I would like to add my two cents here. First, I have had opportunities to
work with the concept of reflowing boards with components on the bottom
side of an assembly. I have tried this with a variety of components
ranging from small chip resistors, 100 leaded 25 mil gull wing packages and
BGAs. In all cases, there was no adverse effects of the second reflow. I
do agree that you want to keep this reflow to a minimum in order to keep
the formation of intermetallics to a minimum, but one additional pass
shouldn't have too drastic effect, provided we aren't talking about
electronics in a harsh environment.
John Guy
ACI / EMPF
714 N. Senate Ave.
Indianapolis, IN 46202-3112
voice: (317) 655-EMPF ext.130
fax: (317) 655-3699
e-mail: [log in to unmask]
-----Original Message-----
From: Eddie Brunker [SMTP:[log in to unmask]]
Sent: Wednesday, November 26, 1997 9:35 AM
To: [log in to unmask]
Subject: [TN] Double Sided Assembly
Dave you wrote|-
Hi Lee - two items to think about as you venture into double sided
assembly: your reflow oven capabilities and the robustness of your pwb
surface finish. Reflow ovens that have both top and bottom side heating
provide some recipe flexibility to keep your "first pass and now bottom
side" of the pwa from reaching reflow temperature. Surface tension will
keep many components attached to the pwa so avoiding bottomside reflow
isn't a requirement but it does keep the solder joint intermetallic
thickness minimized. Secondly your "second pass and now top side" of the
pwa has seen one thermal exposure - the surface finish (OSP, tin/lead,
gold/nickel, etc.) needs to maintain its solderability so that successful
soldering can be completed during the second pass. Good Luck.
Dave Hillman
Rockwell Collins
This simply isn't the way I see things.
Peak reflow temperatures are usually around 212 - 222 centigrade.
Reflow temperature of solder 179 - 183 degrees.
That's 40 degrees difference.
Copper vias the thickness of the board 50 - 60 thou.
How do we create 40 degrees differencial across this distance?
Even if we do, vias with direct connections,( such as vias in pads ) to the
top side pads will be cold, while other pads will have no problem achieving
reflow.
Assemblies need to be heated up completely, especially multi-layer with
lots
of copper planes.
The issues of tombstoning are currently being discussed. Thermal
differences
for whatever reasons, (maybee inadequate heating/convection in IR ovens(
are
likely to be the cause.
Surface tension usually does hold components on underneath, but if it
doesn't then some means of support is needed while it reaches reflow
temperatures. We have used tooling jigs during reflow on our complex
assemblies, while adhesive is simpler solution.
Sorry to argue Dave but I do feel passionate about this point.
Thanks
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