George,
To respond to your aside, my 2 cents worth is that the "mechanical" info on
component data sheets is very poor. Most of the content focusses on the
electrical functionality of the device, but there's precious little help
with mounting the b-----s. A few data sheets have land pattern
recommendations if you're lucky (one diode spec I came across even had
separate land pattern recommendations for both CR and Wave solder -
exceptional! or may be there isn't enough to say about a diode otherwise to
fill a decent data sheet). BUT, there is little or nothing about
"environment/habitat" requirements for where the component is going to
live. How is it to be established in its new home, what sort of neighbours
should it have/not have, how much space it needs from said neighbours, etc.
Particularly tiresome to find at times is lead finish information - it's
available, often on a separate spec for the package, but there's no cross
reference to where this info can be found, and you can be left guessing if
it's not a "standard" package.
Some more assistance to the poor board stuffer would be welcome instead of
giving everything to our ethereal design engineers, who move on a higher
plane. Many board designs, especially aerospace applications, use a great
many components on a board, and to get all the needed information on each
to integrate them all into a working design that can be manufactured is a
herculean task. It could, I'm sure, be made a lot easier with the
co-operation of the component manufacturers, though I guess there's a fair
amount of butt protection involved - the more they say, the more they can
be held liable for. Or am I getting too cynical now?
I suggested a while ago on this forum that perhaps IPC could produce a
guideline about component specs - their format, content, minimum info
requirements for all those involved with using the component. It met with
deafening silence - probably because there is an almost infinite number of
design permutations and it would be impossible to cover all the bases with
hard facts, but better information on how to determine our own solutions
could be included, and not have the specs turn into Encyclopedia
Britannica.
End winge.
Peter
"Franck, George"
<george_h_franck@RAY To: [log in to unmask]
THEON.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask]> Subject: Re: [TN] Use of IPC-7095, -SM-782,
and -7525
01/09/02 10:12 PM
Please respond to
"TechNet E-Mail
Forum."
I have been playing around, blindly, with this topic for a little while.
This is what I have learned, precious little that it is.
The answer depends on the BGA construction. Many/most BGAs do not actually
have balls on them, they have 'bumps' of 63/37 solder. These bumps will
completely melt during reflow, allowing the BGA to float on molten solder
during the soldering process. For these components, you *generally* want
the pad on the board to be the same size as the pad on the component. With
the pads being the same size, the resulting solder joint will be
symmetrical, top to bottom. Since the writers of the Specs, (bless them
all), can only guess at what pad size the manufacturers of future BGAs will
use, it becomes difficult to always have the right answer.
Other BGAs have real 90% Pb balls on them. These do not melt, and the
soldering process for these components create two solder joints. One from
the Component to the 90% Pb ball, and a second from the ball to the board.
In this case, the board pad size will depend on the ball size.
There are other BGA configurations, such as solder columns, which, again,
you have to look at individually.
As an aside question (or complaint), Have people found that the information
the component manufacturers put in their component spec sheets (pad size
and
spacing, and what has to be done to route the pin out....) Have people
found that these follow any kind of DFM process? In my experience, when my
Engineers choose the smallest, most dense component packages, we often end
up with a very interesting Fab and Assembly designs.
George Franck
-----Original Message-----
From: Lou Hart [mailto:[log in to unmask]]
Sent: Tuesday, January 08, 2002 5:10 PM
To: [log in to unmask]
Subject: [TN] Use of IPC-7095, -SM-782, and -7525
TechNetters, I have a question regarding consistency of 7095 (Design and
Assembly Process Implementation for BGAs), SM-782 (the Surface Mount pad
document), and 7525 (Stencil Design Guidelines), resulting from discussion
with process engineers and techs earlier today.
We were looking at a BGA, 20x20 package, with 1.27 mm pitch. The stencil
design guidelines (7525, Table 1 on p. 4), call for an aperture of 0.75 mm.
That table also refers to a 0.80 mm pad. SM-782 (section 14) says a 20x20
BGA will have 0.75 mm balls that should go onto 0.60 mm pads. Do these
documents contradict each other regarding pad size, and is 7525 wrong with
regard to BGA stencils?
Further, the bare board does have 0.60 mm pads for the BGA. 7095, in
6.2.1, p. 33, says "land diameter is usually smaller than the ball
diameter..land size reduction of 20 to 25% has been determined to provide
reliable attachment...".
As always, thanks for any comments. Lou Hart
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