Injection molded solder process for forming solder bumps on substrates
Solder bumps of uniform height are provided on a substrate through the use of injection molded solder. Copper pillars or ball limiting metallurgy are formed over I/O pads within the channels of a patterned layer of photoresist. Solder is injected over the pillars or BLM, filling the channels. The solder, which does not contain flux, is allowed to solidify. It forms a plurality of solder structures (bumps) of equal heights. Solder injection and solidification are preferably carried out in a nitrogen environment or a forming gas environment. Molten solder can be injected in channels formed in round wafers without spillage using a carrier assembly that accommodates such wafers and a fill head.
1. A method comprising:
providing a substrate in the form of a round wafer having a plurality of electrically conductive contact pads;
mounting the round wafer to a carrier and positioning a pair of protective elements in adjoining relation to the wafer, each of the protective elements having a semicircular opening, the diameters of the semicircular openings matching the diameter of the wafer;
providing a layer of photoresist on the substrate;
patterning the layer of photoresist such that a plurality of channels are formed over the contact pads, each of the channels having substantially the same height;
depositing a layer of metal over the contact pads;
passing an injection molded solder fill head containing molten solder over the channels while the substrate is in a low oxygen environment, causing the fill head to engage the layer of photoresist and move translationally with respect to the wafer and the carrier;
injecting molten solder containing no flux into the channels from the fill head;
allowing said solder to solidify, whereby said solder adheres to the metal and forms bumps of substantially the same height; and
removing the layer of photoresist.
2. The method of claim 1 , wherein sufficient solder is injected into the channels such that the bumps extend outside the channels.
3. The method of claim 1 , wherein the step of depositing metal includes electroplating copper over the contact pads to form copper pillars within the channels.
4. The method of claim 3 , wherein the copper pillars have non-uniform heights.
5. The method of claim 1 , further including the step of curing the layer of photoresist by heat treatment prior to injecting molten solder into the channels.
6. The method of claim 1 , wherein the step of providing the layer of photoresist includes laminating a layer of dry film on the substrate.
7. The method of claim 1 , wherein the layer of deposited metal is smaller in height than the height of the channels.
8. The method of claim 1 , further including the steps of mounting the round wafer to the carrier such that the wafer and carrier have coplanar top surfaces and providing the layer of photoresist on the wafer and an adjoining surface of the carrier.
9. The method of claim 8 , wherein the layer of photoresist is provided as a dry film.
10. The method of claim 9 , wherein the fill head includes a slot for injecting the molten solder, the slot having a length that is substantially the same as the diameter of the wafer, further including the step of causing the slot to pass over all of the channels as the fill head moves translationally with respect to the wafer and carrier.
11. The method of claim 9 , further including the step of attaching the wafer to the carrier.