Phil,

An untented or unplugged via hole next to a surface mount land, with mixed
technology, will usually lead to solder migration or solder drain. The wave
solder operation heats the solder on the surface mount land (through the via
connection) and the solder is drawn down the via hole. The quickest fix is
to tent the via holes. However, you must use a high conformance type solder
mask, so that you do not have a mask thickness greater than 0.002 inch. The
LPI masks will not tent vias, so many shops use a plugging operation. This
leads to added expense. Another fix would be to provide a small soldermask
dam between the via hole and the surface mount land. This will prevent the
solder from migrating into the hole, during the wave solder operation.

Last, but not least, is to increase the thermal barrier between the via and
the surface mount land. This is often very difficult in a  design, that is
complete, or extremely dense.

At 04:46 PM 5/15/96 -0400, [log in to unmask] wrote:
>To Technet 
>
>Soldering of mixed technology boards:
>   I have a problem in which the SMDs show random opens at the solder joint
>after the wave soldering of the components that go into the holes.
>
>   The SMD solder connections that are most likely to have opens are those
>which have a short heat path thru the via holes to a land that contacts the
>solder wave.
>
>   Suggestions have been:
>      (1)  Tent or plug all of the holes that contact the wave except
>             those that have lead in the hole.
>
>       (2)  Use a nitrogen blanket during wave soldering
>
>       (3) Flood the top side of the board with flux during wave
>                        soldering so that any connection that is reflowed
>does              not oxidize and will form a new solder joint.
>
>       (4) After wave soldering, flood the top side of the board with
>                    flux and reflow it.
>
>Is there any other solutions,  the nitrogen blanket is a long term fix for
>the assembly shop but not a present fix.
>
>Thanking you in advance for any solutions 
>Phil Hinton
>[log in to unmask]    
>
>
Regards,

Gary Ferrari
Chairman IPC-D-275
Tech Circuits
(203)269-3311
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