Wayne,
of course, you still need to divide by h [solder joint height] to get non-dimensional.
The product does not give you any forces, just a displacement. How the combination of thin solder [with low modulus, but with ductility] and thin IMCs [with high modulus and low ductility] actually works is anybody's guess at this point in time. You can analyze it as purely a solder joint in creep-fatigue, as a brittle fully elastic high-strength loading of the IMCs, some combination thereof. But verification of whichever approach taken is critical.
Werner
-----Original Message-----
From: Thayer, Wayne - IIW <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, May 21, 2010 8:12 am
Subject: Re: [TN] Solder joint height
But if DCTE x DT is in units of delta L/L, and I multiply that by another L,
then final units of your equation are in L.
Something fails when stress (units of FORCE/(L x L)) exceeds a critical value.
To get to this unit you usually use elastic modulus, which is units of (FORCE/(L
x L))/(L/L). In a composite structure such as what we are talking about (part,
board, solder), the overall elastic modulus is usually driven by the lowest
elastic modulus in the system.
For a BGA, it is obvious that elastic modulus is unimportant, because the net
resisting force of the balls (due to both relatively low elastic modulus and
relatively large height) is so much less than the forces developed by the DCTE
stress that we can assume it is negligible: The balls will shear as dictated by
your initial equation. But for the QFN and LCC condition, my intuition causes
me to wonder if the elastic modulus of the solder is also important,
particularly since you pointed out in the paper I referenced earlier that the
intermetallics due to very thin solder joints are much stronger than the bulk
solder.
So should I interpret your response as the strength of the intermetallics is
still not significant when analyzing QFN/LCC mountings?
Wayne
________________________________
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Friday, May 21, 2010 7:35 AM
To: [log in to unmask]; Thayer, Wayne - IIW
Subject: Re: [TN] Solder joint height
DNP stands for distance from neutral point.
Werner
-----Original Message-----
From: Thayer, Wayne - IIW <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, May 21, 2010 7:12 am
Subject: Re: [TN] Solder joint height
Thanks for your reply, Werner.
DNP? I'm trying to figure that one out. Something to do with elastic modulii?
Or is it number of cycles?
I got the 1.5mil number from the Amkor paper, BOARD LEVEL ASSEMBLY AND
RELIABILITY CONSIDERATIONS
FOR QFN TYPE PACKAGES. Someone pointed that paper out on this forum a few
months ago. That's why I decided to try the pin probe technique.
Wayne
________________________________
From: Werner Engelmaier [mailto:[log in to unmask]<mailto:[log in to unmask]>]
Sent: Thursday, May 20, 2010 6:30 PM
To: [log in to unmask]<mailto:[log in to unmask]>; Thayer, Wayne - IIW
Subject: Re: [TN] Solder joint height
Hy Wayne,
Well, those QFNs and LCCs are deja vu all over again. With LLCCCs we had an
obvious huge CTE-mismatch between ceramic and FR-4, with QFNs it is not so
obvious since the CTE-mismatch comes from the die--so die size and die attach
are a big part of the equation.
So it depends on the stresses developed by the thermal expansion mismatch from
the product of DNPxDCTExDT whether the thin SJs will survive long enough for
reliability.
Werner
-----Original Message-----
From: Thayer, Wayne - IIW <[log in to unmask]<mailto:[log in to unmask]>>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Thu, May 20, 2010 4:17 pm
Subject: [TN] Solder joint height
Should there be a spec for minimum solder joint height for leadless components?
An old paper from Werner (mid 90's, I believe) popped up on the Internet when I
queried this, and Werner has a statement in it that joints less than about 50
microns thick are basically all intermetallics, but that is not necessarily a
bad thing since the intermetallics are stronger than the solder (but more
brittle).
I figured since we often try to get additional height for BGA joints to improve
reliability, we should be concerned about QFNs and LCCs.
I grabbed a 1.5mil (37 micron) tungsten rod out of our cleanroom (used for
clearing clogged wirebond capillaries)and started seeing if I could get that
under my leadless components. Couldn't get it under any of them, likewise with
the QFNs.
Checked Technet archives and didn't see an obvious thread, but maybe I didn't
have the correct search parameters--if you just enter "height" you get a couple
of thousand hits.
Wayne Thayer
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