1. Zestron Seminar
Hotel Equatorial
Penang
15th March 2013
Effects of Water Soluble Tacky Flux
Formulation on Solder Ball Attach to a
Cu-OSP Surface Finish in N2 and Air
J. Z. Hussain, S. A. Khoo, C. D. Breach and A. Hawkins
cbreach@kester.com.sg

2. This presentation is about the relative
performance of four tacky fluxes
1. Performance 2. What’s in a Water
Requirements Soluble Tacky Flux?
3. Experimental Results 4. Summary
100000
Flux A
Flux B
80000 Flux C
Apparent Viscosity (Poise)
Flux D
60000 400
Apparent viscosity is Flux A Viscosity Table 1
300 similar at ~50s-1
Flux B Viscosity Table 1
40000 Flux C Viscosity Table 1
200
Flux D Viscosity Table 1
100
20000
0
10 20 40 60 80
0
0.01 0.1 1 10 100 1000
Shear Rate (s-1)

3. 1. Performance Requirements
What’s expected from a tacky flux

4. Tacky fluxes are primarily used for
chip attach and solder ball attach
Source: Yole

5. This is what is currently expected of
tacky fluxes in electronics packaging
1. Hold solder balls/dice firmly in
position at room temperature
2. Wet & spread over the ball/pad
at/above a critical temperature
3. Realign misplaced balls in
AIR
Pre-reflow Post-reflow
4. Remove oxides/coatings and enable
base metal-Sn reactions in AIR

6. At room temperature tacky fluxes must
have good adhesion
At ambient temperature tacky fluxes are viscous and sticky but they
must not spontaneously spread
Viscosity decreases when sheared to allow wetting/spreading
over a substrate

7. At and above a certain temperature
tacky fluxes must wet and spread
Must become fluid and able to wet/spread when heated above a
certain temperature

8. Tacky fluxes must react with tin oxides
on solder balls
SnO and SnO2 may be SnO2 is the more thermodynamically
present on solder balls stable material and is more common
Solder Ball
SnO2
SnO2 is amphoteric i.e. it is
SnO
both acidic and basic
Acidic and basic components of fluxes can react with oxides but
8
acids are more reactive

9. Tacky fluxes must also remove surface
coatings like OSP
Azole Benzimidazole
OSPs (Organic Solderability Preservative)
are azole-derived chemical coatings
OSPs are used to protect the surface of copper
and minimize/inhibit copper oxide growth
Substituted benzimidazole
They form chemical bonds with copper
Substituted
arylphenylimidazole
Cu

10. In summary, this is what a tacky flux has
to do to facilitate reaction of Sn & Cu
A B
C D

11. There is a final (obvious) property of
water soluble tacky fluxes
The flux residue needs be soluble in
DI water or DI water-based solutions
The flux components must be water
soluble
Air reflow may result in oxidation of
residues and reduce water solubility
Flux must be designed to be
oxidation-resistant
OR oxidized flux residue must be
designed to be water soluble

12. 2. What’s in a Water Soluble Tacky Flux?

13. Tacky fluxes are complex mixtures of
components that form organic fluids
Tacky fluxes are similar to
emulsions: often multi-phase
Emulsions are dispersions of liquid
droplets in a continuous liquid
Dispersion size and volume
fraction affects properties

14. These are the kinds of materials that
are mixed together
Vehicles that transport components & react with with metal oxides
Activators that accelerate reaction with oxides and surface finishes
Surfactants to wet/spread the flux and help wetting of surfaces by
molten solder
Solvents to dissolve the components and reduce viscosity
Thickening agents to control the viscosity of fluxes
These components often have more than one function

15. Vehicles
Materials that contain and
transport other components
Vehicles usually have more
than one function

16. Vehicles impart viscosity and react
with oxides
Water soluble tacky fluxes use low
molecular weight water soluble polymers
These materials typically have
multiple functions
Surfactants and activators may
also be used as vehicles

17. Activators
Molecules that stimulate
reactions with metal oxides

18. Activators are chemicals that react
with oxides
Typical activators are carboxylic
acids
Acetic acid Pentanoic acid
Oxalic acid Malonic acid Succinic acid
Acid group
Example: Oxalic acid reaction with SnO
+ SnO Sn + 2CO2 + H2O

