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February 7, 2013

Pin-in-Paste Problems

We are doing pin-in-paste with a Mill-Max pin and are having approx 10% problem with solder getting into the pin and soldering the prongs together at the very end of the prongs. This is a serious issue since it's almost impossible to inspect and find these issues and then when the board is mated to another board the insertion pins get bent. Any suggestions?


Experts Comments

It's difficult to give specific recommendations, since we don't know the exact geometry and materials of construction of the pin. In general, what I think is happening is that some paste is being forced inside the bottom of the pin during insertion. During reflow, the top of the pin will heat up faster than the board. The heat is conducted down the pin, and begins melting the paste. The paste wets the prongs of the pin, and tends to go in the direction of the highest temperature (up the prongs). The gap between the prongs of the pin enhances this flow because of capillary forces.

If my assessment of root cause is correct, then the best solution is to eliminate the paste intrusion during insertion. If you are over-printing and partially filling the hole, you may want to consider doing a surface-only print, or at least limit the amount of paste that is allowed into the hole. This will make achieving the required volume a challenge, and may in fact be impossible, depending on the geometry. Another avenue is to slow down the ramp rate in the reflow profile, near the liquidus temperature of the solder being used. This alone will probably not solve the problem, however.
Fritz Byle
Process Engineer
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.

With a paste solution, you can modify the stencil apertures to prevent the pin from dragging paste and reduce the paste volume.

Another option is to use a flux-coated washer preform that can be used to solder the connector. The preform is 100% solder and is very controllable and repeatable when compared to paste. The flux-coating on the preform eliminates the need to ad flux manually. A preform (1 for each pin) when placed on top and can be designed to deliver the right volume so that the prongs don't see solder.

The preform can also be placed on the bottom of the connector and a hot air gun is used to solder all the washers in 1 shot, so that solder flows away from the prongs. Even though, this does not lend itself to pin-in-paste, the preform solution would be highly repeatable.

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.
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