Page 16 - Chip Scale Review_November December_2022-digital

The displacement around 90μm from
the initial location at 25ºC to the high
temperature of 105ºC is even greater
than the diameter of the size of the
micro-bump at approximately 70µm,
which easily causes contact instability.
I n con side r at ion of t he t he r m al
expansion effect, two separate double-
sided probe heads were designed so
they could be adjusted to accurate
positions by use of fiducial markers and
precision guide pins during assembly.
However, this design still requires
optical re-alignment to check the
precision of the final position at each
testing temperature point to ensure
contact stability and repeatability.
Probe head/thermal control. The
high-speed and fine-pitch probe head
integrated with a high-performance
thermal control system is the most
critical design module of the double-
Figure 4: Double-sided probing system. sided probing system. To reach the
requirement for 112Gbps for the
Alignment. The alignment module aligning the tip of the probe head with pulse amplitude modulation 4 signal
is a dual-charge-coupled device (CCD) the micro-bump on the DUT. (PAM4), every step in the design of the
module combined with a 3-axis X-Y-Θ We used a dummy device to transfer probe head must be carefully checked
heating shuttle design to ensure precise printing of the probe mark of the probe by simulation. Channel simulation
alig n ment when the temperat u re head tip and perform self-calibration results, including that for package
varies. High-resolution dual-CCD by computing the relative distance to substrate, fine-pitch probe head, and
cameras are used to take a picture of ensure the proper alignment of the the loopback interposer, show the
the micro-bump on the top side of the probe tips to the device micro-bumps. behavior of insertion, return loss, and
package to align every package to the However, thermal expansion at high the impedance curve (Figure 6).
accurate position before picking up the temperatures will cause the alignment T he t a rgeted i mped a nce value
device under test (DUT). Moreover, to be further from the original position is 93Ω, so closely achieving the
the heating shuttle plate can reduce the by approximately 90µm from what values and reducing the impedance
soaking time before reaching thermal it was at the initial temperature, as mismatch must be considered for each
equilibrium at the required testing shown in Figure 5. A multiple stacked component. Based on the simulation
temperature; it also plays an important structure design that uses different results, the high-speed requirement
role in auto calibration before each materials causes nonlinear thermal for 112Gbps PAM4 after integration
pick and place action on the package. expansion. It has also been proven with whole channel simulation was
Because the DUT and probe head that the t race of a temperat u re - achieved. Moreover, to prove the
are not on the same side, one issue is dependent probe mark is repeatable.

Figure 5: Optical alignment changes with thermal expansion.

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14 Chip Scale Review November • December • 2022 [ChipScaleReview.com]