View - Micro Systems Engineering

Heatsink Via stack Type Failure [%]
w/o
w/o
2 layer
4 layer
Stripline (Fig. 3a) 0
Coplanar line (Fig. 3b) 0
Constant via diameter: 250 µm (Fig. 2b) 80
Altern. via diameter: 250/130µm (Fig. 2c) 20
Constant via diameter: 250 µm (Fig. 2e) 60
Altern. via diameter: 250/130µm (Fig. 2f) 0
with all types all types (Fig. 2a/d) 0
X-ray analysis revealed no major problems
with heat sink soldering. Heat sinks soldered with
AuSn had small voids (less than 10%) whereas the
SnAgCu solder was almost void free (Fig. 6 and 7).
Fig. 6: X-ray picture of a heat sink soldered with
AuSn (small voids visible)
Fig. 7: X-ray picture of a heat sink soldered with
SnAgCu
Table 1: He fine leak test results
Package RF Performance
The two line configurations (Fig. 3) needed
to be optimized for best wave impedance control [4].
Critical areas are the transitions underneath the wall
to enter the inner cavity. The dielectric cover causes
an impedance step towards lower values. Therefore, a
line width reduction is mandatory for compensation.
Vias, surrounding the lines for shielding purposes and
ground connection, play an important role for the
wide band behaviour of the transition. Their location,
size and pitch determine the maximum possible
frequency with low return loss. Fig. 8 and 9 depict the
structure of the transition for both line types.
CST Microwave Studio was used to simulate
and optimize the transitions.
Fig. 8: Structural model of the stripline
configuration
The outer connection was designed as a
grounded coplanar line allowing direct contacting
with microwave probes (250 µm pitch). The test
packages were measured with a network analyser and
Z-Probes from SÜSS MicroTech up to 50 GHz
between the two outer connections (4 line transitions).
Measurement results are shown in Fig. 10.