A study of solder bridging for the purpose of assembling 3D structures Madhav Rao (ECE) The University of Alabama Research Professors – Dr. Susan L Burkett (ECE) Dr. John C Lusth (CS) Madhav Rao November 3 1/34

Madhav Rao November 3 Initial work: Our research group Source: M. Rao, J. C. Lusth, S. L. Burkett, “Self-assembly solder process to form three-dimensional structures on silicon”, J. Vac. Sci. Technol. B, Vol. 27, No. 1, January D patterns Anchored 3D structures Images showing 3D structures formed from 2D metal patterns. 2/34

Madhav Rao November 3 Initial work: Process flow Conventional metal patterning and dip soldering process Process flow diagram. SiO 2 etched window Chromium adhesive layer deposition Gold seed layer deposition Develop Resist Spin Resist 2.5 µm Nickel electroplating 1.5 µm Copper electroplating Resist strip; Seed layer and adhesive layer etching Resist around patterned structures Dip soldering at 65 ºC Resist and sacrificial layer removal Auto folded structures, after solder reflow Source: M. Rao, J. C. Lusth, S. L. Burkett, “Self-assembly solder process to form three-dimensional structures on silicon”, J. Vac. Sci. Technol. B, Vol. 27, No. 1, January D micro-scale Polyhedron 3/34

Madhav Rao November 3 Aqueous HCL solution Self assembled structure Self Assembly movie Truncated square pyramid of 300 µm base length and 150 µm nominal length dimensions. Real time video captured using Raynox MSN 202, 37 mm macro lens and Cannon SX301S 14.1 MP digital camera in self assembly laboratory in The University of Alabama. 4/34

Madhav Rao November 3 Principle of Surface Tension: Solder Self Assembly Surface tension principles Surface area minimization drives the assembly process. Schematic representation of solder-driven self assembly: before solder reflow. Schematic representation of solder-driven self assembly: after solder reflow. Image redrawn from: K. Harsh, Y.C. Lee, “Modelling for solder self-assembled MEMS, in: Proceedings of the SPIE”, San Jose, CA, 24–30 January 1998, pp. 177–184. 5/34

Madhav Rao November 3 Initial work: Self assembly solder process to form 3D structures Cube Square PyramidTruncated Pyramid Pyramid Truncated Square Pyramid Self Assembled 3D shapes Source: M. Rao, J. C. Lusth, S. L. Burkett, “Self-assembly solder process to form three-dimensional structures on silicon”, J. Vac. Sci. Technol. B, Vol. 27, No. 1, January /34

Madhav Rao November 3 Images showing structures: after Ni–Cu electroplating (column 1), after Cr–Au etch (column 2), after solder reflow (column 3), and representative failures (column 4) Source: M. Rao, J. C. Lusth, S. L. Burkett, “Self-assembly solder process to form three-dimensional structures on silicon”, J. Vac. Sci. Technol. B, Vol. 27, No. 1, January Focusing on failures 7/34

Madhav Rao November 3 Maximum yield: 50 % Yield as a function of polyhedron type. Solder Self Assembled Polyhedra Yield Source: M. Rao, J. C. Lusth, S. L. Burkett, “Self-assembly solder process to form three-dimensional structures on silicon”, J. Vac. Sci. Technol. B, Vol. 27, No. 1, January /34

Madhav Rao November 3 Motivation Can we control solder deposition and solder bridging process to improve the yield ? Solder deposition by dip soldering: Roughness and Thickness Solder bridging: Hinging of metal faces 9/34

Madhav Rao November 3 Last year AVS talk: Analysis of dip soldering process Source: M. Rao, J. C. Lusth, S. L. Burkett, “Analysis of a dip-solder process for self assembly”, J. Vac. Sci. Technol. B, Vol. 29, No. 4, August, Improvement in solder folding yield due to change in process. 10/34

Madhav Rao November 3 Analysis of dip soldering process Parameters Solder Alloy Used Dip Temperature Dip Time Metal Stack Sacrificial Layer SiO 2 type Table 1: Processing parameters used in different SBSA* fabrication procedures. *Keyword: SBSA Solder based self assembly 11/34

Madhav Rao November 3 Solder Bridging studies Wetting Bridging Folding Illustration showing different stages in SBSA process. 12/34

Madhav Rao November 3 Schematic showing gap size between the metal entities and metal thickness of copper metal to be soldered. Gap size Metal thickness Solder Bridging variables 13/34

Madhav Rao November 3 Solder bridging variable: Types of solder coverage Illustration showing different types of soldering. a) Face soldering and b) Edge soldering. b) Edge soldering a) Face soldering 14/34

Madhav Rao November 3 a) Successful bridgingb) Bridging failure SEM images of a) Successfully bridged metal pattern and b) failure in solder bridged metal pattern. 15/34

Madhav Rao November 3 Bridging yield: Face soldering SEM image and b) bridging yield of face soldered 2D template of Box, by varying gap-size and metal thickness. a) SEM imageb) Bridging yield 4.9 µm thick 16/34

