Re: plastic flat-packages with 0.5mm pin spacing (was Cantaloupes)
- From: "Jeff L" <levy_jeff@xxxxxxxxxxx>
- Date: Tue, 16 Aug 2005 00:17:34 GMT
"Winfield Hill" <Winfield_member@xxxxxxxxxxx> wrote in message
news:ddprm60ugj@xxxxxxxxxxxxxxxxxx
> Jeff L wrote...
> >
> > Winfield Hill wrote ...
> >>
> >> What's painful is ICs using these small dimensions, but requiring a
> >> thermal/electrical solder connection to a pad underneath, like Analog
> >> Devices' new DDS chips, which have a 2mm square pad. We're using the
> >> ad9953, just finishing a PCB, and I'm not sure how to deal with it.
> >> See page 30, http://www.analog.com/en/prod/0%2C2877%2CAD9953%2C00.html
> >> and http://www.analog.com/UploadedFiles/Packages/231465055SV_48_1.pdf
> >
> > The proper way to deal with them is with paste and hot air. Then they
> > are not so bad.
>
> In a full automated production environment. We have a hot-air system
> with preheat under, that we use for prototypes. But I don't know how
> much paste to put down under the IC, etc.
Put a little dab of paste on the center pad (not too much or you will have
to remove some, since the IC will not sit low enough for the pins to touch -
push down on the IC, during reflow and a big solder ball should pop out if
that happens), place the part, reflow it using hot air, and make sure it's
centered on the pads using tweezers. When cool, solder the pins by hand.
Thermal stresses should be less with this type of part, when hand soldering,
since you now have the big pad sucking heat away from the die. The legs
basically have a stretched Z shaped bend to absorb different thermal
expansions. If it still bothers you, you can reflow it again after soldering
the pins to relive any stresses.
> I assume all the solder
> has to be melted at once to avoid stress after the part cools down.
>
> > For prototype quantities, some people put a giant via under the pad,
> > so that they can solder it from the back side with a iron.
>
> This is after soldering the 0.5mm pins, right?
Exactly
> When soldering to
> the middle pad through the via, how can one avoid permanent thermal
> stress after the part cools?
See above. You loose a little heat sinking ability if the center pad is not
soldered first, but the large chunk of copper can keep it significantly
cooler then a similar TSSOP without the copper pad, whose pins would be
soldered the same way.
>
> > If you want to have some fun, try some QFN's, or DFN's (same idea as
> > the IC your using, only no leads out the side, they are flush to the
> > package, stuck around the big pad in the middle, on the bottom!).
> > They are, however, not too bad _once_ you get used to them. Or how
> > about CSP's - some are 0.8 mm square with 8 or 9 solder bumps on
> > the bottom - i.e. an 800 um BGA!
>
> We haven't had too much trouble with these, hold in place, hot air
> directly on top, until they suddenly settle down onto the pcb.
>
>
> --
> Thanks,
> - Win
.
- References:
- Cantaloupes
- From: John Larkin
- Re: Cantaloupes
- From: John Fields
- Re: Cantaloupes
- From: John Larkin
- plastic flat-packages with 0.5mm pin spacing (was Cantaloupes)
- From: Winfield Hill
- Re: plastic flat-packages with 0.5mm pin spacing (was Cantaloupes)
- From: Jeff L
- Re: plastic flat-packages with 0.5mm pin spacing (was Cantaloupes)
- From: Winfield Hill
- Cantaloupes
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