Substrates with ultra fine pitch flip chip bumps
A method of attaching a chip to the substrate with an outer layer comprising via pillars embedded in a dielectric such as solder mask, with ends of the via pillars flush with said dielectric, the method comprising the steps of: (o) optionally removing organic varnish, (p) positioning a chip having legs terminated with solder bumps in contact with exposed ends of the via pillars, and (q) applying heat to melt the solder bumps and to wet the ends of the vias with solder.
1. A method of terminating a first side of a multilayer composite structure having an outer layer of copper via posts embedded in a dielectric film, comprising the steps of:
(i) thinning away a surface of the dielectric film to expose the copper via posts;
(ii) sputtering a seed layer of copper over the thinned surface;
(iii) applying, exposing and developing a penultimate pattern of photoresist over the sputtered layer of copper corresponding to a desired pattern for an external feature layer;
(iv) electroplating the external feature layer into the penultimate pattern;
(v) stripping away the penultimate pattern of photoresist;
(vi) applying, exposing and developing an ultimate pattern of photoresist onto the external feature layer corresponding to a desired pattern of micro bumps;
(vii) pattern plating copper via posts into the ultimate pattern of photoresist;
(viii) pattern plating solderable metal over the copper via posts;
(ix) stripping away the ultimate pattern of photoresist;
(x) etching away the seed layer of copper;
(xi) laminating a dielectric outer layer over the external feature layer and via posts;
(xiv) plasma etching the dielectric outer layer to expose a solderable cap of the via post; and
(xv) applying a finishing treatment to the solderable cap.
2. The method of claim 1 wherein the dielectric outer layer is selected from the group consisting of a film dielectric and a dry film solder mask.
3. The method of claim 1 wherein step (xv) comprises applying pressure to the solderable cap along an axis of the via post resulting in a flat coined solderable cap.
4. The method of claim 1 wherein step (xv) comprises applying pressure along an axis of the via post together with heat to cause reflow under pressure, resulting in a flat coined solderable cap.
5. The method of claim 1 wherein step (xv) comprises applying heat to cause reflow of the solderable cap, without applying pressure such that the solderable cap assumes a dome shape due to surface tension.
6. The method of claim 1 wherein step (xiv) of plasma etching comprises exposing the dielectric outer layer to ion bombardment in a low pressure atmosphere produced by ionizing at least one of the gases selected from the group consisting of oxygen, tetrafluoride carbon and fluorine.
7. The method of claim 1 further comprising step (xiii) of applying terminations on a second side of the substrate opposite to the first side.
8. The method of claim 7 , wherein applying terminations comprises:
(a) thinning the second side to expose an end of the copper vias;
(b) sputtering a copper seed layer;
(c) applying, exposing and developing a layer of photoresist over the sputtered copper layer to produce a pattern of pads;
(d) electroplating copper pads into the photoresist;
(e) removing the layer of photoresist; and
(f) depositing a solder mask over the substrate between and overlapping the copper pads.