Phil Hinton asked: >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 first question in my mind is: were they ever soldered at reflow? Your wording seems to imply that they were, and that somehow wave solder opened the solder connection. My guess would be that the joint was never formed at reflow, or formed very poorly. > 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. And this I would attribute to insufficient heating of the joint during reflow due to the "sinking" effect of the metal connection to the via and VCC/ground internal planes, rather than to heating during wavesolder. > Suggestions have been: > (1) Tent or plug all of the holes that contact the wave except > those that have lead in the hole. No objection to this, but it may cost you a buck or so per board. Assuming you have some control over design of the PWB, I'd say make sure that there is a good thermal relief pad design used for all plane connections to the vias. > (2) Use a nitrogen blanket during wave soldering IMO, useless unless you are using no-clean flux, and I can't see how it would help the problem you describe. You don't want lower surface tension of the topside connections, (assuming that they are getting enough heat to melt), you would probably see less solder pullback with higher liquid solder surface tension. > (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. See above. > (4) After wave soldering, flood the top side of the board with > flux and reflow it. A longshot. If solder has wet the lead and the pad, surface tension will be working to keep the metal apart. Flux might "bridge", and help to form a connection between the pad and lead. >Is there any other solutions, the nitrogen blanket is a long term fix for >the assembly shop but not a present fix. Two places I've worked have installed nitrogen for their reflow furnaces only to shut it off after seeing minimal improvements. If I were you, I'd be pretty certain whether the joint formation was good at reflow, then failing at the subsequent wavesolder, or whether you aren't getting a joint during the first reflow. If you are reasonably sure the SMT joints are "opening" during wavesolder, then I'd say speed up the conveyor on the wavesolder, crank down the topside preheat, of narrow the wave contact; any approach so as to transfer less heat to the board and avoid melting the topside joints during wavesolder. And if you don't have a good thermal relief pad on the via to internal plane connections, this could again cause more heat transmission to your pads than you'd like. cheers, Jerry Cupples Interphase Corporation Dallas, TX USA (how 'bout them Rangers?) http://www.iphase.com