19. Surfactants
Molecules that lower surface
tension
Surfactants may also be
thickeners

20. Surfactants reduce the surface tension of
liquids allowing them to wet surfaces
The surface layer of a liquid is like a ‘skin’ of unbalanced forces that
result in a surface ‘tension’
Larger unbalanced forces-larger surface tension
Surfactant molecules reduce liquid surface tension, making it
easier to break the surface layer and allow the liquid to spread

21. Solvents
Organic materials used to reduce
viscosity
Used in combination with
thickeners to control viscosity

22. Solvents are used to dissolve the other
components and control viscosity
Solvents are organic and
used to reduce viscosity
Isopropyl alcohol (IPA) is
a typical solvent
But there are many other
types available

23. Thickeners
Organic materials used to
increase viscosity
Used in combination with
solvents to control viscosity
Surfactants may also be
thickeners

24. Thickening agents provide additional
viscosity control
Thickeners are typically water soluble
polymers/oligomers
Thickeners bind components by weak chemical
interactions (hydrogen bonding)
resin
activator
resin

25. Physiochemical Interactions
Flux component molecules
interact by physical and
chemical reaction
The nature of the interactions
affects the viscosity and water
solubility

26. Tacky fluxes contain components with
low and medium molecular weights
Tacky fluxes are similar to
emulsions: often multi-phase
The different molecular lengths
leads to some components
attracting and others repelling
Dispersion size and volume
fraction affects properties

27. Physical interactions at the molecular
level are determined by chemistry
There is a microstructure in fluxes that depends on the component
chemistry
Polar substances form bonds (H bonds) with other components

28. The various components interact to
form complex structures
Emulsions: microstructure determined by physical and/or chemical
interactions
This is a SIMPLE example of oil, water and a surfactant

29. This is a cryo-TEM image of the
microstructure of an emulsion
Emulsions have a microstructure determined by chemical interactions
Bi-continuous phase (dark lines)
Microstructure strongly affects rheology, wetting, spreading
A Berheim-Grosswasser et al Langmuir 1999 15 5448

30. Types of emulsions are determined by
composition and processing
Emulsions may be unstable and can be stabilised by additives
100 Thermodynamic Stability
10
Droplet size (microns)
Macroemulsions
1
Microemulsions
0.1
Miniemulsions
0.01
0 Mins Hours Days Months Years ∞
Stability
Figure after Klaus Tauer, MPI Colloids and Interfaces, Am Mühlenberg, D-14476 Golm, Germany

31. The structures of emulsions vary but
typically contain dispersed phases
Dispersoid formation and size is determined
partially by chemical interactions
Dispersoid size is affected by shearing during
mixing of components and temperature
Dispersoid size distribution and spatial
distribution affect rheology
Rheology affects printing and dispensing
behaviour

32. Solubility in water means the
components prefer to bond with water
The polar bond with water must be stronger than the polar bond with
other components
Flux components prefer to bond with water rather than other
components/surfaces
+water

33. Solubility in water after air reflow
requires minimal chemical change
Some components of the flux will be lost but what remains should
not react much with oxygen in the air
OR after reflow the reactions with air must not affect water solubility
much
+water
Even with minimal change to the flux after reflow it is still necessary
to use some form of cleaning solution to aid with residue removal

34. The Mixing Process
Image: Physics World

35. The chemical mixing process is critical
and determines the flux properties
A tacky flux is a viscous fluid that starts as a
mixture of fluids and solids
The mixing blades are designed
to impart chaotic mixing
The curves in the figure show the
paths travelled by the blades
With increasing revolutions material
from all over the vessel is mixed Increasing revolutions
Mixer blade and revolutions diagram from Ross (www.mixers.com)

36. The same mixture prepared by different
processes is different
Changing the process (mixing, heating, cooling) can change the
physiochemical properties of the mixture
Dispersoid size distribution and spatial distribution affect rheology

37. 3. Experimental Results

38. This sections shows results of the
following experimental measurements
3.1 Basic Properties 3.2 Thermal Analysis
3.3 Tack Force & Rheology 3.4 Reflow & Ball Shear

