Found a solution after extensive research that increases the durability, tensile strength and performance of interconnects and interfaces used in present-day electronics as part of the JBNSTS, DST Inspire Science Programme.

2. TOUCH.
A Study Of Interconnects and Interfaces.
Brought to you by Group A.

3. INTRODUCTION:
If we're building almost anything that uses
metal, from a nuclear submarine to a
laptop computer, one thing we'll need to be
able to do is join metals together. First
things first, we can’t really join two metals
required for this purpose with adhesive,
not with ordinary glue anyway. There are
two ways to join metals:
 Welding
 Brazing
Welding- Joins two metal parts by heating
the surfaces to the point of melting so that
they fuse and form a secure joint.
Brazing- Joins two metals by heating and
melting a filler (alloy) that bonds to the two
pieces of metal(substrate) and joins them.
WELDING
BRAZING

4. SOLDERING:
 Now we move to a special term known as
‘soldering’ that we’ll be dealing with in this
presentation.
 Soldering is a low-temperature analogue to
brazing . By the American Welding Society's
definition, soldering takes place with fillers
known as solders that melt at below 450°C.
Soldering wire.(Pb+Sn)
SOLDERING

5. PREFERENCE OF SOLDERING:
Now that we’ve seen the means of joining two metals, we
need to pick one. Soldering is preferred because of two
simple reasons:
 It requires a much lower temperature than welding since it
does not involve melting the work pieces. Welding requires
a temperature of 3100 °C whereas in soldering, as
mentioned earlier, the temperature is below 450°C.
 Dissimilar metals can be joined.
 Since we are melting the solder here and not the metals,
they can have different melting points. If we try to weld two
dissimilar metals on the other hand, for example,
Aluminium and Steel, we’d find that Aluminium melts at
less than half the temperature
that steel melts at. So by the time the steel starts melting,
the aluminium would be a pool of liquid.
 Also, the intermetallic compounds (alloys) formed by two
dissimilar metals are brittle which results in poor welding
due to poor bonding.
Soldering a circuit board.

6. WHY APPLY HEAT?
 We might ask ourselves a very simple question- Why
do we need to apply heat to join two metals together by
pressing instead of pressing them together at room
temperature?
Cohesive and adhesive forces, unlike gravitational forces,
do not obey the inverse square law. If the distance
between the molecules is greater than 10-9, the forces of
attraction between them is negligible, but within this limit
the force increases very rapidly as the distance between
the molecules decreases. The maximum distance (=10-9
m) to which two molecules attract each other is called
‘molecular range’. It is usually denoted by c.
The reason why we can’t adhere two pieces of solid
simply by pressing them together is that ordinary pressure
cannot bring the molecules of the two pieces so close (10-
9 m) that adhesive forces may become effective between
them. But if their surfaces in contact are melted by
heating, the molecules in the liquid state fill up the space
between the solid surfaces. Then, on cooling, the
surfaces adhere together.
The Infrared oven running.

7. INTERCONNECTS.
What is solder?
Any of various alloys fused and applied to the joint between metal objects to
unite them without heating the objects to the melting point.
Some properties that an ideal solder must possess-
 High electrical conductivity
 Low Melting Point- Its melting point ought to be below the melting point of
the working piece with the least melting point between the two.
 High Tensile Strength (mechanical Property)
 High Mechanical Fatigue Limit
 Less Brittleness
 Low Surface Tension-so as to able to wet the metallic surface
 Eco-friendliness

8. INTERFACES.
What happens at the interface?
 As the metal reacts with the solder, evil
layers of intermetallic compounds are
formed. We call them “evil” because
they tend to be the most brittle part of
the solder joints. Now some
intermetallics are more brittle than the
other. This has to be kept in mind
during the choice of solder.
A Cu-SAC IMC
interface.

9. WHY ARE IMC LAYERS BRITTLE?
Features like Malleability and Ductility requires change in shape without breaking. This is
possible due to two properties- Slipping and Dislocation.
In either case, the ion layers have to slide over one other and reform attraction with
delocalized electrons in their new positions so that the shape of the metal changes but the
metallic bonds are still present and the material does not break.
Lower is the energy required for dislocation movement, more ductile is the crystal. And if
the bond energy is lower than the dislocation movement energy, applied stress will cause
the bonds to break before we have deformation.
 Result - brittleness.
So what impedes dislocation movement in intermetallics?
The reason is that in intermetallics, each metal atom has a very well defined position in the
lattice. Thus the number of available slip planes for dislocation movement is drastically
reduced causing brittleness.

