That Fourthly point is so true Werner .
Let me just have a little yarn on the fracture itself:
while going through validations i found a relative difference in pull
application angles;
especially with faulty plating ;
that, there was a variation between straight up fracture and angled peel :
that is, relevant vectors of vibrations/G's do differ to "standard" straight
up fracture model scenarios.
Angled peel with faulty wetability gave up very easily ;
and did display difference in separated surface character ("onion" effect),
to pull model .
From than on whenever analyzing ,
peel rather than pull has been the interfacial method for me, say 90d
instead 180d.
Found it quicker, easier, wider variances/more defined detection window .
That is in general , not only TSOP's .
Right or wrong , no idea what you'd think about it .
paul klasek
resmed
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Thursday, 2 March 2000 15:29
To: [log in to unmask]
Subject: Re: [TN] Lead Finishes for Alloy 42 TSOPS
Hi Edwin,
You sure have a mish-mash of opinions, half-truths, and facts. It is not the
lead finishes that are doing it to you. Depending on the details, it is the
solderability--or lack thereof--of the Alloy 42, together with posiible a
large thermal expansion mismatch as well as a lead geometry which is not
really compliant (and thus does not do anything for you).
First, not all Alloy 42 is the same; while all contain about 42% Ni, the
rest
is Fe; Ni is solderable--sort of, it needs much more thermal energy to go
into solution with Sn than does Cu--and Fe is not solderable at all. Manual
soldering delivers higher amounts of heat and therefore give typically
better
results than standard reflow. However, there is some Alloy 42, presumably
coming from a south Korean mill, that is not solderable not matter what you
do; HP-Boise ran into this some years ago and published their negative
experience to their credit.
Second, as you recognize this saying "My experience shows has been that
tin/lead plating has to be reflowed in order to be a protective film. This
type of plating is very porous and the lead oxidizes easily." This is
correct.
Thirdly, Alloy 42 lead-frame TSOPs have a large CTE-mismatch with FR-4 PWBs;
large size die makes things worse yet. Depending of what your product sees
in
terms of burn-in, testing, and severity of use environments, you may or may
not see SJ failures in time frames that matter.
Fourthly, solder joint pull forces are not a good indicator of anything. A
much better understanding of what goes on can be gotten from looking at the
fracture surfaces of this pull test--whether the fracture is interfacial or
goes through the solder tells volumes about the quality of the SJs achieved.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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