Jim,
We also are processing mixed media (SMT + through-hole). But, for just
the reason you raised concern over (unduly stressing the SMT
components),
typically we do not paste-and-reflow. Instead, we stencil a suitable
patch of thermal fast-cure adhesive (in this case, Loctite 3614), and
then IR-cure at a relatively low temp, for a short period of time.
Our experience has been very good: with controlled epoxy patches, we
don't experience epoxy bleed-out. And, since we cure immediately
following pick-and-place, component positioning is fixed for the rest of
the processing.
Just as you do, we then insert the through-hole parts. Finally, we
flow-solder everything in one pass (we do have a reciprocating wave,
which helps a great deal). Works great, and we eliminate one stressor
(the extra temp-ramp).
We feel that the key to the success of this process is, in fact, the
adhesive. Originally, we used a "general-purpose", first-generation
UV/thermal paste. But, it was taking too long to achieve full cure;
and,
the adhesive was prone to drying prior to cure, and could over-cure, and
had low uncured tack. So, we changed to a premixed two-part epoxy, got
really long air time in the bargain, and moderately-high uncured tack --
really important to stabilize the SMT parts during handing up to/through
cure (with this stuff, the parts won't fall off even if the
boards/panels are inverted).
To maintain the kind of consistency in the wave side of the process, we
pay scrupulous attention to the specific gravity of the flux, and we
adjust it as needed. That and attention to profile-based speed and
temperature have given us a highly stable, predictable process, with
very low rework/retouch rates.
$64 question is whether the reduced stress results in reduced field
failures. With only a limited database to work with, the answer thus
far seems to be, "yes." Frankly, I need a larger sample to draw from;
but, our return rate has fallen-off, and while this could be for any one
(or several) other factors, the chief process change has been the
adhesive.
Regards, Sonny Hays
Jim Mathis wrote:
>
> In an effort to decrease defects on a mixed technology assembly with simple
> SMT's on the "solder side" and thru-holes on the "comp side", we have adopted
> the following process: We are first gluing, solder-pasting and then
> reflowing the SMT's. We then flip the board, stuff the thru-holes, and
> wave-solder. This has enabled us to eliminate the "opens" with the reflowed
> solder paste on the SMT's and dramatically reduce the "shorts". However,
> this process means the SMT's are reflowed and then waved.
>
> The big question: Are we introducing potential reliability problems? Has
> anyone out there also done this with no adverse long-term reliability
> problems?
>
> All feedback is welcome, especially good news! Thanks, all.
>
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