10. LET US GO BACK A LITTLE ON THE
TIMELINE….
Once upon a time, not so long ago, Lead was used as a component in
soldering in Pb-Sn alloy (ratio:37-63).
It was, however, banned by the European Union under the RoHS(The
Restriction of Hazardous Substances) Directive effective from 2003 because
of its toxic effects.
We are all aware of phenomena like:-
 Lead poisoning due to leaching
 Contamination of groundwater and crops
 Soil pollution
 Improper disposal
 Extinction of aquatic life
 Miscellaneous health hazards

11. The banning, on the contrary, took a toll on
the manufacturing industries due to the
following reasons:
 The alternatives, SN100C(Sn-Cu-Ni-Ge)
and SAC(Sn-Ag-Cu),
 Even a "good" lead-free solder joint is
grainy and rough.
 Have higher melting point
 The melting point of Pb-Sn is 183°C
whereas the melting point of SAC is within
the range of 217°C to 220°C.
 Are more brittle since the IMC
layers(Cu3Sn and Cu6Sn5) formed are
thicker
 Cannot prevent Tin Whiskers
 Have a range of temperatures at which
they melt as opposed to Lead solder
which a eutectic mixture(a mixture of
substances (in fixed proportions) that
melts and freezes at a single temperature
that is lower than the melting points of the
separate constituents or of any other
mixture of them).

12. NOW LET US ACCELERATE OUR TIMELINE
TO THE TIME OF OUR EXPERIMENT.
 We have conducted a comparative study between two
samples-
 Copper strips with tin foil and SAC
 Copper strips with Pb-Sn solder

13. PREPARATION.
 The copper strips were cut and dipped in
ethyl alcohol containing FeCl3 to remove
the oxide layer.
 The copper strips were polished to
remove any undulation so that the joint
was less brittle.
In case of undulation, the peaks have less
surface area, hence, less activation energy
is required to break the bond as a result of
which the joint becomes weak. The samples
were soldered.
 They were heat exposed in the infrared
oven.

14. EXPERIMENT.
 The temperature-time graph for SAC
for different segments was as follows:
0
50
100
150
200
250
0 8 11 13 33
SAC

15.  The heated samples were cut into fine layers, polished and placed
under an optical microscope and the following images were obtained:
 Inference: The thickness of IMC layer in SAC is greater than Pb-Sn.
Cu-Sn-SAC-Cu Cu-Sn+Pb-Cu

16. TEST OF THE RESISTANCES OF THE
PREPARED SAMPLES.
 Cu-Pb+Sn-Cu Sample:0.13292 Ω
 Cu-Sn+SAC-Cu Sample:0.14133 Ω
INFERENCE: The resistance of Pb-Sn is less than SAC,
0.128
0.13
0.132
0.134
0.136
0.138
0.14
0.142
Resistance
Pb-Sn
SnSAC

17. THE QUENCHING TEST (TEST FOR
MECHANICAL FATIGUE):
 The samples were heated to a
temperature of 400°C with a heat
gun, and then quenched in cold
water at a temperature of 3-4°C.
 The number of cycles of expansion
and contraction (heating and
cooling) after which the samples
broke was recorded.
 SAC broke after 3 cycles whereas
Pb-Sn broke after 5 cycles.
 Inference: SAC has less
mechanical fatigue than Pb-Sn.
Cu-Sn+SAC-Cu fractures.
Cu-Pb-Sn-Cu

18. THE DROP IMPACT TEST.
To test the mechanical strength of the
interfaces.
The samples were dropped from a
height of 2m and examined under the
microscope.
SAC broke after 2 drops whereas
there was no appreciable crack in Pb-
Sn after 5 drops.
Inference: Pb-Sn has more
mechanical strength than SAC.
Sn-SAC fracture surfaces.
Cu-Pb+Sn-Cu section after
drop test. No appreciable
cracks observed.

19. CONCLUSION.
 Pb-Sn has low melting point, more mechanical strength and less
electrical resistance however, SAC is preferred because of its lower
toxicity.

20. ZINC+SAC- AN EXPERIMENTAL
INNOVATION.
 By accelerating the timeline to an
innovative era,
 We tried to reduce the brittleness of the
interface by adding Zinc to SAC(50-50).
 The drop impact test showed results as
follows:
 The sample did not show any visible
change even after 5 drops.
 The resistance of the sample was found
to be 0.14252 Ω
 Inference: Adding Zinc reduces
brittleness.
Before impact test.
After impact test. (repeated 5
times)

21. CREDITS.