Advancements in bumping technologies for flip chip and WLCSP packaging

At R&D level, IZM has advanced stencil printing very close to its technological limits at pitches even down to 50 mu m. Innovative electroformed and laser cut with nano-treatment stencils have been manufactured with an extreme thinness of 20 mu m for bumping wafers at Ultra fine pitches (UFP) of 100 mu m, 80 mu m and 60 mu m. Specifically, for 100 mu m pitch bumping, both type 7 (2-11 mu m) and type 6 (5-15 mu m) pastes of eutectic composition Sn63/Pb37 have been successfully employed Bumping using 25 mu m electroformed stencil thickness has yielded bump heights of 42.3 +/- 3.8 mu m and 43.6 +/- 3.5 mu m for type 7 and type 6 pastes, respectively. A newly prototype developed type 8 paste (2-8 mu m) has been used for the first time to bump chips with peripheral contacts at 80 mu m and 60 mu m pitch. Bumping at 80 mu m pitch with nano-treated laser-cut stencil has yielded bumps of 28 mu m in height. For bumping at 60 mu m pitch, a 20 mu m thick electroformed stencil was used with 35 mu mx80 mu m oblong apertures. Printing at 60 mu m pitch has yielded very promising results and has proved the capability of electroformed technology to manufacture accurate and robust thin stencils. The bump height at 60 mu m pitch was measured to be 28 3 mu m. Paste-in-Resist technology has been developed as an alternative to stencils in order to overcome the manufacturing difficulties of making extremely small apertures. Paste is printed in resist apertures which have been opened by photolithographic processes. In this way, bumping has been demonstrated up to 50 mu m pitches. Complimentary to stencil printing processes, IZM has developed balling technologies up to 400 mu m pitch up to 8" wafers with a thickness of 150 mu m. Solder balling can be achieved either by ''perform ball print" using conventional stencil printers with specially designed stencils or by "ball drop" techniques. Balling technologies have demonstrated the application of 300 mu m and 250 mu m Sn-Pb and Pb-free balls at respective area array pitches of 500 mu m and 400 mu m, the main I/O pitches for WL-CSP bumping.