smt solder paste printing for fine-pitch...smt solder paste printing for fine-pitch satoru akita...

SMT Solder Paste Printing For Fine-Pitch Satoru Akita General Manager

Senju Metal Industry Co., Ltd. Solder Technical Center

2

Insufficient print volume

Stencil accuracy Component tack force

Viscosity Print misalignment

Preheat temp & time control

Atmosphere Insufficient reflow temp & time

Flux heat resistance

Powder size

Warpage

Board accuracy Pad surface condition

Pad design Mount accuracy

Mount load

Size accuracy

Electrode surface condition

In addition to solder paste, it is essential to control the components, equipment, and process.

Effective Factors of Fine Pitch Assembly:

Key Factors that Influence Product Quality

Component PCB Placement

Solder Paste Printing Reflow

Fine Pitch Assembly

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Ensure missing or misaligned components are detected.

Use paste with good print release and stability

- Mounting- ☆ Mount load ☆ Mount accuracy

Use paste with high tack force

- Reflow- ☆ Preheat setup ☆ Peak temp & time ☆ Ramp-up speed

Use paste with high heat-resistant

Ensure print release defects, misalignments, and other abnormalities are detected.

To enable consistent Fine Pitch assembly:

Considerations for Fine Pitch Assembly

Inspection - Printing- ☆ Aperture design ☆ Stencil side-walls

Inspection

Solder Paste Selection ・Solder Powder Size ・FLUX

Stencil Selection

Suitable Printing Parameters

Suitable Reflow Profile

Contents

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5

Large IC and other conventional device

Fine Pitch component

Wide Narrow

Smaller aperture opening area is more difficult to deposit paste as compared to larger devices

It is critical to fill the apertures effectively

Side view of aperture opening:

Stencil Aperture for Fine Pitch Component

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Printability Test: Vo

lum

e(%

)

0

30

60

90

12

15

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

□100*50 □100*75 □100*100 □200*100

Component 内での PasteVo

lum

e(%

)

0

30

60

90

12

15

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

Type

4

Type

5

Type

6

□100*50 □100*75 □100*100 □200*100

Component 内での Paste

When aperture size is small, Type 6 paste can achieve higher print volume and stability.

Effects of different stencil thickness vs. powder size

Stencil Thickness: 40um Stencil Thickness: 60um

Paste under the component Paste under the component

Print Volume Comparison by Powder Size

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Stencil Selection



Contents

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Type4 20 ~ 38um

Type5 15 ~ 25um

Type6 5 ~ 15um

Picture

Surface area ratio per one particle 9 4 1

Powder number ratio per unit volume 8 27 216

Surface area ratio per unit volume 72 108 216

*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

Oxide Film Thickness by Powder Size (Theoretical):

Oxide Film

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Inside Reflow Oven

PCB

O2

O2 (Oxygen)

Paste

<

Incomplete Melting

> + + FLUX FLUX

When powder oxidation increases, flux’s ability to remove oxide layers decreases and causes solder balls and non-wets

Oxidation during Reflow:

Effects of Solder Oxidation

O2

O2

O2 O2

O2

O2

O2 O2 O2 O2

Difficult to achieve with conventional flux

Flux

Oxidized solder powder

Non-oxidized solder powder

Complete Melting

10

Type 5 + Conventional flux Type 6 + Conventional flux

No solder joint issue Solder joint issue found No solder joint issue

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!

Solder Appearance after Reflow Comparison of 0250125 Chip Appearance:

100um

100um

100um 100um

Pad design. Stencil Aperture Size :40um Thickness: 40um

Type 6 + Specialized flux (Senju M705-RGS800 Type6)

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Stencil Selection



Contents

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Standard Laser Process High Performance Laser Process Additive Process ©PROCESS LAB. MICRON

Insufficient Printing

burr

It is critical to use stencils with smooth sidewalls

Different Sidewall Conditions of the Stencil

Supplier A

Supplier B

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120um

60um ●200um

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

Printability by Different Stencil Thickness

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Aspect Ratio = Area of aperture wall

