Strengths and Weaknesses of Current Stencil Technologies · Step Stencil Technology -This paper and presentation was first presented at the 2017 IPC Apex Expo Technical Conference

Strengths and Weaknesses of Current Stencil Technologies

Greg Smith

BlueRing Stencils

[email protected]

Outline• Introduction

• Types of SMT Stencil Technology

• Manufacturing Methods

• Stencil Foil Materials

• Step Technologies

• Nano-Coating Technologies

• Conclusions

Introduction

Brief History of SMT Stencil Manufacturing Methods1980’s thru Late 1990’s: Chemically Etched Stencils, Electroformed Stencils

Late 1990’s thru Early 2000’s: YAG Lasers become the new Normal for laser cutting,

Electroformed Stencils

Introduction

Brief History of SMT Stencil Manufacturing Methods2010’s Until Current: Fiber Laser Cut Stencils, Electroformed Stencils.


Manufacturing Methods

Laser Cut StencilsMost Manufacturers are utilizing Fiber Laser Technology for todays SMT Stencils


Electroformed StencilsThin foil printing, Ultra Fine Component Printing, High Aperture Count Printing Applications

IMAGED MANDREL

• PHOTO RESIST IS APPLIED TO A MANDREL• PHOTO PLOT IS IMAGED INTO RESIST• UNEXPOSED RESIST IS DEVELOPED AWAY



PLATING NICKEL ALLOY

• NICKEL ALLOY IS PLATED ONTO MANDREL TO REQUIRED THICKNESS



FINAL STENCIL FOIL

• NICKEL STENCIL IS SEPARATED FROM MANDREL AND MOUNTED TO FRAME



Electroformed stencils are 100% nickel with superior paste release characteristics

Electroformed SEM


COMPARISON (STRENGTHS & WEAKNESSES) OF MANUFACTURING METHODS

METHOD FOIL THICKNESS APERTURE SIZE TOLERANCE

POSITIONAL ACCURACY METAL THICKNESS

FIBER LASER CUT 2 – 10 MIL,SPECIFIC THICKNESS ONLY

+/- 0.3 MILS 0.4 MILS MAX OVER 31.5 MILS STAINLESS +/- 2% OF FOIL THICKNESS

ELECTROFORMED 1-6 MIL7-8 MIL

+/- 0.3 MILS+/- 0.5 MILS

< 10” X 10” WITHIN 1 MIL> 10” X 10” < 16” X 16” WITHIN 1.5 MILS> 16” X 16” < 20” X 20” WITHIN 2 MILS

+/- 5%

*Tolerances presented are based on presenters data

STENCIL FOIL MATERIALS

IN SMT STENCIL PRINTING, ESPECIALLY LOW AREA RATIO PRINTING….FOIL MATTERS!

Impact of Stencil Foil Type on Solder Paste Transfer Efficiency for Laser Cut SMT Stencils - Originally published in the Proceedings of SMTA International, Rosemont, IL, Sept. 17 – 21, 2017

Material “FG” Description Grain Size Category

1 Yes Stainless A

2 No Stainless B

3 N/A Ni N/A

4 N/A Ni N/A

5 No Stainless C

6 Yes Stainless A

7 Yes Stainless A

Grain Size “A”: 1-5 MicronsGrain Size “B”: 6-10 MicronsGrain Size “C”: >10 MicronsNickel Grain Size: Unknown



Test Vehicle:• 7 Area Ratio Apertures• 5 Mil (125mm) Foil• All square with rounded

corner• 100 Apertures per group





Test Vehicle:• 2 Patterns Per Stencil• 1 Pattern Coated with

Ceramic Nano-Coating• 2 Coupons Per Stencil• Cut Same Day/Same

Laser

Coupon



Transfer Efficiency for Coated and Uncoated Stencils for All Metals with 0.3, 0.4, and 0.5 Area Ratios (Small Area Ratios) Combined.





Transfer Efficiency by Metal by Grain Size for 0.4, 0.5 Area Ratios (Small Area Ratio Printing).




Coefficient of Variation by Metal Type.




