Page 27 - Chip Scale Review_Jul Aug_2022-digital

MB signals included some tight signal- a 50Ω test environment has now been alignment and connectivity are to be
ground spacing that would not allow for implemented on a cViper probe stack- achieved across all sites. Customers
a wider signal probe. Instead, a wider up and PCB layout and measured after expect to be able to swap out probe
ground probe was designed that could assembly to verify that the targeted heads locally, without requiring a time-
be used in selected ground locations to performance is being achieved. consuming realignment process.
reduce the overall impedance of the MB The initial probe head and PCB
cross section. Probe head to probe card alignment design have evolved in several ways
A new ground probe was designed improvements to improve the alignment mechanism.
a n d b u i lt t o m a t c h s e ve r a l ke y When fitting probe heads to probe Monte Carlo analysis techniques were
parameters of the original signal probe, cards for such fine-pitch applications, used to identify the features of the
including test height, spring force at test there is no margin for error if good design that had the greatest impact on
height and crown tip profile where the
probe touches the DUT ball (Figure 4).
The new probe is considerably wider
to reduce overall impedance in the
new cross section, and when tested for
contact resistance (Cres) performance
over time, it showed stable results out to
1M cycles.

Figure 4: a) A standard cViper015, and b) the new
GND probe.

The new probe head cross section
in the simulation model showed that
return loss is now in the expected
range of better than -10dB. As the
probe cross-section impedance was not
a perfect match to 35Ω, the PCB trace
geometry was adjusted to get a closer
overall match.
As mentioned earlier, the direct
RoHS
connection RF signals are routed to P
test equipment via a coaxial connector
and RF cable, which are all matched
for 50Ω. This requires an impedance
transformer to be implemented in the
PCB traces that connect from the DUT
launch area to the RF connector. The
matching of a 35Ω DUT environment to

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