It's difficult to give specific recommendations, since we don'tknow 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 bottomof the pin during insertion. During reflow, the top of the pin will heat upfaster than the board. The heat is conducted down the pin, and begins meltingthe paste. The paste wets the prongs of the pin, and tends to go in thedirection of the highest temperature (up the prongs). The gap between theprongs of the pin enhances this flow because of capillary forces.
If my assessment of root cause is correct, then the best solution is toeliminate the paste intrusion during insertion. If you are over-printing andpartially filling the hole, you may want to consider doing a surface-onlyprint, 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 beimpossible, depending on the geometry. Another avenue is to slow down the ramprate in the reflow profile, near the liquidus temperature of the solder beingused. This alone will probably not solve the problem, however.
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 thestencil apertures to prevent the pin from dragging paste and reduce the pastevolume.
Another option is to use a flux-coated washer preformthat can be used to solder the connector. The preform is 100% solder and isvery controllable and repeatable when compared to paste. The flux-coating onthe 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 theprongs don't see solder.
The preform can also be placed on the bottom of theconnector and a hot air gun is used to solder all the washers in 1 shot, sothat solder flows away from the prongs. Even though, this does not lend itselfto pin-in-paste, the preform solution would be highly repeatable.
Technical Manager - Europe
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.