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March 27, 2013

Standards for Pin-in-Paste

What is the recommended plated hole to pin ration for pin-in-paste? What is the standard for pin-in-paste stencil apertures relative to the annular ring?

P.S.

Experts Comments

Through plated hole to pin ratio, as a lot of other things in our industry, depends on design. There are 2 different scenarios here:
  1. The lead of the component is circular - a recommended hole diameter is 0.015" larger than the pin. Add the lead diameter tolerance to this.
  2. The leas of the component is rectangular - a recommended hole diameter is 0.010" larger than the largest diagonal of the pin. Again, add the lead tolerance to this.
Now for the standard aperture design, you will have to take in consideration the following: the volume of solder paste required for proper soldering condition is twice (x2) the difference between the through hole volume and the lead volume. The paste deposition (brick) can be achieved by both aperture design and volume (stencil thickness).

As a general rule, the stencil thickness should be designed primarily for the finest pitch device on the board. This thickness ranges between 0.005" to 0.008". If the overall thickness cannot fit your needs, there are few things that can help you compensate that:
  • use a step stencil for the through hole components area
  • over print on the board's solder mask
  • additional paste deposition
And when all this is done, here are more parameters to take in consideration:
  • paste type
  • paste viscosity
  • humidity and temperature
  • squeegee parameters: speed, pressure, angle
Definitely a large project - Good luck!
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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
The answer to the pin-to-hole ratio question is usually to make the hole as small as tolerances will allow. We are almost always fighting to get enough paste volume, so minimizing the volume we need to fill is our first objective. In general, take the lead diameter (or diagonal lead dimension for square or rectangular leads) and add 0.007 inches (0.18mm) to get the nominal hole size. This will account for a +/- 0.003" tolerance on hole diameter, and provide a small clearance.  

The answer to the aperture question truly is, "it depends." Specifically, it depends on the characteristics of the specific component, the characteristics of the board, and the hole fill requirements that must be met. If we have a flat (rectangular) lead shape and a thick board, for instance, we will need a large volume of paste, and will definitely need to over-print outside the bounds of the annular ring. When printing outside the annular ring, we need to ensure that the paste will all "pull back" properly. This usually means controlling the PWB design to avoid features that would cause the paste to not pull back properly. When good design practices are followed, it is possible to print up to 0.050" or more from the edge of the land and still have good pull-back.  

I highly recommend using a good spreadsheet-based model for pin-in-paste development which should include the following input parameters at minimum:
  • Lead cross sectional area and maximum dimension (diameter or diagonal)
  • Hole diameter (maximum and minimum)
  • PWB thickness
  • Target and minimum post-reflow hole fill  
Given the above parameters, you can use the spreadsheet to estimate the volume of solder required, and then given the metal loading in the paste (usually just over 50% by volume) you can determine the paste volume needed. You will need to account for both the amount of paste that resides on the surface, and what will partially fill the hole. The hole fill is highly dependent on hole diameter.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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