Coming from you, I'll take that as a true compliment. I'll have to admit
that all my responses are instantaneous and after further reflection, I
was pleased with the simplistic perfection of that response, but after
all as with my name I am a giver and taker of wit.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Fjelstad
Sent: Monday, June 15, 2009 7:31 AM
To: [log in to unmask]
Subject: Re: [TN] Corner ball pad cratering
Brevity is the soul of wit. LOL
Joe
In a message dated 6/15/2009 6:57:55 A.M. Pacific Daylight Time,
[log in to unmask] writes:
Well, its semi related.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, June 15, 2009 6:15 AM
To: [log in to unmask]
Subject: Re: [TN] Corner ball pad cratering
Morning Genny and everyone!
Hope you all had a good weekend! It was HOT here!
Genny, I have the last picture you sent on Friday posted, it's here:
http://stevezeva.homestead.com/files/darker_100x.jpg
I saw something here Saturday afternoon that I think might cause
cratering, I don't know though. So I took a picture of it. What do you
think?
http://stevezeva.homestead.com/files/Pad_Craterer.jpg
Steve
-----Original Message-----
From: Genny Gibbard [mailto:[log in to unmask]]
Sent: Friday, June 12, 2009 4:53 PM
To: TechNet E-Mail Forum
Cc: Steve Gregory
Subject: RE: [TN] Corner ball pad cratering
OK, I may be premature in discounting that that there is possible
improvement to be made in the reflow profile. I will add it to the
list
to check.
The board is still in the proto-type/NPI phase, it is encountering far
more handling than it will once design settles down. Lots of
installation and removal from the chassis, etc, for modifications. The
board is large with high layer count, but not overly thick, the
affected
IC is large, and there are mechanically mounted shields on areas
adjacent to the IC. This particular board passed all tests with flying
colours, including tests over temperature, until it came to the final
system verification check, and then it started failing.
The cross-section only revealed cratering on the corner balls,
unfortunately some of the corner balls are more critical than strictly
redundant pwr/gnd type connections.
So it may have possibly started in reflow, but didn't propagate to a
failure point until the end of the process. Or it may have been
completely induced by mechanical means, due to all the extra handling
and flexing of the board. We do not currently have a cross-section
from
early in the process to compare.
The success already mentioned with corner bonding (by Amol and in the
article link below), encourages me to look in that direction for
improvement, along with reviewing reflow profiles.
One last pic - less bright. I believe this shows a good joint
(immersion Ag, leadfree ball). Please correct me if I'm wrong. Steve,
if I could impose one more time...for Monday morning.
Thanks for all the discussion.
Genny
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
/*
Sent: June 12, 2009 1:07 PM
To: [log in to unmask]
Subject: Re: [TN] Corner ball pad cratering
Hi Genny,
When you say: "We believe the cause is mechanical, not thermal"-what do
you mean by that? Where would these mechanical stresses come from?
Werner
-----Original Message-----
From: Genny
To: [log in to unmask]
Sent: Fri, 12 Jun 2009 12:12 pm
Subject: Re: [TN] Corner ball pad cratering
As always, Steve, you are a wealth of information. That article is
perfect, thank you. This seems to mirror what Amol said about doing a
1/4 of the length. This article said to do an L shaped bead 2-6 balls
along each corner side.
Here are some photos to post. Would you oblige?
BGA is 40mm per side. PCB is ~1 ft sq. A little bigger than what is
found in the typical blackberry, I think... Ball pitch is 1.27mm.
We believe the cause is mechanical, not thermal.
Thanks everyone.
Genny
-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: June 12, 2009 6:35 AM
To: TechNet E-Mail Forum; Genny Gibbard
Subject: RE: [TN] Corner ball pad cratering
Morning Genny,
Since you said you were going to look at corner bonding, here is a
really good article (if you haven't seen it already) that Michael
Kochanowski from Intel and Brian Toleno from Henkel did about it:
http://www.trafalgar2.com/documents/Technical_Articles/6.09-kochanowski.
pdf
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Thursday, June 11, 2009 3:10 PM
To: [log in to unmask]
Subject: [TN] C
orner ball pad cratering
Hello all,
If you have identified a BGA with corner ball pad cratering, what would
be your suggestions to correct this condition?
Our thoughts:
1. We plan to evaluate our handling processes to reduce/remove
excessive flex/strain occurring around this part 2. We plan to
evaluate
corner bonding or underfilling for the BGA.
Which do you feel would be the best direction to go? Do you have
other
suggestions?
Thanks in advance for all your help
Genny
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