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Lead-Free Solder Bump Technologies for Flip-Chip …

Lead-Free Solder Bump Technologies for Flip-Chip packaging Applications Zaheed S. Karim1 and Jim Martin2 1 Advanced Interconnect Technology Ltd. 1901 Sunley Centre, 9 Wing Yin Street, Tsuen Wan, Hong Kong Tel: (852) 2719-5440, Fax: (852) 2358-4766, E-mail: 2 Shipley LLC 455 Forest Street, Marlborough, MA 01752, USA Tel: (516) 868-8800, Fax: (516) 868-4781, E-mail: Abstract We describe the fabrication and characterization of five different types of Lead-Free Solder bump interconnections for use in Flip-Chip electronic packaging applications. Lead-Free Solder bumps were fabricated from pure-tin (Sn), tin-bismuth (Sn:Bi), eutectic tin-copper (Sn:Cu), eutectic tin-silver (Sn:Ag), and ternary tin-silver-copper (Sn:Ag:Cu) alloys.

Lead-Free Solder Bump Technologies for Flip-Chip Packaging Applications Zaheed S. Karim1 and Jim Martin2 1Advanced Interconnect Technology Ltd. 1901 Sunley Centre, 9 Wing Yin Street, Tsuen Wan, Hong Kong

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Transcription of Lead-Free Solder Bump Technologies for Flip-Chip …

1 Lead-Free Solder Bump Technologies for Flip-Chip packaging Applications Zaheed S. Karim1 and Jim Martin2 1 Advanced Interconnect Technology Ltd. 1901 Sunley Centre, 9 Wing Yin Street, Tsuen Wan, Hong Kong Tel: (852) 2719-5440, Fax: (852) 2358-4766, E-mail: 2 Shipley LLC 455 Forest Street, Marlborough, MA 01752, USA Tel: (516) 868-8800, Fax: (516) 868-4781, E-mail: Abstract We describe the fabrication and characterization of five different types of Lead-Free Solder bump interconnections for use in Flip-Chip electronic packaging applications. Lead-Free Solder bumps were fabricated from pure-tin (Sn), tin-bismuth (Sn:Bi), eutectic tin-copper (Sn:Cu), eutectic tin-silver (Sn:Ag), and ternary tin-silver-copper (Sn:Ag:Cu) alloys.

2 The fabrication process consists of the electrolytic plating, using a fountain (cup) plater, of a 5 m thick copper under-bump-metal (UBM) onto which is plated the Lead-Free Solder . The as-plated bumps were subsequently re-flowed in a 5-zone re-flow oven. Due to the characteristic high-tin compositions of the Lead-Free Solder alloys, which can cause the rapid and uneven formation of tin-copper intermetallics at the bump-UBM interface upon re-flow, a unique proprietary nickel cap using a single photolithography process that completely encapsulates the copper UBM was developed. Two different test structures, one with perimeter- and a second with area-distributed Solder bumps , each with bumps of average size 125 m diameter (post-re-flow) were fabricated onto daisy-chain wafers to characterize the Lead-Free Solder bumping and bonding process and to conduct a series of reliability tests.

3 Tests conducted to characterize the properties of the Lead-Free bumps included Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), Auger Electron Spectroscopy (AES), micro-sectioning, and ball shear measurements for which the bumps were re-flowed multiple times and subjected to ball shear tests in-between re-flows to study the formation of intermetallic compounds. Lead-Free Solder bumped daisy-chain test die were also Flip-Chip bonded onto BT-epoxy substrates with patterned copper traces overplated with nickel/gold. The bonded die were underfilled and subjected to environmental tests consisting high-temperature storage, thermal cycling, and accelerated aging.

4 Details of the Lead-Free Solder bump fabrication process together with the performance results including their electrical, mechanical, and reliability characteristics will be presented for all five Lead-Free alloys chosen in this study. Key words: Lead-Free , Solder bumps , Flip-Chip , under-bump-metal Introduction Restrictions on the use of lead-containing solders in electronic products are currently under consideration because it is believed that the lead from such products, which are typically disposed of in landfills, have the potential to leach out and contaminate the drinking water system.

