Shear testing of BGA pads—pads are not as good as they were?
Bob Willis
Shear testing of BGA pads—
pads are not as good as they
were?
Shear force measurement is a common and The joints in Figure 2 show joint frac- on shear testing go to www.dage-group.com.
quick way of destructively assessing solder ture, after which the test load was removed During a lead-free assembly project,
joints and complements pull strength mea- from the sample. Applying defined loads members of the SMART Group Commit-
surements. The use of shear measurement and monitoring the change in resistance tee set-up a production feature assembling
has increased in popularity as a method of is a useful technique with advanced shear conventional and flexible circuit boards
testing joints after some form of environ- measurement. In recent years, the introduc- with different board materials, surface
mental testing because it is easy and quick tion of high speed testing of ball grid array finishes and alloys. During the event, one
to perform. This is particularly true during (BGA) balls has been very beneficial to series of experiments was conducted by
the introduction of lead-free manufacture, examine the impact of nickel gold surface Peter Starkey, Starkey Technical Services
and it is one of the test methods of choice finishes on ball bond reliability and the and Naim Kapadia, JJS Electronics Ltd.
at National Physical Laboratory (NPL) on poor load performance of copper pads on Three dummy BGA panels produced
many of their lead-free studies. lead-free laminates. For further information from different laminates were tested during
a practical demonstration of high-speed
bond testing using a DAGE 4000HS
system. Two of the laminates were specifi-
cally designed for lead-free with a higher
TG and improved time to delamination
plus standard FR4 panel with a TG of
approximately 140˚C. The FR4 laminate
panels were finished with silver, and the
other two with nickel/gold coating. The
dummy BGA test panels were produced by
dispensing solder paste from Multicore on
a Camalot Xyflexpro system. The boards
were reflowed on a BTU Pyramax convec-
Figure 1. Examples of tin/silver/copper joints on OSP after shear testing from the Lead-Free Experience.
tion oven at 60 cm/min with a peak reflow
temperature of 230˚C to form the ball ter-
!
minations. Th
!
e test panels were reflowed
once, twice and three times to simulate
assembly exposure.
The main observation that can be
made is that the incidence of actual bond
failure ball-to-pad is quite low. The solder-
able finish appears to have little effect on
the failure mechanism; depending on the
batch origin of the test pieces, failure oc-
curs either within the solder or by detach-
ment of pad from laminate.
The FR4 laminate appears to have the
Figure 2. Microsections of chip terminations above after controlled load shear testing at the Lead-Free Experience. highest bond strength of pad to laminate,
! !
4 – Global SMT & Packaging – May 2009 www.globalsmt.net
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