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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… and why?
Should there be thermo-grease between the metal plate and the case?
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.
Metal plate: 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 problems.
Should you need more information, please let me know!
Post subject: Posted: Tue Feb 07, 2006 5:24 am
Thanks for your response. You have misunderstood me though.
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.
The problem 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.