Barry,
You don't say how many solder joints each THT component has. Are they
axials or pin grid array connectors (2 or 200 leads per)? They are mounted
on the top, correct? ( I read your message as saying the top is SMT only,
yet there are THT leads sticking out the bottom side?) My decision would
also depend on the board quantities. But even assuming the THT components
are only axials, as long as I had a wave machine with suitable capabilities
(such as a chip wave, laminar wave combo) and an accurate epoxy dispenser
or printer, AND the volumes were decent and would continue for awhile, I
would use glue/cure/stuff THT/wave process for the bottom side SMT (after
reflowing the top side SMT, of course). A very viable option may be to do
a reflow top/reflow bottom and hand add the THT, depending on the above
variables.
I would also consider the glue/cure/stuff THT/wave process ONLY if the
bottom side SMT land geometries were well designed for such a process. The
IPC-SM-782 guidelines have quite a tolerance to them since they aren't
process specific. Pay special attention to the tant cap, SOT-23 (are they
high or low profile?), and SOIC lands.
This, IMO, is a borderline scenario, and I'd probably do a time study on
each process to determine the best course. Your answer should be
predicated on the defect rates YOU typically see with all three processes.
Some have a lot of trouble with the glue/cure/stuff THT/wave process on
0805's, and if so that would slant the decision.
Larry Morse
Celwave R.F.
[log in to unmask]
----------
From: internet
To: Larry Morse
Subject: [log in to unmask]
Date: Wednesday, February 05, 1997 3:21PM
Hi Technetters,
Given design: 2 sided mixed SMT/Thru-hole board. Top is all SMT. Bottom
(wave
side) has approx:
12 SOIC-14
5 SOIC-8
20 SOT-23
10 each of Tantalum A, B, C case sizes
Predominantly 0805 R's and C's
A few 1206 R's and C's
(of course) Leads from 10 thru-hole components to be wave soldered on the
bottom
SMT pad geometry is standard IPC. SMT and Wave process is "No Clean"
Would you:
A. Glue bottom SMTs (no paste) to be wave soldered later, or,
B. Redundantly apply paste (still with glue) and reflow the bottom SMTs,
before wave solder?
Comments explaining your choice, or any technical reference on this matter,
would be much appreciated.
Thanks and good day to all.
Barry Khor
Sr. Staff Specialist, Quality Eng.
Hughes Network Systems
San Diego, CA
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