Ask the Experts
September 4, 2007 - Updated
September 4, 2007 - Originally Posted

Stencil Aperture Ratio

What is the best stencil aperture ratio to use when printing very small IC packages like 0.4mm CSP's?

J. S.

Expert Panel Responses

Follow the IPC Design Guidelines.

Bob Black
North America Sales Manager
Essegi Automation
Mr. Black was the President and Co-Founder of Zevatech in 1977 and introduced first Pick and Place System at Nepcon West 1980. Bob is now the President, CEO and Co-Founder of Juki Automation Systems. He is also a Co-Founder of the SMEMA Council of IPC. He serves as a member of SMTA and SEMI..

The best answer to this question is "it depends"... It depends on the type of paste (Type III, IV etc...), stencil technology (Laser cut, E-Fab etc...), pad orientation, type of equipment, etc... Studies shows, smaller powder size (Type IV or higher) lend itself to breaking the traditional Area Ratio rule of 0.66. With type IV and higher paste type, stencil with area ratio of 0.5 have been successfully used. Bottom line is, you need to run some screening experiment specific to your application to determine the area ratio that works for you.

Dr. Rita Mohanty
Director Advanced Development
Speedline Technologies
Dr. Mohanty, PhD in Chemical Engineering, has over fifteen years experience in both industry and academic relating to engineering and electronic polymers, electronic packaging and assembly. She has experience in SMT process optimization and Lead-free transition.

As far as paste transfer from the stencil aperture the design is determined by the Area Ratio (ratio of the aperture opening to the area of the aperture walls). As this ratio becomes smaller as is the case for small apertures for small packages complete paste release becomes more difficult.

Mike Burgess
Regioanl Sales Manager
ASM Assemby Systems
Mike Burgess is the ASM Assembly Systems PSP Regional Sales Manager, who has numerous years of experience in the electronics industry as a user and supplier. He is responsible for stencils, screens, tooling, chemistries, fabric rolls, and stencil coatings.
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