Electrolytic Deposition of Fine Pitch Sn/Cu Solder Bumps for Flip Chip Packaging

摘要

Current methods for the formation of pre-solder bumps for flip chip attachment use stencil printing techniques withan appropriate alloy solder paste. The continuing trend towards increased miniaturization and the associateddecrease in size of solder resist opening, SRO is causing production difficulties with the stencil printing process.Practical experience of production yields has shown that stencil printing will not be able to meet future requirementsfor solder bump pitch production below 150μm for these applications.This paper describes latest developments in the electrolytic deposition of solder to replace the stencil printingprocess; results from production of 90μm bump pitch solder arrays with tin/copper alloy are given. The solder bumpis produced with a specially developed electrolytic tin process which fills a photo resist defined structure on the SRO.The photoresist dimensions determine the volume of solder produced and the subsequent bump height after reflow.Investigations on the bump reliability after reflow are shown including copper alloy concentration at 0.7% and x-rayinvestigation to confirm uniform metal deposition. The self centering mechanism found in the bump productionprocess during reflow is presented and the capability to correct photoresist registration issues.The solder bumps are shown as deposited onto an electroless nickel/gold or electroless nickel/palladium/gold finalfinish which serves also as a barrier layer to copper diffusion into the solder bump.Discussion of further development work in the production of alloys of tin/copper together with silver are given withfirst test results.