How to Bond PCB and Metal for high power RF boards

[Vineeth_S]

I want to bond a PCB (2 layer & 0.6mm thickness) to a Metal (copper & 4mm thickness). Could anyone help me with the process of bonding it? And there is a flange of power amplifier that has to get soldered with the metal and leads of PA has to be soldered on PCB.

I came to know few options, but still unaware of implementation part.

Like Thermally & Electrically conductive Adhesive (TECA) like coolspan from Rogers.

Is there any other way to bond the metal and PCB? Like soldering (sweat soldering/sandwich soldering)?

Thermal dissipation is ~400W which will be fixed to my mechanical enclosure.

 

[FvM]

In a usual RF power stage, PCB and power transistor are screwed to a flat heat sink surface, the latter through a PCB cutout. PCB thermal conductivity is insufficient to dissipate much power, in so far I don't see a purpose of soldering the PCB to a heat sink.

A sketch of the intended PA geometry might clarify things.

 

[Vineeth_S]

This is the outline of the component, LDMOS Power Amplifier Transistor

Below flange has to be soldered on the heatsink for better performance.

 

[FvM]

Did you review AN181008? They also solder the PCBs to the heat sink.

 

[Vineeth_S]

Yes, I do checked. The PCB fabrication (Metal +PCB) process was missing.
I have found the process to bonding the PCB and metal. It is called sweat soldering process. and solder the PA using solder preforms.
This is most simple and cost effective way i think.

because other fabrication process like bonding using TECA and Metal clad PCB fabrication are expensive than the sweat soldering process.
Adhesive - curing process make the use of adhesive more complex.

 

[ge]

In the 1980s, we soldered PCB to copper plate (and plated aluminum) to prevent gaps under RF traces. Techs would heat up copper plate on a hot plate, add solder ( ? solder paste ? ) and then put board on top.
Push down on the board with a wooden Q-tip swab and see if the RF response changes. It takes a crap load of screws to keep the board in contact with the carrier. At low frequencies, the gaps seem to be not a big deal, ( ? < 100MHz ? ). Maybe it depends on flatness of copper plate? At a few GHz, it can be an issue. Sweat soldering (labor wise) is also expensive and a pain in the butt.

 

[Vineeth_S]

In the 1980s, we soldered PCB to copper plate (and plated aluminum) to prevent gaps under RF traces. Techs would heat up copper plate on a hot plate, add solder ( ? solder paste ? ) and then put board on top.
Push down on the board with a wooden Q-tip swab and see if the RF response changes. It takes a crap load of screws to keep the board in contact with the carrier. At low frequencies, the gaps seem to be not a big deal, ( ? < 100MHz ? ). Maybe it depends on flatness of copper plate? At a few GHz, it can be an issue. Sweat soldering (labor wise) is also expensive and a pain in the butt.

Yes, Do we have any other option to stick the PCB and copper clad?

 

[ge]

If the copper clad is thick/stiff and very flat, use lots of screws so that board and transistor makes constant contact. At MHz frequencies, this is not as big of a deal. If there is danger of board flex, solder. Keep in mind things will move around as they get hot. Substrate and copper likely have different CTE.

 

[biff44]

400 watts?
how about soldering it directly to a big honkin copper plate.