SEM of Uncoated Aperture Sidewall, Material 1Best Performer, FG Material




SEM of Uncoated Aperture Sidewall, Material 3




CONCLUSIONS• Not all Fine Grain (FG) materials perform the same• Material 1 (FG) and Material 2 (Not FG) were determined to

outperform the other 5 materials when comparing Transfer Efficiency and Coeficient of Variation

• Laser cutting the material with the highest transfer efficiency and the lowest coefficient of variation and applying a Ceramic Nano-Coating produces the best printing process

• SEM Analysis shows that base materials cut differently and some materials exhibit smoother sidewalls than others.

STEP TECHNOLOGIES

Step Stencil Technology - This paper and presentation was first presented at the 2017 IPC Apex Expo Technical Conference and published in the 2017 Technical Conference Proceedings.

Step Down Stencils:

Material thickness is

reduced in areas where

smaller components are

placed on the PWB so that

proper Area Ratios are

maintained to obtain good

paste release.*Step Down Stencil

STEP TECHNOLOGIES


Step UP Stencils:

Material thickness is

increased around specific

components such as edge

connectors, large BGA’s,

D-PAK’s and other devices

to increase the volume of

paste deposited.

STEP TECHNOLOGIES


Currently there are three technologies in North America to manufacture

step stencils.

Micro-Machined Technology Welded Technology Etched Technology

STEP TECHNOLOGIES


Chem-Etch Technology• Historically, the primary

method to manufacture step stencils

• Photographic process applies resist to areas that will not be etched.

• Etchant is sprayed onto surface and removes material until the desired thickness is achieved.

Etched Technology

STEP TECHNOLOGIES


Laser Welded Technology• The area of the step is cut from

the stencil.• The required thickness of the

step area is cut from another foil the exact same size.

• The new foil blank is placed into the cut out area and the foil is welded together.

• Apertures are then laser cut.Laser Welded Technology

STEP TECHNOLOGIES


Micro-Machining Technology• Material is attached to cooled

vacuum plate.• CNC based milling machine

removes very small amounts of material at a time until desired thickness is achieved.

• Stepped foil is mounted to frame and apertures are laser cut. Micro-MachiningTechnology

STEP TECHNOLOGIES

METHOD STEP DEPTH / HEIGHT THICKNESS

STEP SIZE TOLERANCE

POSITIONAL ACCURACY STEP DEPTH / HEIGHT TOLERANCE

FOIL SURFACE

LASER WELDING 2 MILS PER WELD, CAN CUT AND REWELD

+/- 0.3 MILS WITHIN 0.4 MILS MAX OVER TRAVEL STAINLESS +/- 2% OF STEP FOIL THICKNESS

SAME AS MIL FINISH

MICRO-MACHINED DEPENDENT ON FOIL THICKNESS

+/- 1 MIL +/- 0.5 MILS +/- 0.5 MILS BURNISHED FROM ROTATING CUT TOOL

CHEM-ETCH DEPENDENT ON FOIL THICKNESS

+/- 2 MILS +/- 0.5 MILS +/- 0.5 MILS SATIN SEMI-SMOOTH SURFACE

COMPARISON (STRENGTHS & WEAKNESSES) OF STEP TECHNOLOGIES

STEP TECHNOLOGIES

COMPARISON (STRENGTHS & WEAKNESSES) OF STEP TECHNOLOGIES

• CHEM ETCH TECHNOLOGY• HIGH NUMBER OF STEP AREAS• RELIEF STEPS IN BOTTOM OF STENCIL, ie. PLATED VIAS • VERY SMALL APERTURES, <100 MILS• VERY LARGE STEP DOWN AREAS• SEMI-SMOOTH ETCHED SURFACE CAN CATCH/ROB FLUX FROM PASTE

• MICRO-MACHINING TECHNOLOGY• MANY SMALL STEP AREAS• STEP DOWN/UP AREAS IN TENTHS OF A MIL THICKNESSES• AREAS LARGER THAN 2” CAN WARP/DEFORM METAL• BURNISHING AT STEPPED IS SMOOTH, DOES NOT CATCH/ROB PASTE FLUX

• WELDED-STEP TECHNOLOGY• MOST STEP UP AND STEP DOWN STENCILS• LIMITED TO 4” X 4” SIZE FOR LARGE STEPS AND 100 MILS FOR SMALL STEP UP OR DOWN AREAS• MILL FOIL SURFACE AND FOIL THICKNESS TOLERANCE ARE BIGGEST ADVANTAGES

NANO-COATING TECHNOLOGIES

Stencil Nano-Coatings-Do They Improve Repeatability and Uniformity in The Print Process? - This paper and presentation was first presented at the 2018 IPC Apex Expo Technical Conference and published in the 2018 Technical Conference Proceedings.

