SMT Solder Paste Printing For Fine-Pitch
Satoru Akita
General Manager
Senju Metal Industry Co., Ltd.
Solder Technical Center
Key Factors that Influence Product Quality
Effective Factors of Fine Pitch Assembly:
Component
Size accuracy
Board accuracy
Electrode
surface condition
Pad design
Mount accuracy
Mount load
Pad surface
condition
Warpage
Insufficient print
volume
Flux heat resistance
Powder size
Placement
PCB
Stencil accuracy
Component tack force
Preheat temp &
time control
Atmosphere
Print misalignment
Fine Pitch
Assembly
Insufficient reflow
temp & time
Viscosity
Solder Paste
Printing
Reflow
In addition to solder paste, it is essential to control
the components, equipment, and process.
2
Considerations for Fine Pitch Assembly
To enable consistent Fine Pitch assembly:
Use paste with good print
release and stability
- Printing☆ Aperture design
☆ Stencil side-walls
Inspection
Use paste with high
tack force
- Mounting☆ Mount load
☆ Mount accuracy
Ensure print release defects,
misalignments, and other
abnormalities are detected.
Use paste with high
heat-resistant
Inspection
- Reflow☆ Preheat setup
☆ Peak temp & time
☆ Ramp-up speed
Ensure missing or misaligned
components are detected.
3
Contents
 Solder Paste Selection
・Solder Powder Size
・FLUX
 Stencil Selection
 Suitable Printing Parameters
 Suitable Reflow Profile
4
Stencil Aperture for Fine Pitch Component
Side view of aperture opening:
Large IC and other
conventional device
Wide
Fine Pitch
component
Narrow
Smaller aperture opening area is more difficult to deposit paste
as compared to larger devices
It is critical to fill the apertures effectively
5
Print Volume Comparison by Powder Size
Printability Test:
Effects of different stencil thickness vs. powder size
Stencil Thickness: 60um
15
12
12
90
90
Volume(%)
15
60
30
□100*50
□100*75
Component 内での Paste
□100*100
Paste under the component
□200*100
□100*50
□100*75
□100*100
Type6
Type5
Type4
Type6
Type5
Type4
Type6
Type5
Type4
Type6
Type5
0
Type4
Type6
Type5
Type4
Type6
Type5
Type4
Type6
Type5
Type4
Type6
Type5
0
60
30
Type4
Volume(%)
Stencil Thickness: 40um
□200*100
Component 内での Paste
Paste under the component
When aperture size is small, Type 6 paste can achieve higher
print volume and stability.
6
Contents
 Solder Paste Selection
・Solder Powder Size
・FLUX
 Stencil Selection
 Suitable Printing Parameters
 Suitable Reflow Profile
7
Oxide Film
Oxide Film Thickness by Powder Size (Theoretical):
Type4
20 ~ 38um
Type5
15 ~ 25um
Type6
5 ~ 15um
9
4
1
8
27
216
72
108
216
Picture
Surface area ratio per
one particle
Powder number ratio
per unit volume
Surface area ratio per
unit volume
*Calculation based on average size of Type4 (30um), Type5(20um), Type6(10um)
Surface area of Type 6 is three times more than Type4 = oxide
film thickness is three times. Oxidation risk is greater with finer
powder size
8
Effects of Solder Oxidation
Oxidation during Reflow:
Inside Reflow Oven
FLUX
Flux
Non-oxidized solder powder
>
+
FLUX
<
+
Oxidized solder powder
O2 (Oxygen)
O2 O2 O2 O2 O2
O2
O2
O2
O2
O2
O2
PCB
Paste
Complete Melting
Incomplete Melting
When powder oxidation increases,
flux’s ability to remove oxide layers
decreases and causes solder balls
and non-wets
Difficult to achieve with conventional flux
9
Solder Appearance after Reflow
Comparison of 0250125 Chip Appearance:
 Stencil Aperture Size :40um
 Thickness: 40um
 Pad design.
100um
100um
100um
100um
Type 5 + Conventional flux
Type 6 + Conventional flux
Type 6 + Specialized flux
No solder joint issue
Solder joint issue found
No solder joint issue
(Senju M705-RGS800 Type6)
When components are mounted, reflowability worsens as flux
also needs to remove the oxidation on the component side.
By optimizing flux, it enables successful soldering!
10
Contents
 Solder Paste Selection
・Solder Powder Size
・FLUX
 Stencil Selection
 Suitable Printing Parameters
 Suitable Reflow Profile
11
Different Sidewall Conditions of the Stencil
burr
Supplier A
Insufficient Printing
Supplier B
©PROCESS LAB. MICRON
Additive Process
Standard Laser Process
High Performance Laser Process
It is critical to use stencils with smooth sidewalls
12
Printability by Different Stencil Thickness
60um
●200um
120um
For small apertures, thicker stencil will produce poor print
definition and transfer volume becomes inconsistent.
