Analysis of a dip-solder process for self-assembly Madhav Rao (ECE) The University of Alabama...

Analysis of a dip-solder process for self-assembly

Madhav Rao (ECE)

The University of Alabama

Research Professors – Dr. John C Lusth (CS)

Dr. Susan L Burkett (ECE)

Madhav Rao October 211/27

http://www.ua.edu/

Madhav Rao October 21

Earlier work: Dr. Gracias Research Group

Source: Timothy G. Leong,, Paul A. Lester, Travis L. Koh, Emma K. Call, and, David H. Gracias, “Surface Tension-Driven Self-Folding Polyhedra”, Langmuir 2007 23 (17), 8747-8751: Supporting information electronic files, ACS Copyrights.

Aqueo

us H

CL solu

tion

Self assembled structure

Self Assembly movie

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Source: Timothy G. Leong,, Paul A. Lester, Travis L. Koh, Emma K. Call, and, David H. Gracias, “Surface Tension-Driven Self-Folding Polyhedra”, Langmuir 2007 23 (17), 8747-8751: Supporting information electronic files, ACS Copyrights.

Snapshots of the video

Free floating 3D structures

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Earlier 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 2009.

2D patterns Anchored 3D structures

Fig 1: Images showing 3D structures formed from 2D metal patterns

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Earlier work: Process flowConventional metal patterning and dip soldering process

Fig 2: Process flow diagram.

SiO2 etched windowChromium adhesive layer depositionGold seed layer depositionDevelop ResistSpin Resist2.5 µm Nickel electroplating1.5 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


3-D micro-scale Polyhedron

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Principle of Surface Tension: Solder Self Assembly

•Surface tension principles

•Surface area minimization drives the assembly process.

Fig 3: Schematic representation of solder-driven self assembly: before solder reflow.Fig 3: 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.

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Earlier work: Self assembly solder process to form 3D structures

Cube

Square Pyramid Truncated Pyramid

Pyramid

Truncated Square Pyramid

Fig 4: Self Assembled 3D shapes


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Fig 5: 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)


Focusing on failures

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Maximum yield: 50 %

Fig 6: Yield as a function of polyhedron type.

Solder Self Assembled Polyhedra Yield

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Motivation

Can we control the solder deposition process and improve the yield ?

Thickness

Roughness

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Dip soldering process

Dip Temperatur

e

Flux Temperatur

e

Solder alloys

Blanket samples

Patterned samples

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Solder alloys

Trade names

StoichiometryM.P.

in C⁰Internal

Designation LMA117 44.7Bi-22.6Pb-8.3Sn-5.3Cd-19.1In 47 SA47

LOW203 52.5Bi-32Pb-15.5Sn 95 SA95

LMA281 58Bi-42Sn 138 SA138

INDALLOY241 95.5Sn-3.8Ag-0.7Cu 217 SA217

Table 1: Different solders used

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Difference: Heating flux and flux at room temperature for blanket samples

Fig 7: Roughness of dip-soldering at low dipping temperatures of SA95 solder alloy

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Blanket samples: Low Dipping temperature roughness data

Fig 8: Roughness of dip-soldering at low dipping temperatures of solder alloys

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Blanket samples: High Dipping temperature roughness data

Fig 9: Roughness of dip-soldering at high dipping temperatures of solder alloys with flux maintained at room temperature

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Madhav Rao October 21Fig 10: Thickness of dip-soldering at low dipping temperatures of solder alloys

Blanket samples: Low dipping temperature thickness data

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Blanket samples: High dipping temperature thickness data

Fig 11: Thickness of dip-soldering at high dipping temperatures of solder alloys with flux maintained at room temperature

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Blanket samples: Experimental analysis

Trend of decreased roughness and thickness is observed as dip-temperature increases. Better uniformity at higher temperature is achieved except for SA217 alloy.

At the higher dipping temperatures, the highest melting point solder results in a significantly thicker layer than the other solders.

Low melting point alloy shows less variation in thickness and roughness. This suggests better uniformity when using low melting-point alloy.

Preheating flux improves uniformity for temperatures only near the melting-point of alloy.

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BarsDividersLinkersPattern designations

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Processing Conditions

Solders Dip TimeDip

TemperaturePattern Names

Average Yield in

%

Standard Error in

%

SA47 90 seconds 50 C

Bars 89.37 4.530Linkers 90.09 6.787

Dividers 97.16 0.565

SA95 2 seconds 50 C

Bars 98.79 0.909Linkers 91.59 0.476


SA138 2 seconds 40 C

Bars 85.95 3.188Linkers 89.84 5.714


Dip soldering process for three different solder alloys

Table 2: Dip soldering process and yield for copper metal patterns

SA95 shows consistent and less varying wetting yield for different patterns

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Wetting yield: Analysis

Blanket samples did not require significant dipping time.

We suspect: •The lowest melting point alloy needs to overcome the resistance provided by the high density of non-wetting regions around the metal pads.

•The higher melting point alloy provided the necessary thermal energy to wet the patterns.

SA47 required 90 seconds of dip-time for complete coverage.

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Fig 12: Thickness of pattern dip-soldering

Thickness measurement of solder alloys on patterned samples

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Patterned samples: Experimental analysis

Thickness of deposited solder remains low for Bar, as compared to other designs: Linker and Divider.

Bar

Linker

Divider

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Roughness measurement of solder alloys on patterned samples

Fig 13: Roughness of pattern dip-soldering

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Conclusions

Blanket Samples

• Better uniformity is obtained at higher dip temperatures, especially for the lowest melting-point alloy.

•Heated flux is preferable for dip-soldering at temperatures near the melting-point of the alloy.

•Dip solder thickness depends on alloy melting-point.

Patterned Samples

•Coverage of individual elements when dip soldered is pattern dependent.

•Consistent coverage is obtained using SA95.

• A change in the dip soldering procedure was needed for patterned samples.

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Future Work

• Bridging and folding study on uniform solder deposited patterned samples.

•Experiments on dipping time over varying density of metal pads.

•Hollow polyhedra with minimum amount of deposited solder.

•Two sided 3D VLSI chips [A. Kamto et.al]

•Can this lead to wireless signal points on a chip ??

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

GND

0

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

GND

0

Vref

FB

Comp

Reset

I-sense

Drain

Source

Shtdwn

Pulse Width Modulator

Vref

FB

Comp

Reset

I-sense

Drain

Source

Shtdwn

Pulse Width Modulator

S1

S8

D

C1 ENBC3C2

Multiplexer

A

H

U/D

Reset

B1

B8

Load

Carry out

ENB

Preload Counter

Receiving Antenna

Transmitting Antenna

Fig 14: Wireless signal transmission prototype.

Source: A. Kamto, Y. Liu, L. Schaper and S. L. Burkett, , “Reliability study of through-silicon via (TSV) copper filled interconnects”, Thin Solid Films , Vol. 518, No. 5, 2009

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Acknowledgments

•College of Engineering, UA

•CAF facilities, UA.

Questions, Comments and Suggestions !!

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Analysis of a dip-solder process for self-assembly Madhav Rao (ECE) The University of Alabama...

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