= t: stencil thickness d: aperture diameter

Area beneath the aperture opening

4t

d

Area beneath the aperture opening

Area of aperture wall

Aperture Opening Size Stencil thickness/ um Aspect ratio/ %

200um

80 1.6

100 2.0

120 2.4

When aspect ratio is larger, printing becomes more difficult

In general, aspect ratio of 2.0 or lower is recommended

Aspect Ratio:

Aperture Opening & Stencil Thickness

150umPitch Stencil opening： 110um

(Aspect ratio : 1.5%)





Stencil : Process Lab Nano Coating Stencil Thickness : 40 um Solder Paste : Senju BPS Type 7 Series Printer: Dek Proflow

Printability of Bumping

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2. Exposure 5. Reflow 1. Dry Film Lamination 3. Opening 4. Paste Printing 6. DF Removing

Dry Film Printing Method

Item Pitch(um) SRO(um) SRT(um) DFO(um) DFT(um)

Size 110 55 20 65 30

■ Substrate size

Dry Film Aperture Opening Size: 200×240mm Dry Film Thickness: 0.3mm Solder Paste : Senju M705-DFS-Series

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Process:

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Stencil Selection



Contents

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Mask

PCB

Make sure there is no scraping leftover in the printing area

Mask

PCB

■ Excess volume for large opening ■ Poor release for small opening

release

release

- To prevent residues - ■ Adjust print pressure ■ Control Squeegee ■ Backup properly

Considerations Inside the Printer

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Parameters Recommended Conditions (Ex.)

Printer Type 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

Printing Conditions: To ensure high printing quality of very small apertures, it is necessary to optimize printing parameters.

Print Parameter Setup

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volu

me(

%)

20

30

40

50

60

70

80

20 30 40 50 100 200

print speed(mm/s)

標準

偏差

0

2

4

6

8

10

20 30 40 50 100 200

print speed(mm/s)

30mm/s

50mm/s

200mm/s

Too fast or too slow speed will cause greater variations in transfer volume

Printing Speed (20, 30, 40, 50, 100, 200mm/s): Print speed affects deposit time and pressure which can impact volume transferability.

Vol distribution

Deviation

Printing Speed

Complete

Paste rolling

Fill the stencil aperture

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volu

me(

%)

20

30

40

50

60

70

80

0.1 0.5 1 5 10

down speed(mm/s)

標準

偏差

0

2

4

6

8

10

0.1 0.5 1 5 10

down speed(mm/s)

0.1mm/s

1mm/s

5mm/s

Vol distribution

Deviation

After Printing

During Release

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

Key is to minimize solder buildup on walls

Slow separation speed can cause poor print definition and lower transfer volume, leading to larger variations

Separation Speed

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Stencil Selection



Contents

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Reflow Profile Guideline:

③

③ Ramp-to-peak ■ Recommended rate (avg): 2~3℃/s

④

①

② Preheat/Soak ■ Recommended temp: 140~170℃ ■ Recommended time: Around 70sec

④ Reflow ■ Recommended peak temp: 230~245℃ ■ Time above liquidus（220℃): Over 30sec

②

It is important to control preheat zone (where oxidation is likely to occur) and ramp-to-peak rate

① Ramp-to-soak ■ Recommended rate (avg): 2~4℃/s

Critical Parameters of Reflow Profile

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are best possibilities

From solder paste approach…

- Jet Dispense Process -

When more solder volume is needed for larger components: When solder joint strength becomes a concern for larger devices,

■ Stencil design change ■ Chip solder

Process to supplement solder volume of printing paste Non-contact Shorter tact time as compared to air dispense

Half-edge (Step Down) Chip Solder

Mixed Component Mounting

Chip solder

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Forc

e(N

)

0

10

20

30

40

50

60

0.12t 0.10t 0.08t 0.08t+Jet

Stencil thickness

Shear Strength by Print Thickness: Effects of strength by adding solder (1608 chip capacitor)

Stencil Thickness Avg Force (N)

0.12t 40.08

0.10t 33.13

0.08t 26.54

0.08t + JET 37.13

Adding solder volume by jet dispensing can potentially increase strength Due to new process, further verification testing is needed

Potential Increase in Strength by Jet Dispensing

*Solder Paste : Senju NXD900ZH series

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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

Conclusion

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http://www.senju-m.co.jp

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smt solder paste printing for fine-pitch...smt solder paste printing for fine-pitch satoru akita...

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