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Ask the Experts

March 27, 2013 - Updated
March 20, 2013 - Originally Posted

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.

Expert Panel Responses

The answer to the pin-to-hole ratio question is usually to makethe hole as small as tolerances will allow. We are almost always fighting toget enough paste volume, so minimizing the volume we need to fill is our firstobjective. In general, take the lead diameter (or diagonal lead dimension forsquare or rectangular leads) and add 0.007 inches (0.18mm) to get the nominalhole size. This will account for a +/- 0.003" tolerance on hole diameter, andprovide a small clearance. The answer to the aperture question truly is, "it depends."Specifically, it depends on the characteristics of the specific component, thecharacteristics 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, wewill need a large volume of paste, and will definitely need to over-printoutside 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 usuallymeans controlling the PWB design to avoid features that would cause the pasteto not pull back properly. When good design practices are followed, it ispossible to print up to 0.050" or more from the edge of the land and still havegood pull-back. I highly recommend using a good spreadsheet-based model forpin-in-paste development which should include the following input parameters atminimum:

Lead cross sectional area and maximum dimension (diameter ordiagonal)
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 thevolume 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 thesurface, and what will partially fill the hole. The hole fill is highlydependent on hole diameter.

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.

Through plated hole to pin ratio, as a lot of other thingsin our industry, depends on design. There are 2 different scenarios here:

The lead of the component is circular - a recommendedhole diameter is 0.015" larger than the pin. Add the lead diametertolerance to this.
The leas of the component isrectangular - a recommended hole diameter is 0.010" larger than thelargest diagonal of the pin. Again, add the lead tolerance to this.

Now for the standard aperture design, you will have totake in consideration the following: the volume of solder paste required forproper soldering condition is twice (x2) the difference between the throughhole volume and the lead volume. The paste deposition (brick) can be achievedby both aperture design and volume (stencil thickness). As a general rule, thestencil thickness should be designed primarily for the finest pitch device onthe board. This thickness ranges between 0.005" to 0.008". If the overallthickness cannot fit your needs, there are few things that can help youcompensate 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 totake in consideration:

paste type
paste viscosity
humidity and temperature
squeegee parameters: speed, pressure, angle

Definitely a large project - Good luck!

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.

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