5-30GHz Attenuator
ATT_PO12748QAG
Application note
The design of the motherboard has a strong impact on the over all performance since the
transition from the motherboard to the package is comparably large. In case of the SMD type
packages of United Monolithic Semiconductors the motherboard should be designed
according to the information given in the following to achieve good performance. Other
configurations are also possible but can lead to different results. If you need advise please
contact United Monolithic Semiconductors for further information.
SMD type packages of UMS should allow design and fabrication of micro- and mm-wave
modules at low cost. Therefore, a suitable motherboard environment has been chosen. All
tests and verifications have been performed on Rogers RO4003. This material exhibits a
permittivity of 3.38 and has been used with a thickness of 200µm [8 mils] and a 1/2oz or less
copper cladding. The corresponding 50 Ohm transmission line has a strip width of about
460µm [approx. 18 mils].
The contact areas on the motherboard for the package connections should be designed
according to the footprint given below. The proper via structure under the ground pad is very
important in order to achieve a good RF and lifetime performance. All tests have been done
by using a grid of plated through vias with a diameter of less than 300µm [12 mils] and a
spacing of less than 700µm [28 mils] from the centres of two adjacent vias. The via grid
should cover the whole space under the ground pad and the vias closest to the RF ports
should be located near the edge of the pad to allow a good RF ground connection. Since the
vias are important for heat transfer, a proper via filling should be guaranteed during the
mounting procedure to get a low thermal resistance between package and heat sink. For
power devices the use of heat slugs in the motherboard instead of a via grid is
recommended.
For the mounting process the SMD type package can be handled as a standard surface
mount component. The use of either solder or conductive epoxy is possible. The solder
thickness after reflow should be typical 50µm [2 mils] and the lateral alignment between the
package and the motherboard should be within 50µm [2 mils]. Caution should be taken to
obtain a good and reliable contact over the whole pad areas. Voids or other improper
connections, in particular, between the ground pads of motherboard and package will lead to
a deterioration of the RF performance and the heat dissipation. The latter effect can reduce
drastically reliability and lifetime of the product.
Ref. : DSATT-PO12748QAG5276 - 3 Oct 05
8/14
Specifications subject to change without notice
Route Départementale 128 , B.P.46 - 91401 ORSAY Cedex - FRANCE
Tel.: +33 (0)1 69 33 03 08 - Fax : +33 (0)1 69 33 03 09
UMS [ UNITED MONOLITHIC SEMICONDUCTORS ]
