31 Jul
2000
31 Jul
'00
4:07 p.m.
I don't know. They were clearly harder to handle
than DIPs,
That is exactly it. I don't think flatpacks ever could be pick-n-placed
effectively. Soldering may have also been a problem, with the long leads.
Every picture I have seen of a flatpack line in a factory involves hand
placed parts soldered with little irons.
SOICs are clearly the winner in the space department as well. The
flatpack thinking had each lead only slightly "gull winged", and rather
long. With SOICs, the lead in preformed and small - great for automation
and space savings.
In such cases the common
approach was to straddle thermal frames with DIPs and use the thermal
frame to conduct the heat to the sides of the enclosure and then design
the enclosure itself to get rid of the heat. The ME types always told
me that this was easier to do with DIPs than with flat-packs, but I
was never clear why a frame bolted on top of a flat-pack was inferior
to a DIP straddling the frame.
The thermal resistance of ceramic DIPs is less than flatpacks. The result
is better heat flow to the frames.
There was also an assertion that
for this reason flat-packs were generally used in short-life
applications, such as seeker or guidance electronics, but I have
no evidence to suggest that this was anything more than legend.
Maybe, I had not heard that one. Possible reasons may be weight, size, or
the fragility of flatpacks.
One thing that I have seen is that really very little military
electronics does use flatpacks - most stuff is made of lots of cerDIPs
(heavy boards!).
William Donzelli
aw288(a)osfn.org