in the substrate, and
a plurality of solder bumps, each solder bump comprising a solder element comprised of a first solder having a first melting point, each solder element being positioned within one of the plurality of recesses, contacting one of the plurality of contact pads and having a top surface coplanar with a top surface of the substrate, each solder bump further comprising a covering layer comprised of a second solder having a second melting point lower than the first melting point, each covering layer contacting one of the solder elements and extending above the top surface of the substrate;
aligning electrically conductive elements of an integrated circuit chip with the solder bumps;
causing reflow of the covering layers of second solder, the solder elements comprised of the first solder remaining in a solid state during reflow of the covering layers of second solder, and
causing the second solder to solidify such that the structure is attached and electrically connected to the integrated circuit chip.
2. The method of claim 1 , wherein each covering layer forms a frustoconical element having a flat top surface.
3. The method of claim 2 , wherein each covering layer completely covers one of the solder elements above the substrate.
4. The method of claim 2 , wherein the substrate is an organic substrate.