→ It is important to select the most suitable stencil thickness
13
Aperture Opening & Stencil Thickness
Aspect Ratio:
Aspect Ratio =
Area of aperture wall
Area beneath the
aperture opening
=
Area of aperture wall
Area beneath the
aperture opening
4t
t: stencil thickness
d: aperture diameter
d
When aspect ratio is larger, printing becomes more difficult
In general, aspect ratio of 2.0 or lower is recommended
Aperture
Opening Size
200um
Stencil thickness/ um
Aspect ratio/ %
80
1.6
100
2.0
120
2.4
14
Printability of Bumping
150umPitch
Stencil opening： 110um
(Aspect ratio : 1.5%)




120umPitch
Stencil opening： 90um
(Aspect ratio : 1.8%)
Stencil : Process Lab Nano Coating
Stencil Thickness : 40 um
Solder Paste : Senju BPS Type 7 Series
Printer: Dek Proflow
120umPitch
Stencil opening： 80um
(Aspect ratio : 2.0%)
15
Dry Film Printing Method
Process:
1. Dry Film Lamination 2. Exposure
3. Opening
4. Paste Printing
5. Reflow
6. DF Removing
■ Substrate size
Item
Pitch(um)
SRO(um)
SRT(um)
DFO(um)
DFT(um)
Size
110
55
20
65
30
 Dry Film Aperture Opening Size: 200×240mm
 Dry Film Thickness: 0.3mm
 Solder Paste : Senju M705-DFS-Series
16
Contents
 Solder Paste Selection
・Solder Powder Size
・FLUX
 Stencil Selection
 Suitable Printing Parameters
 Suitable Reflow Profile
17
Considerations Inside the Printer
Make sure there is no scraping
leftover in the printing area
Mask
release
PCB
■ Excess volume for large opening
■ Poor release for small opening
- To prevent residues ■ Adjust print pressure
■ Control Squeegee
■ Backup properly
Mask
release
PCB
18
Print Parameter Setup
Printing Conditions:
To ensure high printing quality of very small apertures, it is necessary to
optimize printing parameters.
Parameters
Printer Type
Recommended Conditions (Ex.)
Open Squeegee
Squeegee Type
Metal
Squeegee Angle
60°
Print Speed
30-50mm/s
Print Pressure
0.20-0.30N/mm
Separation Speed
1.0-5.0mm/s
Print Environment
22-28℃ 40-60%RH
It is especially important to control print speed and
separation speed
19
Printing Speed
Printing Speed (20, 30, 40, 50, 100, 200mm/s):
Print speed affects deposit time and pressure which can impact volume transferability.
80
70
60
volume(%)
Paste rolling
50
30mm/s
40
30
Vol distribution
20
20
30
40
50
100
200
print speed(mm/s)
10
Fill the stencil aperture
50mm/s
標準偏差
8
6
4
Deviation
2
0
20
30
40
print speed(mm/s)
50
100
200
200mm/s
Complete
Too fast or too slow speed will cause greater variations
in transfer volume
20
Separation Speed
Separation Speed (0.1, 0.5, 1.0, 2.5, 5.0, 10.0mm/sec):
Separation speed affects the adhesion to the aperture walls which can
significantly impact paste release & print definition
80
After Printing
70
volume(%)
60
50
0.1mm/s
40
30
Vol distribution
20
0.1
During Release
0.5
1
5
10
down speed(mm/s)
10
1mm/s
標準偏差
8
6
4
Deviation
2
Key is to minimize solder
buildup on walls
0
0.1
0.5
down speed(mm/s)
1
5
10
5mm/s
Slow separation speed can cause poor print definition and
lower transfer volume, leading to larger variations
21
Contents
 Solder Paste Selection
・Solder Powder Size
・FLUX
 Stencil Selection
 Suitable Printing Parameters
 Suitable Reflow Profile
22
Critical Parameters of Reflow Profile
Reflow Profile Guideline:
④
Reflow
■ Recommended peak temp: 230~245℃
■ Time above liquidus（220℃): Over 30sec
②
④
Preheat/Soak
■ Recommended temp: 140~170℃
■ Recommended time: Around 70sec
②
③
③
Ramp-to-peak
■ Recommended rate (avg): 2~3℃/s
①
①
Ramp-to-soak
■ Recommended rate (avg): 2~4℃/s
It is important to control preheat zone (where oxidation is
likely to occur) and ramp-to-peak rate
23
Mixed Component Mounting
When more solder volume is needed for larger components:
When solder joint strength becomes a
concern for larger devices,
■
■
Stencil design change
Chip solder
are best possibilities
Chip solder
Half-edge (Step Down)
Chip Solder
From solder paste approach…
- Jet Dispense Process Process to supplement solder volume
of printing paste
Non-contact
Shorter tact time as compared to air
dispense
24
Potential Increase in Strength by Jet Dispensing
Shear Strength by Print Thickness:
Effects of strength by adding solder (1608 chip capacitor)
60
Stencil Thickness
Force(N)
50
40
Avg Force (N)
0.12t
40.08
20
0.10t
33.13
10
0.08t
26.54
0.08t + JET
37.13
30
0
0.12t
0.10t
Stencil thickness
0.08t
0.08t+Jet
*Solder Paste : Senju NXD900ZH series
Adding solder volume by jet dispensing can potentially
increase strength
Due to new process, further verification testing is needed
25
Conclusion
To achieve successful Fine Pitch assembly:
 Solder Paste Selection
■ Solder powder : Select fine powder (< Type 6: 5～15um)
■ Flux : Select high oxidation-resistance & high heat-resistance
chemistries
 Stencil Selection
Select stencil with very smooth aperture walls
Select aperture design with 2.0 or lower aspect ratio
 Printing Parameter Setting
Set suitable parameters (especially separation speed)
 PCB Quality (Plating/Resist)
Ensure there is no contamination of the plating and resist is aligned
 Reflow Profile
Set suitable preheat temp/time & ramp-up speed
For Fine Pitch assembly, it is essential to increase overall
accuracy
26
THANK YOU
http://www.senju-m.co.jp
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