Bob, I guess what I didn't say clearly enough that the solder under the
component termination was 1um or less. As to the deformation of the
copper we at first assumed it was excessive PnP pressure on the chip
components. But we also have see it occur with copper "L" lead frames
on SOIC's so this doesn't make sense. As for copper going into solution
underneath the lead frame I like the idea but how to prove it. The
scalloped IMC between the copper and solder doesn't appear excessive for
a convection reflow operation. So thanks for the idea I'll have to let
it rattle around inside my congested head for a while.
Dale
dlh
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, January 03, 2011 4:52 PM
To: [log in to unmask]
Subject: Re: [TN] solder joint height values (h) for leadless SMT
components?
I also find this interesting. I am going out on a limb and assuming the
rate of copper dissolution is exacerbated, the "force" is actually
leaching, not pressure, and the root cause may have been excessive
touch-up or rework. Any opinions or observations otherwise?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, January 03, 2011 3:39 PM
To: [log in to unmask]
Subject: Re: [TN] solder joint height values (h) for leadless SMT
components?
Dale,
That is interesting. I have a possible dumb question:
You mention copper at 1um or less the copper become deformed such that
if
follows the component termination contour.
What is the source of these forces and does this imply the copper is
being
torn from the FR4?
Thanks,
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hart, Dale L CTR
USAF
AFMC AFRL/RXSA
Sent: Monday, January 03, 2011 3:41 PM
To: [log in to unmask]
Subject: Re: [TN] solder joint height values (h) for leadless SMT
components?
John,
I've been doing failure analysis for the Air Force for nearly 30 years.
I've cross sectioned countless chip resistors and capacitors during this
time. If I find the standoff height of solder under a component
termination
greater than 50um it is extremely rare. The average height is between
25 to
40um. Approximately 5 percent of these components have 1um or less. Of
these 1um or less the copper pad underneath the termination is deformed
to
the contour of the component termination.
This has also occurred with both iron/nickel and copper "L" shaped gull
wing
leads. The manufacturers that populate and reflow these boards tell me
that
their stencil was, for example, 7 mils and their paste height was within
their acceptable limits. I have asked if they performed line proofing
with a
DPA of representative components at various locations on their panel the
answer is usually no or that is was done so long ago that none of the
present operators remember it being done.
Have I seen failures of solder joints of this pedigree, the answer is
yes
but not as many you might expect. It depends on the operational
environment
(thermal cycling, shock and vibration). Also these type of solder
joints
are very susceptible to cracking if they have been encapsulated.
Dale Hart
UTC Failure Analysis Engineer
dlh
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nieznanski, John A -
GS
Sent: Monday, January 03, 2011 1:24 PM
To: [log in to unmask]
Subject: [TN] solder joint height values (h) for leadless SMT
components?
Happy 2011 TechNetters!
The solder joint height parameter "h" is a very sensitive parameter in
the
E-W solder fatigue equations per IPC-D-279 and IPC-SM-785, particularly
for
leadless components.
For leadless components, it is common to use half the solder stencil
height
for "h", in lieu of measured values. So for example, given a 6 mil thick
solder stencil, "h" is often assumed to be 3 mils with some acceptably
small
variations, to be quantified and confirmed.
However there are many leadless SMT components that have solder joints
that
can not be characterized by a single "h" value. For example, SMT
resistors
often have a solder fillet that is between 50% and 100% the thickness of
the
component along the outside edge of the terminals and much less under
the
terminals. Similary for leadless MELF diodes and castellated LCC
packages.
Ideally it would be great to build test boards with all these
configurations
to get failure data and correlate these results with DPA measurements of
solder joint cross sections. Are you aware of any published papers that
discuss this type of analysis and/or attempt to derive and correlate an
"effective h" for leadless component types and terminals based on
measurements?
In lieu of any published efforts, does it seems plausible or reasonable
to
develop the concept of an "effective h" or "average h" based on several
design and assembly parameters such as solder pad dimensions, wetted
terminal dimensions, solder stencil thickness, component-to-pad
misregistration, etc? Is this just wishful thinking? Alternatively, I
suppose one could make the "weakest link" argument and say that the
"minimum
h" is going to dominate the behavior of the solder joint, not the
"average
h" or "effective h", since the "minimum h" is where the shear stresses
and
strains will concentrate and initiate wearout.
Thanks in advance for any results, comments, thoughts or suggestions
that
you can provide............
Regards,
John Nieznanski
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