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technological procedure to assemble a power amplifier - RF Cafe
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Below are the old forum threads, including responses to the original posts.
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Post subject: technological procedure to assemble a power
amplifier Posted: Mon Feb 06, 2006 5:12 am
What would be the technological
procedure to assemble a power amplifier when the RF/microwave transistors
have to be soldered to a metal plate for better thermal dissipation?
The whole assembly is as follows: there is a PCB (microwave board
– teflon based or of the ROGER 4003 type); there will be components
which are pick-and-placed on this board; the board has to be attached
(screwed or soldered) to a metal plate and the RF/microwave power transistors
have to be soldered to the same metal plate and their input-output pins
also have to be soldered to the PCB. Then that whole assembly has to
be attached to the case of the amplifier.
So again: what should
be the step by step technological procedure to do this assembly?
Also: what should be the metal of the metal plate? Copper, aluminium…
Should there be thermo-grease between the metal plate and
Post subject: Posted:
Mon Feb 06, 2006 1:41 pm
Joined: Mon Jun
27, 2005 2:02 pm
Your questions are good and important since wrong assembly might
cause problems of over-heat and reduce the efficiency of your PA.
Concerning the PCB: It is best to solder the PCB to the metal plate
to form a complete ground contact and best heat evacuation. The equivalent
to that would be many screws adjacent to each other. If your PCB is
large then it would be lot of screws - this can result in high cost
and assembly burden if you think of mass production.
I always used an Aluminium, because it is lighter than copper and its
density is lower; that means it can absorb and evacuate heat more quickly.
If you use Aluminium you should coat it with Anodize to prevent corosion.
Since your power transistors are thicker than the PCB, there
should be "gums" in the metal plate compatible to their repsective packages.
The pins should be mounted in the same height of the PCB - there should
be minimal tolearnce when designing these "gums". Use a minimal quantity
of silicon grease when you assemble the transistors and screw them with
the right torque to the PCB - the recommended torque should be noted
in the data sheet. Then you can solder the pins (flanges) of the transistors.
For the first prototype unit, I suggest that you bend the flanges in
90 degress and solder the edges to the PCB. If you do it correctly,
there should be no impact on the electrical performance (Mismatching
etc), in this way you could later remove the transistor if there are
Should you need more information, please let me know!
Post subject: Posted: Tue Feb 07, 2006
Thanks for your response. You have misunderstood
I would like to use transistors with packages that
require they to be soldered to the metal plate and in this way to improve
the thermal dissipation. These transistors typically don't have flanges
with screw wholes. Most of the Freescale transistors have 2 types of
packages – one for screwing down and one for soldering.
for me is the technology sequence. Do you need first to pick-and-place
the other components on the PCB and then run them through whatever soldering
process one uses and then assemble the PCB and the transistor with appropriate
soldering material (possibly with lower soldering temperature than the
one used for the SMD components on the PCB) and run this assembly now
through the soldering equipment? Or, do you pick-and –place the components
on the PCB and before soldering them, the PCB is assembled (attached)
with the transistor and its soldering material (paste or pre-form or
whatever) to the metal plate and then the whole assembly is run through
the soldering process?
Post subject: Posted: Tue Feb 07, 2006 10:09 am
IR forgot to
mention one small, but important, detail.. Any Aluminum plate that is
going to be soldered needs to be nickel plated, otherwise the solder
will not adhere.
Post subject: Posted:
Tue Feb 07, 2006 4:59 pm
Joined: Mon Jun
27, 2005 2:02 pm
You will first
assemble your PCB with the transistors to the metal plate and run the
PCB with transistors through the soldering process. The reason for this
is that the soldering of the transitors is done in a higher temperature
than of the other SMD components.
After that you will have to
do the pick-and-place of the SMD components and run it again through
soldering process with the usual temperature for SMD components (Which
should be lower of course than the temperature of the first process).
For information about the recommended soldering temperature for the
transistors cosult with Freescale.