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November 2000

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 16 Nov 2000 09:54:20 EST
Content-Type:
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Hi Bob,
You sure got a problem. Unfortunately, you are not telling us what solder
alloy the solder balls are supposed to be, let alone what they actually are.
>I have a PBGA357 that is exhibiting a strange (to me) soldering condition.
>Two to three balls (not necessarily adjacent, but usually along an edge) do
>not appear to be wetting to the solder paste on the land.
A: You have some solder balls that did not wet, others that did. This means
the minimum reflow temperature was not reached for the alloy of the
non-wetted solder balls. The reason for this could be: (1) these solder balls
are of a different solder alloy with a higher Liquidus temperature than the
others; (2) these locations have a higher heat transfer rateto the cool
interior of the BGA.

>This is what it looks like after reflow: The paste on the land has reflowed
to a
>mound, the ball on the BGA is compressed in shape but not wetted to the
mound
>on the land (so the ball must have reached reflow temperature to get
compressed.)
A: The PCB is always hotter than any of the larger components, so the paste
reflow is no surprise and also no indicator. The non-wetted solder balls
clealt did NOT reach minimum reflow temperature, they clearly reached and
exceeded Solidus for the alloy and may have even exceeded the Liquidus for
the alloy (only the eutectic alloy has the Liquidus equal to the Solidus),
but clearly did not reach minimum solder reflow temperature which is about 20
C above Liquidus. The confusion comes from some sloppy usage of terminology
that has taken place in our industry. At temperature between Solidus and
Liquidus, the solder is pasty which accounts for the ball deformation.

>A clear line of demarcation between the compressed ball and the mound of
>solder on the land is visible.
A: A clear indication that the minimum solder reflow temperature was not
reached.

>Profile run several months ago with thermocouples imbedded in one center
>ball and one edge ball showed max. temp. of 217 (center) and 220 (edge)
>degrees C with time above 183 of 56 seconds. Max slope is 2 (center) and2.8
>(edge) degrees C.
A: For near-eutectic Sn/Pb 217C for about 5 seconds should be adequate.
Unfortunately, "time above 183 of 56 seconds" is not a good indication that
an adequate amount of heat (temperature & time) for wetting reached the
solder ball. The 183C temperature has only real meaning for eutectic Sn/Pb;
the time above 183 is not a good indicator--time above Liquidus + 20C would
be much more informative.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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