5 The electronics industry has been targeted to go Lead-Free despite the fact that less than of all the lead produced annually in the world is used for electronic solders [1]. Laws banning the use of lead in electronics products are due to be enacted in the European Community in January 2008 and similar legislation to restrict the use of lead is pending in both the United States and in Japan [2]. Efforts to identify suitable Lead-Free finishes for electronic components has thus far been focused mainly on printed circuit boards, leadframe packages, and in the selection of Solder pastes. Little attention has being paid to the fabrication of Lead-Free bump interconnections which are required when packaging integrated circuits (ICs) by Flip-Chip bonding despite industry predictions that approximately of all ICs manufactured in the year 2004 will have to be bumped and packaged using Flip-Chip [3].

6 Five different types of Lead-Free alloy bump materials, with tin constituting a major fraction of the Solder , were selected as possible Lead-Free replacements on the basis of: i) worldwide resources and availability of tin; ii) the comparable cost of the Lead-Free alloys to lead-tin Solder ; iii) the bumps can be fabricated using conventional low-cost electroplating techniques; iv) the compatibility of tin-based solders with current re-flow processes, materials, and surface mount equipment, and; v) the familiarity of the electronics manufacturing and assembly industry in handling tin alloys. Tin-bismuth bumps with a composition of 90wt%Sn:10wt%Bi and a melting point (MP) of 200 C were fabricated as a potential replacement for eutectic lead-tin Solder bumps which have a MP of 183 C.

7 Lead-Free bumps made of pure-tin with a MP of 232 C, eutectic tin-copper with a composition of with a MP of 227 C, and tin-silver with a eutectic composition of and a MP of 221 C were fabrcated as Lead-Free replacements for both eutectic and high melting point high-lead Solder bumps . Ternary composition tin-silver-copper bumps with an alloy composition of and a MP of 216 C were also fabricated by electroplating using a unique process of sequential plating from two separate binary Sn:Cu and Sn:Ag plating solutions. Lead-Free Solder Bump Fabrication Process Lead-Free bumps of average size 125 m in diameter (post-re-flow) were plated onto silicon wafers patterned with perimeter- and area-distributed daisy-chain test structures (Figs.)

8 1 and 2). Primary steps used in the fabrication of the Lead-Free bumps consisted of: a) A sputter cleaning step to remove the naturally formed oxide layer on the aluminum bond pads followed by sputter deposition of chrome (thickness of 500 ) and copper (thickness of 5,000 ). These two sputtered metals effectively form the adhesion, barrier to indiffusion, and electrical buss layers for plating. b) Patterning of the wafer with an 80 m thick positive-tone liquid photoresist, positive chrome photomask, UV exposure (using a mask aligner), and developing the thick photoresist to define the areas to be plated.

9 C) Mounting of the wafer in a cup plater and application of current (using contacts at the edges of the wafer) to the underlying copper layer for plating of a 5 m thick copper under-bump-metal (UBM) layer which acts as a "wettable" foundation to the Lead-Free Solder bump upon re-flow. d) Electroplating of Lead-Free Solder bumps using the appropriate plating solution, anodes, and direct current (DC) or pulse-plating in a fountain (cup) plater system. e) Removal of the thick photoresist and chemical etching of the sputtered copper and chrome layers. f) Application of flux and re-flow of the as-plated bumps in a 5-zone re-flow oven to form the characteristic spherical Solder bump shape.

10 Experimental Results Difficulties encountered in the fabrication of the Lead-Free Solder bumps consisted mainly in the control and measurement of the copper composition in the eutectic tin-copper plating bath and the plated deposit since the target value for the copper concentration was only Control of the copper concentration was achieved by continuosly adjusting and measuring the copper in the plating bath solution (provided by Shipley) and the plated deposit and by using pulse plating instead of direct current. Plating solutions were analysed for their copper content using Atomic Absorption Spectroscopy (AAS).


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