Madhav Rao November 3 Truncated Square Pyramid Truncated PyramidSquare Pyramid Spire Pyramid Inverted Square Pyramid Closed Box Box SEM image of face soldered 2D patterns of 8 polyhedra 17/34

Madhav Rao November 3 Truncated Square Pyramid Truncated Pyramid Square Pyramid Spire Pyramid Closed BoxBox Inverted Square Pyramid Bridging yield of face soldered 2D patterns of 8 polyhedra. 18/34

Madhav Rao November 3 a) SEM image and b) bridging yield varying gap-size of 2D template of Edge soldered box for a fixed metal thickness of 4.9 µm. Bridging yield: Edge soldering a) SEM imageb) Bridging yield Maximum yield 19/34

Madhav Rao November 3 Truncated Square Pyramid Truncated PyramidSquare Pyramid Spire Closed Box Box Pyramid Inverted Square Pyramid 20/34 Bridging yield of edge soldered 2D patterns of 8 polyhedra.

Madhav Rao November 3 a)b) SEM images of a) Successfully folded 3D structure and b) unsuccessful folded structure. 21/34

Madhav Rao November 3 2D patterns of Polyhedra Dimensions Base length (µm) Side Length (µm) Linear length (µm) Face Area (µm 2 ) Angle (in °) Truncated Square Pyramid Square Pyramid Pyramid Truncated Pyramid Box Spire Closed Box Inverted Square Pyramid Table 2: Dimensions of 2D precursors of different polyhedra attempted. 22/34

Madhav Rao November 3 Face soldering: Folding yield of various 3D structures Folding yield of different face soldered 3D structures. Low face area influences improved yield If solder bridged successfully, folding yield remains same regardless of gap size. 23/34

Madhav Rao November 3 Edge soldering: Folding yield of various 3D structures Folding yield of different edge soldered 3D structures. Low face area influences improved yield: similar trend Low gap size showed higher folding yield, suggesting influence of solder bridging on folding yield. 24/34

Madhav Rao November 3 Folding yield due to different soldering types Folding yield due to different types of soldering for a fixed 7.5 µm gap size between metal faces. Low folding yield was obtained for 4 edge soldered structures. Hence low folding yield is attributed to slightly reduced bridging of edge soldered structures. Solder bridging showed slightly less than 100% and variations for edge soldered patterns 25/34

Madhav Rao November 3 Analysis on solder bridging Solder bridging influences the success of 3D folding. Higher face area deters folding, regardless of the success of solder bridging. Low gap size and increased metal thickness favors the success of solder bridging. 26/34

Madhav Rao November 3 Number of faces: 2 to 8 Face length: 75, 150, 225 and 300 µm Solder coverage: %, 40.5 %, %, % and 100 % Other variables that influence solder bridging 27/34

Madhav Rao November 3 Face soldering: Number of faces and face length a) Bridging yield b) Folding yield a) Bridging and b) folding yield of face soldered patterns of different face length versus number of faces 150 µm face length and above shows improved solder bridging for 7.5 µm gap and 4.9 µm metal thick. Trend of reduced bridging yield is observed as number of faces increases. Folding yield was evaluated by considering only successfully bridged patterns. Lower face length once bridged showed higher folding yield. 28/34

Madhav Rao November 3 Bridging and folding yield of 300 µm face soldered patterned length versus number of faces. Trend of reduced folding yield is observed as number of faces increases. 2 possible reasons: 1. Increase in number of metals to fold, thereby increasing the competition while folding. 2. Higher solder quantity required to fold, for increased number of faces. Face soldering: Number of faces and face length 29/34

Madhav Rao November 3 Bridging and folding yield of edge soldered patterns of different face length versus number of faces Unexpected dependence of face length on bridging yield of edge soldered patterns. a) Bridging yieldb) Folding yield Edge soldering: Number of faces and face length Similar trend: Lower face length once bridged showed higher folding yield. Trend of reduced folding yield is observed as number of faces increases. 30/34

Madhav Rao November 3 Solder coverage Bridging and folding yield of different solder covered metal patterns versus number of faces. a) Bridging yieldb) Folding yield Low bridging yield for lowest solder covered patterns. Increased solder coverage favored success in folding. 31/34

Madhav Rao November 3 Comparison of Truncated square pyramid (TSP) with nominal length of 150 µm and box with four faces of 150 µm face length. Truncation of faces designed helps in folding mechanism and not in bridging. a) Face solderingb) Edge soldering 32/34 Truncation

Madhav Rao November 3 Conclusions 150 µm face length and above shows improved solder bridging. Increased number of faces decreases the bridging and folding yield for both types of soldering. Lower face length once bridged showed higher folding yield for both types of soldering. Increased solder coverage showed improved folding yield. Truncation of faces designed helps in folding mechanism. 33/34

Madhav Rao November 3 Acknowledgments College of Engineering, UA CAF facilities, UA. Army research laboratory (Cooperative agreement no: W911NF ) Questions, Comments and Suggestions !! 34/34