Guy,
I wouldn't be too quick to disagree with Richard. If you make mechanical measurements I think you will find that a gull wing lead with a heel fillet is stronger than one without. Remember if you lose 0.020" length of bottom area you will make up most of the lost area by the area you gain between the solder in the heel fillet and the lead.
Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Thursday, August 25, 2011 3:04 PM
To: [log in to unmask]
Subject: Re: [TN] FW: [TN] Gull Wing Lead - no heel fillet
Rarely do I disagree with Richard.
But, in this case, I do. The strength is in the bond between the lead and
the land. The larger the bond area the stronger. Sacrificing the bond area
to provide a heel fillet would, I predict, be counter productive, when
considering mechanical shock. I do agree with Dewey. Compliance is your
friend when it comes to CTE mismatches. So, if we tested with thermal
cycling, and if there was a mismatch, a thick joint might outlast a large
overlap.
So, without a reply, Doug is right . . . it depends.
Guy
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, August 25, 2011 12:28 PM
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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