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Smart Chemistry Towards Highly Efficient Solder Material FormulationYarong Shi, Ph.DKester [email protected]
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Outline/Agenda
• Introduction of Paste and Fluxing Chemistry• Activator Properties• Paste Performance• Conclusions• Acknowledgements • Q & A
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Paste in SMT Process
Resin/Rosin
ActivatorOther
Solvent
Rheology Modifiers
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Fluxing Chemistry
4
Cu2O + 2RCOOH/HX = Cu(RCOO)2/CuX2 + Cu + H2OCuO + 2RCOOH/HX = Cu(RCOO)2/CuX2 + H2O
Eact
Heat TransferSolvent Evpn
TIME
HEA
T
Clean oxide Prevent re-oxidation Wet the surface
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Cu Oxidation
Cu CuO Cu2O
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Copper Oxide Removal by Halide
CuCl2 (blue)
CuCl (green)
CuO + 2HCl → CuCl2 + H2O
Cu2O + 2HCl → CuCl2 + Cu + H2O
Aqueous solutions of "copper(II) chloride". Greenish when high in [Cl-], more blue when lower in [Cl-].
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Oxide Removal by Organic Acid
CuO + 2RCOOH = Cu(RCOO)2 + H2OCu2O + 2RCOOH = Cu(RCOO)2 + Cu + H2O
Malonic Succinic Glutaric Adipic
Fluxing reaction byproducts have different stabilities
Dicarboxylic acid Malonic Succinic Glutaric AdipicCu complex dissociation
constant (pKMA) 5.80 3.48 3.00 3.02
Acid dissociation constant (pKa)
2.83, 5.69
4.20, 5.61
4.31, 5.41
4.43, 5.41
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Copper Coordination
Coordination modes of the carboxylate Fluxing reaction byproducts can have distinct structures based on the activator component, board and reflow condition
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Activator Study
• Two common activators A&B are chosen• A & B are combined in different ratios to give activators
Act1-5• Act1-5 are studied for their physical properties and
solderability• Act1-5 are formulated to give Paste P1-5
A BAct1
Act2Act3
Act4Act5
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Thermal Stability of Act1-5
0
20
40
60
80
100
120
Weig
ht (%
)
0 50 100 150 200 250 300 350 400Temperature (°C)
Act1––––––– Act2– – – – Act3––––– · Act4––– – – Act5––– –––
Universal V4.7A TA Inst
Thermal stability:Initial stage (preheat and soaking): Act1 > Act2 > Act3 ≈ Act4 ≈ Act5 Second stage (reflow): Act3 > Act4 > Act 2 >Act5 > Act1
Act3
Act4
Act2Act5
Act1
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Conductivity of Act1-5 Solution1
0
200
400
600
800
1000
Act1 Act2 Act3 Act4 Act5
Con
duct
ivity
(mS/
m)
Conductivities of Act1-5 change dramatically Act4 shows the strongest ionic mobility
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Surface Tension of Act1-5 Solution1
64
65
66
67
68
69
70
71
72
73
74
surf
ace
tensi
on
Act1 Act2 Act3 Act4 Act5
Activators
Each Pair
Student's t
0.05
Oneway Analysis of surface tension By Activators
Act1-5 changed the surface tension of solvent 1 High surface tension limits the flux spread and coverage on
PCB, thus may impact wetting
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Spread Test of Act1-5
Activator Residue color
Act1 blue
Act2 green
Act3 black
Act4 black
Act5 clear
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Spread Area of Solution 1
Spr
ead
area
usi
ng s
olut
ion
1
100
150
200
250
300
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
5
1-Low 2-Mid 3-High
Activators within Preheating Temp
Variability Chart for Spread area using solution 1
Preheat and soak impact:• Low: all activators are close• Mid: Act3-5 spread over more area• High: Larger spread area of Act1 correlates with its high thermal stability
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Solder Spread-Solvent ImpactS
prea
d ar
ea u
sing
sol
utio
n2
100
150
200
250
300
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
51-Low 2-Mid 3-High
Activator within Preheating Temp
Variability Chart for Spread area using solution2
• Act3-5 start to activate at low preheat temperature
• More degradation at high preheatingS
prea
d ar
ea u
sing
sol
utio
n 1
100
150
200
250
300
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
5
Act
1
Act
2
Act
3
Act
4
Act
5
1-Low 2-Mid 3-High
Activators within Preheating Temp
Variability Chart for Spread area using solution 1
in solvent 1
Formulation solvent is a critical component of performance: mediate chemical reactions
during the initial heating stage
physical barrier against oxidation and thermal degradation
in solvent 2
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Model Paste Formulation
Flux:
Paste:SAC305, T4, 88.5%
Material % WeightSolvent 30~60Rosin 30~60Activators 5Rheology modifier
0~10
Additives 0~2
The formulation was not designed for optimal performance, but to study the activator impacts
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Flux Thermal Stability
Flux Flux1 Flux2 Flux3 Flux4 Flux5Flux weight loss on
TGA at 150 °C 18.7% 17.4% 19.2% 16.0% 19.8%
Flux weight loss on TGA at 217 °C 46.8% 46.7% 46.7% 46.3% 48.4%
Flux1-4 show same level stability
Flux5 shows slightly lower thermal stability
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Paste Tackiness
In A&B mix, complex interactions between activators, other flux additives and the solder particles result in byproducts having a strong influence on the rheology of these pastes.
