Hi Rick, hi John,
I am indeed on line—are not all consultants working 24/7?
Whenever the shear/pull stength question comes up, my first reaction is
'Why?'
Here is my standard answer:
No standards on shear/pull tests exist—nor should they exist. And there are
good reasons none exist.
First, there cannot be such thing as a true 'shear/pull test;' while you are
of course stressing the solder joint, what you are doing to the solder joint
in most cases is primarily peeling it—the proper reference would be a 'peeling'
test.
Second, the solder joint strength, as determined by a lead peeling test, has
no bearing on the reliability of the solder joint, provided the peeling test
does not reveal inadequate wetting or poor metallization, e.g., 'Black Pad.’
Third, in a peeling test you always need to observe the whole peel-load
history for the whole peeling process; e.g., with a leaded solder joint the largest
load will be at the initial portion of peeling through the heel fillet, with
lower loads subsequently depending on whether or not (or to what extent) side
fillets are present.
Fourth, the fracture surfaces of the peeled solder joints give typically more
information than do the peel-load histories, because the ONLY really
important finding is whether or not adequate wetting has taken place, i.e. the
separation is mostly in the solder vs. being interfacial; or whether or not the
metallization is the 'weakest' link, i.e. the separation is between metallization
layers. Good quality solder joints frequently cause failure between the
soldering pad and the component/PCB resin matrix.
Fifth, for leaded solder joints, the reliability in actual use does to a
large extent depend on the heel fillet and the 'bottom flat' wetting; if the foot
length is <3W, than also on the presence of a toe fillet, because short feet
'rock' during thermal cycling. Side fillets are less important, unless you have
wetting problems.
Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com
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