Benefits of Nano-Coatings• Hydrophobic-repel water based chemistry (flux)

• Oleophobic-repel oil based chemistry (flux)

• Improved Transfer Efficiency (Ceramic)

• Reduced Underside Cleaning Frequency(Ceramic and Self-Assembled Monolayer)

• Reduced Bridging after print (Ceramic and Self-Assembled Monolayer)


Types of Nano-Coatings

• Self Assembled Monolayer• Manually applied to the board side of stencil

• Thickness is 2-4 nano meters

• Clear-no color

• Validation done by testing surface energy

• Can be reapplied

• Primary benefits are reduced underside cleaning and reduced bridging




• Self Assembled Monolayer-Testing Surface Energy




• Ceramic • Applied with custom, precision spray equipment• Thickness is 2-4 microns• Color and UV dye• Cured after coating to create a hard, durable coating• Lower Coefficient of Variation in Print Process• Primary benefits are reduced underside cleaning,

reduced bridging, and increased transfer efficiency on small aperture printing




Self Assembled Monolayer Ceramic-Spray Coat and Cure




Test Vehicle

Three stencils, one with SAM coating, one with Ceramic coating and one is uncoated



Print Parameters

• 50 Boards Printed

• Boards 1, 10, 20, 30, 40 and 50 were measured with a 3D solder paste inspection system (SPI)

• Volume, Height and Area data were collected

Experimental Methodology



Results



Results

Printed Solder Paste Transfer Efficiency

• Tukey-Kramer HSD Analysis on Small Area Ratio components

• Evaluates data to determine if it is significantly different

• Ceramic coating mean TE is highest

• SAM coating mean TE is lower than the uncoated stencil



Results



Results

Printed Solder Paste HeightTukey-Kramer HSD Analysis:

• 0.4 CSP and 01005 component mean print height SAM coating shows significantly different (lower) print height results when compared to the uncoated and Ceramic coated stencil.

• Ceramic coating mean height results are not significantly different than the uncoated stencil.



Results



SEM Analysis

Uncoated ApertureCeramic Nano-Coated Aperture10%-25% TE Improvement



• There are 2 types of nano-coatings currently being used.

• Self-Assembled Monolayer (SAM)

• Ceramic

• When looking at printed paste volume, Ceramic nano-coatings improve transfer efficiency for 0.66 area ratio apertures and smaller (0.4 CSP and 01005) and SAM nano-coatings decrease printed paste volume when compared to uncoated stencils.

• When area ratios are larger than 0.66, adding SAM nano-coating and Ceramic nano-coating can result in slight increases in printed paste volume when compared to uncoated stencils.

• When area ratios are less than 0.66 (0.4 CSP and 01005), SAM nano-coating decreases the printed height when compared to uncoated stencils. Both Ceramic and uncoated stencil printed height are similar.

Conclusions



Conclusions

• For components larger than 01005, SAM and Ceramic nano-coatings produced printed paste height slightly higher than the uncoated stencil.

• Printed paste area was higher for the 01005 component on the uncoated stencil.

• Printed paste area was higher for the ceramic nano-coated stencil on the 0.4 CSP, 0.4 QFN and 0.4 QFP components than both the SAM nano-coated stencil and the uncoated stencil.

• When area ratios are less than 0.66 (0.4 CSP and 01005), it is recommended that Ceramic nano-coatings are used to improve repeatability and uniformity in the print process.

• When area ratios are more than 0.66, it is recommended that either Ceramic or SAM nano-coatings are used to improve repeatability and uniformity in the print process.

Thank You for Your Attention!Any questions?

Greg SmithBlueRing Stencils

[email protected]

Documents

Strengths and Weaknesses of Current Stencil Technologies · Step Stencil Technology -This paper and presentation was first presented at the 2017 IPC Apex Expo Technical Conference