39. 3.1 Basic Properties

40. These data compare some fundamental
properties of the fluxes
Measurement Flux
A B C D
Acid No. (mg KOH/g) 40 39 41 32
pH 3.5 7.3 6.8 4.7
Malcom Viscosity (Poise) 1740 3950 4510 3390
Solubility in DI Water High High Low High
Acid number: Fluxes A-C were similar and Flux D had a
significantly lower acid number
Significant variation in pH with Fluxes A and D on the low side
Malcom viscosity: Flux A had the lowest viscosity
Solubility in DI water: Flux C had the lowest solubility

41. The significance of acid number is…
Acid number measures of the concentration of
bound acidic groups in organic molecules
Higher concentration of acidic groups
Flux
D B A C
Flux D - lowest acid number of all the fluxes: a non-chemist might
think lower acid number = poor flux activity
Acid number says nothing about acid STRENGTH!

42. pH tells more about how water reacts
with flux components
The relative concentration of hydronium only tells us the extent to
which water reacts with the flux
Stronger Acid in Aqueous Solution
Flux
B C D A
Fluxes A and D are stronger acids in aqueous solution than fluxes
B and C

43. The significance of pH is…
pH is an indication of the hydronium concentration i.e. the acidity of a
solution
Hydronium ions are small and relatively mobile in aqueous solutions
Water soluble tacky fluxes are usually not aqueous systems and acidic
components are not expected to dissociate without water
Hydronium ions are not expected in tacky flux in the absence of
water

44. An acid is a substance that donates a proton and
dissociates in water to produce hydronium ions
Simple acids react with water and release hydronium ions:
Acetic acid
⎯⎯ →
CH 3CO 2 H + H 2O ←⎯ CH 3CO-2 + H 3O +
⎯
pK a = 4.75 ; this is a weak acid
Ethanol
⎯⎯
→
CH 3CH 2OH + H 2O ←⎯ CH 3CO 2O − + H 3O +
⎯
pK a = 16 ; this is a weaker acid than acetic acid
pKa is an index that characterizes the strength of an acid in water
Smaller pKa : stronger aqueous acid (higher H3O+ concentration)
The strength of acid is different in other solvents

45. Solid compounds can be acids even
when not in aqueous solution
Acidity of organic groups is determined by the distribution of
charge around molecules
More electronegative atoms are atoms that have more power to
attract negative charges
Increasing acidity
Electronegativity F>Cl>Br>I>C
Acetic acid Iodoacetic acid Bromoacetic acid Chloroacetic acid Fluoroethanoic acid
pKa=4.76 pKa=3.15 pKa=2.86 pKa=2.81 pKa=2.15
More acidic molecules have a smaller pKa but pKa only measures
acid strength in aqueous solutions
When mixed with other solvents the acid strength will differ

46. 4.2 Thermal Analysis

47. Thermal analysis was performed with
DSC and TGA
Homogeneity, solvent loss and
phase separation can be assessed
by DSC
TGA (Thermogravimetric Analysis)
measures mass loss during heating

48. The graphs below show DSC data for
Flux B and Flux C
Flux B appeared inhomogeneous,
possibly mixing related
Phase transitions did not shift to higher
temperature with increased heating rate
Flux C also appeared inhomogeneous,
possibly mixing related
Phase transitions showed erratic shifts
with increased heating rate

49. The graphs below show DSC data for
each Flux A and Flux D
Flux A appeared relatively
homogeneous
Phase transitions shifted to higher
temperature with increased heating rate
Flux D appeared relatively
homogeneous
Phase transitions did not shift to higher
temperature with increased heating rate

50. The TGA weight loss curves in N2 and
air are shown in the graphs below
At 260°C Flux Din air shows the least weight loss of all fluxes relative
to N2

51. The DTGA weight loss curves in air are
shown in the graphs below
The derivative curves show multiple chemical reactions
The behaviour of Flux D is similar in air and nitrogen but the
other fluxes show significant differences starting at 220°C

52. Rheology & Tack Force

53. Tack force measures a combination of
adhesion and flux viscoelastic properties
Tack force tests measure a combination of adhesion
and viscoelastic properties

54. Rheology and tack force are relevant
to the pin transfer process
The pin transfer process begins with coating the pins with flux
The pins
descend
As the pins The pins
retract the flux penetrate to a
deforms & fixed depth
Each pin is
coated with a fractures
uniform film of
flux
The flux is viscoelastic
and starts to fracture

