Without any doubt, the two best options are:
a) to respin the design/fix the footprint or
b) to get the part leads formed to fit the existing footprint.
Somehow they have calculated it will take millions of thermal cycles
before failure...
If this were true, then why would we ever need heel fillets. I am very
skeptical of the calculation and the model validity.
On one hand I have a Class 3 defect, and only a mathematical calculation
indicating it is reliable....
What is that saying....."In God we trust, all others bring test data!"
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, August 25, 2011 9:28 AM
To: [log in to unmask]
Subject: [TN] FW: [TN] Gull Wing Lead - no heel fillet
-----Original Message-----
From: Stadem, Richard D.
Sent: Thursday, August 25, 2011 10:01 AM
To: 'Amol Kane'
Subject: RE: [TN] Gull Wing Lead - no heel fillet
Phil explained that his pad does not provide enough length to form
either a heel or toe fillet, that is, the pad is the same exact length
of the component foot. His question was, is the solder connection along
the bottom surface enough, or will there be certain reliability issues
without a heel fillet.
So my feeling is, if the gull wing component leads can be "re-formed" to
slide the foot outwards, perhaps .020" or so, this would allow the heel
of the lead to rest inside the pad, and a heel fillet could be formed.
While the toe would now overhang the front of the pad, this is redundant
as the toe fillet provides very little to the overall finished solder
surface of the component lead (the plated inside bottom surface area of
the component lead is intended to be the solderable surface, not the
unplated oxidized exposed copper edges. That is why the heel fillet is
considered to be so important). While this would provide a .020" x "W"
smaller planar area for solder wetting, it would be more than offset by
allowing the solder volume up the inside of the heel area.
Then you would be able to get a heel fillet that is a minimum of either
1 lead thickness in height (class 3) or at least 1/2 the lead thickness
(Class 2).
-----Original Message-----
From: Amol Kane [mailto:[log in to unmask]]
Sent: Thursday, August 25, 2011 9:04 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: [TN] Gull Wing Lead - no heel fillet
Richard,
Can you elucidate "I would reform the leads outward to obtain a minimum
acceptable heel fillet (1/2 H)"?
Thanks,
Amol
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, August 25, 2011 8:54 AM
To: [log in to unmask]
Subject: Re: [TN] Gull Wing Lead - no heel fillet
I would reform the leads outward to obtain a minimum acceptable heel
fillet (1/2 H). If the toe fillet is not there anyway, or is redundant,
then you have improved the reliability to the equivalent "good" pad
dimension.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Wednesday, August 24, 2011 8:39 PM
To: [log in to unmask]
Subject: Re: [TN] Gull Wing Lead - no heel fillet
Thanks Bev. Not under pressure, just surprised anyone would suggest hi
rel w/o heel fillets!
----- Original Message -----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Wednesday, August 24, 2011 05:08 PM
To: 'TechNet E-Mail Forum' <[log in to unmask]>; Bavaro, Phillip @ MWG - TW
Subject: RE: [TN] Gull Wing Lead - no heel fillet
Phil,
First, I am a chemist not a mech eng.
Second, I never heard of the Steinburg equation before your e-mail.
Third, the companies I have worked for (well the 2 big, reputable ones)
built and build class 2 product.
That being said, I googled Steinburg equation and came across the
abstract
below. Sounds to me that I would not be wanting to hang my hat, let
alone
my product, on an equation, Steinburg or Guinan-Steinburg that is a tad
too
simplistic.
Sounds to me that what you really need is thermal cycling, but my guess
is
that you are under some time pressure for a decision. I do not envy
you.
My 2.0242 cents.
Bev
RIM
Shear modulus at all pressures: Generalized Guinan-Steinberg formula
Purchase
References and further reading may be available for this article. To
view
references and further reading you must purchase this article.
Leonid Burakovsky, a, and Dean L. Prestona,
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Available online 21 August 2006.
Abstract
The Guinan-Steinberg (GS) formula for the shear modulus at all pressures
is
widely used in material strength studies. As we demonstrate here, the GS
formula predicts a value for the shear modulus that is higher than its
actual value at low to moderate compressions, even if it has the correct
ultrahigh pressure limit. We show that the reason for this shortcoming
is
insufficient negative curvature in the GS shear modulus as a function of
pressure, and propose a generalized GS formula which corrects this
inaccuracy. Both the (standard) GS formula and new generalized formula
are
compared to the experimental data and ab initio calculations on the
shear
moduli of gold, aluminum, cobalt and beryllium. While the standard
formula
fails, the generalized one is in good agreement with data in each of the
four cases.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Wednesday, August 24, 2011 4:58 PM
To: [log in to unmask]
Subject: [TN] Gull Wing Lead - no heel fillet
Hypothetical situation:
Gull wing lead, land pattern error, zero heel (also zero toe) fillet.
Hardware would not meet Class 3 workmanship requirements.
An equation is being used to decide if the pad to gull wing lead lap
solder joint is adequate to survive the intended thermal cycling
environment.
This would have been a question for Werner but I am sure there are
others here who can shed some light on this for me.
I know that part of the justification for why we have set our IPC
standards where they are is partly based on years of testing.
How could a mathematical calculation such as "Steinberg equation"
justify a lack of Class 3 gull wing lead heel fillet?
Any help is appreciated.
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