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A Comparative Study: Thermosonic Bonding of Flip Chips with Gold Stud Bumps to a Variety of Organic Substrates
Keywords: Flip Cip on organic substrates, organic material, flip chip reliability data
Thermosonic flip chip bonding to ceramic substrates is a well understood and reliable connection method for lower I/O devices that facilitates smaller package sizes and improved electrical performance over traditional wire bonding processes. Recently there has been increased interest among package designers to expand the choices of substrates to include organic materials. This is to improve the cost-competitiveness of the package while maintaining high performance. This paper will define the requirements necessary for gold to gold bonding of a flip chip test die to organic substrates, discuss the purpose of this study and to investigate the effect and reliability of various organic substrate materials characteristics such as FR4, BT and (high temp. substrate) compared with a ceramic substrate that will serve as a process control under different process conditions using Au-to Au interconnect flip chip process. The flip chip assembly process covering the pick & place, Au to Au interconnection as well as the underfill process is investigated with special emphasis on process reliability for each type of organic substrate which was mention above. Characterization of the critical GGI process parameters such as heating temperature, bonding pressure, ultrasonic power setting and duration time for optimal bonding condition is established. The reliability evaluation is concentrating on thermo-mechanical robustness and electrically functional performance of the final assembly. Performed tests are ball bond shear test, electrical test, die shear test, temperature cycling and humidity storage. X-ray inspection for Au bumps accuracy position with respect to the Au bond pads. Underfill quality is revealed by C-SAM inspection for void, crack and de-lamination defects. The evaluation will compare low cost flip-chip mounting processes using 70um diameter gold stud bump connections on organic boards such as FR4, BT, high performance, high Tg with Au/Ni/Cu material plating. Flip Chip samples consisting of 7mm x 7mm test chips bumped with 112 gold stud bumps with bump pitch down to 150um, mounted on three different kinds of organic substrate materials will be used in this test. The dies were prepared with coined bumps of 1mil Au-wire. The assembly methodology consists of four primary steps: 1) Bumping of the silicon IC utilizing common wire bonding equipment, 2) Connection of the bumped IC directly to the organic substrate by gold to gold thermosonic interconnects. 3) Underfill post-process and 4) Reliability Testing under JEDEC specifications. With the recent improvements in thermosonic bond horn design along with high Tg substrates, the ability to bond to organic substrates will be shown. In particular, the bonding is accomplished by applying the heat required by means of a heated bond head, allowing the organic substrate with a significantly lower Youngs modulus (as compared with ceramic ) to remain relatively cool and hence capable of forming the mono-metalic Au joint.
Nick Stolper, Engineer
Panasonic Factory Solutions Company of America
Buffalo Grove, IL

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