Wayne,
The thing about the copper layout suggested by Linear is that on the same component the upper right corner has copper defined pads, whereas all the pads in the Cu planes are solder mask defined. So on one hand you have neat 25 mils square pads and on the other you have whatever size the solder mask gives you and positioned wherever the tolerances for applying the solder mask send you.
Hence non-uniform joint volume across the component, eventually component non-planar, so an even lower stand-off on the side of the Cu planes.
Not to mention the unbalanced thermal needs of the two categories of pads. Richard already mentioned the no-clean might not fully activate under the part, now we're adding tons of Cu, just to make it more difficult...
I respect component manufacturers for their work, they really are a driving force in our field and they face tougher challenges than most assemblers. But at times short term profit seems to get the edge on the rest of the industry.
Thanks,
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 : Thayer, Wayne [mailto:[log in to unmask]]
Envoyé : 16 avril 2009 13:44
À : [log in to unmask]
Objet : 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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