20-33GHz High Gain Buffer Amplifier

CHA2098RBF

Ref. : DSCHA2098RBF2057 -26-Feb.-02

5/6

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

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 above. 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 plenty plated through vias

with a diameter of less than 200µm [8 mils] and a spacing of less than 400µm [16

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.