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 mum. Innovative electroformed and laser-cut with nano-treatment stencils have been manufactured with an extreme thinness of 20 mum for bumping wafers at ultra fine pitches (UFP) of 100 mum, 80 mum and 60 mum. Specifically, for 100 mum pitch bumping, both type 7 (2-11 mum) and type 6 (5-15 mum) pastes of eutectic composition Sn63/Pb37 have been successfully employed. Bumping using 25 mum electroformed stencil thickness has yielded bump heights of 42.3plusmn3.8 mum and 43.6plusmn3.5 mum for type 7 and type 6 pastes, respectively. A newly prototype developed type 8 paste (2-8 mum) has been used for the first time to bump chips with peripheral contacts at 80 mum and 60 mum pitch. Bumping at 80 mum pitch with nano-treated laser-cut stencil has yielded bumps of 28 mum in height. For bumping at 60 mum pitch, a 20 mum thick electroformed stencil was used with 35 mum times 80 mum oblong apertures. Printing at 60 mum 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 mum pitch was measured to be 28 plusmn3 mum. 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 mum pitches. Complimentary to stencil printing processes, IZM has developed balling technologies up to 400 mum pitch up to 8rdquo wafers with a thickness of 150 mum. Solder balling can be achieved either by ldquoperform ball printrdquo using conventional stencil printers with specially designed stencils or by ldquoball droprdquo techniques. Balling technologies have demonstrated the application of 300 mum and 250 mum Sn-Pb and Pb-f- ree balls at respective area array pitches of 500 mum and 400 mum, the main I/O pitches for WL-CSP bumping.