Due to the need for smaller components such as 01005, CSP and

Micro BGA’s, proper printer setup has become critical to the

overall quality aspect of the production process.

Circuits Assembly, “On-line Enhancements of the Stencil Printing Process”

Pan, J., . Critical variables of solder paste stencil printing for Micro-BGA and fine

pitch QFP. IEEEI/CPMT International Electronics Manufacturing Technology Symposium

Feldmann, K. and Sturm, J. ) Closed loop quality control in printed circuit assembly.

IEEE Transactions on Components, Hybrids and Manufacturing technology, Part A, Vol. 17,No.2

“According to estimates, 50 to 70 percent of total defects in surface

mount assembly lines are related to the stencil printing process, and

approximately 30 to 50 percent of the total manufacturing cost is due

to test and rework”

There are more than 60 Variables found within the screen printing

process. By implementing a proactive quality initiative to better

understand each variables impact will guarantee reliable TE.

• Printing Process - Setup

• Solder Paste Material

• Tooling and Equipment

• Design

• Environment

• Human Interaction

• Speed of Print - Setup

– Under 1 to 12 Inches/Sec

• Shear Thinning – High Speed – 4”-8”

• Squeegee Pressure - Setup

– 1 to 1.5 lbs per/linear inch of blade

• Too high – Scooping / Excessive wear

• Too low – poor fill /left over paste on stencil

• Paste Application - Setup

– Roll diameter of 3/8 to 1/2 inch

– 1/2 inch beyond end of blade

• Allow Paste to Flow Properly

– Reprint first 5 boards*

• Cleaning Frequency

– Under wipe/Vacuum – 1 to 20 prints

• Stencil to PCB

– Proper Gasketing

• Poor Alignment

• PWB Shrink

• Aperture Size

• Paste Under Stencil

• Solder Mask on Pad

• HASL Dome

• Silk Screen Layer

• Paste Properties

– Metal % / Alloy

– Powder Size

• Type 1 – 6

– Lead Free

– No Clean

– Water Soluble

• Storage and Handling

– Time out of Storage

– Time on Stencil

– Expiration of Paste

• Flux Reduction

– Viscosity Increase

– Poor Roll

– Poor Aperture Fill

– Reflow Issues

• Stencil

– Wear/Damage

– Enhancements

• Nano-Coatings*

– Material

• Metal / Plastic

• Squeegee

– Wear / Damage

– Blade Type*

• Rubber /Polyurethane

• Metal –Finer Pitch

• PCB Planarity

– Supplier Issue

– Board Support*

• Pads

– SMD or NSMD

• Spacing

• Complexity

– Low, Mid, High*

• Board Layout

– Single

– Panelized

• Stencil Technology

– Aperture

• Density

• Shape

– Round

– Square

» Radius corners

• Reduction

• Stencil Technology

– Area Ratio*

• >0.66

– <0.66 Type 6 paste*

– Step Stencil

– Nano-Coating

– Pitch Dimension

• Controlled Environment

– Temperature

– Relative Humidity

– Seasonal

– Air Flow*

• Inside Printer

– Air Movement

» Dries Out Paste

• Time of Day

– Good day / Bad day*

• Shift Changeover

• Operator

– Skill / Error*

– Diligence

– Access Level to Printer

• Solder Bridging

• Insufficient Solder

• Poor Alignment

• Poor Print Deposition

• Accept the fact that 1 size does not fit all

– 5 mil stencil rarely yields 5 mils of paste

• Determine what % of defects are print related

– Most likely very high percentage – corrective actions

• Understanding “First-Pass” Yield

– Be honest – Improved print process = reduced rework

• Implement proactive 3D Solder Paste Inspection

– Fast ROI - Pay dirt all the way!

• Accurate / Repeatable

– .10 Mil / GR&R under 25%

• Multiple Application Use

– Adhesives

– Thick Films

– Grease

• Lowest Cost of Ownership

– Starting at under $15,000.

• Highly Accurate / Repeatable – .07 Mil / GR&R under 10%

• 2 Levels of Analysis – Quantitative and Qualitative

• True 3D – Height / Volume – Higher Resolution

• Pixel by Pixel Data Points

• FOV / Camera Resolution

• Sample Based Inspection – Focus on Critical Components

• Strong Price / Performance – Starting at under $25,000.

Post-print $ 0.50

Post-reflow $ 5.00

Post-ICT $ 50.00

Field Failure $ 500.00

Example: Multiple Line EMS Company

Target Stencil Thickness = 5 Mils

Measured Solder Paste Thickness = 7 Mils

Excessive Solder = 2 Mils

Percentage Over-Print = 40%

Annual Paste Expense Per Year = $50,000

Estimated Yearly Cost Savings Due to SPI Control = $20,000

** Typical Offline SPI System Cost = Less than $25,000 **

Questions?