Years ago in a former life we made LGA Dc-Dc converters and I performed
extensive power temp cycling. Temperature cycling was 0 - 100C every 15
minutes and power cycling 1 minute on, 1 minute off from 0 power to
approximately 1 watt in a package a bit bigger than 1/2" square. Solder used
was Sn63 and the test board used 4 oz copper on outer layers and 2 oz copper
on inner layers. After a few thousand thermal cycles (>4000) we had more
problems with vias and PTH than solder joint failures. I cannot comment on
cleaning due to the fact we were using no clean flux. The solder joints
exhibited minimal cracks in DPA (no acoustic microscope available) and we
only had an X-ray radiography system.
Perhaps Werner could comment on power temp cycling.
Regards,
John Maxwell
----- Original Message -----
From: "Thayer, Wayne" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, April 16, 2009 10:44 AM
Subject: Re: [TN] LGAs in a military environment?
Linear Tech engineers aren't total dummies. My bet is that this part is
already on an epoxy-glass board with high copper loading (gnd planes).
Doesn't excuse them from not putting a link to reliability studies for that
design on the datasheet.
My major concern would be the cleaning problem. Worst case, I would stencil
"fat" amounts of solder paste on the bare part and reflow to get domed
deposits. Then print "fat" paste on the board and set the part in the wet
paste. Might get at least 3 mils of joint height that way, so cleaning
might be OK.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Thursday, April 16, 2009 11:25 AM
To: [log in to unmask]
Subject: Re: [TN] LGAs in a military environment?
What am I going to do with this one then? I have to assemble 3 of them on a
fairly large 14 layers RoHS board...
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1778,C1764,P60551,D26185
Please also note the "wise" layout suggested on page 19. Does anybody think
about us assemblers?
Regards,
Ioan Tempea, ing.
t : 450-967-7100 ext : 244
[log in to unmask]
www.digico.cc
P N'imprimer que si nécessaire - Print only if you must -----Message
d'origine----- De : Stadem, Richard D. [mailto:[log in to unmask]]
Envoyé : 16 avril 2009 11:11
À : [log in to unmask]
Objet : Re: [TN] LGAs in a military environment?
Bev's assessment is right on.
The particular component that Graham's designer wishes to use is a real
"problem child" from a long term reliability standpoint, not only because of
the fact that it is an LGA with very large delta CTE between component and
substrate, but also from the standpoint of having a lot of connections
underneath, which reduces cleanability even more.
And no-clean fluxes typically don't completely activate underneath a package
of this type, even if it were able to be used.
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