(1)Heel fillet height would be a problem if you change the device from L-lead to Gull wing. The pad design and layout may be affected by the shape of the leads.
(2) Leads coplanarity got a lot to do with the sufficient filler height, therefore, the solder paste thickness and flux type should be taking into consideration.
(3) Assume all the PCB layout are perfectly balanced by Master Gurus, with "no potato chip"... for SOIC TTSOP....;-). Minimalist always require the best of the best...
jk (my 2 cents)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: February 14, 2008 2:36 PM
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] Minimum Heel Fillet Height
I agree with both of you.
We infer a good solder joint has been made by looking at the joint using IPC
visual acceptance criteria. An inference like this is only a little better
than an assumption.
That said, most shops will do okay applying the standard given the IPC
design guidelines for land patterns and stencils, with solderable parts in
good condition and good solderable boards, processed with appropriate time
temperature profiles.
They get into trouble when they attempt to apply the criteria as some kind
of hard and fast requirement, not knowing that they are merely, generally
applicable.
This is why, in my opinion, the standard says defects should be
dispositioned.
The subject of this thread is not ceramics and CTE, but the heel joint,
presumably of an SOIC, TTSOP, or some such component. A memory device,
without enough solder to over come a coplanarity problem like the once
described, one would have to question the capability of the process to
produce consistently reliable memory modules.
Wouldn't you agree?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Thursday, February 14, 2008 12:56 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] Minimum Heel Fillet Height
Guru Richard,
I don't quite understand your philosophy. We don't pay much attention to the
fillet heel HEIGHT, but rather the wetting quality. A 'high' fillet has
sometimes proven to be false, with nearly no true adhesion to the component
(in this case we had issues with ceramic caps). Furthermore, high fillets
are negative when soldering chip components on RF Teflon boards with metal
back, i.e. the CTE mismatch can cause component cracks. We have produced
hundreds of thousands boards with the requirement, that the fillets should
be as 'low' as possible, the reliability and joint strength based on good
adhesion between the component's projection on the board, rather than fat
fillets. If the physical conditions of the two parts, board pad and
component termination, are good, the capillary force will provide for a
suitable and nice fillet, very often only some 25 percent of the total
height. I have a whole book based on wetting mechanisms, and the authors
underline what I said above. There is no math for deciding fillet heel
height in the book. If all I say here is wrong, my soldering universe will
disappear under my feet, and I'll fall free in the cyber solder pot.
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Stadem, Richard D.
Skickat: den 14 februari 2008 14:26
Till: [log in to unmask]
Ämne: Re: [TN] Minimum Heel Fillet Height
Herminia,
Because the heel fillet is the only portion of the solder joint
(typically) that is made to the finished (flat) portion of the component
lead, it is the most critical. The toe and side fillets are formed to
exposed copper. While it is desirable to have good toe and heel fillets,
the heel is the most important.
With that in mind, a decision was made to require a heel fillet at least
half a lead thickness in height.
Having sufficient amounts of solder paste certainly improves one's
chances of obtaining a Class 3 fillet, but certainly does not gaurantee
it.
Just because you have proof that your solder thickness sufficient to
form a good solder fillet isn't good enough. Solder does not always wet
where it is supposed to, or this forum would never have existed.
JEDEC standards exist for coplanarity of component leads.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Herminia Guevarra
Sent: Wednesday, February 13, 2008 5:01 PM
To: [log in to unmask]
Subject: [TN] Minimum Heel Fillet Height
Hi, folks,
Can anybody please explain in details how the minimum heel fillet height
criteria for Class 2 was computed and specified?
F= solder thickness (G) + 50% lead thickness
Can solder thickness be solder paste height prior reflow?
What about if leads are not coplanar? What would be the spec for co
planarity? How can they be measured?
Need reply...
Thanks,
Herminia
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