Component mix is another issue, Freescale, for example, in their LGA
application notes has a target solder volume of 7000 mil^3 and a min of
4800 mil^3. Trying to get to the min with a typical 6 mil stencil would
mean overprinting the pad somewhat @ their recommended pad diameter. A
thicker stencil may not be an option depending on the pitch/size of
other parts on the assembly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, March 19, 2008 9:23 AM
To: [log in to unmask]
Subject: Re: [TN] LGA Reliability
Actually, I am in agreement with Werner on LGAs. It is another one of
those "it depends" issues.
Larger LGAs with arrayed terminations on the bottom of the package are
susceptible to process constraints such as the amount of solder
deposited on the pads. If you can print enough solder volume to
essentially replace the missing solder balls, then you improve the
reliability. Even if you use a standard 6 mil thick stencil you end up
with very flat solder joints that have very little compliancy to take up
the difference in the CTE between the LGA package and the PWB. If the
pads on the PWB are .030" diameter or larger, I have noticed that the
number of extraneous solder balls under the LGA begins to increase when
you are trying to get enough solder under the part.
On the smaller types of LGAs with only bottom terminations on the
periphery, the problems I have been seeing is trying to keep all of the
solder bricks uniform enough so that when they go into reflow all of the
terminations are wetted. Sometimes one or two of the pads have a little
bit less solder, and no physical contact is made between these shorter
bricks and the LGA pads, because all of the rest of the bricks are
holding the part up.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Wednesday, March 19, 2008 8:20 AM
To: [log in to unmask]
Subject: Re: [TN] LGA Reliability
Hi Phil,
The devil is in the details-however, 2-to-2.5mils standoffs =
2-to-2.5mils solder joint height are of concern even with small
delta-CTEs [I am not even going near the cleaning issues].
With large thermal slugs, these packages may also be thermally
non-symmetric, which means that they warp out-of-plane both during
soldering operations and operation with any kind of power dissipation.
Ask them if their "reliability claims" are based on IPC-9701 testing.
Werner
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