MICROWAVE PACKAGES

Abstract: A microwave package comprising a base plate, a side wall member and a lid which are welded together after assembly of the microwave device and substrate onto the base on which it can be tested prior to welding the side wall member and lid and base to form a hermetically sealed package.

Description: MICROWAVE PACKAGES The present invention relates to microwave packages and more particularly to packages which are hermetically sealed. Known microwave packages comprise a box into which a microwave circuit is placed, a lid being screwed into position to seal the package. These packages suffer from a number of disadvantages. In particular they are not suitable for a wide frequency range of operation which is now possible with gallium arsenide integrated circuits. The present invention seeks to provide a microwave package which can operate with good microwave performance over a wide range of frequencies and also which has good thermal properties allowing operation at higher power ratings than previously obtainable. According to the present invention there is provided a microwave package including an active microwave device connected to substrate, the substrate being positioned on a base plate of the package in good thermal contact therewith, a wall member and a lid member for the package being welded to the base plate to form a hermetically sealed package in which the substrate and base plate have similar coefficients of - la - expansion and in which contact is made to the active microwave device via glass to metal seals in the wall me ber. Preferably the substrate has electrically conductive tracks printed on it at one end of which the microwave device is connected and at the other end of which contact pads are provided for connection to leads passing through the glass to metal seals in which the complete package is constructed such that the height of the lead above the base member is substantially equivalent to the height of the contact pad above the base member such that the lead and contact member can be connected together without any substantial distance - 2 - between them. The importance of the above feature is that a large distance between the two would necessitate a joining wire or very large solder blob which would introduce microphony effects into the circuit. According to the present invention there is also provided a microwave package including a base plate of tungsten copper a wall member of nickel/iron or Kovar and a lid of nickel/iron or Kovar, in which the microwave circuit is mounted on a beryllium oxide substrate which is positioned on the base in good thermal contact therewith and in which the leads to the microwave device pass through the walls of the package via glass to metal seals and the lid, wall member and base are welded together to form a hermetically sealed package. The use of tungsten copper as a base gives extremely good heat conduction away from the substrate via preferably a gold intermediate layer and therefore allows the microwave device or devices mounted on the substrate to operate at higher power levels than previously. The use of tungsten copper as a base also allows the package to be welded by a laser since the increase in temperature within the package is limited to 120°C OMPI - 3 - which does not damage the microwave devices. In addition the welding of both wall member to base and lid to wall member and the use of glass to metal seals provides an extremely good hermetically sealed microwave package which is capable of operating over a large frequency range. Embodiments of the present invention will now be described with reference to the accompanying drawings in which:- Figure 1 shows a microwave package according to the present invention in cross sectional elevation and Figure 2 shows in greater detail a portion of the package of figure 1 illustrating the glass to metal seal and contact with the substrate by the input lead. Referring now to figure 1, there is shown a microwave package including an active microwave device 10 mounted on a substrate 12 preferably made of beryllium oxide which has a relatively high thermal conductivity of 2.5 W/cm °K. Contact is made to the device 10 via leads 14 which pass through glass to metal seals 16 in a wall member 18. The substrate 12 is fixed to a base plate 20 preferably via an intermediate conducting layer 22 which is preferably of gold. The wall member 18 fits into a recess 24 in the base plate 20 and is welded all round to the base member 24 using a laser which form a weld 26. _ 4 _ A lid 28 is placed on top of the wall member 18 and welded all round to the wall member 18 by a laser as shown at 30. The base, wall member and lid are preferably rectangular. With reference to figure 2 there is shown in greater detail the connection to the device 10 from the through lead 14. The lead 14 passes through the glass seal 16 at a height above the base member such that it is only slightly above a conductive track 32 on the substrate and such that it can be soldered directly to the track 32 as shown at 34. The substrate is gold backed 22 and the base 20 is plated with nickel and gold all over or on the inner package surface 36 as shown. The base of the microwave package is made from tungsten copper which has a coefficient of thermal ' expansion of 10.1 pp K""-- which is similar to the thermal expansion of beryllium oxide (approx 6 ppm K-^). The thermal conductivity of tungsten copper is 240 m"! K"^ which compares favourably with copper (400) and is substantially better than Kovar (17). The presence of the gold backing for the highly thermally conductive substrate and the nickel and gold plating, 36, of the base means that the microwave device 10 is provided with a good heat sink and can therefore be run at a higher power than with previous - 5 - designs of microwave package. In assembly the microwave device 10 is attached to the substrate 12 by, for example, soldering and then the gold backed alumina substrate 12 is soldered to the base 20 by means of an entectic. The wall member 18 made of Kovar or nickel iron is either fitted with the glass to metal seals or is provided with appropriate holes for a seal to be added later. The complete bonding of the microwave device and testing may be carried out before the attachment of the walls if desired. The walls have the same thickness as the glass seals (1.15 mm approx) and are welded to the base by the use of a laser, as is the lid 28 of the package. In a practical embodiment the laser used to weld both base and lid was a pulsed solid state Nd/YAG laser of high average power with a computer machine control. The computer was programmed to weld round all four sides in an argon atmosphere, the laser beams being focussed on the metal joints. In an alternative embodiment using an alumina ceramic substrate (thermal expansion approx 8 ppm K~ ) a hole is cut through the ceramic substrate to accommodate the microwave device which is soldered directly on to the base to give the required high thermal conductivity. This is necessary because the - 6 - alumina ceramic substrate does not have a sufficiently high thermal conductivity to enable the microwave device to be run at high power levels if it were positioned on top of the alumina ceramic substrate. In practise the alumina substrate which carries the microwave stripline has a hole cut out of it preferably by laser. The substrate is soldered to the base and the device to the base in the hole made in the substrate. The principal features of the present invention are that the use of tungsten copper base provides a good heat sink for the microwave device. The welding of the walls to the base using a laser technique does not cause an increase in temperature of more than 120°C thus preventing damage to the components. The advantage of laser welding the base to the walls is that the chip and stripline can be assembled, bonded and tested before the walls are put in position. Normally the base must be soldered to the walls and all the above processes carried out within the confinement of the walls. The good thermal properties of the package permit the use of gallium microwave devices operating over a wide frequency range. Packages using integral glass-to-metal 50 ohm hermetic - 7 - seals have good microwave performance up to at least 10 GHz, and in some forms, up to 18 GHz. This performance is also broadband, i.e. there is continuous operation from DC upwards. Many ceramic based packages of the same physical dimensions as the present invention suffer from narrow band performance due to resonances between the metal or metallised lid and the connecting lines through the package walls. The all-metal package also gives a superior performance with respect to "below waveguide cut off" performance whereby the transmission of unwanted signals can be suppressed by keeping the width of the package to a maximum dimension. This is of particular importance where the gallium arsenide integrated circuits, to be contained in the package, have high gain bandwidth product (e.g. 50 GHz or greater). The metal base affords a satisfactory RF ground for the IC chips. The reproducibility of RF performance depends on the exploitation of microstrip on lumped element components whereby soldering or epoxying the chips to the ground plane is used to produce repeatable characteristics or parasitic elements respectively.

