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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