Hello Chan,

Yesterday I was reading a book written by John H. Lau and one chapter was
about Reliability of BGA Solder Joints.  It is fairly comprehensive.

There was one case history which indicated that the root cause of some BGA
cracked solder joints was due to brittle intermetallic at the chip to ball
interface and that the failure mechanism was the reflow temp maximum
temperature. I was relieved to see that the problems occurred when the temp
was 285°C and higher.

Our process maximum is 220°C (passive components are allowed 230°C max) so
your peak temp of 215°C is fine.  Don't forget that these parts are
susceptible to popcorning so make sure your ramps up and down are less than
3°C/sec.

I have more concern over the short dwell time during the liquid phase
relative to the internal voids in the finished connections.  You might want
to increase this time rather than shorten it up depending on your results
after reliability testing.



To the Technetters out there,

Has anyone out there overcome the problem of creating voids within the bga
balls yet?

If so, what mechanism generates the voids?  Is it the flux, is it the
topography of the Sn 63 balls before assembly/reflow, is it dwell time
while molten?

Xray shows that there are not internal voids in the balls before reflow,
yet after assembly the voids are definitely there.

By the way, we are not trying to get rid of the voids, just trying to
understand the mechanism better.





TThanks,

Phil Bavaro
At 8:49 AM -0500 1/28/98, Chan, Marcelo wrote:
>Based on the responses received, I may have been too vague.... My solder
>paste vendor recommends approximately 30 seconds from the time the
>solder becomes liquidus, 183 deg. C, to the peak temperature which in
>most cases is from 210 to 235 deg. C.... In my case our peak temperature
>is 215 deg. C.... My question is in regards to this 30 seconds...If I
>know that I have wetting within 10 seconds or so, what are the
>consequences to decreasing the reflow time from 30 to say 20 or even
>10.... If intermettalics formation is an issue, please highlight how
>thick should it be for BGA assemblies and methods to measure it....The
>surface finish on this PWB is Im. Au...
>
>Thanks
>Marcelo
>


Phillip A. Bavaro
Senior Manufacturing Engineer
[log in to unmask]
(619) 658-2542

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