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October 6, 2008

Stencil Aperture Change Due to Board Finish?

Is there any reason to alter apertures on stencils if board finishes change from ENIG to OSP?



R.W.

Experts Comments

Typically there is no change in stencil apertures when changing from ENIG to OSP PCB surface finish

Flux activity and paste transfer volume efficiency (function of flux chemistry that determines how much paste gets transferred from the stencil to the board as opposed to getting trapped in the stencil) are crucial in wetting and spreading on OSP pads

  • good flux activity means better spread
  • high paste transfer efficiency means more paste and more flux especially for small apertures

This becomes even more important with Pb-free pastes.

Nitrogen reflow could improve spread but the cost adder is so high that normally the right solder paste is selected for optimum results in air reflow

With OSP and Pb-free no-clean pastes where there is normally a stencil aperture reduction, solder spread is not complete - this is normal

See photos of Indium 5.8ls Pb-free no-clean solder paste reflowed in air on OSP finishes

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Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

Stencil apertures should not be affected by the change from ENIG to OSP. Aperture dimensions are related to the size and volume of paste applied rather than the board coating.

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Kris Roberson
Manager of Assembly Technology
IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

This will depend on your solder paste and inspection criteria. Some solder pastes do not wet out to the corners of pads when used with OSP finishes.

If your inspection criteria require full pad coverage then you may have to go to full pad or near full pad printing. This can cause mid-chip solderballing issues if overdone.

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John Vivari
Application Engineering Supervisor
Nordson EFD
Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.
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