Hi Steve
Sounds like a fun one.
I've got a similar design, and the designer would/could not lay it out so I could use selective soldering pallets in the wave for the full bottom side. What I ended up doing is a partial selective soldering, where my process is:
- screen paste (eutectic) on most of the SMT (90% of it)
- dot dispense glue where needed (the stencil does not screen paste in these locations)
- install parts and reflow / cure the glue.
Wave soldering is done with a pallet that exposes the areas of the board with through-hole parts, plus whatever SMT parts are too close to through-hole parts.
Originally the process was to glue all the parts on. With this process the cycle time is a couple of minutes less per board, the yield is an order of magnitude better, and the test guys have stopped glaring at me. Life is good!
If that isn't possible, I'd go with screening the glue as others have brought up already. Better cycle time, accuracy, etc.
Good luck!
regards
Graham Collins
Process Engineer,
Northrop Grumman
Atlantic Facility of Litton Systems Canada
(902) 873-2000 ext 6215
>>> [log in to unmask] 09/06/02 03:44PM >>>
Is anyone successfully using a high temperature paste to do bottom side SMT reflow, thereby avoiding the glue/cure/wave process? The reason I ask is that we may be asked to build a board with roughly 1800 bottom side SMT (none topside, mostly 0603s, 0805's, maybe 150 SOIC's from 8-16) with a few hundred top side pth, axial and radial.
It's a long story as to why I would ask this, and it's not something I wish to get into here (suffice it to say that the people that needed to read Earl's book (see, I knew I could fit it in somewhere) didn't, but the long and short of it is I cringe at both the throughput issues with the dispenser (15k/hour) and the yield problems associated with gluing and waving rather than reflowing. We do NOT have AOI, for what that's worth.
Have a nice weekend, all.....
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