55. Rheology and tack force are also
relevant pin to substrate flux transfer
The pin transfer process ends with flux deposited at the bond site
The pins
descend
As the pins The pins reach a
retract the flux pre-set height
Each bond site deforms &
is coated with a fractures
uniform amount
of flux
The flux is viscoelastic
and starts to fracture

56. Tack force changes with pin speed
and specimen thickness
Tack force increase with speed: typical viscoelastic behaviour
At 2mil specimen thickness fluxes A, C and D showed similar behaviour
At 4mil specimen thickness fluxes there were clearer differences between
the fluxes
56

57. At 1Hz the fluxes have different
viscoelasticity / viscoplasticity
Flux A: almost linear
response, low stress
Flux B: nonlinear response,
high stress
Flux C: nonlinear response,
high stress, similar shape to B
Flux D: nonlinear response,
medium stress
Stiffness of the fluids falls in this order:
Flux B≈Flux C>Flux D>Flux A

58. The effects frequency on viscoelastic
behaviour support the stress-strain data
With increasing frequency the elastic and viscous components of
shear modulus increase
Fluxes B, C and D are the ‘stiffest’ and have the highest viscous
components
Flux A is very ‘soft’ and is the most ‘liquid like’

59. The effect of shear rate on apparent
viscosity (AV) is shown below
Peak in AV followed by shear thinning
Slight increase in AV with At 40-50s-1 the AVs
shear rate at low shear rates are similar
Flux A has very low initial AV
compared with other fluxes
At high enough shear rates all fluxes show almost the same response

60. Reflow and Ball Shear

61. Reflow was characterized using a
miniature reflow oven
Tacky flux was printed on OSP coated
Cu PCB
Highly oxidized solder spheres were used
to compare the efficacy of the fluxes

62. The videos below show how each flux
attacks oxides in N2
A heavily oxidized solder ball was used to test flux performance
Flux A Flux B
Flux C Flux D

63. The videos below show how each flux
attacks oxides in Air
A heavily oxidized solder ball was used to test flux performance
Flux A Flux B
Flux C Flux D

64. Two of the fluxes showed good capability
to realign misplaced spheres
Excellent
Flux D
Flux B
Poor
Flux C
Flux A
64

65. Ball Shear Data-Air Reflow
The distribution of ball shear strength may be an indirect indication of how well
a flux removes OSP to allow contact between molten solder and copper
Fluxes A, C and D achieved good results
65

66. Cleaning

67. Tacky fluxes were simply printed on
copper pads and reflowed in air
During reflow when the flux is activated it attacks the
OSP
At the same time the flux is attacked by oxygen

68. Cleaning was assessed in a very
simple way
Reflowed specimens were placed in DI water + 10
minutes in an ultrasonic bath
Rinsed with DI water and examined
with an optical microscope

69. Discolouration (tarnishing) after air
reflow is an indicator of cleanliness
Flux A Less discolouration
Cleaner surface
Flux B More easily removable
residues
Flux C
Flux D
Discolouration
69

70. Discolouration is related to thermal
stability
Degradation of the flux in air renders the flux unable to protect the
copper from oxidation

71. The rate of loss of flux and flux
oxidation reactions affect tarnishing
More flux reaction with air equates to higher rate of
degradation

72. Faster reaction with air may mean lack
of protection for the copper
Faster thinning of flux=thinner barrier and increased
chances of Cu oxidation

73. Summary

74. Summary/1
Flux D exhibited the least mass loss in air relative to N2
indicating minimal reaction with air

75. Summary/2
Flux D had the lowest acid number but was the most
effective at removing oxides during reflow in air
High acid number is not necessary for optimum flux
performance
Measurement Flux
A B C D
Acid No. (mg KOH/g) 40 39 41 32
Flux C Flux D

76. Summary/3
Flux D exhibited the best realignment of misplaced balls due
to flux activity and the ability to remove OSP and oxides and
obtain contact between molten solder and bare CU
Excellent
Flux D
Flux B
Poor
Flux C
Flux A
76

77. Summary/4
Higher ball shear was obtained with Flux D implying more
bonded interfacial area and easier flow of solder through the
flux OR easier displacement of the flux by the molten solder

78. Summary/4
The least tarnishing of Cu pads after was achieved with Flux
D indicating minimal penetration of air to the Cu
Flux A
Flux B
Flux C
Flux D