Wayne,
Awfully close to the board! And I think someone already mentioned the forest of solder joints under the part. And people are going to use a water-based cleaner? What about surface tension? (This is where CFCs shone, but I'm glad they are gone.) I wouldn't be comfortable with an OA solder paste process for this part unless I worked closely with my cleaning company and had an extensive study where you either pried off a ton of parts after cleaning trials or used glass parts with the same size and stand-off as your LGA for your cleaning qualification of this part.
Bev
RIM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Thursday, April 16, 2009 1:44 PM
To: [log in to unmask]
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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