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November 2003

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Subject:
From:
Guenter Grossmann <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 18 Nov 2003 10:54:50 +0100
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Danny

Solder strength is not defined quantitatively.  That's why you don't
find standards. Usually the ENIG-solder interface is the week point in a
solder joint. But, as Werner already pointed out, this bond is good (the
chemical reaction of the solder with the substrate took place) or it
isn't. All you can do is an analysis of this interface. Werner pointed
one out. A crude possibility is ripping of the components with a screw
driver ---> But don't use any comments of the force necessary as a
criteria. After removal of the components you need to look close (SEM,
500x magnification) at the interface. If the bond was ok you will see
signs of plastic deformation and, in BGA joints the path of the rupture
is along the interface of the component. If the solder connection is
week the rupture is on the PCB side and the surface will be flat with
the appearance of dried mud. As I said, this is a 0-order approach to
find the worst cases since the rupturing behaviour depends on the stress
applied. You might also find plastic deformation and brittle fracture on
one pad indication that only part of the pad has alloyed with the
solder. Well, that's  the point of a decision whether you can live with
that situation at the moment. It is in any case a sign that the ENIG
process runs in a critical  zone and you need to speak with your PCB
vendor who might advice you to define more sophisticated analysis or
help to optimize your soldering process.

Guenter

EMPA
Swiss Federal Laboratories  for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1 823 4054
mail:     [log in to unmask]

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