Sounds like insufficient solder to me.
The solder volume at a lead to land interface is not merely a function of
solder paste height. I'm sure I'm not telling you something you didn't
already know.
You can increase the heel fillet height by printing larger area on the pad.
I have even seen stencil designs that printed off the pad under the part for
this purpose.
You better test the paste for this application first. Many pastes will leave
little fines under the part if you do this to it. But, some coalesce very
well leaving a target class three solder connection.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Herminia Guevarra
Sent: Thursday, February 14, 2008 10:24 AM
To: [log in to unmask]
Subject: Re: [TN] Minimum Heel Fillet Height
Thanks everyone for all great ideas...the problem found is mainly IC
coplanarity.
Solder did not wick up on 1 to 2 of 66 TSOP lead bends but when cross
sectioned the IC coplanarity measured was within IC MFR's spec. The
solder paste height was measured within spec too prior reflow.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Thursday, February 14, 2008 8:30 AM
To: [log in to unmask]
Subject: Re: [TN] Minimum Heel Fillet Height
Essentially, it says the solder fillet needs to be 50% of the thickness
of
the heel bend or lead thickness (depending on the package). The "G"
thickness is there to prevent someone from saying, "It would have been
50%
of the lead thickness, if the lead were coplanar".
None of these are intended to be measured. They are visual criteria. In
my
opinion, if companies out there using measuring tools trying prove
acceptance are miss using the standard.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Herminia Guevarra
Sent: Wednesday, February 13, 2008 6:01 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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