A thick IMF does not rule out BP. A P-rich layer will continue to grow
through an IMF after reflow, and causes latent failures. This process is
sped up or exacerbated by consecutive thermal cycles during service.
http://ieeexplore.ieee.org/iel5/9983/32057/01491025.pdf?arnumber=1491025
Also read "The Black Pad Failure Mechanism-From Beginning to End" by
metallurgists at Alpha Metals. They have a good explanation of the
continuing growth of the P layer long after reflow. This can continue
until the BGAs literally fall off of the board.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Wednesday, June 18, 2008 9:33 AM
To: [log in to unmask]
Subject: Re: [LF] [TN] SN100 for Reflow Application
Hi Pete,
We will not be using the DICY cured laminate in a LF process. It wont
withstand LF process temperatures. yes, we did observe a P rich area and
a rough Ni surface. However, a presence of a thick intermetallic layer
rules out black pad. it does reflect a plating bath that Is potentially
unbalanced wrt the chemical composition. However, the SnPb version of
the board is also ENIG and from the same board supplier. Therefore all
things being equal (plating wise), we see cracks only with the LF
assembly.
All the obvious SMT process steps (Solder deposition, solder ht, reflow
temps etc.,) were checked to ensure accuracy.
Regards,
Amol
-----Original Message-----
From: Pete Houwen [mailto:[log in to unmask]]
Sent: Wednesday, June 18, 2008 9:53 AM
To: [log in to unmask]; Kane, Amol (349)
Subject: Re: [TN] SN100 for Reflow Application
Amol,
If you are joints that much more prone to failure, don't rule out your
process just because they aren't failing during the process.
Are these ENiG plated boards? When you did your cross sections, did you
do any analysis of the interfacial? Are you getting P-enriched zones?
Are there voids - large or micro, in the joints or balls? Are the
soldermask openings and registration the same on both versions? Do you
inspect the paste deposition to make sure the stencil is releasing the
lead free solder as it did with tin/lead?
While the fractures aren't showing up until you apply mechanical stress,
the process itself could be weakening the joints, and that doesn't
always show up on all of the balls, or even the same ones consistently.
Though pad cratering indicates that at least at those pads, the joint
was stronger than the laminate.
Be careful using dicy cured epoxies with lead free process temperatures.
A whole 'nother can o' worms.
Pete
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