Claims: - 8 - CLAIMS 1. A microwave package including an active microwave device connected to a substrate, the substrate being positioned on a base plate of the package in good 5 thermal contact therewith, a wall member and a lid member for the package being welded to the base plate to form a hermetically sealed package in which the substrate and base plate have similar coefficients of expansion and in which contact is made to the active 10 microwave device via glass to metal seals in the wall member. 2. A microwave package as claimed in claim 1 in which the substrate has electrically conductive tracks printed on it at one end of which the microwave device 15 is connected and at the other end of which contact pads are provided for connection to leads passing through the glass to metal seals in which the complete package is constructed such that the height of the lead above the base member is substantially equivalent to the 20 height of the contact pad above the base member such that the lead and contact member can be connected together without any substantial distance between -? them. 3. A microwave package as claimed in claim 1 or 25 claim 2 in which the base plate is made of tungsten copper, the wall member of nickel iron or kovar and the lid of nickel iron or kovar, in which the microwave OMPI - 9 - circuit is mounted on a beryllium oxide substrate which is positioned on the base in good thermal contact therewith. 4. A microwave package as claimed in claim 1 or claim 2 in which the base plate is made of tungsten copper, the wall member of nickel iron or kovar and the lid of nickel iron or kovar and in which the microwave circuit is mounted directly onto the base via a hole formed in an alumina substrate. 5. A microwave package as claimed in any one of claims 1 to 3 in which the base plate is formed with a recess around its periphery into which recess the wall member is located prior to welding. 6. A microwave package substantially as described with reference to the accompanying drawings.

Inventors: SPARROW JOHN ANDREW, SPARROW, JOHN, ANDREW
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1983
Main classification: H01L23
IPC codes: H01L23