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The bowing analysis used ANSYS of the test area would exceed a target was also extended between each site area
simulation tools to model the stresses limit of 50μm. To reduce bowing, the and with these changes, the bowing within
created by the 8 x 600 probes in the probe attachment points for the probe head were the DUT area was reduced to less than the
head design. The design uses a one- moved closer to the DUT area (Figure 50μm threshold.
piece ceramic body to house the probes, 9). This creates additional challenges Initial feasibility studies for the denser
surrounded by a stainless-steel frame, but for the PCB routing, so it is important to 8-site layout confirm it is possible to
even with this, the initial analysis indicated define the two designs in parallel to avoid include all the required RF functionality
that maximum deformation in the center surprises later! The stainless-steel frame and manage the increased bowing stresses
on the probe head. Follow-on work can
include a complete routing and fabrication
of the new design, but the possibility
to create a 16-site pattern is also being
explored. The final design would need
to accommodate all the signal routing
requirements and respect the layout
restrictions created by the probe head
attachment and alignment features. The
RF cabling used is being reviewed, to see
if shorter lengths will be usable in the new
layout to help minimize insertion loss
contribution.

Summary
A complex probe card design has been
deployed at multiple customer locations and
successfully used for validation and test of
new mmWave transducers. Because nothing
stands still in our technology-driven world,
the design has evolved in several areas to
meet new customer requirements, improve
Figure 8: New PCB layout. usability and reduce COT.
Mu lt iple de sig n cha nge s we re
implemented to match 35Ω impedance
of the mmWave MB signals and improve
alignment of the probe head and probe card
PCB. Options to increase the density of the
multi-site test pattern have been validated for
feasibility and development has been started.

Acknowledgements
The author thanks the project teams
across Cohu and Synergie-CAD for their
extensive contributions to this project.
This article is based on a presentation
originally given at SWTest 2022.
Reference
1. J. Mroczkowski, “55GHz octal-
site wafer test probe card for 5G
mmWave devices,” SWTest, 2021.
Figure 9: Bowing study results.

Biography
Peter Cockburn is a Senior Product Marketing Manager at Cohu, Inc., Verwood, UK. He received a BSc
from the U. of Southampton in UK. He has over 30 years of experience in the ATE industry, including software
development, product marketing and program management. Email: peter.cockburn@cohu.com

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