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IPC Slump
Paste SlumpIPC-TM-650 2.4.35
P1 0.30, 0.33P2 0.15, 0.33P3 0.25, 0.56P4 0.30, 0.63P5 0.30, 0.56
P2 shows the best hot-slump resistance Besides rheological impact, the reaction between the activators
and the solder alloy at elevated temperature structure the pastes deposit
Different reaction mechanism at room temp vs. high temp
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Paste Shelf-Life
P1, P4 and P5 are remarkably stable over 3 month storage at room temperature no separation or viscosity increase, no degradation in solderabilityP2 started to dry out after one week followed by P3drying out after two weeks
A&B mix in Act2 and Act3 interact much faster with the solder powders under ambient conditions.
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Reflow Profiles
Harsh profile in air to stress the flux chemistry
Profiles Max(+) slope
Max(-) slope
Time 50-
220°C
Time >217°
C
Peak Temp
217°C/peak
Peak/217°C
Lead-free long
2.03 -2.47 204 73 250.0 0.80 -1.19
Lead-free extra soak
1.14 -1.77 249 99 255.0 0.59 -1.21
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Large Pad Dwetting
Dewetting performance on 6.5 mm pad:Lead-free long: P4 ≈ P3 ≈ P1 ≈ P2 > P5Lead-free extra-soak: P2 ≈ P1 ≈ P4 ≈ P3 > P5
Profile P1 P2 P3 P4 P5
Lead-free long
71% 68% 72% 74% 60%
Lead-free
extra-soak
58% 59% 55% 57% 31%
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Solderballing
Paste P1 P2 P3 P4 P5
Lead-free long
L3 L2 L1 L1 L4
Lead-free extra-soak
L3 L4 L4 L2 L5
Solderballing performance:Lead-free long: P3 ≈ P4 > P2 > P1 > P5Lead-free extra-soak: P4 > P1 ≈ P2 ≈ P3 > P5
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Coalescence
Paste P1 P2 P3 P4 P5
Lead-free long
0.15mm BGA 0.15mm BGA 0.15mm BGA 0.15mm BGA 0.15mm BGA
Lead-free extra-soak
0.30mm BGA 0.45mm BGA 0.45mm BGA 0.45mm BGA 0.40mm BGA
Coalescence performance:Lead-free long: All pastes have show excellent coalescenceLead-free extra-soak: P1 > P5 > P2 ≈ P3 ≈ P4
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Correlations
Under “lead-free long” profile
Wetting and solderballing performance correlate with the thermal stability of activators during reflow
*Performance level: 1-5 scale, 5 is the best
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Paste SummaryPaste P1 P2 P3 P4 P5
Tackiness 4 5 5 3 2IPC slump 3 4 2 1 1Shelf-life 5 0 1 5 5
Reflow profile1 (lead-free long)Solderballing 3 4 5 5 2
Coalescence 5 5 5 5 5
Dewetting 4 3 4 4 3Reflow profile2 (lead-free extra soak)
Solderballing 3 2 2 4 0
Coalescence 2 0 0 0 1
Dewetting 3 3 3 3 1
Total 75 47 52 72 53
*Performance level, 1-5 scale, 5 is the best.
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Conclusions
• The physical properties, thermal stability and solderabilityof these activators can be tuned by adjusting the ratios between activators
• Paste physical properties and performance depend on complex interactions within the formula
• Studies on reliability and the reaction mechanisms are desired to provide further scientific guidance to improve the performance of solder paste formulations
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Acknowledgements
Kester R&D team• Michelle Wilson for her great support to paste
preparation and testing• Jim Lowe for the thermogravimetric analyses • Kyle Loomis for the reflow profile development support
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